Update copyright for 2014

[postgresql] / src / backend / access / transam / xlog.c

/*-------------------------------------------------------------------------
 *
 * xlog.c
 *              PostgreSQL transaction log manager
 *
 *
 * Portions Copyright (c) 1996-2014, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/backend/access/transam/xlog.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include <ctype.h>
#include <time.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <unistd.h>

#include "access/clog.h"
#include "access/multixact.h"
#include "access/subtrans.h"
#include "access/timeline.h"
#include "access/transam.h"
#include "access/tuptoaster.h"
#include "access/twophase.h"
#include "access/xact.h"
#include "access/xlog_internal.h"
#include "access/xlogreader.h"
#include "access/xlogutils.h"
#include "catalog/catversion.h"
#include "catalog/pg_control.h"
#include "catalog/pg_database.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "postmaster/bgwriter.h"
#include "postmaster/startup.h"
#include "replication/walreceiver.h"
#include "replication/walsender.h"
#include "storage/barrier.h"
#include "storage/bufmgr.h"
#include "storage/fd.h"
#include "storage/ipc.h"
#include "storage/latch.h"
#include "storage/pmsignal.h"
#include "storage/predicate.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "storage/reinit.h"
#include "storage/smgr.h"
#include "storage/spin.h"
#include "utils/builtins.h"
#include "utils/guc.h"
#include "utils/ps_status.h"
#include "utils/relmapper.h"
#include "utils/snapmgr.h"
#include "utils/timestamp.h"
#include "pg_trace.h"

extern uint32 bootstrap_data_checksum_version;

/* File path names (all relative to $PGDATA) */
#define RECOVERY_COMMAND_FILE   "recovery.conf"
#define RECOVERY_COMMAND_DONE   "recovery.done"
#define PROMOTE_SIGNAL_FILE             "promote"
#define FALLBACK_PROMOTE_SIGNAL_FILE "fallback_promote"


/* User-settable parameters */
int                     CheckPointSegments = 3;
int                     wal_keep_segments = 0;
int                     XLOGbuffers = -1;
int                     XLogArchiveTimeout = 0;
bool            XLogArchiveMode = false;
char       *XLogArchiveCommand = NULL;
bool            EnableHotStandby = false;
bool            fullPageWrites = true;
bool            wal_log_hints = false;
bool            log_checkpoints = false;
int                     sync_method = DEFAULT_SYNC_METHOD;
int                     wal_level = WAL_LEVEL_MINIMAL;
int                     CommitDelay = 0;        /* precommit delay in microseconds */
int                     CommitSiblings = 5; /* # concurrent xacts needed to sleep */
int                     num_xloginsert_slots = 8;

#ifdef WAL_DEBUG
bool            XLOG_DEBUG = false;
#endif

/*
 * XLOGfileslop is the maximum number of preallocated future XLOG segments.
 * When we are done with an old XLOG segment file, we will recycle it as a
 * future XLOG segment as long as there aren't already XLOGfileslop future
 * segments; else we'll delete it.  This could be made a separate GUC
 * variable, but at present I think it's sufficient to hardwire it as
 * 2*CheckPointSegments+1.      Under normal conditions, a checkpoint will free
 * no more than 2*CheckPointSegments log segments, and we want to recycle all
 * of them; the +1 allows boundary cases to happen without wasting a
 * delete/create-segment cycle.
 */
#define XLOGfileslop    (2*CheckPointSegments + 1)


/*
 * GUC support
 */
const struct config_enum_entry sync_method_options[] = {
        {"fsync", SYNC_METHOD_FSYNC, false},
#ifdef HAVE_FSYNC_WRITETHROUGH
        {"fsync_writethrough", SYNC_METHOD_FSYNC_WRITETHROUGH, false},
#endif
#ifdef HAVE_FDATASYNC
        {"fdatasync", SYNC_METHOD_FDATASYNC, false},
#endif
#ifdef OPEN_SYNC_FLAG
        {"open_sync", SYNC_METHOD_OPEN, false},
#endif
#ifdef OPEN_DATASYNC_FLAG
        {"open_datasync", SYNC_METHOD_OPEN_DSYNC, false},
#endif
        {NULL, 0, false}
};

/*
 * Statistics for current checkpoint are collected in this global struct.
 * Because only the background writer or a stand-alone backend can perform
 * checkpoints, this will be unused in normal backends.
 */
CheckpointStatsData CheckpointStats;

/*
 * ThisTimeLineID will be same in all backends --- it identifies current
 * WAL timeline for the database system.
 */
TimeLineID      ThisTimeLineID = 0;

/*
 * Are we doing recovery from XLOG?
 *
 * This is only ever true in the startup process; it should be read as meaning
 * "this process is replaying WAL records", rather than "the system is in
 * recovery mode".  It should be examined primarily by functions that need
 * to act differently when called from a WAL redo function (e.g., to skip WAL
 * logging).  To check whether the system is in recovery regardless of which
 * process you're running in, use RecoveryInProgress() but only after shared
 * memory startup and lock initialization.
 */
bool            InRecovery = false;

/* Are we in Hot Standby mode? Only valid in startup process, see xlog.h */
HotStandbyState standbyState = STANDBY_DISABLED;

static XLogRecPtr LastRec;

/* Local copy of WalRcv->receivedUpto */
static XLogRecPtr receivedUpto = 0;
static TimeLineID receiveTLI = 0;

/*
 * During recovery, lastFullPageWrites keeps track of full_page_writes that
 * the replayed WAL records indicate. It's initialized with full_page_writes
 * that the recovery starting checkpoint record indicates, and then updated
 * each time XLOG_FPW_CHANGE record is replayed.
 */
static bool lastFullPageWrites;

/*
 * Local copy of SharedRecoveryInProgress variable. True actually means "not
 * known, need to check the shared state".
 */
static bool LocalRecoveryInProgress = true;

/*
 * Local copy of SharedHotStandbyActive variable. False actually means "not
 * known, need to check the shared state".
 */
static bool LocalHotStandbyActive = false;

/*
 * Local state for XLogInsertAllowed():
 *              1: unconditionally allowed to insert XLOG
 *              0: unconditionally not allowed to insert XLOG
 *              -1: must check RecoveryInProgress(); disallow until it is false
 * Most processes start with -1 and transition to 1 after seeing that recovery
 * is not in progress.  But we can also force the value for special cases.
 * The coding in XLogInsertAllowed() depends on the first two of these states
 * being numerically the same as bool true and false.
 */
static int      LocalXLogInsertAllowed = -1;

/*
 * When ArchiveRecoveryRequested is set, archive recovery was requested,
 * ie. recovery.conf file was present. When InArchiveRecovery is set, we are
 * currently recovering using offline XLOG archives. These variables are only
 * valid in the startup process.
 *
 * When ArchiveRecoveryRequested is true, but InArchiveRecovery is false, we're
 * currently performing crash recovery using only XLOG files in pg_xlog, but
 * will switch to using offline XLOG archives as soon as we reach the end of
 * WAL in pg_xlog.
*/
bool            ArchiveRecoveryRequested = false;
bool            InArchiveRecovery = false;

/* Was the last xlog file restored from archive, or local? */
static bool restoredFromArchive = false;

/* options taken from recovery.conf for archive recovery */
char       *recoveryRestoreCommand = NULL;
static char *recoveryEndCommand = NULL;
static char *archiveCleanupCommand = NULL;
static RecoveryTargetType recoveryTarget = RECOVERY_TARGET_UNSET;
static bool recoveryTargetInclusive = true;
static bool recoveryPauseAtTarget = true;
static TransactionId recoveryTargetXid;
static TimestampTz recoveryTargetTime;
static char *recoveryTargetName;
static int min_recovery_apply_delay = 0;
static TimestampTz recoveryDelayUntilTime;

/* options taken from recovery.conf for XLOG streaming */
static bool StandbyModeRequested = false;
static char *PrimaryConnInfo = NULL;
static char *TriggerFile = NULL;

/* are we currently in standby mode? */
bool            StandbyMode = false;

/* whether request for fast promotion has been made yet */
static bool fast_promote = false;

/* if recoveryStopsHere returns true, it saves actual stop xid/time/name here */
static TransactionId recoveryStopXid;
static TimestampTz recoveryStopTime;
static char recoveryStopName[MAXFNAMELEN];
static bool recoveryStopAfter;

/*
 * During normal operation, the only timeline we care about is ThisTimeLineID.
 * During recovery, however, things are more complicated.  To simplify life
 * for rmgr code, we keep ThisTimeLineID set to the "current" timeline as we
 * scan through the WAL history (that is, it is the line that was active when
 * the currently-scanned WAL record was generated).  We also need these
 * timeline values:
 *
 * recoveryTargetTLI: the desired timeline that we want to end in.
 *
 * recoveryTargetIsLatest: was the requested target timeline 'latest'?
 *
 * expectedTLEs: a list of TimeLineHistoryEntries for recoveryTargetTLI and the timelines of
 * its known parents, newest first (so recoveryTargetTLI is always the
 * first list member).  Only these TLIs are expected to be seen in the WAL
 * segments we read, and indeed only these TLIs will be considered as
 * candidate WAL files to open at all.
 *
 * curFileTLI: the TLI appearing in the name of the current input WAL file.
 * (This is not necessarily the same as ThisTimeLineID, because we could
 * be scanning data that was copied from an ancestor timeline when the current
 * file was created.)  During a sequential scan we do not allow this value
 * to decrease.
 */
static TimeLineID recoveryTargetTLI;
static bool recoveryTargetIsLatest = false;
static List *expectedTLEs;
static TimeLineID curFileTLI;

/*
 * ProcLastRecPtr points to the start of the last XLOG record inserted by the
 * current backend.  It is updated for all inserts.  XactLastRecEnd points to
 * end+1 of the last record, and is reset when we end a top-level transaction,
 * or start a new one; so it can be used to tell if the current transaction has
 * created any XLOG records.
 */
static XLogRecPtr ProcLastRecPtr = InvalidXLogRecPtr;

XLogRecPtr      XactLastRecEnd = InvalidXLogRecPtr;

/*
 * RedoRecPtr is this backend's local copy of the REDO record pointer
 * (which is almost but not quite the same as a pointer to the most recent
 * CHECKPOINT record).  We update this from the shared-memory copy,
 * XLogCtl->Insert.RedoRecPtr, whenever we can safely do so (ie, when we
 * hold an insertion slot).  See XLogInsert for details.  We are also allowed
 * to update from XLogCtl->RedoRecPtr if we hold the info_lck;
 * see GetRedoRecPtr.  A freshly spawned backend obtains the value during
 * InitXLOGAccess.
 */
static XLogRecPtr RedoRecPtr;

/*
 * RedoStartLSN points to the checkpoint's REDO location which is specified
 * in a backup label file, backup history file or control file. In standby
 * mode, XLOG streaming usually starts from the position where an invalid
 * record was found. But if we fail to read even the initial checkpoint
 * record, we use the REDO location instead of the checkpoint location as
 * the start position of XLOG streaming. Otherwise we would have to jump
 * backwards to the REDO location after reading the checkpoint record,
 * because the REDO record can precede the checkpoint record.
 */
static XLogRecPtr RedoStartLSN = InvalidXLogRecPtr;

/*----------
 * Shared-memory data structures for XLOG control
 *
 * LogwrtRqst indicates a byte position that we need to write and/or fsync
 * the log up to (all records before that point must be written or fsynced).
 * LogwrtResult indicates the byte positions we have already written/fsynced.
 * These structs are identical but are declared separately to indicate their
 * slightly different functions.
 *
 * To read XLogCtl->LogwrtResult, you must hold either info_lck or
 * WALWriteLock.  To update it, you need to hold both locks.  The point of
 * this arrangement is that the value can be examined by code that already
 * holds WALWriteLock without needing to grab info_lck as well.  In addition
 * to the shared variable, each backend has a private copy of LogwrtResult,
 * which is updated when convenient.
 *
 * The request bookkeeping is simpler: there is a shared XLogCtl->LogwrtRqst
 * (protected by info_lck), but we don't need to cache any copies of it.
 *
 * info_lck is only held long enough to read/update the protected variables,
 * so it's a plain spinlock.  The other locks are held longer (potentially
 * over I/O operations), so we use LWLocks for them.  These locks are:
 *
 * WALBufMappingLock: must be held to replace a page in the WAL buffer cache.
 * It is only held while initializing and changing the mapping.  If the
 * contents of the buffer being replaced haven't been written yet, the mapping
 * lock is released while the write is done, and reacquired afterwards.
 *
 * WALWriteLock: must be held to write WAL buffers to disk (XLogWrite or
 * XLogFlush).
 *
 * ControlFileLock: must be held to read/update control file or create
 * new log file.
 *
 * CheckpointLock: must be held to do a checkpoint or restartpoint (ensures
 * only one checkpointer at a time; currently, with all checkpoints done by
 * the checkpointer, this is just pro forma).
 *
 *----------
 */

typedef struct XLogwrtRqst
{
        XLogRecPtr      Write;                  /* last byte + 1 to write out */
        XLogRecPtr      Flush;                  /* last byte + 1 to flush */
} XLogwrtRqst;

typedef struct XLogwrtResult
{
        XLogRecPtr      Write;                  /* last byte + 1 written out */
        XLogRecPtr      Flush;                  /* last byte + 1 flushed */
} XLogwrtResult;


/*
 * A slot for inserting to the WAL. This is similar to an LWLock, the main
 * difference is that there is an extra xlogInsertingAt field that is protected
 * by the same mutex. Unlike an LWLock, a slot can only be acquired in
 * exclusive mode.
 *
 * The xlogInsertingAt field is used to advertise to other processes how far
 * the slot owner has progressed in inserting the record. When a backend
 * acquires a slot, it initializes xlogInsertingAt to 1, because it doesn't
 * yet know where it's going to insert the record. That's conservative
 * but correct; the new insertion is certainly going to go to a byte position
 * greater than 1. If another backend needs to flush the WAL, it will have to
 * wait for the new insertion. xlogInsertingAt is updated after finishing the
 * insert or when crossing a page boundary, which will wake up anyone waiting
 * for it, whether the wait was necessary in the first place or not.
 *
 * A process can wait on a slot in two modes: LW_EXCLUSIVE or
 * LW_WAIT_UNTIL_FREE. LW_EXCLUSIVE works like in an lwlock; when the slot is
 * released, the first LW_EXCLUSIVE waiter in the queue is woken up. Processes
 * waiting in LW_WAIT_UNTIL_FREE mode are woken up whenever the slot is
 * released, or xlogInsertingAt is updated. In other words, a process in
 * LW_WAIT_UNTIL_FREE mode is woken up whenever the inserter makes any progress
 * copying the record in place. LW_WAIT_UNTIL_FREE waiters are always added to
 * the front of the queue, while LW_EXCLUSIVE waiters are appended to the end.
 *
 * To join the wait queue, a process must set MyProc->lwWaitMode to the mode
 * it wants to wait in, MyProc->lwWaiting to true, and link MyProc to the head
 * or tail of the wait queue. The same mechanism is used to wait on an LWLock,
 * see lwlock.c for details.
 */
typedef struct
{
        slock_t         mutex;                  /* protects the below fields */
        XLogRecPtr      xlogInsertingAt; /* insert has completed up to this point */

        PGPROC     *owner;                      /* for debugging purposes */

        bool            releaseOK;              /* T if ok to release waiters */
        char            exclusive;              /* # of exclusive holders (0 or 1) */
        PGPROC     *head;                       /* head of list of waiting PGPROCs */
        PGPROC     *tail;                       /* tail of list of waiting PGPROCs */
        /* tail is undefined when head is NULL */
} XLogInsertSlot;

/*
 * All the slots are allocated as an array in shared memory. We force the
 * array stride to be a power of 2, which saves a few cycles in indexing, but
 * more importantly also ensures that individual slots don't cross cache line
 * boundaries.  (Of course, we have to also ensure that the array start
 * address is suitably aligned.)
 */
typedef union XLogInsertSlotPadded
{
        XLogInsertSlot slot;
        char            pad[CACHE_LINE_SIZE];
} XLogInsertSlotPadded;

/*
 * Shared state data for XLogInsert.
 */
typedef struct XLogCtlInsert
{
        slock_t         insertpos_lck;  /* protects CurrBytePos and PrevBytePos */

        /*
         * CurrBytePos is the end of reserved WAL. The next record will be inserted
         * at that position. PrevBytePos is the start position of the previously
         * inserted (or rather, reserved) record - it is copied to the prev-link
         * of the next record. These are stored as "usable byte positions" rather
         * than XLogRecPtrs (see XLogBytePosToRecPtr()).
         */
        uint64          CurrBytePos;
        uint64          PrevBytePos;

        /*
         * Make sure the above heavily-contended spinlock and byte positions are
         * on their own cache line. In particular, the RedoRecPtr and full page
         * write variables below should be on a different cache line. They are
         * read on every WAL insertion, but updated rarely, and we don't want
         * those reads to steal the cache line containing Curr/PrevBytePos.
         */
        char            pad[CACHE_LINE_SIZE];

        /*
         * fullPageWrites is the master copy used by all backends to determine
         * whether to write full-page to WAL, instead of using process-local one.
         * This is required because, when full_page_writes is changed by SIGHUP,
         * we must WAL-log it before it actually affects WAL-logging by backends.
         * Checkpointer sets at startup or after SIGHUP.
         *
         * To read these fields, you must hold an insertion slot. To modify them,
         * you must hold ALL the slots.
         */
        XLogRecPtr      RedoRecPtr;             /* current redo point for insertions */
        bool            forcePageWrites;        /* forcing full-page writes for PITR? */
        bool            fullPageWrites;

        /*
         * exclusiveBackup is true if a backup started with pg_start_backup() is
         * in progress, and nonExclusiveBackups is a counter indicating the number
         * of streaming base backups currently in progress. forcePageWrites is set
         * to true when either of these is non-zero. lastBackupStart is the latest
         * checkpoint redo location used as a starting point for an online backup.
         */
        bool            exclusiveBackup;
        int                     nonExclusiveBackups;
        XLogRecPtr      lastBackupStart;

        /* insertion slots, see XLogInsertSlot struct above for details */
        XLogInsertSlotPadded *insertSlots;
} XLogCtlInsert;

/*
 * Total shared-memory state for XLOG.
 */
typedef struct XLogCtlData
{
        XLogCtlInsert Insert;

        /* Protected by info_lck: */
        XLogwrtRqst LogwrtRqst;
        XLogRecPtr      RedoRecPtr;             /* a recent copy of Insert->RedoRecPtr */
        uint32          ckptXidEpoch;   /* nextXID & epoch of latest checkpoint */
        TransactionId ckptXid;
        XLogRecPtr      asyncXactLSN;   /* LSN of newest async commit/abort */
        XLogSegNo       lastRemovedSegNo;               /* latest removed/recycled XLOG
                                                                                 * segment */

        /* Fake LSN counter, for unlogged relations. Protected by ulsn_lck. */
        XLogRecPtr      unloggedLSN;
        slock_t         ulsn_lck;

        /* Time of last xlog segment switch. Protected by WALWriteLock. */
        pg_time_t       lastSegSwitchTime;

        /*
         * Protected by info_lck and WALWriteLock (you must hold either lock to
         * read it, but both to update)
         */
        XLogwrtResult LogwrtResult;

        /*
         * Latest initialized page in the cache (last byte position + 1).
         *
         * To change the identity of a buffer (and InitializedUpTo), you need to
         * hold WALBufMappingLock.  To change the identity of a buffer that's still
         * dirty, the old page needs to be written out first, and for that you
         * need WALWriteLock, and you need to ensure that there are no in-progress
         * insertions to the page by calling WaitXLogInsertionsToFinish().
         */
        XLogRecPtr      InitializedUpTo;

        /*
         * These values do not change after startup, although the pointed-to pages
         * and xlblocks values certainly do.  xlblock values are protected by
         * WALBufMappingLock.
         */
        char       *pages;                      /* buffers for unwritten XLOG pages */
        XLogRecPtr *xlblocks;           /* 1st byte ptr-s + XLOG_BLCKSZ */
        int                     XLogCacheBlck;  /* highest allocated xlog buffer index */

        /*
         * Shared copy of ThisTimeLineID. Does not change after end-of-recovery.
         * If we created a new timeline when the system was started up,
         * PrevTimeLineID is the old timeline's ID that we forked off from.
         * Otherwise it's equal to ThisTimeLineID.
         */
        TimeLineID      ThisTimeLineID;
        TimeLineID      PrevTimeLineID;

        /*
         * archiveCleanupCommand is read from recovery.conf but needs to be in
         * shared memory so that the checkpointer process can access it.
         */
        char            archiveCleanupCommand[MAXPGPATH];

        /*
         * SharedRecoveryInProgress indicates if we're still in crash or archive
         * recovery.  Protected by info_lck.
         */
        bool            SharedRecoveryInProgress;

        /*
         * SharedHotStandbyActive indicates if we're still in crash or archive
         * recovery.  Protected by info_lck.
         */
        bool            SharedHotStandbyActive;

        /*
         * WalWriterSleeping indicates whether the WAL writer is currently in
         * low-power mode (and hence should be nudged if an async commit occurs).
         * Protected by info_lck.
         */
        bool            WalWriterSleeping;

        /*
         * recoveryWakeupLatch is used to wake up the startup process to continue
         * WAL replay, if it is waiting for WAL to arrive or failover trigger file
         * to appear.
         */
        Latch           recoveryWakeupLatch;

        /*
         * During recovery, we keep a copy of the latest checkpoint record here.
         * Used by the background writer when it wants to create a restartpoint.
         *
         * Protected by info_lck.
         */
        XLogRecPtr      lastCheckPointRecPtr;
        CheckPoint      lastCheckPoint;

        /*
         * lastReplayedEndRecPtr points to end+1 of the last record successfully
         * replayed. When we're currently replaying a record, ie. in a redo
         * function, replayEndRecPtr points to the end+1 of the record being
         * replayed, otherwise it's equal to lastReplayedEndRecPtr.
         */
        XLogRecPtr      lastReplayedEndRecPtr;
        TimeLineID      lastReplayedTLI;
        XLogRecPtr      replayEndRecPtr;
        TimeLineID      replayEndTLI;
        /* timestamp of last COMMIT/ABORT record replayed (or being replayed) */
        TimestampTz recoveryLastXTime;
        /* current effective recovery target timeline */
        TimeLineID      RecoveryTargetTLI;

        /*
         * timestamp of when we started replaying the current chunk of WAL data,
         * only relevant for replication or archive recovery
         */
        TimestampTz currentChunkStartTime;
        /* Are we requested to pause recovery? */
        bool            recoveryPause;

        /*
         * lastFpwDisableRecPtr points to the start of the last replayed
         * XLOG_FPW_CHANGE record that instructs full_page_writes is disabled.
         */
        XLogRecPtr      lastFpwDisableRecPtr;

        slock_t         info_lck;               /* locks shared variables shown above */
} XLogCtlData;

static XLogCtlData *XLogCtl = NULL;

/*
 * We maintain an image of pg_control in shared memory.
 */
static ControlFileData *ControlFile = NULL;

/*
 * Calculate the amount of space left on the page after 'endptr'. Beware
 * multiple evaluation!
 */
#define INSERT_FREESPACE(endptr)        \
        (((endptr) % XLOG_BLCKSZ == 0) ? 0 : (XLOG_BLCKSZ - (endptr) % XLOG_BLCKSZ))

/* Macro to advance to next buffer index. */
#define NextBufIdx(idx)         \
                (((idx) == XLogCtl->XLogCacheBlck) ? 0 : ((idx) + 1))

/*
 * XLogRecPtrToBufIdx returns the index of the WAL buffer that holds, or
 * would hold if it was in cache, the page containing 'recptr'.
 */
#define XLogRecPtrToBufIdx(recptr)      \
        (((recptr) / XLOG_BLCKSZ) % (XLogCtl->XLogCacheBlck + 1))

/*
 * These are the number of bytes in a WAL page and segment usable for WAL data.
 */
#define UsableBytesInPage (XLOG_BLCKSZ - SizeOfXLogShortPHD)
#define UsableBytesInSegment ((XLOG_SEG_SIZE / XLOG_BLCKSZ) * UsableBytesInPage - (SizeOfXLogLongPHD - SizeOfXLogShortPHD))

/*
 * Private, possibly out-of-date copy of shared LogwrtResult.
 * See discussion above.
 */
static XLogwrtResult LogwrtResult = {0, 0};

/*
 * Codes indicating where we got a WAL file from during recovery, or where
 * to attempt to get one.
 */
typedef enum
{
        XLOG_FROM_ANY = 0,                      /* request to read WAL from any source */
        XLOG_FROM_ARCHIVE,                      /* restored using restore_command */
        XLOG_FROM_PG_XLOG,                      /* existing file in pg_xlog */
        XLOG_FROM_STREAM,                       /* streamed from master */
} XLogSource;

/* human-readable names for XLogSources, for debugging output */
static const char *xlogSourceNames[] = {"any", "archive", "pg_xlog", "stream"};

/*
 * openLogFile is -1 or a kernel FD for an open log file segment.
 * When it's open, openLogOff is the current seek offset in the file.
 * openLogSegNo identifies the segment.  These variables are only
 * used to write the XLOG, and so will normally refer to the active segment.
 */
static int      openLogFile = -1;
static XLogSegNo openLogSegNo = 0;
static uint32 openLogOff = 0;

/*
 * These variables are used similarly to the ones above, but for reading
 * the XLOG.  Note, however, that readOff generally represents the offset
 * of the page just read, not the seek position of the FD itself, which
 * will be just past that page. readLen indicates how much of the current
 * page has been read into readBuf, and readSource indicates where we got
 * the currently open file from.
 */
static int      readFile = -1;
static XLogSegNo readSegNo = 0;
static uint32 readOff = 0;
static uint32 readLen = 0;
static XLogSource readSource = 0;               /* XLOG_FROM_* code */

/*
 * Keeps track of which source we're currently reading from. This is
 * different from readSource in that this is always set, even when we don't
 * currently have a WAL file open. If lastSourceFailed is set, our last
 * attempt to read from currentSource failed, and we should try another source
 * next.
 */
static XLogSource currentSource = 0;    /* XLOG_FROM_* code */
static bool lastSourceFailed = false;

typedef struct XLogPageReadPrivate
{
        int                     emode;
        bool            fetching_ckpt;  /* are we fetching a checkpoint record? */
        bool            randAccess;
} XLogPageReadPrivate;

/*
 * These variables track when we last obtained some WAL data to process,
 * and where we got it from.  (XLogReceiptSource is initially the same as
 * readSource, but readSource gets reset to zero when we don't have data
 * to process right now.  It is also different from currentSource, which
 * also changes when we try to read from a source and fail, while
 * XLogReceiptSource tracks where we last successfully read some WAL.)
 */
static TimestampTz XLogReceiptTime = 0;
static XLogSource XLogReceiptSource = 0;                /* XLOG_FROM_* code */

/* State information for XLOG reading */
static XLogRecPtr ReadRecPtr;   /* start of last record read */
static XLogRecPtr EndRecPtr;    /* end+1 of last record read */

static XLogRecPtr minRecoveryPoint;             /* local copy of
                                                                                 * ControlFile->minRecoveryPoint */
static TimeLineID minRecoveryPointTLI;
static bool updateMinRecoveryPoint = true;

/*
 * Have we reached a consistent database state? In crash recovery, we have
 * to replay all the WAL, so reachedConsistency is never set. During archive
 * recovery, the database is consistent once minRecoveryPoint is reached.
 */
bool            reachedConsistency = false;

static bool InRedo = false;

/* Have we launched bgwriter during recovery? */
static bool bgwriterLaunched = false;

/* For WALInsertSlotAcquire/Release functions */
static int      MySlotNo = 0;
static bool holdingAllSlots = false;

static void readRecoveryCommandFile(void);
static void exitArchiveRecovery(TimeLineID endTLI, XLogSegNo endLogSegNo);
static bool recoveryStopsHere(XLogRecord *record, bool *includeThis, bool *delayThis);
static void recoveryPausesHere(void);
static void recoveryApplyDelay(void);
static bool SetRecoveryDelayUntilTime(TimestampTz xtime);
static void SetLatestXTime(TimestampTz xtime);
static void SetCurrentChunkStartTime(TimestampTz xtime);
static void CheckRequiredParameterValues(void);
static void XLogReportParameters(void);
static void checkTimeLineSwitch(XLogRecPtr lsn, TimeLineID newTLI,
                                        TimeLineID prevTLI);
static void LocalSetXLogInsertAllowed(void);
static void CreateEndOfRecoveryRecord(void);
static void CheckPointGuts(XLogRecPtr checkPointRedo, int flags);
static void KeepLogSeg(XLogRecPtr recptr, XLogSegNo *logSegNo);

static bool XLogCheckBuffer(XLogRecData *rdata, bool holdsExclusiveLock,
                                XLogRecPtr *lsn, BkpBlock *bkpb);
static Buffer RestoreBackupBlockContents(XLogRecPtr lsn, BkpBlock bkpb,
                                                 char *blk, bool get_cleanup_lock, bool keep_buffer);
static void AdvanceXLInsertBuffer(XLogRecPtr upto, bool opportunistic);
static bool XLogCheckpointNeeded(XLogSegNo new_segno);
static void XLogWrite(XLogwrtRqst WriteRqst, bool flexible);
static bool InstallXLogFileSegment(XLogSegNo *segno, char *tmppath,
                                           bool find_free, int *max_advance,
                                           bool use_lock);
static int XLogFileRead(XLogSegNo segno, int emode, TimeLineID tli,
                         int source, bool notexistOk);
static int      XLogFileReadAnyTLI(XLogSegNo segno, int emode, int source);
static int XLogPageRead(XLogReaderState *xlogreader, XLogRecPtr targetPagePtr,
                         int reqLen, XLogRecPtr targetRecPtr, char *readBuf,
                         TimeLineID *readTLI);
static bool WaitForWALToBecomeAvailable(XLogRecPtr RecPtr, bool randAccess,
                                                        bool fetching_ckpt, XLogRecPtr tliRecPtr);
static int      emode_for_corrupt_record(int emode, XLogRecPtr RecPtr);
static void XLogFileClose(void);
static void PreallocXlogFiles(XLogRecPtr endptr);
static void RemoveOldXlogFiles(XLogSegNo segno, XLogRecPtr endptr);
static void UpdateLastRemovedPtr(char *filename);
static void ValidateXLOGDirectoryStructure(void);
static void CleanupBackupHistory(void);
static void UpdateMinRecoveryPoint(XLogRecPtr lsn, bool force);
static XLogRecord *ReadRecord(XLogReaderState *xlogreader, XLogRecPtr RecPtr,
                   int emode, bool fetching_ckpt);
static void CheckRecoveryConsistency(void);
static XLogRecord *ReadCheckpointRecord(XLogReaderState *xlogreader,
                                         XLogRecPtr RecPtr, int whichChkpti, bool report);
static bool rescanLatestTimeLine(void);
static void WriteControlFile(void);
static void ReadControlFile(void);
static char *str_time(pg_time_t tnow);
static bool CheckForStandbyTrigger(void);

#ifdef WAL_DEBUG
static void xlog_outrec(StringInfo buf, XLogRecord *record);
#endif
static void pg_start_backup_callback(int code, Datum arg);
static bool read_backup_label(XLogRecPtr *checkPointLoc,
                                  bool *backupEndRequired, bool *backupFromStandby);
static void rm_redo_error_callback(void *arg);
static int      get_sync_bit(int method);

static void CopyXLogRecordToWAL(int write_len, bool isLogSwitch,
                                  XLogRecData *rdata,
                                  XLogRecPtr StartPos, XLogRecPtr EndPos);
static void ReserveXLogInsertLocation(int size, XLogRecPtr *StartPos,
                                                  XLogRecPtr *EndPos, XLogRecPtr *PrevPtr);
static bool ReserveXLogSwitch(XLogRecPtr *StartPos, XLogRecPtr *EndPos,
                                  XLogRecPtr *PrevPtr);
static XLogRecPtr WaitXLogInsertionsToFinish(XLogRecPtr upto);
static void WakeupWaiters(XLogRecPtr EndPos);
static char *GetXLogBuffer(XLogRecPtr ptr);
static XLogRecPtr XLogBytePosToRecPtr(uint64 bytepos);
static XLogRecPtr XLogBytePosToEndRecPtr(uint64 bytepos);
static uint64 XLogRecPtrToBytePos(XLogRecPtr ptr);

static void WALInsertSlotAcquire(bool exclusive);
static void WALInsertSlotAcquireOne(int slotno);
static void WALInsertSlotRelease(void);
static void WALInsertSlotReleaseOne(int slotno);

/*
 * Insert an XLOG record having the specified RMID and info bytes,
 * with the body of the record being the data chunk(s) described by
 * the rdata chain (see xlog.h for notes about rdata).
 *
 * Returns XLOG pointer to end of record (beginning of next record).
 * This can be used as LSN for data pages affected by the logged action.
 * (LSN is the XLOG point up to which the XLOG must be flushed to disk
 * before the data page can be written out.  This implements the basic
 * WAL rule "write the log before the data".)
 *
 * NB: this routine feels free to scribble on the XLogRecData structs,
 * though not on the data they reference.  This is OK since the XLogRecData
 * structs are always just temporaries in the calling code.
 */
XLogRecPtr
XLogInsert(RmgrId rmid, uint8 info, XLogRecData *rdata)
{
        XLogCtlInsert *Insert = &XLogCtl->Insert;
        XLogRecData *rdt;
        XLogRecData *rdt_lastnormal;
        Buffer          dtbuf[XLR_MAX_BKP_BLOCKS];
        bool            dtbuf_bkp[XLR_MAX_BKP_BLOCKS];
        BkpBlock        dtbuf_xlg[XLR_MAX_BKP_BLOCKS];
        XLogRecPtr      dtbuf_lsn[XLR_MAX_BKP_BLOCKS];
        XLogRecData dtbuf_rdt1[XLR_MAX_BKP_BLOCKS];
        XLogRecData dtbuf_rdt2[XLR_MAX_BKP_BLOCKS];
        XLogRecData dtbuf_rdt3[XLR_MAX_BKP_BLOCKS];
        XLogRecData hdr_rdt;
        pg_crc32        rdata_crc;
        uint32          len,
                                write_len;
        unsigned        i;
        bool            doPageWrites;
        bool            isLogSwitch = (rmid == RM_XLOG_ID && info == XLOG_SWITCH);
        bool            inserted;
        uint8           info_orig = info;
        static XLogRecord *rechdr;
        XLogRecPtr      StartPos;
        XLogRecPtr      EndPos;

        if (rechdr == NULL)
        {
                rechdr = malloc(SizeOfXLogRecord);
                if (rechdr == NULL)
                        elog(ERROR, "out of memory");
                MemSet(rechdr, 0, SizeOfXLogRecord);
        }

        /* cross-check on whether we should be here or not */
        if (!XLogInsertAllowed())
                elog(ERROR, "cannot make new WAL entries during recovery");

        /* info's high bits are reserved for use by me */
        if (info & XLR_INFO_MASK)
                elog(PANIC, "invalid xlog info mask %02X", info);

        TRACE_POSTGRESQL_XLOG_INSERT(rmid, info);

        /*
         * In bootstrap mode, we don't actually log anything but XLOG resources;
         * return a phony record pointer.
         */
        if (IsBootstrapProcessingMode() && rmid != RM_XLOG_ID)
        {
                EndPos = SizeOfXLogLongPHD;             /* start of 1st chkpt record */
                return EndPos;
        }

        /*
         * Here we scan the rdata chain, to determine which buffers must be backed
         * up.
         *
         * We may have to loop back to here if a race condition is detected below.
         * We could prevent the race by doing all this work while holding an
         * insertion slot, but it seems better to avoid doing CRC calculations
         * while holding one.
         *
         * We add entries for backup blocks to the chain, so that they don't need
         * any special treatment in the critical section where the chunks are
         * copied into the WAL buffers. Those entries have to be unlinked from the
         * chain if we have to loop back here.
         */
begin:;
        for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
        {
                dtbuf[i] = InvalidBuffer;
                dtbuf_bkp[i] = false;
        }

        /*
         * Decide if we need to do full-page writes in this XLOG record: true if
         * full_page_writes is on or we have a PITR request for it.  Since we
         * don't yet have an insertion slot, fullPageWrites and forcePageWrites
         * could change under us, but we'll recheck them once we have a slot.
         */
        doPageWrites = Insert->fullPageWrites || Insert->forcePageWrites;

        len = 0;
        for (rdt = rdata;;)
        {
                if (rdt->buffer == InvalidBuffer)
                {
                        /* Simple data, just include it */
                        len += rdt->len;
                }
                else
                {
                        /* Find info for buffer */
                        for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
                        {
                                if (rdt->buffer == dtbuf[i])
                                {
                                        /* Buffer already referenced by earlier chain item */
                                        if (dtbuf_bkp[i])
                                        {
                                                rdt->data = NULL;
                                                rdt->len = 0;
                                        }
                                        else if (rdt->data)
                                                len += rdt->len;
                                        break;
                                }
                                if (dtbuf[i] == InvalidBuffer)
                                {
                                        /* OK, put it in this slot */
                                        dtbuf[i] = rdt->buffer;
                                        if (doPageWrites && XLogCheckBuffer(rdt, true,
                                                                                   &(dtbuf_lsn[i]), &(dtbuf_xlg[i])))
                                        {
                                                dtbuf_bkp[i] = true;
                                                rdt->data = NULL;
                                                rdt->len = 0;
                                        }
                                        else if (rdt->data)
                                                len += rdt->len;
                                        break;
                                }
                        }
                        if (i >= XLR_MAX_BKP_BLOCKS)
                                elog(PANIC, "can backup at most %d blocks per xlog record",
                                         XLR_MAX_BKP_BLOCKS);
                }
                /* Break out of loop when rdt points to last chain item */
                if (rdt->next == NULL)
                        break;
                rdt = rdt->next;
        }

        /*
         * NOTE: We disallow len == 0 because it provides a useful bit of extra
         * error checking in ReadRecord.  This means that all callers of
         * XLogInsert must supply at least some not-in-a-buffer data.  However, we
         * make an exception for XLOG SWITCH records because we don't want them to
         * ever cross a segment boundary.
         */
        if (len == 0 && !isLogSwitch)
                elog(PANIC, "invalid xlog record length %u", len);

        /*
         * Make additional rdata chain entries for the backup blocks, so that we
         * don't need to special-case them in the write loop.  This modifies the
         * original rdata chain, but we keep a pointer to the last regular entry,
         * rdt_lastnormal, so that we can undo this if we have to loop back to the
         * beginning.
         *
         * At the exit of this loop, write_len includes the backup block data.
         *
         * Also set the appropriate info bits to show which buffers were backed
         * up. The XLR_BKP_BLOCK(N) bit corresponds to the N'th distinct buffer
         * value (ignoring InvalidBuffer) appearing in the rdata chain.
         */
        rdt_lastnormal = rdt;
        write_len = len;
        for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
        {
                BkpBlock   *bkpb;
                char       *page;

                if (!dtbuf_bkp[i])
                        continue;

                info |= XLR_BKP_BLOCK(i);

                bkpb = &(dtbuf_xlg[i]);
                page = (char *) BufferGetBlock(dtbuf[i]);

                rdt->next = &(dtbuf_rdt1[i]);
                rdt = rdt->next;

                rdt->data = (char *) bkpb;
                rdt->len = sizeof(BkpBlock);
                write_len += sizeof(BkpBlock);

                rdt->next = &(dtbuf_rdt2[i]);
                rdt = rdt->next;

                if (bkpb->hole_length == 0)
                {
                        rdt->data = page;
                        rdt->len = BLCKSZ;
                        write_len += BLCKSZ;
                        rdt->next = NULL;
                }
                else
                {
                        /* must skip the hole */
                        rdt->data = page;
                        rdt->len = bkpb->hole_offset;
                        write_len += bkpb->hole_offset;

                        rdt->next = &(dtbuf_rdt3[i]);
                        rdt = rdt->next;

                        rdt->data = page + (bkpb->hole_offset + bkpb->hole_length);
                        rdt->len = BLCKSZ - (bkpb->hole_offset + bkpb->hole_length);
                        write_len += rdt->len;
                        rdt->next = NULL;
                }
        }

        /*
         * Calculate CRC of the data, including all the backup blocks
         *
         * Note that the record header isn't added into the CRC initially since we
         * don't know the prev-link yet.  Thus, the CRC will represent the CRC of
         * the whole record in the order: rdata, then backup blocks, then record
         * header.
         */
        INIT_CRC32(rdata_crc);
        for (rdt = rdata; rdt != NULL; rdt = rdt->next)
                COMP_CRC32(rdata_crc, rdt->data, rdt->len);

        /*
         * Construct record header (prev-link is filled in later, after reserving
         * the space for the record), and make that the first chunk in the chain.
         *
         * The CRC calculated for the header here doesn't include prev-link,
         * because we don't know it yet. It will be added later.
         */
        rechdr->xl_xid = GetCurrentTransactionIdIfAny();
        rechdr->xl_tot_len = SizeOfXLogRecord + write_len;
        rechdr->xl_len = len;           /* doesn't include backup blocks */
        rechdr->xl_info = info;
        rechdr->xl_rmid = rmid;
        rechdr->xl_prev = InvalidXLogRecPtr;
        COMP_CRC32(rdata_crc, ((char *) rechdr), offsetof(XLogRecord, xl_prev));

        hdr_rdt.next = rdata;
        hdr_rdt.data = (char *) rechdr;
        hdr_rdt.len = SizeOfXLogRecord;
        write_len += SizeOfXLogRecord;

        /*----------
         *
         * We have now done all the preparatory work we can without holding a
         * lock or modifying shared state. From here on, inserting the new WAL
         * record to the shared WAL buffer cache is a two-step process:
         *
         * 1. Reserve the right amount of space from the WAL. The current head of
         *    reserved space is kept in Insert->CurrBytePos, and is protected by
         *    insertpos_lck.
         *
         * 2. Copy the record to the reserved WAL space. This involves finding the
         *    correct WAL buffer containing the reserved space, and copying the
         *    record in place. This can be done concurrently in multiple processes.
         *
         * To keep track of which insertions are still in-progress, each concurrent
         * inserter allocates an "insertion slot", which tells others how far the
         * inserter has progressed. There is a small fixed number of insertion
         * slots, determined by the num_xloginsert_slots GUC. When an inserter
         * finishes, it updates the xlogInsertingAt of its slot to the end of the
         * record it inserted, to let others know that it's done. xlogInsertingAt
         * is also updated when crossing over to a new WAL buffer, to allow the
         * the previous buffer to be flushed.
         *
         * Holding onto a slot also protects RedoRecPtr and fullPageWrites from
         * changing until the insertion is finished.
         *
         * Step 2 can usually be done completely in parallel. If the required WAL
         * page is not initialized yet, you have to grab WALBufMappingLock to
         * initialize it, but the WAL writer tries to do that ahead of insertions
         * to avoid that from happening in the critical path.
         *
         *----------
         */
        START_CRIT_SECTION();
        WALInsertSlotAcquire(isLogSwitch);

        /*
         * Check to see if my RedoRecPtr is out of date.  If so, may have to go
         * back and recompute everything.  This can only happen just after a
         * checkpoint, so it's better to be slow in this case and fast otherwise.
         *
         * If we aren't doing full-page writes then RedoRecPtr doesn't actually
         * affect the contents of the XLOG record, so we'll update our local copy
         * but not force a recomputation.
         */
        if (RedoRecPtr != Insert->RedoRecPtr)
        {
                Assert(RedoRecPtr < Insert->RedoRecPtr);
                RedoRecPtr = Insert->RedoRecPtr;

                if (doPageWrites)
                {
                        for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
                        {
                                if (dtbuf[i] == InvalidBuffer)
                                        continue;
                                if (dtbuf_bkp[i] == false &&
                                        dtbuf_lsn[i] <= RedoRecPtr)
                                {
                                        /*
                                         * Oops, this buffer now needs to be backed up, but we
                                         * didn't think so above.  Start over.
                                         */
                                        WALInsertSlotRelease();
                                        END_CRIT_SECTION();
                                        rdt_lastnormal->next = NULL;
                                        info = info_orig;
                                        goto begin;
                                }
                        }
                }
        }

        /*
         * Also check to see if fullPageWrites or forcePageWrites was just turned
         * on; if we weren't already doing full-page writes then go back and
         * recompute. (If it was just turned off, we could recompute the record
         * without full pages, but we choose not to bother.)
         */
        if ((Insert->fullPageWrites || Insert->forcePageWrites) && !doPageWrites)
        {
                /* Oops, must redo it with full-page data. */
                WALInsertSlotRelease();
                END_CRIT_SECTION();
                rdt_lastnormal->next = NULL;
                info = info_orig;
                goto begin;
        }

        /*
         * Reserve space for the record in the WAL. This also sets the xl_prev
         * pointer.
         */
        if (isLogSwitch)
                inserted = ReserveXLogSwitch(&StartPos, &EndPos, &rechdr->xl_prev);
        else
        {
                ReserveXLogInsertLocation(write_len, &StartPos, &EndPos,
                                                                  &rechdr->xl_prev);
                inserted = true;
        }

        if (inserted)
        {
                /*
                 * Now that xl_prev has been filled in, finish CRC calculation of the
                 * record header.
                 */
                COMP_CRC32(rdata_crc, ((char *) &rechdr->xl_prev), sizeof(XLogRecPtr));
                FIN_CRC32(rdata_crc);
                rechdr->xl_crc = rdata_crc;

                /*
                 * All the record data, including the header, is now ready to be
                 * inserted. Copy the record in the space reserved.
                 */
                CopyXLogRecordToWAL(write_len, isLogSwitch, &hdr_rdt, StartPos, EndPos);
        }
        else
        {
                /*
                 * This was an xlog-switch record, but the current insert location was
                 * already exactly at the beginning of a segment, so there was no need
                 * to do anything.
                 */
        }

        /*
         * Done! Let others know that we're finished.
         */
        WALInsertSlotRelease();

        MarkCurrentTransactionIdLoggedIfAny();

        END_CRIT_SECTION();

        /*
         * Update shared LogwrtRqst.Write, if we crossed page boundary.
         */
        if (StartPos / XLOG_BLCKSZ != EndPos / XLOG_BLCKSZ)
        {
                /* use volatile pointer to prevent code rearrangement */
                volatile XLogCtlData *xlogctl = XLogCtl;

                SpinLockAcquire(&xlogctl->info_lck);
                /* advance global request to include new block(s) */
                if (xlogctl->LogwrtRqst.Write < EndPos)
                        xlogctl->LogwrtRqst.Write = EndPos;
                /* update local result copy while I have the chance */
                LogwrtResult = xlogctl->LogwrtResult;
                SpinLockRelease(&xlogctl->info_lck);
        }

        /*
         * If this was an XLOG_SWITCH record, flush the record and the empty
         * padding space that fills the rest of the segment, and perform
         * end-of-segment actions (eg, notifying archiver).
         */
        if (isLogSwitch)
        {
                TRACE_POSTGRESQL_XLOG_SWITCH();
                XLogFlush(EndPos);
                /*
                 * Even though we reserved the rest of the segment for us, which is
                 * reflected in EndPos, we return a pointer to just the end of the
                 * xlog-switch record.
                 */
                if (inserted)
                {
                        EndPos = StartPos + SizeOfXLogRecord;
                        if (StartPos / XLOG_BLCKSZ != EndPos / XLOG_BLCKSZ)
                        {
                                if (EndPos % XLOG_SEG_SIZE == EndPos % XLOG_BLCKSZ)
                                        EndPos += SizeOfXLogLongPHD;
                                else
                                        EndPos += SizeOfXLogShortPHD;
                        }
                }
        }

#ifdef WAL_DEBUG
        if (XLOG_DEBUG)
        {
                StringInfoData buf;

                initStringInfo(&buf);
                appendStringInfo(&buf, "INSERT @ %X/%X: ",
                                                 (uint32) (EndPos >> 32), (uint32) EndPos);
                xlog_outrec(&buf, rechdr);
                if (rdata->data != NULL)
                {
                        appendStringInfoString(&buf, " - ");
                        RmgrTable[rechdr->xl_rmid].rm_desc(&buf, rechdr->xl_info, rdata->data);
                }
                elog(LOG, "%s", buf.data);
                pfree(buf.data);
        }
#endif

        /*
         * Update our global variables
         */
        ProcLastRecPtr = StartPos;
        XactLastRecEnd = EndPos;

        return EndPos;
}

/*
 * Reserves the right amount of space for a record of given size from the WAL.
 * *StartPos is set to the beginning of the reserved section, *EndPos to
 * its end+1. *PrevPtr is set to the beginning of the previous record; it is
 * used to set the xl_prev of this record.
 *
 * This is the performance critical part of XLogInsert that must be serialized
 * across backends. The rest can happen mostly in parallel. Try to keep this
 * section as short as possible, insertpos_lck can be heavily contended on a
 * busy system.
 *
 * NB: The space calculation here must match the code in CopyXLogRecordToWAL,
 * where we actually copy the record to the reserved space.
 */
static void
ReserveXLogInsertLocation(int size, XLogRecPtr *StartPos, XLogRecPtr *EndPos,
                                                  XLogRecPtr *PrevPtr)
{
        volatile XLogCtlInsert *Insert = &XLogCtl->Insert;
        uint64          startbytepos;
        uint64          endbytepos;
        uint64          prevbytepos;

        size = MAXALIGN(size);

        /* All (non xlog-switch) records should contain data. */
        Assert(size > SizeOfXLogRecord);

        /*
         * The duration the spinlock needs to be held is minimized by minimizing
         * the calculations that have to be done while holding the lock. The
         * current tip of reserved WAL is kept in CurrBytePos, as a byte position
         * that only counts "usable" bytes in WAL, that is, it excludes all WAL
         * page headers. The mapping between "usable" byte positions and physical
         * positions (XLogRecPtrs) can be done outside the locked region, and
         * because the usable byte position doesn't include any headers, reserving
         * X bytes from WAL is almost as simple as "CurrBytePos += X".
         */
        SpinLockAcquire(&Insert->insertpos_lck);

        startbytepos = Insert->CurrBytePos;
        endbytepos = startbytepos + size;
        prevbytepos = Insert->PrevBytePos;
        Insert->CurrBytePos = endbytepos;
        Insert->PrevBytePos = startbytepos;

        SpinLockRelease(&Insert->insertpos_lck);

        *StartPos = XLogBytePosToRecPtr(startbytepos);
        *EndPos = XLogBytePosToEndRecPtr(endbytepos);
        *PrevPtr = XLogBytePosToRecPtr(prevbytepos);

        /*
         * Check that the conversions between "usable byte positions" and
         * XLogRecPtrs work consistently in both directions.
         */
        Assert(XLogRecPtrToBytePos(*StartPos) == startbytepos);
        Assert(XLogRecPtrToBytePos(*EndPos) == endbytepos);
        Assert(XLogRecPtrToBytePos(*PrevPtr) == prevbytepos);
}

/*
 * Like ReserveXLogInsertLocation(), but for an xlog-switch record.
 *
 * A log-switch record is handled slightly differently. The rest of the
 * segment will be reserved for this insertion, as indicated by the returned
 * *EndPos value. However, if we are already at the beginning of the current
 * segment, *StartPos and *EndPos are set to the current location without
 * reserving any space, and the function returns false.
*/
static bool
ReserveXLogSwitch(XLogRecPtr *StartPos, XLogRecPtr *EndPos, XLogRecPtr *PrevPtr)
{
        volatile XLogCtlInsert *Insert = &XLogCtl->Insert;
        uint64          startbytepos;
        uint64          endbytepos;
        uint64          prevbytepos;
        uint32          size = SizeOfXLogRecord;
        XLogRecPtr      ptr;
        uint32          segleft;

        /*
         * These calculations are a bit heavy-weight to be done while holding a
         * spinlock, but since we're holding all the WAL insertion slots, there
         * are no other inserters competing for it. GetXLogInsertRecPtr() does
         * compete for it, but that's not called very frequently.
         */
        SpinLockAcquire(&Insert->insertpos_lck);

        startbytepos = Insert->CurrBytePos;

        ptr = XLogBytePosToEndRecPtr(startbytepos);
        if (ptr % XLOG_SEG_SIZE == 0)
        {
                SpinLockRelease(&Insert->insertpos_lck);
                *EndPos = *StartPos = ptr;
                return false;
        }

        endbytepos = startbytepos + size;
        prevbytepos = Insert->PrevBytePos;

        *StartPos = XLogBytePosToRecPtr(startbytepos);
        *EndPos = XLogBytePosToEndRecPtr(endbytepos);

        segleft = XLOG_SEG_SIZE - ((*EndPos) % XLOG_SEG_SIZE);
        if (segleft != XLOG_SEG_SIZE)
        {
                /* consume the rest of the segment */
                *EndPos += segleft;
                endbytepos = XLogRecPtrToBytePos(*EndPos);
        }
        Insert->CurrBytePos = endbytepos;
        Insert->PrevBytePos = startbytepos;

        SpinLockRelease(&Insert->insertpos_lck);

        *PrevPtr = XLogBytePosToRecPtr(prevbytepos);

        Assert((*EndPos) % XLOG_SEG_SIZE == 0);
        Assert(XLogRecPtrToBytePos(*EndPos) == endbytepos);
        Assert(XLogRecPtrToBytePos(*StartPos) == startbytepos);
        Assert(XLogRecPtrToBytePos(*PrevPtr) == prevbytepos);

        return true;
}

/*
 * Subroutine of XLogInsert.  Copies a WAL record to an already-reserved
 * area in the WAL.
 */
static void
CopyXLogRecordToWAL(int write_len, bool isLogSwitch, XLogRecData *rdata,
                                        XLogRecPtr StartPos, XLogRecPtr EndPos)
{
        char       *currpos;
        int                     freespace;
        int                     written;
        XLogRecPtr      CurrPos;
        XLogPageHeader pagehdr;

        /* The first chunk is the record header */
        Assert(rdata->len == SizeOfXLogRecord);

        /*
         * Get a pointer to the right place in the right WAL buffer to start
         * inserting to.
         */
        CurrPos = StartPos;
        currpos = GetXLogBuffer(CurrPos);
        freespace = INSERT_FREESPACE(CurrPos);

        /*
         * there should be enough space for at least the first field (xl_tot_len)
         * on this page.
         */
        Assert(freespace >= sizeof(uint32));

        /* Copy record data */
        written = 0;
        while (rdata != NULL)
        {
                char       *rdata_data = rdata->data;
                int                     rdata_len = rdata->len;

                while (rdata_len > freespace)
                {
                        /*
                         * Write what fits on this page, and continue on the next page.
                         */
                        Assert(CurrPos % XLOG_BLCKSZ >= SizeOfXLogShortPHD || freespace == 0);
                        memcpy(currpos, rdata_data, freespace);
                        rdata_data += freespace;
                        rdata_len -= freespace;
                        written += freespace;
                        CurrPos += freespace;

                        /*
                         * Get pointer to beginning of next page, and set the xlp_rem_len
                         * in the page header. Set XLP_FIRST_IS_CONTRECORD.
                         *
                         * It's safe to set the contrecord flag and xlp_rem_len without a
                         * lock on the page. All the other flags were already set when the
                         * page was initialized, in AdvanceXLInsertBuffer, and we're the
                         * only backend that needs to set the contrecord flag.
                         */
                        currpos = GetXLogBuffer(CurrPos);
                        pagehdr = (XLogPageHeader) currpos;
                        pagehdr->xlp_rem_len = write_len - written;
                        pagehdr->xlp_info |= XLP_FIRST_IS_CONTRECORD;

                        /* skip over the page header */
                        if (CurrPos % XLogSegSize == 0)
                        {
                                CurrPos += SizeOfXLogLongPHD;
                                currpos += SizeOfXLogLongPHD;
                        }
                        else
                        {
                                CurrPos += SizeOfXLogShortPHD;
                                currpos += SizeOfXLogShortPHD;
                        }
                        freespace = INSERT_FREESPACE(CurrPos);
                }

                Assert(CurrPos % XLOG_BLCKSZ >= SizeOfXLogShortPHD || rdata_len == 0);
                memcpy(currpos, rdata_data, rdata_len);
                currpos += rdata_len;
                CurrPos += rdata_len;
                freespace -= rdata_len;
                written += rdata_len;

                rdata = rdata->next;
        }
        Assert(written == write_len);

        /* Align the end position, so that the next record starts aligned */
        CurrPos = MAXALIGN64(CurrPos);

        /*
         * If this was an xlog-switch, it's not enough to write the switch record,
         * we also have to consume all the remaining space in the WAL segment.
         * We have already reserved it for us, but we still need to make sure it's
         * allocated and zeroed in the WAL buffers so that when the caller (or
         * someone else) does XLogWrite(), it can really write out all the zeros.
         */
        if (isLogSwitch && CurrPos % XLOG_SEG_SIZE != 0)
        {
                /* An xlog-switch record doesn't contain any data besides the header */
                Assert(write_len == SizeOfXLogRecord);

                /*
                 * We do this one page at a time, to make sure we don't deadlock
                 * against ourselves if wal_buffers < XLOG_SEG_SIZE.
                 */
                Assert(EndPos % XLogSegSize == 0);

                /* Use up all the remaining space on the first page */
                CurrPos += freespace;

                while (CurrPos < EndPos)
                {
                        /* initialize the next page (if not initialized already) */
                        WakeupWaiters(CurrPos);
                        AdvanceXLInsertBuffer(CurrPos, false);
                        CurrPos += XLOG_BLCKSZ;
                }
        }

        if (CurrPos != EndPos)
                elog(PANIC, "space reserved for WAL record does not match what was written");
}

/*
 * Allocate a slot for insertion.
 *
 * In exclusive mode, all slots are reserved for the current process. That
 * blocks all concurrent insertions.
 */
static void
WALInsertSlotAcquire(bool exclusive)
{
        int                     i;

        if (exclusive)
        {
                for (i = 0; i < num_xloginsert_slots; i++)
                        WALInsertSlotAcquireOne(i);
                holdingAllSlots = true;
        }
        else
                WALInsertSlotAcquireOne(-1);
}

/*
 * Workhorse of WALInsertSlotAcquire. Acquires the given slot, or an arbitrary
 * one if slotno == -1. The index of the slot that was acquired is stored in
 * MySlotNo.
 *
 * This is more or less equivalent to LWLockAcquire().
 */
static void
WALInsertSlotAcquireOne(int slotno)
{
        volatile XLogInsertSlot *slot;
        PGPROC     *proc = MyProc;
        bool            retry = false;
        int                     extraWaits = 0;
        static int      slotToTry = -1;

        /*
         * Try to use the slot we used last time. If the system isn't particularly
         * busy, it's a good bet that it's available, and it's good to have some
         * affinity to a particular slot so that you don't unnecessarily bounce
         * cache lines between processes when there is no contention.
         *
         * If this is the first time through in this backend, pick a slot
         * (semi-)randomly. This allows the slots to be used evenly if you have a
         * lot of very short connections.
         */
        if (slotno != -1)
                MySlotNo = slotno;
        else
        {
                if (slotToTry == -1)
                        slotToTry = MyProc->pgprocno % num_xloginsert_slots;
                MySlotNo = slotToTry;
        }

        /*
         * We can't wait if we haven't got a PGPROC.  This should only occur
         * during bootstrap or shared memory initialization.  Put an Assert here
         * to catch unsafe coding practices.
         */
        Assert(MyProc != NULL);

        /*
         * Lock out cancel/die interrupts until we exit the code section protected
         * by the slot.  This ensures that interrupts will not interfere with
         * manipulations of data structures in shared memory. There is no cleanup
         * mechanism to release the slot if the backend dies while holding one,
         * so make this a critical section.
         */
        START_CRIT_SECTION();

        /*
         * Loop here to try to acquire slot after each time we are signaled by
         * WALInsertSlotRelease.
         */
        for (;;)
        {
                bool            mustwait;

                slot = &XLogCtl->Insert.insertSlots[MySlotNo].slot;

                /* Acquire mutex.  Time spent holding mutex should be short! */
                SpinLockAcquire(&slot->mutex);

                /* If retrying, allow WALInsertSlotRelease to release waiters again */
                if (retry)
                        slot->releaseOK = true;

                /* If I can get the slot, do so quickly. */
                if (slot->exclusive == 0)
                {
                        slot->exclusive++;
                        mustwait = false;
                }
                else
                        mustwait = true;

                if (!mustwait)
                        break;                          /* got the lock */

                Assert(slot->owner != MyProc);

                /*
                 * Add myself to wait queue.
                 */
                proc->lwWaiting = true;
                proc->lwWaitMode = LW_EXCLUSIVE;
                proc->lwWaitLink = NULL;
                if (slot->head == NULL)
                        slot->head = proc;
                else
                        slot->tail->lwWaitLink = proc;
                slot->tail = proc;

                /* Can release the mutex now */
                SpinLockRelease(&slot->mutex);

                /*
                 * Wait until awakened.
                 *
                 * Since we share the process wait semaphore with the regular lock
                 * manager and ProcWaitForSignal, and we may need to acquire a slot
                 * while one of those is pending, it is possible that we get awakened
                 * for a reason other than being signaled by WALInsertSlotRelease. If
                 * so, loop back and wait again.  Once we've gotten the slot,
                 * re-increment the sema by the number of additional signals received,
                 * so that the lock manager or signal manager will see the received
                 * signal when it next waits.
                 */
                for (;;)
                {
                        /* "false" means cannot accept cancel/die interrupt here. */
                        PGSemaphoreLock(&proc->sem, false);
                        if (!proc->lwWaiting)
                                break;
                        extraWaits++;
                }

                /* Now loop back and try to acquire lock again. */
                retry = true;
        }

        slot->owner = proc;

        /*
         * Normally, we initialize the xlogInsertingAt value of the slot to 1,
         * because we don't yet know where in the WAL we're going to insert. It's
         * not critical what it points to right now - leaving it to a too small
         * value just means that WaitXlogInsertionsToFinish() might wait on us
         * unnecessarily, until we update the value (when we finish the insert or
         * move to next page).
         *
         * If we're grabbing all the slots, however, stamp all but the last one
         * with InvalidXLogRecPtr, meaning there is no insert in progress. The last
         * slot is the one that we will update as we proceed with the insert, the
         * rest are held just to keep off other inserters.
         */
        if (slotno != -1 && slotno != num_xloginsert_slots - 1)
                slot->xlogInsertingAt = InvalidXLogRecPtr;
        else
                slot->xlogInsertingAt = 1;

        /* We are done updating shared state of the slot itself. */
        SpinLockRelease(&slot->mutex);

        /*
         * Fix the process wait semaphore's count for any absorbed wakeups.
         */
        while (extraWaits-- > 0)
                PGSemaphoreUnlock(&proc->sem);

        /*
         * If we couldn't get the slot immediately, try another slot next time.
         * On a system with more insertion slots than concurrent inserters, this
         * causes all the inserters to eventually migrate to a slot that no-one
         * else is using. On a system with more inserters than slots, it still
         * causes the inserters to be distributed quite evenly across the slots.
         */
        if (slotno != -1 && retry)
                slotToTry = (slotToTry + 1) % num_xloginsert_slots;
}

/*
 * Wait for the given slot to become free, or for its xlogInsertingAt location
 * to change to something else than 'waitptr'. In other words, wait for the
 * inserter using the given slot to finish its insertion, or to at least make
 * some progress.
 */
static void
WaitOnSlot(volatile XLogInsertSlot *slot, XLogRecPtr waitptr)
{
        PGPROC     *proc = MyProc;
        int                     extraWaits = 0;

        /*
         * Lock out cancel/die interrupts while we sleep on the slot. There is
         * no cleanup mechanism to remove us from the wait queue if we got
         * interrupted.
         */
        HOLD_INTERRUPTS();

        /*
         * Loop here to try to acquire lock after each time we are signaled.
         */
        for (;;)
        {
                bool            mustwait;

                /* Acquire mutex.  Time spent holding mutex should be short! */
                SpinLockAcquire(&slot->mutex);

                /* If I can get the lock, do so quickly. */
                if (slot->exclusive == 0 || slot->xlogInsertingAt != waitptr)
                        mustwait = false;
                else
                        mustwait = true;

                if (!mustwait)
                        break;                          /* the lock was free */

                Assert(slot->owner != MyProc);

                /*
                 * Add myself to wait queue.
                 */
                proc->lwWaiting = true;
                proc->lwWaitMode = LW_WAIT_UNTIL_FREE;
                proc->lwWaitLink = NULL;

                /* waiters are added to the front of the queue */
                proc->lwWaitLink = slot->head;
                if (slot->head == NULL)
                        slot->tail = proc;
                slot->head = proc;

                /* Can release the mutex now */
                SpinLockRelease(&slot->mutex);

                /*
                 * Wait until awakened.
                 *
                 * Since we share the process wait semaphore with other things, like
                 * the regular lock manager and ProcWaitForSignal, and we may need to
                 * acquire an LWLock while one of those is pending, it is possible that
                 * we get awakened for a reason other than being signaled by
                 * LWLockRelease. If so, loop back and wait again.  Once we've gotten
                 * the LWLock, re-increment the sema by the number of additional
                 * signals received, so that the lock manager or signal manager will
                 * see the received signal when it next waits.
                 */
                for (;;)
                {
                        /* "false" means cannot accept cancel/die interrupt here. */
                        PGSemaphoreLock(&proc->sem, false);
                        if (!proc->lwWaiting)
                                break;
                        extraWaits++;
                }

                /* Now loop back and try to acquire lock again. */
        }

        /* We are done updating shared state of the lock itself. */
        SpinLockRelease(&slot->mutex);

        /*
         * Fix the process wait semaphore's count for any absorbed wakeups.
         */
        while (extraWaits-- > 0)
                PGSemaphoreUnlock(&proc->sem);

        /*
         * Now okay to allow cancel/die interrupts.
         */
        RESUME_INTERRUPTS();
}

/*
 * Wake up all processes waiting for us with WaitOnSlot(). Sets our
 * xlogInsertingAt value to EndPos, without releasing the slot.
 */
static void
WakeupWaiters(XLogRecPtr EndPos)
{
        volatile XLogInsertSlot *slot = &XLogCtl->Insert.insertSlots[MySlotNo].slot;
        PGPROC     *head;
        PGPROC     *proc;
        PGPROC     *next;

        /*
         * If we have already reported progress up to the same point, do nothing.
         * No other process can modify xlogInsertingAt, so we can check this before
         * grabbing the spinlock.
         */
        if (slot->xlogInsertingAt == EndPos)
                return;
        /* xlogInsertingAt should not go backwards */
        Assert(slot->xlogInsertingAt < EndPos);

        /* Acquire mutex.  Time spent holding mutex should be short! */
        SpinLockAcquire(&slot->mutex);

        /* we should own the slot */
        Assert(slot->exclusive == 1 && slot->owner == MyProc);

        slot->xlogInsertingAt = EndPos;

        /*
         * See if there are any waiters that need to be woken up.
         */
        head = slot->head;

        if (head != NULL)
        {
                proc = head;

                /* LW_WAIT_UNTIL_FREE waiters are always in the front of the queue */
                next = proc->lwWaitLink;
                while (next && next->lwWaitMode == LW_WAIT_UNTIL_FREE)
                {
                        proc = next;
                        next = next->lwWaitLink;
                }

                /* proc is now the last PGPROC to be released */
                slot->head = next;
                proc->lwWaitLink = NULL;
        }

        /* We are done updating shared state of the lock itself. */
        SpinLockRelease(&slot->mutex);

        /*
         * Awaken any waiters I removed from the queue.
         */
        while (head != NULL)
        {
                proc = head;
                head = proc->lwWaitLink;
                proc->lwWaitLink = NULL;
                proc->lwWaiting = false;
                PGSemaphoreUnlock(&proc->sem);
        }
}

/*
 * Release our insertion slot (or slots, if we're holding them all).
 */
static void
WALInsertSlotRelease(void)
{
        int                     i;

        if (holdingAllSlots)
        {
                for (i = 0; i < num_xloginsert_slots; i++)
                        WALInsertSlotReleaseOne(i);
                holdingAllSlots = false;
        }
        else
                WALInsertSlotReleaseOne(MySlotNo);
}

static void
WALInsertSlotReleaseOne(int slotno)
{
        volatile XLogInsertSlot *slot = &XLogCtl->Insert.insertSlots[slotno].slot;
        PGPROC     *head;
        PGPROC     *proc;

        /* Acquire mutex.  Time spent holding mutex should be short! */
        SpinLockAcquire(&slot->mutex);

        /* we must be holding it */
        Assert(slot->exclusive == 1 && slot->owner == MyProc);

        slot->xlogInsertingAt = InvalidXLogRecPtr;

        /* Release my hold on the slot */
        slot->exclusive = 0;
        slot->owner = NULL;

        /*
         * See if I need to awaken any waiters..
         */
        head = slot->head;
        if (head != NULL)
        {
                if (slot->releaseOK)
                {
                        /*
                         * Remove the to-be-awakened PGPROCs from the queue.
                         */
                        bool            releaseOK = true;

                        proc = head;

                        /*
                         * First wake up any backends that want to be woken up without
                         * acquiring the lock. These are always in the front of the queue.
                         */
                        while (proc->lwWaitMode == LW_WAIT_UNTIL_FREE && proc->lwWaitLink)
                                proc = proc->lwWaitLink;

                        /*
                         * Awaken the first exclusive-waiter, if any.
                         */
                        if (proc->lwWaitLink)
                        {
                                Assert(proc->lwWaitLink->lwWaitMode == LW_EXCLUSIVE);
                                proc = proc->lwWaitLink;
                                releaseOK = false;
                        }
                        /* proc is now the last PGPROC to be released */
                        slot->head = proc->lwWaitLink;
                        proc->lwWaitLink = NULL;

                        slot->releaseOK = releaseOK;
                }
                else
                        head = NULL;
        }

        /* We are done updating shared state of the slot itself. */
        SpinLockRelease(&slot->mutex);

        /*
         * Awaken any waiters I removed from the queue.
         */
        while (head != NULL)
        {
                proc = head;
                head = proc->lwWaitLink;
                proc->lwWaitLink = NULL;
                proc->lwWaiting = false;
                PGSemaphoreUnlock(&proc->sem);
        }

        /*
         * Now okay to allow cancel/die interrupts.
         */
        END_CRIT_SECTION();
}


/*
 * Wait for any WAL insertions < upto to finish.
 *
 * Returns the location of the oldest insertion that is still in-progress.
 * Any WAL prior to that point has been fully copied into WAL buffers, and
 * can be flushed out to disk. Because this waits for any insertions older
 * than 'upto' to finish, the return value is always >= 'upto'.
 *
 * Note: When you are about to write out WAL, you must call this function
 * *before* acquiring WALWriteLock, to avoid deadlocks. This function might
 * need to wait for an insertion to finish (or at least advance to next
 * uninitialized page), and the inserter might need to evict an old WAL buffer
 * to make room for a new one, which in turn requires WALWriteLock.
 */
static XLogRecPtr
WaitXLogInsertionsToFinish(XLogRecPtr upto)
{
        uint64          bytepos;
        XLogRecPtr      reservedUpto;
        XLogRecPtr      finishedUpto;
        volatile XLogCtlInsert *Insert = &XLogCtl->Insert;
        int                     i;

        if (MyProc == NULL)
                elog(PANIC, "cannot wait without a PGPROC structure");

        /* Read the current insert position */
        SpinLockAcquire(&Insert->insertpos_lck);
        bytepos = Insert->CurrBytePos;
        SpinLockRelease(&Insert->insertpos_lck);
        reservedUpto = XLogBytePosToEndRecPtr(bytepos);

        /*
         * No-one should request to flush a piece of WAL that hasn't even been
         * reserved yet. However, it can happen if there is a block with a bogus
         * LSN on disk, for example. XLogFlush checks for that situation and
         * complains, but only after the flush. Here we just assume that to mean
         * that all WAL that has been reserved needs to be finished. In this
         * corner-case, the return value can be smaller than 'upto' argument.
         */
        if (upto > reservedUpto)
        {
                elog(LOG, "request to flush past end of generated WAL; request %X/%X, currpos %X/%X",
                         (uint32) (upto >> 32), (uint32) upto,
                         (uint32) (reservedUpto >> 32), (uint32) reservedUpto);
                upto = reservedUpto;
        }

        /*
         * finishedUpto is our return value, indicating the point upto which
         * all the WAL insertions have been finished. Initialize it to the head
         * of reserved WAL, and as we iterate through the insertion slots, back it
         * out for any insertion that's still in progress.
         */
        finishedUpto = reservedUpto;

        /*
         * Loop through all the slots, sleeping on any in-progress insert older
         * than 'upto'.
         */
        for (i = 0; i < num_xloginsert_slots; i++)
        {
                volatile XLogInsertSlot *slot = &XLogCtl->Insert.insertSlots[i].slot;
                XLogRecPtr insertingat;

        retry:
                /*
                 * We can check if the slot is in use without grabbing the spinlock.
                 * The spinlock acquisition of insertpos_lck before this loop acts
                 * as a memory barrier. If someone acquires the slot after that, it
                 * can't possibly be inserting to anything < reservedUpto. If it was
                 * acquired before that, an unlocked test will return true.
                 */
                if (!slot->exclusive)
                        continue;

                SpinLockAcquire(&slot->mutex);
                /* re-check now that we have the lock */
                if (!slot->exclusive)
                {
                        SpinLockRelease(&slot->mutex);
                        continue;
                }
                insertingat = slot->xlogInsertingAt;
                SpinLockRelease(&slot->mutex);

                if (insertingat == InvalidXLogRecPtr)
                {
                        /*
                         * slot is reserved just to hold off other inserters, there is no
                         * actual insert in progress.
                         */
                        continue;
                }

                /*
                 * This insertion is still in progress. Do we need to wait for it?
                 *
                 * When an inserter acquires a slot, it doesn't reset 'insertingat', so
                 * it will initially point to the old value of some already-finished
                 * insertion. The inserter will update the value as soon as it finishes
                 * the insertion, moves to the next page, or has to do I/O to flush an
                 * old dirty buffer. That means that when we see a slot with
                 * insertingat value < upto, we don't know if that insertion is still
                 * truly in progress, or if the slot is reused by a new inserter that
                 * hasn't updated the insertingat value yet. We have to assume it's the
                 * latter, and wait.
                 */
                if (insertingat < upto)
                {
                        WaitOnSlot(slot, insertingat);
                        goto retry;
                }
                else
                {
                        /*
                         * We don't need to wait for this insertion, but update the
                         * return value.
                         */
                        if (insertingat < finishedUpto)
                                finishedUpto = insertingat;
                }
        }
        return finishedUpto;
}

/*
 * Get a pointer to the right location in the WAL buffer containing the
 * given XLogRecPtr.
 *
 * If the page is not initialized yet, it is initialized. That might require
 * evicting an old dirty buffer from the buffer cache, which means I/O.
 *
 * The caller must ensure that the page containing the requested location
 * isn't evicted yet, and won't be evicted. The way to ensure that is to
 * hold onto an XLogInsertSlot with the xlogInsertingAt position set to
 * something <= ptr. GetXLogBuffer() will update xlogInsertingAt if it needs
 * to evict an old page from the buffer. (This means that once you call
 * GetXLogBuffer() with a given 'ptr', you must not access anything before
 * that point anymore, and must not call GetXLogBuffer() with an older 'ptr'
 * later, because older buffers might be recycled already)
 */
static char *
GetXLogBuffer(XLogRecPtr ptr)
{
        int                     idx;
        XLogRecPtr      endptr;
        static uint64 cachedPage = 0;
        static char *cachedPos = NULL;
        XLogRecPtr      expectedEndPtr;

        /*
         * Fast path for the common case that we need to access again the same
         * page as last time.
         */
        if (ptr / XLOG_BLCKSZ == cachedPage)
        {
                Assert(((XLogPageHeader) cachedPos)->xlp_magic == XLOG_PAGE_MAGIC);
                Assert(((XLogPageHeader) cachedPos)->xlp_pageaddr == ptr - (ptr % XLOG_BLCKSZ));
                return cachedPos + ptr % XLOG_BLCKSZ;
        }

        /*
         * The XLog buffer cache is organized so that a page is always loaded
         * to a particular buffer.  That way we can easily calculate the buffer
         * a given page must be loaded into, from the XLogRecPtr alone.
         */
        idx = XLogRecPtrToBufIdx(ptr);

        /*
         * See what page is loaded in the buffer at the moment. It could be the
         * page we're looking for, or something older. It can't be anything newer
         * - that would imply the page we're looking for has already been written
         * out to disk and evicted, and the caller is responsible for making sure
         * that doesn't happen.
         *
         * However, we don't hold a lock while we read the value. If someone has
         * just initialized the page, it's possible that we get a "torn read" of
         * the XLogRecPtr if 64-bit fetches are not atomic on this platform. In
         * that case we will see a bogus value. That's ok, we'll grab the mapping
         * lock (in AdvanceXLInsertBuffer) and retry if we see anything else than
         * the page we're looking for. But it means that when we do this unlocked
         * read, we might see a value that appears to be ahead of the page we're
         * looking for. Don't PANIC on that, until we've verified the value while
         * holding the lock.
         */
        expectedEndPtr = ptr;
        expectedEndPtr += XLOG_BLCKSZ - ptr % XLOG_BLCKSZ;

        endptr = XLogCtl->xlblocks[idx];
        if (expectedEndPtr != endptr)
        {
                /*
                 * Let others know that we're finished inserting the record up
                 * to the page boundary.
                 */
                WakeupWaiters(expectedEndPtr - XLOG_BLCKSZ);

                AdvanceXLInsertBuffer(ptr, false);
                endptr = XLogCtl->xlblocks[idx];

                if (expectedEndPtr != endptr)
                        elog(PANIC, "could not find WAL buffer for %X/%X",
                                 (uint32) (ptr >> 32) , (uint32) ptr);
        }
        else
        {
                /*
                 * Make sure the initialization of the page is visible to us, and
                 * won't arrive later to overwrite the WAL data we write on the page.
                 */
                pg_memory_barrier();
        }

        /*
         * Found the buffer holding this page. Return a pointer to the right
         * offset within the page.
         */
        cachedPage = ptr / XLOG_BLCKSZ;
        cachedPos = XLogCtl->pages + idx * (Size) XLOG_BLCKSZ;

        Assert(((XLogPageHeader) cachedPos)->xlp_magic == XLOG_PAGE_MAGIC);
        Assert(((XLogPageHeader) cachedPos)->xlp_pageaddr == ptr - (ptr % XLOG_BLCKSZ));

        return cachedPos + ptr % XLOG_BLCKSZ;
}

/*
 * Converts a "usable byte position" to XLogRecPtr. A usable byte position
 * is the position starting from the beginning of WAL, excluding all WAL
 * page headers.
 */
static XLogRecPtr
XLogBytePosToRecPtr(uint64 bytepos)
{
        uint64          fullsegs;
        uint64          fullpages;
        uint64          bytesleft;
        uint32          seg_offset;
        XLogRecPtr      result;

        fullsegs = bytepos / UsableBytesInSegment;
        bytesleft = bytepos % UsableBytesInSegment;

        if (bytesleft < XLOG_BLCKSZ - SizeOfXLogLongPHD)
        {
                /* fits on first page of segment */
                seg_offset = bytesleft + SizeOfXLogLongPHD;
        }
        else
        {
                /* account for the first page on segment with long header */
                seg_offset = XLOG_BLCKSZ;
                bytesleft -= XLOG_BLCKSZ - SizeOfXLogLongPHD;

                fullpages = bytesleft / UsableBytesInPage;
                bytesleft = bytesleft % UsableBytesInPage;

                seg_offset += fullpages * XLOG_BLCKSZ + bytesleft + SizeOfXLogShortPHD;
        }

        XLogSegNoOffsetToRecPtr(fullsegs, seg_offset, result);

        return result;
}

/*
 * Like XLogBytePosToRecPtr, but if the position is at a page boundary,
 * returns a pointer to the beginning of the page (ie. before page header),
 * not to where the first xlog record on that page would go to. This is used
 * when converting a pointer to the end of a record.
 */
static XLogRecPtr
XLogBytePosToEndRecPtr(uint64 bytepos)
{
        uint64          fullsegs;
        uint64          fullpages;
        uint64          bytesleft;
        uint32          seg_offset;
        XLogRecPtr      result;

        fullsegs = bytepos / UsableBytesInSegment;
        bytesleft = bytepos % UsableBytesInSegment;

        if (bytesleft < XLOG_BLCKSZ - SizeOfXLogLongPHD)
        {
                /* fits on first page of segment */
                if (bytesleft == 0)
                        seg_offset = 0;
                else
                        seg_offset = bytesleft + SizeOfXLogLongPHD;
        }
        else
        {
                /* account for the first page on segment with long header */
                seg_offset = XLOG_BLCKSZ;
                bytesleft -= XLOG_BLCKSZ - SizeOfXLogLongPHD;

                fullpages = bytesleft / UsableBytesInPage;
                bytesleft = bytesleft % UsableBytesInPage;

                if (bytesleft == 0)
                        seg_offset += fullpages * XLOG_BLCKSZ + bytesleft;
                else
                        seg_offset += fullpages * XLOG_BLCKSZ + bytesleft + SizeOfXLogShortPHD;
        }

        XLogSegNoOffsetToRecPtr(fullsegs, seg_offset, result);

        return result;
}

/*
 * Convert an XLogRecPtr to a "usable byte position".
 */
static uint64
XLogRecPtrToBytePos(XLogRecPtr ptr)
{
        uint64          fullsegs;
        uint32          fullpages;
        uint32          offset;
        uint64          result;

        XLByteToSeg(ptr, fullsegs);

        fullpages = (ptr % XLOG_SEG_SIZE) / XLOG_BLCKSZ;
        offset = ptr % XLOG_BLCKSZ;

        if (fullpages == 0)
        {
                result = fullsegs * UsableBytesInSegment;
                if (offset > 0)
                {
                        Assert(offset >= SizeOfXLogLongPHD);
                        result += offset - SizeOfXLogLongPHD;
                }
        }
        else
        {
                result = fullsegs * UsableBytesInSegment +
                        (XLOG_BLCKSZ - SizeOfXLogLongPHD) +  /* account for first page */
                        (fullpages - 1) * UsableBytesInPage; /* full pages */
                if (offset > 0)
                {
                        Assert(offset >= SizeOfXLogShortPHD);
                        result += offset - SizeOfXLogShortPHD;
                }
        }

        return result;
}

/*
 * Determine whether the buffer referenced by an XLogRecData item has to
 * be backed up, and if so fill a BkpBlock struct for it.  In any case
 * save the buffer's LSN at *lsn.
 */
static bool
XLogCheckBuffer(XLogRecData *rdata, bool holdsExclusiveLock,
                                XLogRecPtr *lsn, BkpBlock *bkpb)
{
        Page            page;

        page = BufferGetPage(rdata->buffer);

        /*
         * We assume page LSN is first data on *every* page that can be passed to
         * XLogInsert, whether it has the standard page layout or not. We don't
         * need to take the buffer header lock for PageGetLSN if we hold an
         * exclusive lock on the page and/or the relation.
         */
        if (holdsExclusiveLock)
                *lsn = PageGetLSN(page);
        else
                *lsn = BufferGetLSNAtomic(rdata->buffer);

        if (*lsn <= RedoRecPtr)
        {
                /*
                 * The page needs to be backed up, so set up *bkpb
                 */
                BufferGetTag(rdata->buffer, &bkpb->node, &bkpb->fork, &bkpb->block);

                if (rdata->buffer_std)
                {
                        /* Assume we can omit data between pd_lower and pd_upper */
                        uint16          lower = ((PageHeader) page)->pd_lower;
                        uint16          upper = ((PageHeader) page)->pd_upper;

                        if (lower >= SizeOfPageHeaderData &&
                                upper > lower &&
                                upper <= BLCKSZ)
                        {
                                bkpb->hole_offset = lower;
                                bkpb->hole_length = upper - lower;
                        }
                        else
                        {
                                /* No "hole" to compress out */
                                bkpb->hole_offset = 0;
                                bkpb->hole_length = 0;
                        }
                }
                else
                {
                        /* Not a standard page header, don't try to eliminate "hole" */
                        bkpb->hole_offset = 0;
                        bkpb->hole_length = 0;
                }

                return true;                    /* buffer requires backup */
        }

        return false;                           /* buffer does not need to be backed up */
}

/*
 * Initialize XLOG buffers, writing out old buffers if they still contain
 * unwritten data, upto the page containing 'upto'. Or if 'opportunistic' is
 * true, initialize as many pages as we can without having to write out
 * unwritten data. Any new pages are initialized to zeros, with pages headers
 * initialized properly.
 */
static void
AdvanceXLInsertBuffer(XLogRecPtr upto, bool opportunistic)
{
        XLogCtlInsert *Insert = &XLogCtl->Insert;
        int                     nextidx;
        XLogRecPtr      OldPageRqstPtr;
        XLogwrtRqst WriteRqst;
        XLogRecPtr      NewPageEndPtr = InvalidXLogRecPtr;
        XLogRecPtr      NewPageBeginPtr;
        XLogPageHeader NewPage;
        int                     npages = 0;

        LWLockAcquire(WALBufMappingLock, LW_EXCLUSIVE);

        /*
         * Now that we have the lock, check if someone initialized the page
         * already.
         */
        while (upto >= XLogCtl->InitializedUpTo || opportunistic)
        {
                nextidx = XLogRecPtrToBufIdx(XLogCtl->InitializedUpTo);

                /*
                 * Get ending-offset of the buffer page we need to replace (this may
                 * be zero if the buffer hasn't been used yet).  Fall through if it's
                 * already written out.
                 */
                OldPageRqstPtr = XLogCtl->xlblocks[nextidx];
                if (LogwrtResult.Write < OldPageRqstPtr)
                {
                        /*
                         * Nope, got work to do. If we just want to pre-initialize as much
                         * as we can without flushing, give up now.
                         */
                        if (opportunistic)
                                break;

                        /* Before waiting, get info_lck and update LogwrtResult */
                        {
                                /* use volatile pointer to prevent code rearrangement */
                                volatile XLogCtlData *xlogctl = XLogCtl;

                                SpinLockAcquire(&xlogctl->info_lck);
                                if (xlogctl->LogwrtRqst.Write < OldPageRqstPtr)
                                        xlogctl->LogwrtRqst.Write = OldPageRqstPtr;
                                LogwrtResult = xlogctl->LogwrtResult;
                                SpinLockRelease(&xlogctl->info_lck);
                        }

                        /*
                         * Now that we have an up-to-date LogwrtResult value, see if we
                         * still need to write it or if someone else already did.
                         */
                        if (LogwrtResult.Write < OldPageRqstPtr)
                        {
                                /*
                                 * Must acquire write lock. Release WALBufMappingLock first,
                                 * to make sure that all insertions that we need to wait for
                                 * can finish (up to this same position). Otherwise we risk
                                 * deadlock.
                                 */
                                LWLockRelease(WALBufMappingLock);

                                WaitXLogInsertionsToFinish(OldPageRqstPtr);

                                LWLockAcquire(WALWriteLock, LW_EXCLUSIVE);

                                LogwrtResult = XLogCtl->LogwrtResult;
                                if (LogwrtResult.Write >= OldPageRqstPtr)
                                {
                                        /* OK, someone wrote it already */
                                        LWLockRelease(WALWriteLock);
                                }
                                else
                                {
                                        /* Have to write it ourselves */
                                        TRACE_POSTGRESQL_WAL_BUFFER_WRITE_DIRTY_START();
                                        WriteRqst.Write = OldPageRqstPtr;
                                        WriteRqst.Flush = 0;
                                        XLogWrite(WriteRqst, false);
                                        LWLockRelease(WALWriteLock);
                                        TRACE_POSTGRESQL_WAL_BUFFER_WRITE_DIRTY_DONE();
                                }
                                /* Re-acquire WALBufMappingLock and retry */
                                LWLockAcquire(WALBufMappingLock, LW_EXCLUSIVE);
                                continue;
                        }
                }

                /*
                 * Now the next buffer slot is free and we can set it up to be the next
                 * output page.
                 */
                NewPageBeginPtr = XLogCtl->InitializedUpTo;
                NewPageEndPtr = NewPageBeginPtr + XLOG_BLCKSZ;

                Assert(XLogRecPtrToBufIdx(NewPageBeginPtr) == nextidx);

                NewPage = (XLogPageHeader) (XLogCtl->pages + nextidx * (Size) XLOG_BLCKSZ);

                /*
                 * Be sure to re-zero the buffer so that bytes beyond what we've
                 * written will look like zeroes and not valid XLOG records...
                 */
                MemSet((char *) NewPage, 0, XLOG_BLCKSZ);

                /*
                 * Fill the new page's header
                 */
                NewPage   ->xlp_magic = XLOG_PAGE_MAGIC;

                /* NewPage->xlp_info = 0; */    /* done by memset */
                NewPage   ->xlp_tli = ThisTimeLineID;
                NewPage   ->xlp_pageaddr = NewPageBeginPtr;
                /* NewPage->xlp_rem_len = 0; */         /* done by memset */

                /*
                 * If online backup is not in progress, mark the header to indicate
                 * that* WAL records beginning in this page have removable backup
                 * blocks.  This allows the WAL archiver to know whether it is safe to
                 * compress archived WAL data by transforming full-block records into
                 * the non-full-block format.  It is sufficient to record this at the
                 * page level because we force a page switch (in fact a segment switch)
                 * when starting a backup, so the flag will be off before any records
                 * can be written during the backup.  At the end of a backup, the last
                 * page will be marked as all unsafe when perhaps only part is unsafe,
                 * but at worst the archiver would miss the opportunity to compress a
                 * few records.
                 */
                if (!Insert->forcePageWrites)
                        NewPage   ->xlp_info |= XLP_BKP_REMOVABLE;

                /*
                 * If first page of an XLOG segment file, make it a long header.
                 */
                if ((NewPage->xlp_pageaddr % XLogSegSize) == 0)
                {
                        XLogLongPageHeader NewLongPage = (XLogLongPageHeader) NewPage;

                        NewLongPage->xlp_sysid = ControlFile->system_identifier;
                        NewLongPage->xlp_seg_size = XLogSegSize;
                        NewLongPage->xlp_xlog_blcksz = XLOG_BLCKSZ;
                        NewPage   ->xlp_info |= XLP_LONG_HEADER;
                }

                /*
                 * Make sure the initialization of the page becomes visible to others
                 * before the xlblocks update. GetXLogBuffer() reads xlblocks without
                 * holding a lock.
                 */
                pg_write_barrier();

                *((volatile XLogRecPtr *) &XLogCtl->xlblocks[nextidx]) = NewPageEndPtr;

                XLogCtl->InitializedUpTo = NewPageEndPtr;

                npages++;
        }
        LWLockRelease(WALBufMappingLock);

#ifdef WAL_DEBUG
        if (npages > 0)
        {
                elog(DEBUG1, "initialized %d pages, upto %X/%X",
                         npages, (uint32) (NewPageEndPtr >> 32), (uint32) NewPageEndPtr);
        }
#endif
}

/*
 * Check whether we've consumed enough xlog space that a checkpoint is needed.
 *
 * new_segno indicates a log file that has just been filled up (or read
 * during recovery). We measure the distance from RedoRecPtr to new_segno
 * and see if that exceeds CheckPointSegments.
 *
 * Note: it is caller's responsibility that RedoRecPtr is up-to-date.
 */
static bool
XLogCheckpointNeeded(XLogSegNo new_segno)
{
        XLogSegNo       old_segno;

        XLByteToSeg(RedoRecPtr, old_segno);

        if (new_segno >= old_segno + (uint64) (CheckPointSegments - 1))
                return true;
        return false;
}

/*
 * Write and/or fsync the log at least as far as WriteRqst indicates.
 *
 * If flexible == TRUE, we don't have to write as far as WriteRqst, but
 * may stop at any convenient boundary (such as a cache or logfile boundary).
 * This option allows us to avoid uselessly issuing multiple writes when a
 * single one would do.
 *
 * Must be called with WALWriteLock held. WaitXLogInsertionsToFinish(WriteRqst)
 * must be called before grabbing the lock, to make sure the data is ready to
 * write.
 */
static void
XLogWrite(XLogwrtRqst WriteRqst, bool flexible)
{
        bool            ispartialpage;
        bool            last_iteration;
        bool            finishing_seg;
        bool            use_existent;
        int                     curridx;
        int                     npages;
        int                     startidx;
        uint32          startoffset;

        /* We should always be inside a critical section here */
        Assert(CritSectionCount > 0);

        /*
         * Update local LogwrtResult (caller probably did this already, but...)
         */
        LogwrtResult = XLogCtl->LogwrtResult;

        /*
         * Since successive pages in the xlog cache are consecutively allocated,
         * we can usually gather multiple pages together and issue just one
         * write() call.  npages is the number of pages we have determined can be
         * written together; startidx is the cache block index of the first one,
         * and startoffset is the file offset at which it should go. The latter
         * two variables are only valid when npages > 0, but we must initialize
         * all of them to keep the compiler quiet.
         */
        npages = 0;
        startidx = 0;
        startoffset = 0;

        /*
         * Within the loop, curridx is the cache block index of the page to
         * consider writing.  Begin at the buffer containing the next unwritten
         * page, or last partially written page.
         */
        curridx = XLogRecPtrToBufIdx(LogwrtResult.Write);

        while (LogwrtResult.Write < WriteRqst.Write)
        {
                /*
                 * Make sure we're not ahead of the insert process.  This could happen
                 * if we're passed a bogus WriteRqst.Write that is past the end of the
                 * last page that's been initialized by AdvanceXLInsertBuffer.
                 */
                XLogRecPtr EndPtr = XLogCtl->xlblocks[curridx];
                if (LogwrtResult.Write >= EndPtr)
                        elog(PANIC, "xlog write request %X/%X is past end of log %X/%X",
                                 (uint32) (LogwrtResult.Write >> 32),
                                 (uint32) LogwrtResult.Write,
                                 (uint32) (EndPtr >> 32), (uint32) EndPtr);

                /* Advance LogwrtResult.Write to end of current buffer page */
                LogwrtResult.Write = EndPtr;
                ispartialpage = WriteRqst.Write < LogwrtResult.Write;

                if (!XLByteInPrevSeg(LogwrtResult.Write, openLogSegNo))
                {
                        /*
                         * Switch to new logfile segment.  We cannot have any pending
                         * pages here (since we dump what we have at segment end).
                         */
                        Assert(npages == 0);
                        if (openLogFile >= 0)
                                XLogFileClose();
                        XLByteToPrevSeg(LogwrtResult.Write, openLogSegNo);

                        /* create/use new log file */
                        use_existent = true;
                        openLogFile = XLogFileInit(openLogSegNo, &use_existent, true);
                        openLogOff = 0;
                }

                /* Make sure we have the current logfile open */
                if (openLogFile < 0)
                {
                        XLByteToPrevSeg(LogwrtResult.Write, openLogSegNo);
                        openLogFile = XLogFileOpen(openLogSegNo);
                        openLogOff = 0;
                }

                /* Add current page to the set of pending pages-to-dump */
                if (npages == 0)
                {
                        /* first of group */
                        startidx = curridx;
                        startoffset = (LogwrtResult.Write - XLOG_BLCKSZ) % XLogSegSize;
                }
                npages++;

                /*
                 * Dump the set if this will be the last loop iteration, or if we are
                 * at the last page of the cache area (since the next page won't be
                 * contiguous in memory), or if we are at the end of the logfile
                 * segment.
                 */
                last_iteration = WriteRqst.Write <= LogwrtResult.Write;

                finishing_seg = !ispartialpage &&
                        (startoffset + npages * XLOG_BLCKSZ) >= XLogSegSize;

                if (last_iteration ||
                        curridx == XLogCtl->XLogCacheBlck ||
                        finishing_seg)
                {
                        char       *from;
                        Size            nbytes;
                        Size            nleft;
                        int                     written;

                        /* Need to seek in the file? */
                        if (openLogOff != startoffset)
                        {
                                if (lseek(openLogFile, (off_t) startoffset, SEEK_SET) < 0)
                                        ereport(PANIC,
                                                        (errcode_for_file_access(),
                                         errmsg("could not seek in log file %s to offset %u: %m",
                                                        XLogFileNameP(ThisTimeLineID, openLogSegNo),
                                                        startoffset)));
                                openLogOff = startoffset;
                        }

                        /* OK to write the page(s) */
                        from = XLogCtl->pages + startidx * (Size) XLOG_BLCKSZ;
                        nbytes = npages * (Size) XLOG_BLCKSZ;
                        nleft = nbytes;
                        do
                        {
                                errno = 0;
                                written  = write(openLogFile, from, nleft);
                                if (written <= 0)
                                {
                                        if (errno == EINTR)
                                                continue;
                                        ereport(PANIC,
                                                        (errcode_for_file_access(),
                                                         errmsg("could not write to log file %s "
                                                                        "at offset %u, length %lu: %m",
                                                                        XLogFileNameP(ThisTimeLineID, openLogSegNo),
                                                                        openLogOff, (unsigned long) nbytes)));
                                }
                                nleft -= written;
                                from += written;
                        } while (nleft > 0);

                        /* Update state for write */
                        openLogOff += nbytes;
                        npages = 0;

                        /*
                         * If we just wrote the whole last page of a logfile segment,
                         * fsync the segment immediately.  This avoids having to go back
                         * and re-open prior segments when an fsync request comes along
                         * later. Doing it here ensures that one and only one backend will
                         * perform this fsync.
                         *
                         * This is also the right place to notify the Archiver that the
                         * segment is ready to copy to archival storage, and to update the
                         * timer for archive_timeout, and to signal for a checkpoint if
                         * too many logfile segments have been used since the last
                         * checkpoint.
                         */
                        if (finishing_seg)
                        {
                                issue_xlog_fsync(openLogFile, openLogSegNo);

                                /* signal that we need to wakeup walsenders later */
                                WalSndWakeupRequest();

                                LogwrtResult.Flush = LogwrtResult.Write;                /* end of page */

                                if (XLogArchivingActive())
                                        XLogArchiveNotifySeg(openLogSegNo);

                                XLogCtl->lastSegSwitchTime = (pg_time_t) time(NULL);

                                /*
                                 * Request a checkpoint if we've consumed too much xlog since
                                 * the last one.  For speed, we first check using the local
                                 * copy of RedoRecPtr, which might be out of date; if it looks
                                 * like a checkpoint is needed, forcibly update RedoRecPtr and
                                 * recheck.
                                 */
                                if (IsUnderPostmaster && XLogCheckpointNeeded(openLogSegNo))
                                {
                                        (void) GetRedoRecPtr();
                                        if (XLogCheckpointNeeded(openLogSegNo))
                                                RequestCheckpoint(CHECKPOINT_CAUSE_XLOG);
                                }
                        }
                }

                if (ispartialpage)
                {
                        /* Only asked to write a partial page */
                        LogwrtResult.Write = WriteRqst.Write;
                        break;
                }
                curridx = NextBufIdx(curridx);

                /* If flexible, break out of loop as soon as we wrote something */
                if (flexible && npages == 0)
                        break;
        }

        Assert(npages == 0);

        /*
         * If asked to flush, do so
         */
        if (LogwrtResult.Flush < WriteRqst.Flush &&
                LogwrtResult.Flush < LogwrtResult.Write)

        {
                /*
                 * Could get here without iterating above loop, in which case we might
                 * have no open file or the wrong one.  However, we do not need to
                 * fsync more than one file.
                 */
                if (sync_method != SYNC_METHOD_OPEN &&
                        sync_method != SYNC_METHOD_OPEN_DSYNC)
                {
                        if (openLogFile >= 0 &&
                                !XLByteInPrevSeg(LogwrtResult.Write, openLogSegNo))
                                XLogFileClose();
                        if (openLogFile < 0)
                        {
                                XLByteToPrevSeg(LogwrtResult.Write, openLogSegNo);
                                openLogFile = XLogFileOpen(openLogSegNo);
                                openLogOff = 0;
                        }

                        issue_xlog_fsync(openLogFile, openLogSegNo);
                }

                /* signal that we need to wakeup walsenders later */
                WalSndWakeupRequest();

                LogwrtResult.Flush = LogwrtResult.Write;
        }

        /*
         * Update shared-memory status
         *
         * We make sure that the shared 'request' values do not fall behind the
         * 'result' values.  This is not absolutely essential, but it saves some
         * code in a couple of places.
         */
        {
                /* use volatile pointer to prevent code rearrangement */
                volatile XLogCtlData *xlogctl = XLogCtl;

                SpinLockAcquire(&xlogctl->info_lck);
                xlogctl->LogwrtResult = LogwrtResult;
                if (xlogctl->LogwrtRqst.Write < LogwrtResult.Write)
                        xlogctl->LogwrtRqst.Write = LogwrtResult.Write;
                if (xlogctl->LogwrtRqst.Flush < LogwrtResult.Flush)
                        xlogctl->LogwrtRqst.Flush = LogwrtResult.Flush;
                SpinLockRelease(&xlogctl->info_lck);
        }
}

/*
 * Record the LSN for an asynchronous transaction commit/abort
 * and nudge the WALWriter if there is work for it to do.
 * (This should not be called for synchronous commits.)
 */
void
XLogSetAsyncXactLSN(XLogRecPtr asyncXactLSN)
{
        XLogRecPtr      WriteRqstPtr = asyncXactLSN;
        bool            sleeping;

        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;

        SpinLockAcquire(&xlogctl->info_lck);
        LogwrtResult = xlogctl->LogwrtResult;
        sleeping = xlogctl->WalWriterSleeping;
        if (xlogctl->asyncXactLSN < asyncXactLSN)
                xlogctl->asyncXactLSN = asyncXactLSN;
        SpinLockRelease(&xlogctl->info_lck);

        /*
         * If the WALWriter is sleeping, we should kick it to make it come out of
         * low-power mode.      Otherwise, determine whether there's a full page of
         * WAL available to write.
         */
        if (!sleeping)
        {
                /* back off to last completed page boundary */
                WriteRqstPtr -= WriteRqstPtr % XLOG_BLCKSZ;

                /* if we have already flushed that far, we're done */
                if (WriteRqstPtr <= LogwrtResult.Flush)
                        return;
        }

        /*
         * Nudge the WALWriter: it has a full page of WAL to write, or we want it
         * to come out of low-power mode so that this async commit will reach disk
         * within the expected amount of time.
         */
        if (ProcGlobal->walwriterLatch)
                SetLatch(ProcGlobal->walwriterLatch);
}

/*
 * Advance minRecoveryPoint in control file.
 *
 * If we crash during recovery, we must reach this point again before the
 * database is consistent.
 *
 * If 'force' is true, 'lsn' argument is ignored. Otherwise, minRecoveryPoint
 * is only updated if it's not already greater than or equal to 'lsn'.
 */
static void
UpdateMinRecoveryPoint(XLogRecPtr lsn, bool force)
{
        /* Quick check using our local copy of the variable */
        if (!updateMinRecoveryPoint || (!force && lsn <= minRecoveryPoint))
                return;

        LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);

        /* update local copy */
        minRecoveryPoint = ControlFile->minRecoveryPoint;
        minRecoveryPointTLI = ControlFile->minRecoveryPointTLI;

        /*
         * An invalid minRecoveryPoint means that we need to recover all the WAL,
         * i.e., we're doing crash recovery.  We never modify the control file's
         * value in that case, so we can short-circuit future checks here too.
         */
        if (minRecoveryPoint == 0)
                updateMinRecoveryPoint = false;
        else if (force || minRecoveryPoint < lsn)
        {
                /* use volatile pointer to prevent code rearrangement */
                volatile XLogCtlData *xlogctl = XLogCtl;
                XLogRecPtr      newMinRecoveryPoint;
                TimeLineID      newMinRecoveryPointTLI;

                /*
                 * To avoid having to update the control file too often, we update it
                 * all the way to the last record being replayed, even though 'lsn'
                 * would suffice for correctness.  This also allows the 'force' case
                 * to not need a valid 'lsn' value.
                 *
                 * Another important reason for doing it this way is that the passed
                 * 'lsn' value could be bogus, i.e., past the end of available WAL, if
                 * the caller got it from a corrupted heap page.  Accepting such a
                 * value as the min recovery point would prevent us from coming up at
                 * all.  Instead, we just log a warning and continue with recovery.
                 * (See also the comments about corrupt LSNs in XLogFlush.)
                 */
                SpinLockAcquire(&xlogctl->info_lck);
                newMinRecoveryPoint = xlogctl->replayEndRecPtr;
                newMinRecoveryPointTLI = xlogctl->replayEndTLI;
                SpinLockRelease(&xlogctl->info_lck);

                if (!force && newMinRecoveryPoint < lsn)
                        elog(WARNING,
                           "xlog min recovery request %X/%X is past current point %X/%X",
                                 (uint32) (lsn >> 32), (uint32) lsn,
                                 (uint32) (newMinRecoveryPoint >> 32),
                                 (uint32) newMinRecoveryPoint);

                /* update control file */
                if (ControlFile->minRecoveryPoint < newMinRecoveryPoint)
                {
                        ControlFile->minRecoveryPoint = newMinRecoveryPoint;
                        ControlFile->minRecoveryPointTLI = newMinRecoveryPointTLI;
                        UpdateControlFile();
                        minRecoveryPoint = newMinRecoveryPoint;
                        minRecoveryPointTLI = newMinRecoveryPointTLI;

                        ereport(DEBUG2,
                                (errmsg("updated min recovery point to %X/%X on timeline %u",
                                                (uint32) (minRecoveryPoint >> 32),
                                                (uint32) minRecoveryPoint,
                                                newMinRecoveryPointTLI)));
                }
        }
        LWLockRelease(ControlFileLock);
}

/*
 * Ensure that all XLOG data through the given position is flushed to disk.
 *
 * NOTE: this differs from XLogWrite mainly in that the WALWriteLock is not
 * already held, and we try to avoid acquiring it if possible.
 */
void
XLogFlush(XLogRecPtr record)
{
        XLogRecPtr      WriteRqstPtr;
        XLogwrtRqst WriteRqst;

        /*
         * During REDO, we are reading not writing WAL.  Therefore, instead of
         * trying to flush the WAL, we should update minRecoveryPoint instead. We
         * test XLogInsertAllowed(), not InRecovery, because we need checkpointer
         * to act this way too, and because when it tries to write the
         * end-of-recovery checkpoint, it should indeed flush.
         */
        if (!XLogInsertAllowed())
        {
                UpdateMinRecoveryPoint(record, false);
                return;
        }

        /* Quick exit if already known flushed */
        if (record <= LogwrtResult.Flush)
                return;

#ifdef WAL_DEBUG
        if (XLOG_DEBUG)
                elog(LOG, "xlog flush request %X/%X; write %X/%X; flush %X/%X",
                         (uint32) (record >> 32), (uint32) record,
                         (uint32) (LogwrtResult.Write >> 32), (uint32) LogwrtResult.Write,
                   (uint32) (LogwrtResult.Flush >> 32), (uint32) LogwrtResult.Flush);
#endif

        START_CRIT_SECTION();

        /*
         * Since fsync is usually a horribly expensive operation, we try to
         * piggyback as much data as we can on each fsync: if we see any more data
         * entered into the xlog buffer, we'll write and fsync that too, so that
         * the final value of LogwrtResult.Flush is as large as possible. This
         * gives us some chance of avoiding another fsync immediately after.
         */

        /* initialize to given target; may increase below */
        WriteRqstPtr = record;

        /*
         * Now wait until we get the write lock, or someone else does the flush
         * for us.
         */
        for (;;)
        {
                /* use volatile pointer to prevent code rearrangement */
                volatile XLogCtlData *xlogctl = XLogCtl;
                XLogRecPtr      insertpos;

                /* read LogwrtResult and update local state */
                SpinLockAcquire(&xlogctl->info_lck);
                if (WriteRqstPtr < xlogctl->LogwrtRqst.Write)
                        WriteRqstPtr = xlogctl->LogwrtRqst.Write;
                LogwrtResult = xlogctl->LogwrtResult;
                SpinLockRelease(&xlogctl->info_lck);

                /* done already? */
                if (record <= LogwrtResult.Flush)
                        break;

                /*
                 * Before actually performing the write, wait for all in-flight
                 * insertions to the pages we're about to write to finish.
                 */
                insertpos = WaitXLogInsertionsToFinish(WriteRqstPtr);

                /*
                 * Try to get the write lock. If we can't get it immediately, wait
                 * until it's released, and recheck if we still need to do the flush
                 * or if the backend that held the lock did it for us already. This
                 * helps to maintain a good rate of group committing when the system
                 * is bottlenecked by the speed of fsyncing.
                 */
                if (!LWLockAcquireOrWait(WALWriteLock, LW_EXCLUSIVE))
                {
                        /*
                         * The lock is now free, but we didn't acquire it yet. Before we
                         * do, loop back to check if someone else flushed the record for
                         * us already.
                         */
                        continue;
                }

                /* Got the lock; recheck whether request is satisfied */
                LogwrtResult = XLogCtl->LogwrtResult;
                if (record <= LogwrtResult.Flush)
                {
                        LWLockRelease(WALWriteLock);
                        break;
                }

                /*
                 * Sleep before flush! By adding a delay here, we may give further
                 * backends the opportunity to join the backlog of group commit
                 * followers; this can significantly improve transaction throughput,
                 * at the risk of increasing transaction latency.
                 *
                 * We do not sleep if enableFsync is not turned on, nor if there are
                 * fewer than CommitSiblings other backends with active transactions.
                 */
                if (CommitDelay > 0 && enableFsync &&
                        MinimumActiveBackends(CommitSiblings))
                {
                        pg_usleep(CommitDelay);

                        /*
                         * Re-check how far we can now flush the WAL. It's generally not
                         * safe to call WaitXLogInsetionsToFinish while holding
                         * WALWriteLock, because an in-progress insertion might need to
                         * also grab WALWriteLock to make progress. But we know that all
                         * the insertions up to insertpos have already finished, because
                         * that's what the earlier WaitXLogInsertionsToFinish() returned.
                         * We're only calling it again to allow insertpos to be moved
                         * further forward, not to actually wait for anyone.
                         */
                        insertpos = WaitXLogInsertionsToFinish(insertpos);
                }

                /* try to write/flush later additions to XLOG as well */
                WriteRqst.Write = insertpos;
                WriteRqst.Flush = insertpos;

                XLogWrite(WriteRqst, false);

                LWLockRelease(WALWriteLock);
                /* done */
                break;
        }

        END_CRIT_SECTION();

        /* wake up walsenders now that we've released heavily contended locks */
        WalSndWakeupProcessRequests();

        /*
         * If we still haven't flushed to the request point then we have a
         * problem; most likely, the requested flush point is past end of XLOG.
         * This has been seen to occur when a disk page has a corrupted LSN.
         *
         * Formerly we treated this as a PANIC condition, but that hurts the
         * system's robustness rather than helping it: we do not want to take down
         * the whole system due to corruption on one data page.  In particular, if
         * the bad page is encountered again during recovery then we would be
         * unable to restart the database at all!  (This scenario actually
         * happened in the field several times with 7.1 releases.)      As of 8.4, bad
         * LSNs encountered during recovery are UpdateMinRecoveryPoint's problem;
         * the only time we can reach here during recovery is while flushing the
         * end-of-recovery checkpoint record, and we don't expect that to have a
         * bad LSN.
         *
         * Note that for calls from xact.c, the ERROR will be promoted to PANIC
         * since xact.c calls this routine inside a critical section.  However,
         * calls from bufmgr.c are not within critical sections and so we will not
         * force a restart for a bad LSN on a data page.
         */
        if (LogwrtResult.Flush < record)
                elog(ERROR,
                "xlog flush request %X/%X is not satisfied --- flushed only to %X/%X",
                         (uint32) (record >> 32), (uint32) record,
                   (uint32) (LogwrtResult.Flush >> 32), (uint32) LogwrtResult.Flush);
}

/*
 * Flush xlog, but without specifying exactly where to flush to.
 *
 * We normally flush only completed blocks; but if there is nothing to do on
 * that basis, we check for unflushed async commits in the current incomplete
 * block, and flush through the latest one of those.  Thus, if async commits
 * are not being used, we will flush complete blocks only.      We can guarantee
 * that async commits reach disk after at most three cycles; normally only
 * one or two.  (When flushing complete blocks, we allow XLogWrite to write
 * "flexibly", meaning it can stop at the end of the buffer ring; this makes a
 * difference only with very high load or long wal_writer_delay, but imposes
 * one extra cycle for the worst case for async commits.)
 *
 * This routine is invoked periodically by the background walwriter process.
 *
 * Returns TRUE if we flushed anything.
 */
bool
XLogBackgroundFlush(void)
{
        XLogRecPtr      WriteRqstPtr;
        bool            flexible = true;
        bool            wrote_something = false;

        /* XLOG doesn't need flushing during recovery */
        if (RecoveryInProgress())
                return false;

        /* read LogwrtResult and update local state */
        {
                /* use volatile pointer to prevent code rearrangement */
                volatile XLogCtlData *xlogctl = XLogCtl;

                SpinLockAcquire(&xlogctl->info_lck);
                LogwrtResult = xlogctl->LogwrtResult;
                WriteRqstPtr = xlogctl->LogwrtRqst.Write;
                SpinLockRelease(&xlogctl->info_lck);
        }

        /* back off to last completed page boundary */
        WriteRqstPtr -= WriteRqstPtr % XLOG_BLCKSZ;

        /* if we have already flushed that far, consider async commit records */
        if (WriteRqstPtr <= LogwrtResult.Flush)
        {
                /* use volatile pointer to prevent code rearrangement */
                volatile XLogCtlData *xlogctl = XLogCtl;

                SpinLockAcquire(&xlogctl->info_lck);
                WriteRqstPtr = xlogctl->asyncXactLSN;
                SpinLockRelease(&xlogctl->info_lck);
                flexible = false;               /* ensure it all gets written */
        }

        /*
         * If already known flushed, we're done. Just need to check if we are
         * holding an open file handle to a logfile that's no longer in use,
         * preventing the file from being deleted.
         */
        if (WriteRqstPtr <= LogwrtResult.Flush)
        {
                if (openLogFile >= 0)
                {
                        if (!XLByteInPrevSeg(LogwrtResult.Write, openLogSegNo))
                        {
                                XLogFileClose();
                        }
                }
                return false;
        }

#ifdef WAL_DEBUG
        if (XLOG_DEBUG)
                elog(LOG, "xlog bg flush request %X/%X; write %X/%X; flush %X/%X",
                         (uint32) (WriteRqstPtr >> 32), (uint32) WriteRqstPtr,
                         (uint32) (LogwrtResult.Write >> 32), (uint32) LogwrtResult.Write,
                   (uint32) (LogwrtResult.Flush >> 32), (uint32) LogwrtResult.Flush);
#endif

        START_CRIT_SECTION();

        /* now wait for any in-progress insertions to finish and get write lock */
        WaitXLogInsertionsToFinish(WriteRqstPtr);
        LWLockAcquire(WALWriteLock, LW_EXCLUSIVE);
        LogwrtResult = XLogCtl->LogwrtResult;
        if (WriteRqstPtr > LogwrtResult.Flush)
        {
                XLogwrtRqst WriteRqst;

                WriteRqst.Write = WriteRqstPtr;
                WriteRqst.Flush = WriteRqstPtr;
                XLogWrite(WriteRqst, flexible);
                wrote_something = true;
        }
        LWLockRelease(WALWriteLock);

        END_CRIT_SECTION();

        /* wake up walsenders now that we've released heavily contended locks */
        WalSndWakeupProcessRequests();

        /*
         * Great, done. To take some work off the critical path, try to initialize
         * as many of the no-longer-needed WAL buffers for future use as we can.
         */
        AdvanceXLInsertBuffer(InvalidXLogRecPtr, true);

        return wrote_something;
}

/*
 * Test whether XLOG data has been flushed up to (at least) the given position.
 *
 * Returns true if a flush is still needed.  (It may be that someone else
 * is already in process of flushing that far, however.)
 */
bool
XLogNeedsFlush(XLogRecPtr record)
{
        /*
         * During recovery, we don't flush WAL but update minRecoveryPoint
         * instead. So "needs flush" is taken to mean whether minRecoveryPoint
         * would need to be updated.
         */
        if (RecoveryInProgress())
        {
                /* Quick exit if already known updated */
                if (record <= minRecoveryPoint || !updateMinRecoveryPoint)
                        return false;

                /*
                 * Update local copy of minRecoveryPoint. But if the lock is busy,
                 * just return a conservative guess.
                 */
                if (!LWLockConditionalAcquire(ControlFileLock, LW_SHARED))
                        return true;
                minRecoveryPoint = ControlFile->minRecoveryPoint;
                minRecoveryPointTLI = ControlFile->minRecoveryPointTLI;
                LWLockRelease(ControlFileLock);

                /*
                 * An invalid minRecoveryPoint means that we need to recover all the
                 * WAL, i.e., we're doing crash recovery.  We never modify the control
                 * file's value in that case, so we can short-circuit future checks
                 * here too.
                 */
                if (minRecoveryPoint == 0)
                        updateMinRecoveryPoint = false;

                /* check again */
                if (record <= minRecoveryPoint || !updateMinRecoveryPoint)
                        return false;
                else
                        return true;
        }

        /* Quick exit if already known flushed */
        if (record <= LogwrtResult.Flush)
                return false;

        /* read LogwrtResult and update local state */
        {
                /* use volatile pointer to prevent code rearrangement */
                volatile XLogCtlData *xlogctl = XLogCtl;

                SpinLockAcquire(&xlogctl->info_lck);
                LogwrtResult = xlogctl->LogwrtResult;
                SpinLockRelease(&xlogctl->info_lck);
        }

        /* check again */
        if (record <= LogwrtResult.Flush)
                return false;

        return true;
}

/*
 * Create a new XLOG file segment, or open a pre-existing one.
 *
 * log, seg: identify segment to be created/opened.
 *
 * *use_existent: if TRUE, OK to use a pre-existing file (else, any
 * pre-existing file will be deleted).  On return, TRUE if a pre-existing
 * file was used.
 *
 * use_lock: if TRUE, acquire ControlFileLock while moving file into
 * place.  This should be TRUE except during bootstrap log creation.  The
 * caller must *not* hold the lock at call.
 *
 * Returns FD of opened file.
 *
 * Note: errors here are ERROR not PANIC because we might or might not be
 * inside a critical section (eg, during checkpoint there is no reason to
 * take down the system on failure).  They will promote to PANIC if we are
 * in a critical section.
 */
int
XLogFileInit(XLogSegNo logsegno, bool *use_existent, bool use_lock)
{
        char            path[MAXPGPATH];
        char            tmppath[MAXPGPATH];
        char       *zbuffer;
        XLogSegNo       installed_segno;
        int                     max_advance;
        int                     fd;
        int                     nbytes;

        XLogFilePath(path, ThisTimeLineID, logsegno);

        /*
         * Try to use existent file (checkpoint maker may have created it already)
         */
        if (*use_existent)
        {
                fd = BasicOpenFile(path, O_RDWR | PG_BINARY | get_sync_bit(sync_method),
                                                   S_IRUSR | S_IWUSR);
                if (fd < 0)
                {
                        if (errno != ENOENT)
                                ereport(ERROR,
                                                (errcode_for_file_access(),
                                                 errmsg("could not open file \"%s\": %m", path)));
                }
                else
                        return fd;
        }

        /*
         * Initialize an empty (all zeroes) segment.  NOTE: it is possible that
         * another process is doing the same thing.  If so, we will end up
         * pre-creating an extra log segment.  That seems OK, and better than
         * holding the lock throughout this lengthy process.
         */
        elog(DEBUG2, "creating and filling new WAL file");

        snprintf(tmppath, MAXPGPATH, XLOGDIR "/xlogtemp.%d", (int) getpid());

        unlink(tmppath);

        /*
         * Allocate a buffer full of zeros. This is done before opening the file
         * so that we don't leak the file descriptor if palloc fails.
         *
         * Note: palloc zbuffer, instead of just using a local char array, to
         * ensure it is reasonably well-aligned; this may save a few cycles
         * transferring data to the kernel.
         */
        zbuffer = (char *) palloc0(XLOG_BLCKSZ);

        /* do not use get_sync_bit() here --- want to fsync only at end of fill */
        fd = BasicOpenFile(tmppath, O_RDWR | O_CREAT | O_EXCL | PG_BINARY,
                                           S_IRUSR | S_IWUSR);
        if (fd < 0)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not create file \"%s\": %m", tmppath)));

        /*
         * Zero-fill the file.  We have to do this the hard way to ensure that all
         * the file space has really been allocated --- on platforms that allow
         * "holes" in files, just seeking to the end doesn't allocate intermediate
         * space.  This way, we know that we have all the space and (after the
         * fsync below) that all the indirect blocks are down on disk.  Therefore,
         * fdatasync(2) or O_DSYNC will be sufficient to sync future writes to the
         * log file.
         */
        for (nbytes = 0; nbytes < XLogSegSize; nbytes += XLOG_BLCKSZ)
        {
                errno = 0;
                if ((int) write(fd, zbuffer, XLOG_BLCKSZ) != (int) XLOG_BLCKSZ)
                {
                        int                     save_errno = errno;

                        /*
                         * If we fail to make the file, delete it to release disk space
                         */
                        unlink(tmppath);

                        close(fd);

                        /* if write didn't set errno, assume problem is no disk space */
                        errno = save_errno ? save_errno : ENOSPC;

                        ereport(ERROR,
                                        (errcode_for_file_access(),
                                         errmsg("could not write to file \"%s\": %m", tmppath)));
                }
        }
        pfree(zbuffer);

        if (pg_fsync(fd) != 0)
        {
                close(fd);
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not fsync file \"%s\": %m", tmppath)));
        }

        if (close(fd))
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not close file \"%s\": %m", tmppath)));

        /*
         * Now move the segment into place with its final name.
         *
         * If caller didn't want to use a pre-existing file, get rid of any
         * pre-existing file.  Otherwise, cope with possibility that someone else
         * has created the file while we were filling ours: if so, use ours to
         * pre-create a future log segment.
         */
        installed_segno = logsegno;
        max_advance = XLOGfileslop;
        if (!InstallXLogFileSegment(&installed_segno, tmppath,
                                                                *use_existent, &max_advance,
                                                                use_lock))
        {
                /*
                 * No need for any more future segments, or InstallXLogFileSegment()
                 * failed to rename the file into place. If the rename failed, opening
                 * the file below will fail.
                 */
                unlink(tmppath);
        }

        /* Set flag to tell caller there was no existent file */
        *use_existent = false;

        /* Now open original target segment (might not be file I just made) */
        fd = BasicOpenFile(path, O_RDWR | PG_BINARY | get_sync_bit(sync_method),
                                           S_IRUSR | S_IWUSR);
        if (fd < 0)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not open file \"%s\": %m", path)));

        elog(DEBUG2, "done creating and filling new WAL file");

        return fd;
}

/*
 * Create a new XLOG file segment by copying a pre-existing one.
 *
 * destsegno: identify segment to be created.
 *
 * srcTLI, srclog, srcseg: identify segment to be copied (could be from
 *              a different timeline)
 *
 * Currently this is only used during recovery, and so there are no locking
 * considerations.      But we should be just as tense as XLogFileInit to avoid
 * emplacing a bogus file.
 */
static void
XLogFileCopy(XLogSegNo destsegno, TimeLineID srcTLI, XLogSegNo srcsegno)
{
        char            path[MAXPGPATH];
        char            tmppath[MAXPGPATH];
        char            buffer[XLOG_BLCKSZ];
        int                     srcfd;
        int                     fd;
        int                     nbytes;

        /*
         * Open the source file
         */
        XLogFilePath(path, srcTLI, srcsegno);
        srcfd = OpenTransientFile(path, O_RDONLY | PG_BINARY, 0);
        if (srcfd < 0)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not open file \"%s\": %m", path)));

        /*
         * Copy into a temp file name.
         */
        snprintf(tmppath, MAXPGPATH, XLOGDIR "/xlogtemp.%d", (int) getpid());

        unlink(tmppath);

        /* do not use get_sync_bit() here --- want to fsync only at end of fill */
        fd = OpenTransientFile(tmppath, O_RDWR | O_CREAT | O_EXCL | PG_BINARY,
                                                   S_IRUSR | S_IWUSR);
        if (fd < 0)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not create file \"%s\": %m", tmppath)));

        /*
         * Do the data copying.
         */
        for (nbytes = 0; nbytes < XLogSegSize; nbytes += sizeof(buffer))
        {
                errno = 0;
                if ((int) read(srcfd, buffer, sizeof(buffer)) != (int) sizeof(buffer))
                {
                        if (errno != 0)
                                ereport(ERROR,
                                                (errcode_for_file_access(),
                                                 errmsg("could not read file \"%s\": %m", path)));
                        else
                                ereport(ERROR,
                                                (errmsg("not enough data in file \"%s\"", path)));
                }
                errno = 0;
                if ((int) write(fd, buffer, sizeof(buffer)) != (int) sizeof(buffer))
                {
                        int                     save_errno = errno;

                        /*
                         * If we fail to make the file, delete it to release disk space
                         */
                        unlink(tmppath);
                        /* if write didn't set errno, assume problem is no disk space */
                        errno = save_errno ? save_errno : ENOSPC;

                        ereport(ERROR,
                                        (errcode_for_file_access(),
                                         errmsg("could not write to file \"%s\": %m", tmppath)));
                }
        }

        if (pg_fsync(fd) != 0)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not fsync file \"%s\": %m", tmppath)));

        if (CloseTransientFile(fd))
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not close file \"%s\": %m", tmppath)));

        CloseTransientFile(srcfd);

        /*
         * Now move the segment into place with its final name.
         */
        if (!InstallXLogFileSegment(&destsegno, tmppath, false, NULL, false))
                elog(ERROR, "InstallXLogFileSegment should not have failed");
}

/*
 * Install a new XLOG segment file as a current or future log segment.
 *
 * This is used both to install a newly-created segment (which has a temp
 * filename while it's being created) and to recycle an old segment.
 *
 * *segno: identify segment to install as (or first possible target).
 * When find_free is TRUE, this is modified on return to indicate the
 * actual installation location or last segment searched.
 *
 * tmppath: initial name of file to install.  It will be renamed into place.
 *
 * find_free: if TRUE, install the new segment at the first empty segno
 * number at or after the passed numbers.  If FALSE, install the new segment
 * exactly where specified, deleting any existing segment file there.
 *
 * *max_advance: maximum number of segno slots to advance past the starting
 * point.  Fail if no free slot is found in this range.  On return, reduced
 * by the number of slots skipped over.  (Irrelevant, and may be NULL,
 * when find_free is FALSE.)
 *
 * use_lock: if TRUE, acquire ControlFileLock while moving file into
 * place.  This should be TRUE except during bootstrap log creation.  The
 * caller must *not* hold the lock at call.
 *
 * Returns TRUE if the file was installed successfully.  FALSE indicates that
 * max_advance limit was exceeded, or an error occurred while renaming the
 * file into place.
 */
static bool
InstallXLogFileSegment(XLogSegNo *segno, char *tmppath,
                                           bool find_free, int *max_advance,
                                           bool use_lock)
{
        char            path[MAXPGPATH];
        struct stat stat_buf;

        XLogFilePath(path, ThisTimeLineID, *segno);

        /*
         * We want to be sure that only one process does this at a time.
         */
        if (use_lock)
                LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);

        if (!find_free)
        {
                /* Force installation: get rid of any pre-existing segment file */
                unlink(path);
        }
        else
        {
                /* Find a free slot to put it in */
                while (stat(path, &stat_buf) == 0)
                {
                        if (*max_advance <= 0)
                        {
                                /* Failed to find a free slot within specified range */
                                if (use_lock)
                                        LWLockRelease(ControlFileLock);
                                return false;
                        }
                        (*segno)++;
                        (*max_advance)--;
                        XLogFilePath(path, ThisTimeLineID, *segno);
                }
        }

        /*
         * Prefer link() to rename() here just to be really sure that we don't
         * overwrite an existing logfile.  However, there shouldn't be one, so
         * rename() is an acceptable substitute except for the truly paranoid.
         */
#if HAVE_WORKING_LINK
        if (link(tmppath, path) < 0)
        {
                if (use_lock)
                        LWLockRelease(ControlFileLock);
                ereport(LOG,
                                (errcode_for_file_access(),
                                 errmsg("could not link file \"%s\" to \"%s\" (initialization of log file): %m",
                                                tmppath, path)));
                return false;
        }
        unlink(tmppath);
#else
        if (rename(tmppath, path) < 0)
        {
                if (use_lock)
                        LWLockRelease(ControlFileLock);
                ereport(LOG,
                                (errcode_for_file_access(),
                                 errmsg("could not rename file \"%s\" to \"%s\" (initialization of log file): %m",
                                                tmppath, path)));
                return false;
        }
#endif

        if (use_lock)
                LWLockRelease(ControlFileLock);

        return true;
}

/*
 * Open a pre-existing logfile segment for writing.
 */
int
XLogFileOpen(XLogSegNo segno)
{
        char            path[MAXPGPATH];
        int                     fd;

        XLogFilePath(path, ThisTimeLineID, segno);

        fd = BasicOpenFile(path, O_RDWR | PG_BINARY | get_sync_bit(sync_method),
                                           S_IRUSR | S_IWUSR);
        if (fd < 0)
                ereport(PANIC,
                                (errcode_for_file_access(),
                                 errmsg("could not open transaction log file \"%s\": %m", path)));

        return fd;
}

/*
 * Open a logfile segment for reading (during recovery).
 *
 * If source == XLOG_FROM_ARCHIVE, the segment is retrieved from archive.
 * Otherwise, it's assumed to be already available in pg_xlog.
 */
static int
XLogFileRead(XLogSegNo segno, int emode, TimeLineID tli,
                         int source, bool notfoundOk)
{
        char            xlogfname[MAXFNAMELEN];
        char            activitymsg[MAXFNAMELEN + 16];
        char            path[MAXPGPATH];
        int                     fd;

        XLogFileName(xlogfname, tli, segno);

        switch (source)
        {
                case XLOG_FROM_ARCHIVE:
                        /* Report recovery progress in PS display */
                        snprintf(activitymsg, sizeof(activitymsg), "waiting for %s",
                                         xlogfname);
                        set_ps_display(activitymsg, false);

                        restoredFromArchive = RestoreArchivedFile(path, xlogfname,
                                                                                                          "RECOVERYXLOG",
                                                                                                          XLogSegSize,
                                                                                                          InRedo);
                        if (!restoredFromArchive)
                                return -1;
                        break;

                case XLOG_FROM_PG_XLOG:
                case XLOG_FROM_STREAM:
                        XLogFilePath(path, tli, segno);
                        restoredFromArchive = false;
                        break;

                default:
                        elog(ERROR, "invalid XLogFileRead source %d", source);
        }

        /*
         * If the segment was fetched from archival storage, replace the existing
         * xlog segment (if any) with the archival version.
         */
        if (source == XLOG_FROM_ARCHIVE)
        {
                KeepFileRestoredFromArchive(path, xlogfname);

                /*
                 * Set path to point at the new file in pg_xlog.
                 */
                snprintf(path, MAXPGPATH, XLOGDIR "/%s", xlogfname);
        }

        fd = BasicOpenFile(path, O_RDONLY | PG_BINARY, 0);
        if (fd >= 0)
        {
                /* Success! */
                curFileTLI = tli;

                /* Report recovery progress in PS display */
                snprintf(activitymsg, sizeof(activitymsg), "recovering %s",
                                 xlogfname);
                set_ps_display(activitymsg, false);

                /* Track source of data in assorted state variables */
                readSource = source;
                XLogReceiptSource = source;
                /* In FROM_STREAM case, caller tracks receipt time, not me */
                if (source != XLOG_FROM_STREAM)
                        XLogReceiptTime = GetCurrentTimestamp();

                return fd;
        }
        if (errno != ENOENT || !notfoundOk) /* unexpected failure? */
                ereport(PANIC,
                                (errcode_for_file_access(),
                                 errmsg("could not open file \"%s\": %m", path)));
        return -1;
}

/*
 * Open a logfile segment for reading (during recovery).
 *
 * This version searches for the segment with any TLI listed in expectedTLEs.
 */
static int
XLogFileReadAnyTLI(XLogSegNo segno, int emode, int source)
{
        char            path[MAXPGPATH];
        ListCell   *cell;
        int                     fd;
        List       *tles;

        /*
         * Loop looking for a suitable timeline ID: we might need to read any of
         * the timelines listed in expectedTLEs.
         *
         * We expect curFileTLI on entry to be the TLI of the preceding file in
         * sequence, or 0 if there was no predecessor.  We do not allow curFileTLI
         * to go backwards; this prevents us from picking up the wrong file when a
         * parent timeline extends to higher segment numbers than the child we
         * want to read.
         *
         * If we haven't read the timeline history file yet, read it now, so that
         * we know which TLIs to scan.  We don't save the list in expectedTLEs,
         * however, unless we actually find a valid segment.  That way if there is
         * neither a timeline history file nor a WAL segment in the archive, and
         * streaming replication is set up, we'll read the timeline history file
         * streamed from the master when we start streaming, instead of recovering
         * with a dummy history generated here.
         */
        if (expectedTLEs)
                tles = expectedTLEs;
        else
                tles = readTimeLineHistory(recoveryTargetTLI);

        foreach(cell, tles)
        {
                TimeLineID      tli = ((TimeLineHistoryEntry *) lfirst(cell))->tli;

                if (tli < curFileTLI)
                        break;                          /* don't bother looking at too-old TLIs */

                if (source == XLOG_FROM_ANY || source == XLOG_FROM_ARCHIVE)
                {
                        fd = XLogFileRead(segno, emode, tli,
                                                          XLOG_FROM_ARCHIVE, true);
                        if (fd != -1)
                        {
                                elog(DEBUG1, "got WAL segment from archive");
                                if (!expectedTLEs)
                                        expectedTLEs = tles;
                                return fd;
                        }
                }

                if (source == XLOG_FROM_ANY || source == XLOG_FROM_PG_XLOG)
                {
                        fd = XLogFileRead(segno, emode, tli,
                                                          XLOG_FROM_PG_XLOG, true);
                        if (fd != -1)
                        {
                                if (!expectedTLEs)
                                        expectedTLEs = tles;
                                return fd;
                        }
                }
        }

        /* Couldn't find it.  For simplicity, complain about front timeline */
        XLogFilePath(path, recoveryTargetTLI, segno);
        errno = ENOENT;
        ereport(emode,
                        (errcode_for_file_access(),
                         errmsg("could not open file \"%s\": %m", path)));
        return -1;
}

/*
 * Close the current logfile segment for writing.
 */
static void
XLogFileClose(void)
{
        Assert(openLogFile >= 0);

        /*
         * WAL segment files will not be re-read in normal operation, so we advise
         * the OS to release any cached pages.  But do not do so if WAL archiving
         * or streaming is active, because archiver and walsender process could
         * use the cache to read the WAL segment.
         */
#if defined(USE_POSIX_FADVISE) && defined(POSIX_FADV_DONTNEED)
        if (!XLogIsNeeded())
                (void) posix_fadvise(openLogFile, 0, 0, POSIX_FADV_DONTNEED);
#endif

        if (close(openLogFile))
                ereport(PANIC,
                                (errcode_for_file_access(),
                                 errmsg("could not close log file %s: %m",
                                                XLogFileNameP(ThisTimeLineID, openLogSegNo))));
        openLogFile = -1;
}

/*
 * Preallocate log files beyond the specified log endpoint.
 *
 * XXX this is currently extremely conservative, since it forces only one
 * future log segment to exist, and even that only if we are 75% done with
 * the current one.  This is only appropriate for very low-WAL-volume systems.
 * High-volume systems will be OK once they've built up a sufficient set of
 * recycled log segments, but the startup transient is likely to include
 * a lot of segment creations by foreground processes, which is not so good.
 */
static void
PreallocXlogFiles(XLogRecPtr endptr)
{
        XLogSegNo       _logSegNo;
        int                     lf;
        bool            use_existent;

        XLByteToPrevSeg(endptr, _logSegNo);
        if ((endptr - 1) % XLogSegSize >= (uint32) (0.75 * XLogSegSize))
        {
                _logSegNo++;
                use_existent = true;
                lf = XLogFileInit(_logSegNo, &use_existent, true);
                close(lf);
                if (!use_existent)
                        CheckpointStats.ckpt_segs_added++;
        }
}

/*
 * Throws an error if the given log segment has already been removed or
 * recycled. The caller should only pass a segment that it knows to have
 * existed while the server has been running, as this function always
 * succeeds if no WAL segments have been removed since startup.
 * 'tli' is only used in the error message.
 */
void
CheckXLogRemoved(XLogSegNo segno, TimeLineID tli)
{
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;
        XLogSegNo       lastRemovedSegNo;

        SpinLockAcquire(&xlogctl->info_lck);
        lastRemovedSegNo = xlogctl->lastRemovedSegNo;
        SpinLockRelease(&xlogctl->info_lck);

        if (segno <= lastRemovedSegNo)
        {
                char            filename[MAXFNAMELEN];

                XLogFileName(filename, tli, segno);
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("requested WAL segment %s has already been removed",
                                                filename)));
        }
}

/*
 * Update the last removed segno pointer in shared memory, to reflect
 * that the given XLOG file has been removed.
 */
static void
UpdateLastRemovedPtr(char *filename)
{
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;
        uint32          tli;
        XLogSegNo       segno;

        XLogFromFileName(filename, &tli, &segno);

        SpinLockAcquire(&xlogctl->info_lck);
        if (segno > xlogctl->lastRemovedSegNo)
                xlogctl->lastRemovedSegNo = segno;
        SpinLockRelease(&xlogctl->info_lck);
}

/*
 * Recycle or remove all log files older or equal to passed segno
 *
 * endptr is current (or recent) end of xlog; this is used to determine
 * whether we want to recycle rather than delete no-longer-wanted log files.
 */
static void
RemoveOldXlogFiles(XLogSegNo segno, XLogRecPtr endptr)
{
        XLogSegNo       endlogSegNo;
        int                     max_advance;
        DIR                *xldir;
        struct dirent *xlde;
        char            lastoff[MAXFNAMELEN];
        char            path[MAXPGPATH];

#ifdef WIN32
        char            newpath[MAXPGPATH];
#endif
        struct stat statbuf;

        /*
         * Initialize info about where to try to recycle to.  We allow recycling
         * segments up to XLOGfileslop segments beyond the current XLOG location.
         */
        XLByteToPrevSeg(endptr, endlogSegNo);
        max_advance = XLOGfileslop;

        xldir = AllocateDir(XLOGDIR);
        if (xldir == NULL)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not open transaction log directory \"%s\": %m",
                                                XLOGDIR)));

        /*
         * Construct a filename of the last segment to be kept. The timeline ID
         * doesn't matter, we ignore that in the comparison. (During recovery,
         * ThisTimeLineID isn't set, so we can't use that.)
         */
        XLogFileName(lastoff, 0, segno);

        elog(DEBUG2, "attempting to remove WAL segments older than log file %s",
                 lastoff);

        while ((xlde = ReadDir(xldir, XLOGDIR)) != NULL)
        {
                /*
                 * We ignore the timeline part of the XLOG segment identifiers in
                 * deciding whether a segment is still needed.  This ensures that we
                 * won't prematurely remove a segment from a parent timeline. We could
                 * probably be a little more proactive about removing segments of
                 * non-parent timelines, but that would be a whole lot more
                 * complicated.
                 *
                 * We use the alphanumeric sorting property of the filenames to decide
                 * which ones are earlier than the lastoff segment.
                 */
                if (strlen(xlde->d_name) == 24 &&
                        strspn(xlde->d_name, "0123456789ABCDEF") == 24 &&
                        strcmp(xlde->d_name + 8, lastoff + 8) <= 0)
                {
                        if (XLogArchiveCheckDone(xlde->d_name))
                        {
                                snprintf(path, MAXPGPATH, XLOGDIR "/%s", xlde->d_name);

                                /* Update the last removed location in shared memory first */
                                UpdateLastRemovedPtr(xlde->d_name);

                                /*
                                 * Before deleting the file, see if it can be recycled as a
                                 * future log segment. Only recycle normal files, pg_standby
                                 * for example can create symbolic links pointing to a
                                 * separate archive directory.
                                 */
                                if (lstat(path, &statbuf) == 0 && S_ISREG(statbuf.st_mode) &&
                                        InstallXLogFileSegment(&endlogSegNo, path,
                                                                                   true, &max_advance, true))
                                {
                                        ereport(DEBUG2,
                                                        (errmsg("recycled transaction log file \"%s\"",
                                                                        xlde->d_name)));
                                        CheckpointStats.ckpt_segs_recycled++;
                                        /* Needn't recheck that slot on future iterations */
                                        if (max_advance > 0)
                                        {
                                                endlogSegNo++;
                                                max_advance--;
                                        }
                                }
                                else
                                {
                                        /* No need for any more future segments... */
                                        int                     rc;

                                        ereport(DEBUG2,
                                                        (errmsg("removing transaction log file \"%s\"",
                                                                        xlde->d_name)));

#ifdef WIN32

                                        /*
                                         * On Windows, if another process (e.g another backend)
                                         * holds the file open in FILE_SHARE_DELETE mode, unlink
                                         * will succeed, but the file will still show up in
                                         * directory listing until the last handle is closed. To
                                         * avoid confusing the lingering deleted file for a live
                                         * WAL file that needs to be archived, rename it before
                                         * deleting it.
                                         *
                                         * If another process holds the file open without
                                         * FILE_SHARE_DELETE flag, rename will fail. We'll try
                                         * again at the next checkpoint.
                                         */
                                        snprintf(newpath, MAXPGPATH, "%s.deleted", path);
                                        if (rename(path, newpath) != 0)
                                        {
                                                ereport(LOG,
                                                                (errcode_for_file_access(),
                                                                 errmsg("could not rename old transaction log file \"%s\": %m",
                                                                                path)));
                                                continue;
                                        }
                                        rc = unlink(newpath);
#else
                                        rc = unlink(path);
#endif
                                        if (rc != 0)
                                        {
                                                ereport(LOG,
                                                                (errcode_for_file_access(),
                                                                 errmsg("could not remove old transaction log file \"%s\": %m",
                                                                                path)));
                                                continue;
                                        }
                                        CheckpointStats.ckpt_segs_removed++;
                                }

                                XLogArchiveCleanup(xlde->d_name);
                        }
                }
        }

        FreeDir(xldir);
}

/*
 * Verify whether pg_xlog and pg_xlog/archive_status exist.
 * If the latter does not exist, recreate it.
 *
 * It is not the goal of this function to verify the contents of these
 * directories, but to help in cases where someone has performed a cluster
 * copy for PITR purposes but omitted pg_xlog from the copy.
 *
 * We could also recreate pg_xlog if it doesn't exist, but a deliberate
 * policy decision was made not to.  It is fairly common for pg_xlog to be
 * a symlink, and if that was the DBA's intent then automatically making a
 * plain directory would result in degraded performance with no notice.
 */
static void
ValidateXLOGDirectoryStructure(void)
{
        char            path[MAXPGPATH];
        struct stat stat_buf;

        /* Check for pg_xlog; if it doesn't exist, error out */
        if (stat(XLOGDIR, &stat_buf) != 0 ||
                !S_ISDIR(stat_buf.st_mode))
                ereport(FATAL,
                                (errmsg("required WAL directory \"%s\" does not exist",
                                                XLOGDIR)));

        /* Check for archive_status */
        snprintf(path, MAXPGPATH, XLOGDIR "/archive_status");
        if (stat(path, &stat_buf) == 0)
        {
                /* Check for weird cases where it exists but isn't a directory */
                if (!S_ISDIR(stat_buf.st_mode))
                        ereport(FATAL,
                                        (errmsg("required WAL directory \"%s\" does not exist",
                                                        path)));
        }
        else
        {
                ereport(LOG,
                                (errmsg("creating missing WAL directory \"%s\"", path)));
                if (mkdir(path, S_IRWXU) < 0)
                        ereport(FATAL,
                                        (errmsg("could not create missing directory \"%s\": %m",
                                                        path)));
        }
}

/*
 * Remove previous backup history files.  This also retries creation of
 * .ready files for any backup history files for which XLogArchiveNotify
 * failed earlier.
 */
static void
CleanupBackupHistory(void)
{
        DIR                *xldir;
        struct dirent *xlde;
        char            path[MAXPGPATH];

        xldir = AllocateDir(XLOGDIR);
        if (xldir == NULL)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not open transaction log directory \"%s\": %m",
                                                XLOGDIR)));

        while ((xlde = ReadDir(xldir, XLOGDIR)) != NULL)
        {
                if (strlen(xlde->d_name) > 24 &&
                        strspn(xlde->d_name, "0123456789ABCDEF") == 24 &&
                        strcmp(xlde->d_name + strlen(xlde->d_name) - strlen(".backup"),
                                   ".backup") == 0)
                {
                        if (XLogArchiveCheckDone(xlde->d_name))
                        {
                                ereport(DEBUG2,
                                (errmsg("removing transaction log backup history file \"%s\"",
                                                xlde->d_name)));
                                snprintf(path, MAXPGPATH, XLOGDIR "/%s", xlde->d_name);
                                unlink(path);
                                XLogArchiveCleanup(xlde->d_name);
                        }
                }
        }

        FreeDir(xldir);
}

/*
 * Restore a full-page image from a backup block attached to an XLOG record.
 *
 * lsn: LSN of the XLOG record being replayed
 * record: the complete XLOG record
 * block_index: which backup block to restore (0 .. XLR_MAX_BKP_BLOCKS - 1)
 * get_cleanup_lock: TRUE to get a cleanup rather than plain exclusive lock
 * keep_buffer: TRUE to return the buffer still locked and pinned
 *
 * Returns the buffer number containing the page.  Note this is not terribly
 * useful unless keep_buffer is specified as TRUE.
 *
 * Note: when a backup block is available in XLOG, we restore it
 * unconditionally, even if the page in the database appears newer.
 * This is to protect ourselves against database pages that were partially
 * or incorrectly written during a crash.  We assume that the XLOG data
 * must be good because it has passed a CRC check, while the database
 * page might not be.  This will force us to replay all subsequent
 * modifications of the page that appear in XLOG, rather than possibly
 * ignoring them as already applied, but that's not a huge drawback.
 *
 * If 'get_cleanup_lock' is true, a cleanup lock is obtained on the buffer,
 * else a normal exclusive lock is used.  During crash recovery, that's just
 * pro forma because there can't be any regular backends in the system, but
 * in hot standby mode the distinction is important.
 *
 * If 'keep_buffer' is true, return without releasing the buffer lock and pin;
 * then caller is responsible for doing UnlockReleaseBuffer() later.  This
 * is needed in some cases when replaying XLOG records that touch multiple
 * pages, to prevent inconsistent states from being visible to other backends.
 * (Again, that's only important in hot standby mode.)
 */
Buffer
RestoreBackupBlock(XLogRecPtr lsn, XLogRecord *record, int block_index,
                                   bool get_cleanup_lock, bool keep_buffer)
{
        BkpBlock        bkpb;
        char       *blk;
        int                     i;

        /* Locate requested BkpBlock in the record */
        blk = (char *) XLogRecGetData(record) + record->xl_len;
        for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
        {
                if (!(record->xl_info & XLR_BKP_BLOCK(i)))
                        continue;

                memcpy(&bkpb, blk, sizeof(BkpBlock));
                blk += sizeof(BkpBlock);

                if (i == block_index)
                {
                        /* Found it, apply the update */
                        return RestoreBackupBlockContents(lsn, bkpb, blk, get_cleanup_lock,
                                                                                          keep_buffer);
                }

                blk += BLCKSZ - bkpb.hole_length;
        }

        /* Caller specified a bogus block_index */
        elog(ERROR, "failed to restore block_index %d", block_index);
        return InvalidBuffer;           /* keep compiler quiet */
}

/*
 * Workhorse for RestoreBackupBlock usable without an xlog record
 *
 * Restores a full-page image from BkpBlock and a data pointer.
 */
static Buffer
RestoreBackupBlockContents(XLogRecPtr lsn, BkpBlock bkpb, char *blk,
                                                   bool get_cleanup_lock, bool keep_buffer)
{
        Buffer          buffer;
        Page            page;

        buffer = XLogReadBufferExtended(bkpb.node, bkpb.fork, bkpb.block,
                                                                        RBM_ZERO);
        Assert(BufferIsValid(buffer));
        if (get_cleanup_lock)
                LockBufferForCleanup(buffer);
        else
                LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);

        page = (Page) BufferGetPage(buffer);

        if (bkpb.hole_length == 0)
        {
                memcpy((char *) page, blk, BLCKSZ);
        }
        else
        {
                memcpy((char *) page, blk, bkpb.hole_offset);
                /* must zero-fill the hole */
                MemSet((char *) page + bkpb.hole_offset, 0, bkpb.hole_length);
                memcpy((char *) page + (bkpb.hole_offset + bkpb.hole_length),
                           blk + bkpb.hole_offset,
                           BLCKSZ - (bkpb.hole_offset + bkpb.hole_length));
        }

        /*
         * The checksum value on this page is currently invalid. We don't need to
         * reset it here since it will be set before being written.
         */

        PageSetLSN(page, lsn);
        MarkBufferDirty(buffer);

        if (!keep_buffer)
                UnlockReleaseBuffer(buffer);

        return buffer;
}

/*
 * Attempt to read an XLOG record.
 *
 * If RecPtr is not NULL, try to read a record at that position.  Otherwise
 * try to read a record just after the last one previously read.
 *
 * If no valid record is available, returns NULL, or fails if emode is PANIC.
 * (emode must be either PANIC, LOG). In standby mode, retries until a valid
 * record is available.
 *
 * The record is copied into readRecordBuf, so that on successful return,
 * the returned record pointer always points there.
 */
static XLogRecord *
ReadRecord(XLogReaderState *xlogreader, XLogRecPtr RecPtr, int emode,
                   bool fetching_ckpt)
{
        XLogRecord *record;
        XLogPageReadPrivate *private = (XLogPageReadPrivate *) xlogreader->private_data;

        /* Pass through parameters to XLogPageRead */
        private->fetching_ckpt = fetching_ckpt;
        private->emode = emode;
        private->randAccess = (RecPtr != InvalidXLogRecPtr);

        /* This is the first attempt to read this page. */
        lastSourceFailed = false;

        for (;;)
        {
                char       *errormsg;

                record = XLogReadRecord(xlogreader, RecPtr, &errormsg);
                ReadRecPtr = xlogreader->ReadRecPtr;
                EndRecPtr = xlogreader->EndRecPtr;
                if (record == NULL)
                {
                        if (readFile >= 0)
                        {
                                close(readFile);
                                readFile = -1;
                        }

                        /*
                         * We only end up here without a message when XLogPageRead()
                         * failed - in that case we already logged something. In
                         * StandbyMode that only happens if we have been triggered, so we
                         * shouldn't loop anymore in that case.
                         */
                        if (errormsg)
                                ereport(emode_for_corrupt_record(emode,
                                                                                                 RecPtr ? RecPtr : EndRecPtr),
                                (errmsg_internal("%s", errormsg) /* already translated */ ));
                }

                /*
                 * Check page TLI is one of the expected values.
                 */
                else if (!tliInHistory(xlogreader->latestPageTLI, expectedTLEs))
                {
                        char            fname[MAXFNAMELEN];
                        XLogSegNo       segno;
                        int32           offset;

                        XLByteToSeg(xlogreader->latestPagePtr, segno);
                        offset = xlogreader->latestPagePtr % XLogSegSize;
                        XLogFileName(fname, xlogreader->readPageTLI, segno);
                        ereport(emode_for_corrupt_record(emode,
                                                                                         RecPtr ? RecPtr : EndRecPtr),
                        (errmsg("unexpected timeline ID %u in log segment %s, offset %u",
                                        xlogreader->latestPageTLI,
                                        fname,
                                        offset)));
                        record = NULL;
                }

                if (record)
                {
                        /* Great, got a record */
                        return record;
                }
                else
                {
                        /* No valid record available from this source */
                        lastSourceFailed = true;

                        /*
                         * If archive recovery was requested, but we were still doing
                         * crash recovery, switch to archive recovery and retry using the
                         * offline archive. We have now replayed all the valid WAL in
                         * pg_xlog, so we are presumably now consistent.
                         *
                         * We require that there's at least some valid WAL present in
                         * pg_xlog, however (!fetch_ckpt). We could recover using the WAL
                         * from the archive, even if pg_xlog is completely empty, but we'd
                         * have no idea how far we'd have to replay to reach consistency.
                         * So err on the safe side and give up.
                         */
                        if (!InArchiveRecovery && ArchiveRecoveryRequested &&
                                !fetching_ckpt)
                        {
                                ereport(DEBUG1,
                                                (errmsg_internal("reached end of WAL in pg_xlog, entering archive recovery")));
                                InArchiveRecovery = true;
                                if (StandbyModeRequested)
                                        StandbyMode = true;

                                /* initialize minRecoveryPoint to this record */
                                LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
                                ControlFile->state = DB_IN_ARCHIVE_RECOVERY;
                                if (ControlFile->minRecoveryPoint < EndRecPtr)
                                {
                                        ControlFile->minRecoveryPoint = EndRecPtr;
                                        ControlFile->minRecoveryPointTLI = ThisTimeLineID;
                                }
                                /* update local copy */
                                minRecoveryPoint = ControlFile->minRecoveryPoint;
                                minRecoveryPointTLI = ControlFile->minRecoveryPointTLI;

                                UpdateControlFile();
                                LWLockRelease(ControlFileLock);

                                CheckRecoveryConsistency();

                                /*
                                 * Before we retry, reset lastSourceFailed and currentSource
                                 * so that we will check the archive next.
                                 */
                                lastSourceFailed = false;
                                currentSource = 0;

                                continue;
                        }

                        /* In standby mode, loop back to retry. Otherwise, give up. */
                        if (StandbyMode && !CheckForStandbyTrigger())
                                continue;
                        else
                                return NULL;
                }
        }
}

/*
 * Scan for new timelines that might have appeared in the archive since we
 * started recovery.
 *
 * If there are any, the function changes recovery target TLI to the latest
 * one and returns 'true'.
 */
static bool
rescanLatestTimeLine(void)
{
        List       *newExpectedTLEs;
        bool            found;
        ListCell   *cell;
        TimeLineID      newtarget;
        TimeLineID      oldtarget = recoveryTargetTLI;
        TimeLineHistoryEntry *currentTle = NULL;

        newtarget = findNewestTimeLine(recoveryTargetTLI);
        if (newtarget == recoveryTargetTLI)
        {
                /* No new timelines found */
                return false;
        }

        /*
         * Determine the list of expected TLIs for the new TLI
         */

        newExpectedTLEs = readTimeLineHistory(newtarget);

        /*
         * If the current timeline is not part of the history of the new timeline,
         * we cannot proceed to it.
         */
        found = false;
        foreach(cell, newExpectedTLEs)
        {
                currentTle = (TimeLineHistoryEntry *) lfirst(cell);

                if (currentTle->tli == recoveryTargetTLI)
                {
                        found = true;
                        break;
                }
        }
        if (!found)
        {
                ereport(LOG,
                                (errmsg("new timeline %u is not a child of database system timeline %u",
                                                newtarget,
                                                ThisTimeLineID)));
                return false;
        }

        /*
         * The current timeline was found in the history file, but check that the
         * next timeline was forked off from it *after* the current recovery
         * location.
         */
        if (currentTle->end < EndRecPtr)
        {
                ereport(LOG,
                                (errmsg("new timeline %u forked off current database system timeline %u before current recovery point %X/%X",
                                                newtarget,
                                                ThisTimeLineID,
                                                (uint32) (EndRecPtr >> 32), (uint32) EndRecPtr)));
                return false;
        }

        /* The new timeline history seems valid. Switch target */
        recoveryTargetTLI = newtarget;
        list_free_deep(expectedTLEs);
        expectedTLEs = newExpectedTLEs;

        /*
         * As in StartupXLOG(), try to ensure we have all the history files
         * between the old target and new target in pg_xlog.
         */
        restoreTimeLineHistoryFiles(oldtarget + 1, newtarget);

        ereport(LOG,
                        (errmsg("new target timeline is %u",
                                        recoveryTargetTLI)));

        return true;
}

/*
 * I/O routines for pg_control
 *
 * *ControlFile is a buffer in shared memory that holds an image of the
 * contents of pg_control.      WriteControlFile() initializes pg_control
 * given a preloaded buffer, ReadControlFile() loads the buffer from
 * the pg_control file (during postmaster or standalone-backend startup),
 * and UpdateControlFile() rewrites pg_control after we modify xlog state.
 *
 * For simplicity, WriteControlFile() initializes the fields of pg_control
 * that are related to checking backend/database compatibility, and
 * ReadControlFile() verifies they are correct.  We could split out the
 * I/O and compatibility-check functions, but there seems no need currently.
 */
static void
WriteControlFile(void)
{
        int                     fd;
        char            buffer[PG_CONTROL_SIZE];                /* need not be aligned */

        /*
         * Initialize version and compatibility-check fields
         */
        ControlFile->pg_control_version = PG_CONTROL_VERSION;
        ControlFile->catalog_version_no = CATALOG_VERSION_NO;

        ControlFile->maxAlign = MAXIMUM_ALIGNOF;
        ControlFile->floatFormat = FLOATFORMAT_VALUE;

        ControlFile->blcksz = BLCKSZ;
        ControlFile->relseg_size = RELSEG_SIZE;
        ControlFile->xlog_blcksz = XLOG_BLCKSZ;
        ControlFile->xlog_seg_size = XLOG_SEG_SIZE;

        ControlFile->nameDataLen = NAMEDATALEN;
        ControlFile->indexMaxKeys = INDEX_MAX_KEYS;

        ControlFile->toast_max_chunk_size = TOAST_MAX_CHUNK_SIZE;

#ifdef HAVE_INT64_TIMESTAMP
        ControlFile->enableIntTimes = true;
#else
        ControlFile->enableIntTimes = false;
#endif
        ControlFile->float4ByVal = FLOAT4PASSBYVAL;
        ControlFile->float8ByVal = FLOAT8PASSBYVAL;

        /* Contents are protected with a CRC */
        INIT_CRC32(ControlFile->crc);
        COMP_CRC32(ControlFile->crc,
                           (char *) ControlFile,
                           offsetof(ControlFileData, crc));
        FIN_CRC32(ControlFile->crc);

        /*
         * We write out PG_CONTROL_SIZE bytes into pg_control, zero-padding the
         * excess over sizeof(ControlFileData).  This reduces the odds of
         * premature-EOF errors when reading pg_control.  We'll still fail when we
         * check the contents of the file, but hopefully with a more specific
         * error than "couldn't read pg_control".
         */
        if (sizeof(ControlFileData) > PG_CONTROL_SIZE)
                elog(PANIC, "sizeof(ControlFileData) is larger than PG_CONTROL_SIZE; fix either one");

        memset(buffer, 0, PG_CONTROL_SIZE);
        memcpy(buffer, ControlFile, sizeof(ControlFileData));

        fd = BasicOpenFile(XLOG_CONTROL_FILE,
                                           O_RDWR | O_CREAT | O_EXCL | PG_BINARY,
                                           S_IRUSR | S_IWUSR);
        if (fd < 0)
                ereport(PANIC,
                                (errcode_for_file_access(),
                                 errmsg("could not create control file \"%s\": %m",
                                                XLOG_CONTROL_FILE)));

        errno = 0;
        if (write(fd, buffer, PG_CONTROL_SIZE) != PG_CONTROL_SIZE)
        {
                /* if write didn't set errno, assume problem is no disk space */
                if (errno == 0)
                        errno = ENOSPC;
                ereport(PANIC,
                                (errcode_for_file_access(),
                                 errmsg("could not write to control file: %m")));
        }

        if (pg_fsync(fd) != 0)
                ereport(PANIC,
                                (errcode_for_file_access(),
                                 errmsg("could not fsync control file: %m")));

        if (close(fd))
                ereport(PANIC,
                                (errcode_for_file_access(),
                                 errmsg("could not close control file: %m")));
}

static void
ReadControlFile(void)
{
        pg_crc32        crc;
        int                     fd;

        /*
         * Read data...
         */
        fd = BasicOpenFile(XLOG_CONTROL_FILE,
                                           O_RDWR | PG_BINARY,
                                           S_IRUSR | S_IWUSR);
        if (fd < 0)
                ereport(PANIC,
                                (errcode_for_file_access(),
                                 errmsg("could not open control file \"%s\": %m",
                                                XLOG_CONTROL_FILE)));

        if (read(fd, ControlFile, sizeof(ControlFileData)) != sizeof(ControlFileData))
                ereport(PANIC,
                                (errcode_for_file_access(),
                                 errmsg("could not read from control file: %m")));

        close(fd);

        /*
         * Check for expected pg_control format version.  If this is wrong, the
         * CRC check will likely fail because we'll be checking the wrong number
         * of bytes.  Complaining about wrong version will probably be more
         * enlightening than complaining about wrong CRC.
         */

        if (ControlFile->pg_control_version != PG_CONTROL_VERSION && ControlFile->pg_control_version % 65536 == 0 && ControlFile->pg_control_version / 65536 != 0)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                                 errdetail("The database cluster was initialized with PG_CONTROL_VERSION %d (0x%08x),"
                 " but the server was compiled with PG_CONTROL_VERSION %d (0x%08x).",
                        ControlFile->pg_control_version, ControlFile->pg_control_version,
                                                   PG_CONTROL_VERSION, PG_CONTROL_VERSION),
                                 errhint("This could be a problem of mismatched byte ordering.  It looks like you need to initdb.")));

        if (ControlFile->pg_control_version != PG_CONTROL_VERSION)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                                 errdetail("The database cluster was initialized with PG_CONTROL_VERSION %d,"
                                  " but the server was compiled with PG_CONTROL_VERSION %d.",
                                                ControlFile->pg_control_version, PG_CONTROL_VERSION),
                                 errhint("It looks like you need to initdb.")));

        /* Now check the CRC. */
        INIT_CRC32(crc);
        COMP_CRC32(crc,
                           (char *) ControlFile,
                           offsetof(ControlFileData, crc));
        FIN_CRC32(crc);

        if (!EQ_CRC32(crc, ControlFile->crc))
                ereport(FATAL,
                                (errmsg("incorrect checksum in control file")));

        /*
         * Do compatibility checking immediately.  If the database isn't
         * compatible with the backend executable, we want to abort before we can
         * possibly do any damage.
         */
        if (ControlFile->catalog_version_no != CATALOG_VERSION_NO)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                                 errdetail("The database cluster was initialized with CATALOG_VERSION_NO %d,"
                                  " but the server was compiled with CATALOG_VERSION_NO %d.",
                                                ControlFile->catalog_version_no, CATALOG_VERSION_NO),
                                 errhint("It looks like you need to initdb.")));
        if (ControlFile->maxAlign != MAXIMUM_ALIGNOF)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                   errdetail("The database cluster was initialized with MAXALIGN %d,"
                                         " but the server was compiled with MAXALIGN %d.",
                                         ControlFile->maxAlign, MAXIMUM_ALIGNOF),
                                 errhint("It looks like you need to initdb.")));
        if (ControlFile->floatFormat != FLOATFORMAT_VALUE)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                                 errdetail("The database cluster appears to use a different floating-point number format than the server executable."),
                                 errhint("It looks like you need to initdb.")));
        if (ControlFile->blcksz != BLCKSZ)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                         errdetail("The database cluster was initialized with BLCKSZ %d,"
                                           " but the server was compiled with BLCKSZ %d.",
                                           ControlFile->blcksz, BLCKSZ),
                                 errhint("It looks like you need to recompile or initdb.")));
        if (ControlFile->relseg_size != RELSEG_SIZE)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                errdetail("The database cluster was initialized with RELSEG_SIZE %d,"
                                  " but the server was compiled with RELSEG_SIZE %d.",
                                  ControlFile->relseg_size, RELSEG_SIZE),
                                 errhint("It looks like you need to recompile or initdb.")));
        if (ControlFile->xlog_blcksz != XLOG_BLCKSZ)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                errdetail("The database cluster was initialized with XLOG_BLCKSZ %d,"
                                  " but the server was compiled with XLOG_BLCKSZ %d.",
                                  ControlFile->xlog_blcksz, XLOG_BLCKSZ),
                                 errhint("It looks like you need to recompile or initdb.")));
        if (ControlFile->xlog_seg_size != XLOG_SEG_SIZE)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                                 errdetail("The database cluster was initialized with XLOG_SEG_SIZE %d,"
                                           " but the server was compiled with XLOG_SEG_SIZE %d.",
                                                   ControlFile->xlog_seg_size, XLOG_SEG_SIZE),
                                 errhint("It looks like you need to recompile or initdb.")));
        if (ControlFile->nameDataLen != NAMEDATALEN)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                errdetail("The database cluster was initialized with NAMEDATALEN %d,"
                                  " but the server was compiled with NAMEDATALEN %d.",
                                  ControlFile->nameDataLen, NAMEDATALEN),
                                 errhint("It looks like you need to recompile or initdb.")));
        if (ControlFile->indexMaxKeys != INDEX_MAX_KEYS)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                                 errdetail("The database cluster was initialized with INDEX_MAX_KEYS %d,"
                                          " but the server was compiled with INDEX_MAX_KEYS %d.",
                                                   ControlFile->indexMaxKeys, INDEX_MAX_KEYS),
                                 errhint("It looks like you need to recompile or initdb.")));
        if (ControlFile->toast_max_chunk_size != TOAST_MAX_CHUNK_SIZE)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                                 errdetail("The database cluster was initialized with TOAST_MAX_CHUNK_SIZE %d,"
                                " but the server was compiled with TOAST_MAX_CHUNK_SIZE %d.",
                          ControlFile->toast_max_chunk_size, (int) TOAST_MAX_CHUNK_SIZE),
                                 errhint("It looks like you need to recompile or initdb.")));

#ifdef HAVE_INT64_TIMESTAMP
        if (ControlFile->enableIntTimes != true)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                                 errdetail("The database cluster was initialized without HAVE_INT64_TIMESTAMP"
                                  " but the server was compiled with HAVE_INT64_TIMESTAMP."),
                                 errhint("It looks like you need to recompile or initdb.")));
#else
        if (ControlFile->enableIntTimes != false)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                                 errdetail("The database cluster was initialized with HAVE_INT64_TIMESTAMP"
                           " but the server was compiled without HAVE_INT64_TIMESTAMP."),
                                 errhint("It looks like you need to recompile or initdb.")));
#endif

#ifdef USE_FLOAT4_BYVAL
        if (ControlFile->float4ByVal != true)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                                 errdetail("The database cluster was initialized without USE_FLOAT4_BYVAL"
                                          " but the server was compiled with USE_FLOAT4_BYVAL."),
                                 errhint("It looks like you need to recompile or initdb.")));
#else
        if (ControlFile->float4ByVal != false)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                errdetail("The database cluster was initialized with USE_FLOAT4_BYVAL"
                                  " but the server was compiled without USE_FLOAT4_BYVAL."),
                                 errhint("It looks like you need to recompile or initdb.")));
#endif

#ifdef USE_FLOAT8_BYVAL
        if (ControlFile->float8ByVal != true)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                                 errdetail("The database cluster was initialized without USE_FLOAT8_BYVAL"
                                          " but the server was compiled with USE_FLOAT8_BYVAL."),
                                 errhint("It looks like you need to recompile or initdb.")));
#else
        if (ControlFile->float8ByVal != false)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                errdetail("The database cluster was initialized with USE_FLOAT8_BYVAL"
                                  " but the server was compiled without USE_FLOAT8_BYVAL."),
                                 errhint("It looks like you need to recompile or initdb.")));
#endif

        /* Make the fixed  settings visible as GUC variables, too */
        SetConfigOption("data_checksums", DataChecksumsEnabled() ? "yes" : "no",
                                        PGC_INTERNAL, PGC_S_OVERRIDE);
}

void
UpdateControlFile(void)
{
        int                     fd;

        INIT_CRC32(ControlFile->crc);
        COMP_CRC32(ControlFile->crc,
                           (char *) ControlFile,
                           offsetof(ControlFileData, crc));
        FIN_CRC32(ControlFile->crc);

        fd = BasicOpenFile(XLOG_CONTROL_FILE,
                                           O_RDWR | PG_BINARY,
                                           S_IRUSR | S_IWUSR);
        if (fd < 0)
                ereport(PANIC,
                                (errcode_for_file_access(),
                                 errmsg("could not open control file \"%s\": %m",
                                                XLOG_CONTROL_FILE)));

        errno = 0;
        if (write(fd, ControlFile, sizeof(ControlFileData)) != sizeof(ControlFileData))
        {
                /* if write didn't set errno, assume problem is no disk space */
                if (errno == 0)
                        errno = ENOSPC;
                ereport(PANIC,
                                (errcode_for_file_access(),
                                 errmsg("could not write to control file: %m")));
        }

        if (pg_fsync(fd) != 0)
                ereport(PANIC,
                                (errcode_for_file_access(),
                                 errmsg("could not fsync control file: %m")));

        if (close(fd))
                ereport(PANIC,
                                (errcode_for_file_access(),
                                 errmsg("could not close control file: %m")));
}

/*
 * Returns the unique system identifier from control file.
 */
uint64
GetSystemIdentifier(void)
{
        Assert(ControlFile != NULL);
        return ControlFile->system_identifier;
}

/*
 * Are checksums enabled for data pages?
 */
bool
DataChecksumsEnabled(void)
{
        Assert(ControlFile != NULL);
        return (ControlFile->data_checksum_version > 0);
}

/*
 * Returns a fake LSN for unlogged relations.
 *
 * Each call generates an LSN that is greater than any previous value
 * returned. The current counter value is saved and restored across clean
 * shutdowns, but like unlogged relations, does not survive a crash. This can
 * be used in lieu of real LSN values returned by XLogInsert, if you need an
 * LSN-like increasing sequence of numbers without writing any WAL.
 */
XLogRecPtr
GetFakeLSNForUnloggedRel(void)
{
        XLogRecPtr      nextUnloggedLSN;

        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;

        /* increment the unloggedLSN counter, need SpinLock */
        SpinLockAcquire(&xlogctl->ulsn_lck);
        nextUnloggedLSN = xlogctl->unloggedLSN++;
        SpinLockRelease(&xlogctl->ulsn_lck);

        return nextUnloggedLSN;
}

/*
 * Auto-tune the number of XLOG buffers.
 *
 * The preferred setting for wal_buffers is about 3% of shared_buffers, with
 * a maximum of one XLOG segment (there is little reason to think that more
 * is helpful, at least so long as we force an fsync when switching log files)
 * and a minimum of 8 blocks (which was the default value prior to PostgreSQL
 * 9.1, when auto-tuning was added).
 *
 * This should not be called until NBuffers has received its final value.
 */
static int
XLOGChooseNumBuffers(void)
{
        int                     xbuffers;

        xbuffers = NBuffers / 32;
        if (xbuffers > XLOG_SEG_SIZE / XLOG_BLCKSZ)
                xbuffers = XLOG_SEG_SIZE / XLOG_BLCKSZ;
        if (xbuffers < 8)
                xbuffers = 8;
        return xbuffers;
}

/*
 * GUC check_hook for wal_buffers
 */
bool
check_wal_buffers(int *newval, void **extra, GucSource source)
{
        /*
         * -1 indicates a request for auto-tune.
         */
        if (*newval == -1)
        {
                /*
                 * If we haven't yet changed the boot_val default of -1, just let it
                 * be.  We'll fix it when XLOGShmemSize is called.
                 */
                if (XLOGbuffers == -1)
                        return true;

                /* Otherwise, substitute the auto-tune value */
                *newval = XLOGChooseNumBuffers();
        }

        /*
         * We clamp manually-set values to at least 4 blocks.  Prior to PostgreSQL
         * 9.1, a minimum of 4 was enforced by guc.c, but since that is no longer
         * the case, we just silently treat such values as a request for the
         * minimum.  (We could throw an error instead, but that doesn't seem very
         * helpful.)
         */
        if (*newval < 4)
                *newval = 4;

        return true;
}

/*
 * Initialization of shared memory for XLOG
 */
Size
XLOGShmemSize(void)
{
        Size            size;

        /*
         * If the value of wal_buffers is -1, use the preferred auto-tune value.
         * This isn't an amazingly clean place to do this, but we must wait till
         * NBuffers has received its final value, and must do it before using the
         * value of XLOGbuffers to do anything important.
         */
        if (XLOGbuffers == -1)
        {
                char            buf[32];

                snprintf(buf, sizeof(buf), "%d", XLOGChooseNumBuffers());
                SetConfigOption("wal_buffers", buf, PGC_POSTMASTER, PGC_S_OVERRIDE);
        }
        Assert(XLOGbuffers > 0);

        /* XLogCtl */
        size = sizeof(XLogCtlData);

        /* xlog insertion slots, plus alignment */
        size = add_size(size, mul_size(sizeof(XLogInsertSlotPadded), num_xloginsert_slots + 1));
        /* xlblocks array */
        size = add_size(size, mul_size(sizeof(XLogRecPtr), XLOGbuffers));
        /* extra alignment padding for XLOG I/O buffers */
        size = add_size(size, XLOG_BLCKSZ);
        /* and the buffers themselves */
        size = add_size(size, mul_size(XLOG_BLCKSZ, XLOGbuffers));

        /*
         * Note: we don't count ControlFileData, it comes out of the "slop factor"
         * added by CreateSharedMemoryAndSemaphores.  This lets us use this
         * routine again below to compute the actual allocation size.
         */

        return size;
}

void
XLOGShmemInit(void)
{
        bool            foundCFile,
                                foundXLog;
        char       *allocptr;
        int                     i;

        ControlFile = (ControlFileData *)
                ShmemInitStruct("Control File", sizeof(ControlFileData), &foundCFile);
        XLogCtl = (XLogCtlData *)
                ShmemInitStruct("XLOG Ctl", XLOGShmemSize(), &foundXLog);

        if (foundCFile || foundXLog)
        {
                /* both should be present or neither */
                Assert(foundCFile && foundXLog);
                return;
        }
        memset(XLogCtl, 0, sizeof(XLogCtlData));

        /*
         * Since XLogCtlData contains XLogRecPtr fields, its sizeof should be a
         * multiple of the alignment for same, so no extra alignment padding is
         * needed here.
         */
        allocptr = ((char *) XLogCtl) + sizeof(XLogCtlData);
        XLogCtl->xlblocks = (XLogRecPtr *) allocptr;
        memset(XLogCtl->xlblocks, 0, sizeof(XLogRecPtr) * XLOGbuffers);
        allocptr += sizeof(XLogRecPtr) * XLOGbuffers;

        /* Xlog insertion slots. Ensure they're aligned to the full padded size */
        allocptr += sizeof(XLogInsertSlotPadded) -
                ((uintptr_t) allocptr) % sizeof(XLogInsertSlotPadded);
        XLogCtl->Insert.insertSlots = (XLogInsertSlotPadded *) allocptr;
        allocptr += sizeof(XLogInsertSlotPadded) * num_xloginsert_slots;

        /*
         * Align the start of the page buffers to a full xlog block size boundary.
         * This simplifies some calculations in XLOG insertion. It is also required
         * for O_DIRECT.
         */
        allocptr = (char *) TYPEALIGN(XLOG_BLCKSZ, allocptr);
        XLogCtl->pages = allocptr;
        memset(XLogCtl->pages, 0, (Size) XLOG_BLCKSZ * XLOGbuffers);

        /*
         * Do basic initialization of XLogCtl shared data. (StartupXLOG will fill
         * in additional info.)
         */
        XLogCtl->XLogCacheBlck = XLOGbuffers - 1;
        XLogCtl->SharedRecoveryInProgress = true;
        XLogCtl->SharedHotStandbyActive = false;
        XLogCtl->WalWriterSleeping = false;

        for (i = 0; i < num_xloginsert_slots; i++)
        {
                XLogInsertSlot *slot = &XLogCtl->Insert.insertSlots[i].slot;
                SpinLockInit(&slot->mutex);
                slot->xlogInsertingAt = InvalidXLogRecPtr;
                slot->owner = NULL;

                slot->releaseOK = true;
                slot->exclusive = 0;
                slot->head = NULL;
                slot->tail = NULL;
        }

        SpinLockInit(&XLogCtl->Insert.insertpos_lck);
        SpinLockInit(&XLogCtl->info_lck);
        SpinLockInit(&XLogCtl->ulsn_lck);
        InitSharedLatch(&XLogCtl->recoveryWakeupLatch);

        /*
         * If we are not in bootstrap mode, pg_control should already exist. Read
         * and validate it immediately (see comments in ReadControlFile() for the
         * reasons why).
         */
        if (!IsBootstrapProcessingMode())
                ReadControlFile();
}

/*
 * This func must be called ONCE on system install.  It creates pg_control
 * and the initial XLOG segment.
 */
void
BootStrapXLOG(void)
{
        CheckPoint      checkPoint;
        char       *buffer;
        XLogPageHeader page;
        XLogLongPageHeader longpage;
        XLogRecord *record;
        bool            use_existent;
        uint64          sysidentifier;
        struct timeval tv;
        pg_crc32        crc;

        /*
         * Select a hopefully-unique system identifier code for this installation.
         * We use the result of gettimeofday(), including the fractional seconds
         * field, as being about as unique as we can easily get.  (Think not to
         * use random(), since it hasn't been seeded and there's no portable way
         * to seed it other than the system clock value...)  The upper half of the
         * uint64 value is just the tv_sec part, while the lower half is the XOR
         * of tv_sec and tv_usec.  This is to ensure that we don't lose uniqueness
         * unnecessarily if "uint64" is really only 32 bits wide.  A person
         * knowing this encoding can determine the initialization time of the
         * installation, which could perhaps be useful sometimes.
         */
        gettimeofday(&tv, NULL);
        sysidentifier = ((uint64) tv.tv_sec) << 32;
        sysidentifier |= (uint32) (tv.tv_sec | tv.tv_usec);

        /* First timeline ID is always 1 */
        ThisTimeLineID = 1;

        /* page buffer must be aligned suitably for O_DIRECT */
        buffer = (char *) palloc(XLOG_BLCKSZ + XLOG_BLCKSZ);
        page = (XLogPageHeader) TYPEALIGN(XLOG_BLCKSZ, buffer);
        memset(page, 0, XLOG_BLCKSZ);

        /*
         * Set up information for the initial checkpoint record
         *
         * The initial checkpoint record is written to the beginning of the WAL
         * segment with logid=0 logseg=1. The very first WAL segment, 0/0, is not
         * used, so that we can use 0/0 to mean "before any valid WAL segment".
         */
        checkPoint.redo = XLogSegSize + SizeOfXLogLongPHD;
        checkPoint.ThisTimeLineID = ThisTimeLineID;
        checkPoint.PrevTimeLineID = ThisTimeLineID;
        checkPoint.fullPageWrites = fullPageWrites;
        checkPoint.nextXidEpoch = 0;
        checkPoint.nextXid = FirstNormalTransactionId;
        checkPoint.nextOid = FirstBootstrapObjectId;
        checkPoint.nextMulti = FirstMultiXactId;
        checkPoint.nextMultiOffset = 0;
        checkPoint.oldestXid = FirstNormalTransactionId;
        checkPoint.oldestXidDB = TemplateDbOid;
        checkPoint.oldestMulti = FirstMultiXactId;
        checkPoint.oldestMultiDB = TemplateDbOid;
        checkPoint.time = (pg_time_t) time(NULL);
        checkPoint.oldestActiveXid = InvalidTransactionId;

        ShmemVariableCache->nextXid = checkPoint.nextXid;
        ShmemVariableCache->nextOid = checkPoint.nextOid;
        ShmemVariableCache->oidCount = 0;
        MultiXactSetNextMXact(checkPoint.nextMulti, checkPoint.nextMultiOffset);
        SetTransactionIdLimit(checkPoint.oldestXid, checkPoint.oldestXidDB);
        SetMultiXactIdLimit(checkPoint.oldestMulti, checkPoint.oldestMultiDB);

        /* Set up the XLOG page header */
        page->xlp_magic = XLOG_PAGE_MAGIC;
        page->xlp_info = XLP_LONG_HEADER;
        page->xlp_tli = ThisTimeLineID;
        page->xlp_pageaddr = XLogSegSize;
        longpage = (XLogLongPageHeader) page;
        longpage->xlp_sysid = sysidentifier;
        longpage->xlp_seg_size = XLogSegSize;
        longpage->xlp_xlog_blcksz = XLOG_BLCKSZ;

        /* Insert the initial checkpoint record */
        record = (XLogRecord *) ((char *) page + SizeOfXLogLongPHD);
        record->xl_prev = 0;
        record->xl_xid = InvalidTransactionId;
        record->xl_tot_len = SizeOfXLogRecord + sizeof(checkPoint);
        record->xl_len = sizeof(checkPoint);
        record->xl_info = XLOG_CHECKPOINT_SHUTDOWN;
        record->xl_rmid = RM_XLOG_ID;
        memcpy(XLogRecGetData(record), &checkPoint, sizeof(checkPoint));

        INIT_CRC32(crc);
        COMP_CRC32(crc, &checkPoint, sizeof(checkPoint));
        COMP_CRC32(crc, (char *) record, offsetof(XLogRecord, xl_crc));
        FIN_CRC32(crc);
        record->xl_crc = crc;

        /* Create first XLOG segment file */
        use_existent = false;
        openLogFile = XLogFileInit(1, &use_existent, false);

        /* Write the first page with the initial record */
        errno = 0;
        if (write(openLogFile, page, XLOG_BLCKSZ) != XLOG_BLCKSZ)
        {
                /* if write didn't set errno, assume problem is no disk space */
                if (errno == 0)
                        errno = ENOSPC;
                ereport(PANIC,
                                (errcode_for_file_access(),
                          errmsg("could not write bootstrap transaction log file: %m")));
        }

        if (pg_fsync(openLogFile) != 0)
                ereport(PANIC,
                                (errcode_for_file_access(),
                          errmsg("could not fsync bootstrap transaction log file: %m")));

        if (close(openLogFile))
                ereport(PANIC,
                                (errcode_for_file_access(),
                          errmsg("could not close bootstrap transaction log file: %m")));

        openLogFile = -1;

        /* Now create pg_control */

        memset(ControlFile, 0, sizeof(ControlFileData));
        /* Initialize pg_control status fields */
        ControlFile->system_identifier = sysidentifier;
        ControlFile->state = DB_SHUTDOWNED;
        ControlFile->time = checkPoint.time;
        ControlFile->checkPoint = checkPoint.redo;
        ControlFile->checkPointCopy = checkPoint;
        ControlFile->unloggedLSN = 1;

        /* Set important parameter values for use when replaying WAL */
        ControlFile->MaxConnections = MaxConnections;
        ControlFile->max_worker_processes = max_worker_processes;
        ControlFile->max_prepared_xacts = max_prepared_xacts;
        ControlFile->max_locks_per_xact = max_locks_per_xact;
        ControlFile->wal_level = wal_level;
        ControlFile->wal_log_hints = wal_log_hints;
        ControlFile->data_checksum_version = bootstrap_data_checksum_version;

        /* some additional ControlFile fields are set in WriteControlFile() */

        WriteControlFile();

        /* Bootstrap the commit log, too */
        BootStrapCLOG();
        BootStrapSUBTRANS();
        BootStrapMultiXact();

        pfree(buffer);
}

static char *
str_time(pg_time_t tnow)
{
        static char buf[128];

        pg_strftime(buf, sizeof(buf),
                                "%Y-%m-%d %H:%M:%S %Z",
                                pg_localtime(&tnow, log_timezone));

        return buf;
}

/*
 * See if there is a recovery command file (recovery.conf), and if so
 * read in parameters for archive recovery and XLOG streaming.
 *
 * The file is parsed using the main configuration parser.
 */
static void
readRecoveryCommandFile(void)
{
        FILE       *fd;
        TimeLineID      rtli = 0;
        bool            rtliGiven = false;
        ConfigVariable *item,
                           *head = NULL,
                           *tail = NULL;

        fd = AllocateFile(RECOVERY_COMMAND_FILE, "r");
        if (fd == NULL)
        {
                if (errno == ENOENT)
                        return;                         /* not there, so no archive recovery */
                ereport(FATAL,
                                (errcode_for_file_access(),
                                 errmsg("could not open recovery command file \"%s\": %m",
                                                RECOVERY_COMMAND_FILE)));
        }

        /*
         * Since we're asking ParseConfigFp() to report errors as FATAL, there's
         * no need to check the return value.
         */
        (void) ParseConfigFp(fd, RECOVERY_COMMAND_FILE, 0, FATAL, &head, &tail);

        FreeFile(fd);

        for (item = head; item; item = item->next)
        {
                if (strcmp(item->name, "restore_command") == 0)
                {
                        recoveryRestoreCommand = pstrdup(item->value);
                        ereport(DEBUG2,
                                        (errmsg_internal("restore_command = '%s'",
                                                                         recoveryRestoreCommand)));
                }
                else if (strcmp(item->name, "recovery_end_command") == 0)
                {
                        recoveryEndCommand = pstrdup(item->value);
                        ereport(DEBUG2,
                                        (errmsg_internal("recovery_end_command = '%s'",
                                                                         recoveryEndCommand)));
                }
                else if (strcmp(item->name, "archive_cleanup_command") == 0)
                {
                        archiveCleanupCommand = pstrdup(item->value);
                        ereport(DEBUG2,
                                        (errmsg_internal("archive_cleanup_command = '%s'",
                                                                         archiveCleanupCommand)));
                }
                else if (strcmp(item->name, "pause_at_recovery_target") == 0)
                {
                        if (!parse_bool(item->value, &recoveryPauseAtTarget))
                                ereport(ERROR,
                                                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                                                 errmsg("parameter \"%s\" requires a Boolean value", "pause_at_recovery_target")));
                        ereport(DEBUG2,
                                        (errmsg_internal("pause_at_recovery_target = '%s'",
                                                                         item->value)));
                }
                else if (strcmp(item->name, "recovery_target_timeline") == 0)
                {
                        rtliGiven = true;
                        if (strcmp(item->value, "latest") == 0)
                                rtli = 0;
                        else
                        {
                                errno = 0;
                                rtli = (TimeLineID) strtoul(item->value, NULL, 0);
                                if (errno == EINVAL || errno == ERANGE)
                                        ereport(FATAL,
                                                        (errmsg("recovery_target_timeline is not a valid number: \"%s\"",
                                                                        item->value)));
                        }
                        if (rtli)
                                ereport(DEBUG2,
                                   (errmsg_internal("recovery_target_timeline = %u", rtli)));
                        else
                                ereport(DEBUG2,
                                         (errmsg_internal("recovery_target_timeline = latest")));
                }
                else if (strcmp(item->name, "recovery_target_xid") == 0)
                {
                        errno = 0;
                        recoveryTargetXid = (TransactionId) strtoul(item->value, NULL, 0);
                        if (errno == EINVAL || errno == ERANGE)
                                ereport(FATAL,
                                 (errmsg("recovery_target_xid is not a valid number: \"%s\"",
                                                 item->value)));
                        ereport(DEBUG2,
                                        (errmsg_internal("recovery_target_xid = %u",
                                                                         recoveryTargetXid)));
                        recoveryTarget = RECOVERY_TARGET_XID;
                }
                else if (strcmp(item->name, "recovery_target_time") == 0)
                {
                        /*
                         * if recovery_target_xid or recovery_target_name specified, then
                         * this overrides recovery_target_time
                         */
                        if (recoveryTarget == RECOVERY_TARGET_XID ||
                                recoveryTarget == RECOVERY_TARGET_NAME)
                                continue;
                        recoveryTarget = RECOVERY_TARGET_TIME;

                        /*
                         * Convert the time string given by the user to TimestampTz form.
                         */
                        recoveryTargetTime =
                                DatumGetTimestampTz(DirectFunctionCall3(timestamptz_in,
                                                                                                CStringGetDatum(item->value),
                                                                                                ObjectIdGetDatum(InvalidOid),
                                                                                                                Int32GetDatum(-1)));
                        ereport(DEBUG2,
                                        (errmsg_internal("recovery_target_time = '%s'",
                                                                   timestamptz_to_str(recoveryTargetTime))));
                }
                else if (strcmp(item->name, "recovery_target_name") == 0)
                {
                        /*
                         * if recovery_target_xid specified, then this overrides
                         * recovery_target_name
                         */
                        if (recoveryTarget == RECOVERY_TARGET_XID)
                                continue;
                        recoveryTarget = RECOVERY_TARGET_NAME;

                        recoveryTargetName = pstrdup(item->value);
                        if (strlen(recoveryTargetName) >= MAXFNAMELEN)
                                ereport(FATAL,
                                                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                                                 errmsg("recovery_target_name is too long (maximum %d characters)",
                                                                MAXFNAMELEN - 1)));

                        ereport(DEBUG2,
                                        (errmsg_internal("recovery_target_name = '%s'",
                                                                         recoveryTargetName)));
                }
                else if (strcmp(item->name, "recovery_target_inclusive") == 0)
                {
                        /*
                         * does nothing if a recovery_target is not also set
                         */
                        if (!parse_bool(item->value, &recoveryTargetInclusive))
                                ereport(ERROR,
                                                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                                                 errmsg("parameter \"%s\" requires a Boolean value",
                                                                "recovery_target_inclusive")));
                        ereport(DEBUG2,
                                        (errmsg_internal("recovery_target_inclusive = %s",
                                                                         item->value)));
                }
                else if (strcmp(item->name, "standby_mode") == 0)
                {
                        if (!parse_bool(item->value, &StandbyModeRequested))
                                ereport(ERROR,
                                                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                                                 errmsg("parameter \"%s\" requires a Boolean value",
                                                                "standby_mode")));
                        ereport(DEBUG2,
                                        (errmsg_internal("standby_mode = '%s'", item->value)));
                }
                else if (strcmp(item->name, "primary_conninfo") == 0)
                {
                        PrimaryConnInfo = pstrdup(item->value);
                        ereport(DEBUG2,
                                        (errmsg_internal("primary_conninfo = '%s'",
                                                                         PrimaryConnInfo)));
                }
                else if (strcmp(item->name, "trigger_file") == 0)
                {
                        TriggerFile = pstrdup(item->value);
                        ereport(DEBUG2,
                                        (errmsg_internal("trigger_file = '%s'",
                                                                         TriggerFile)));
                }
                else if (strcmp(item->name, "min_recovery_apply_delay") == 0)
                {
                        const char *hintmsg;

                        if (!parse_int(item->value, &min_recovery_apply_delay, GUC_UNIT_MS,
                                        &hintmsg))
                                ereport(ERROR,
                                                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                                                 errmsg("parameter \"%s\" requires a temporal value", "min_recovery_apply_delay"),
                                                 hintmsg ? errhint("%s", _(hintmsg)) : 0));
                        ereport(DEBUG2,
                                        (errmsg("min_recovery_apply_delay = '%s'", item->value)));
                }
                else
                        ereport(FATAL,
                                        (errmsg("unrecognized recovery parameter \"%s\"",
                                                        item->name)));
        }

        /*
         * Check for compulsory parameters
         */
        if (StandbyModeRequested)
        {
                if (PrimaryConnInfo == NULL && recoveryRestoreCommand == NULL)
                        ereport(WARNING,
                                        (errmsg("recovery command file \"%s\" specified neither primary_conninfo nor restore_command",
                                                        RECOVERY_COMMAND_FILE),
                                         errhint("The database server will regularly poll the pg_xlog subdirectory to check for files placed there.")));
        }
        else
        {
                if (recoveryRestoreCommand == NULL)
                        ereport(FATAL,
                                        (errmsg("recovery command file \"%s\" must specify restore_command when standby mode is not enabled",
                                                        RECOVERY_COMMAND_FILE)));
        }

        /* Enable fetching from archive recovery area */
        ArchiveRecoveryRequested = true;

        /*
         * If user specified recovery_target_timeline, validate it or compute the
         * "latest" value.      We can't do this until after we've gotten the restore
         * command and set InArchiveRecovery, because we need to fetch timeline
         * history files from the archive.
         */
        if (rtliGiven)
        {
                if (rtli)
                {
                        /* Timeline 1 does not have a history file, all else should */
                        if (rtli != 1 && !existsTimeLineHistory(rtli))
                                ereport(FATAL,
                                                (errmsg("recovery target timeline %u does not exist",
                                                                rtli)));
                        recoveryTargetTLI = rtli;
                        recoveryTargetIsLatest = false;
                }
                else
                {
                        /* We start the "latest" search from pg_control's timeline */
                        recoveryTargetTLI = findNewestTimeLine(recoveryTargetTLI);
                        recoveryTargetIsLatest = true;
                }
        }

        FreeConfigVariables(head);
}

/*
 * Exit archive-recovery state
 */
static void
exitArchiveRecovery(TimeLineID endTLI, XLogSegNo endLogSegNo)
{
        char            recoveryPath[MAXPGPATH];
        char            xlogpath[MAXPGPATH];

        /*
         * We are no longer in archive recovery state.
         */
        InArchiveRecovery = false;

        /*
         * Update min recovery point one last time.
         */
        UpdateMinRecoveryPoint(InvalidXLogRecPtr, true);

        /*
         * If the ending log segment is still open, close it (to avoid problems on
         * Windows with trying to rename or delete an open file).
         */
        if (readFile >= 0)
        {
                close(readFile);
                readFile = -1;
        }

        /*
         * If we are establishing a new timeline, we have to copy data from the
         * last WAL segment of the old timeline to create a starting WAL segment
         * for the new timeline.
         *
         * Notify the archiver that the last WAL segment of the old timeline is
         * ready to copy to archival storage. Otherwise, it is not archived for a
         * while.
         */
        if (endTLI != ThisTimeLineID)
        {
                XLogFileCopy(endLogSegNo, endTLI, endLogSegNo);

                if (XLogArchivingActive())
                {
                        XLogFileName(xlogpath, endTLI, endLogSegNo);
                        XLogArchiveNotify(xlogpath);
                }
        }

        /*
         * Let's just make real sure there are not .ready or .done flags posted
         * for the new segment.
         */
        XLogFileName(xlogpath, ThisTimeLineID, endLogSegNo);
        XLogArchiveCleanup(xlogpath);

        /*
         * Since there might be a partial WAL segment named RECOVERYXLOG, get rid
         * of it.
         */
        snprintf(recoveryPath, MAXPGPATH, XLOGDIR "/RECOVERYXLOG");
        unlink(recoveryPath);           /* ignore any error */

        /* Get rid of any remaining recovered timeline-history file, too */
        snprintf(recoveryPath, MAXPGPATH, XLOGDIR "/RECOVERYHISTORY");
        unlink(recoveryPath);           /* ignore any error */

        /*
         * Rename the config file out of the way, so that we don't accidentally
         * re-enter archive recovery mode in a subsequent crash.
         */
        unlink(RECOVERY_COMMAND_DONE);
        if (rename(RECOVERY_COMMAND_FILE, RECOVERY_COMMAND_DONE) != 0)
                ereport(FATAL,
                                (errcode_for_file_access(),
                                 errmsg("could not rename file \"%s\" to \"%s\": %m",
                                                RECOVERY_COMMAND_FILE, RECOVERY_COMMAND_DONE)));

        ereport(LOG,
                        (errmsg("archive recovery complete")));
}

/*
 * For point-in-time recovery, this function decides whether we want to
 * stop applying the XLOG at or after the current record.
 *
 * Returns TRUE if we are stopping, FALSE otherwise.  On TRUE return,
 * *includeThis is set TRUE if we should apply this record before stopping.
 *
 * We also track the timestamp of the latest applied COMMIT/ABORT
 * record in XLogCtl->recoveryLastXTime, for logging purposes.
 * Also, some information is saved in recoveryStopXid et al for use in
 * annotating the new timeline's history file; and recoveryDelayUntilTime
 * is updated, for time-delayed standbys.
 */
static bool
recoveryStopsHere(XLogRecord *record, bool *includeThis, bool *delayThis)
{
        bool            stopsHere;
        uint8           record_info;
        TimestampTz recordXtime;
        char            recordRPName[MAXFNAMELEN];

        /* We only consider stopping at COMMIT, ABORT or RESTORE POINT records */
        if (record->xl_rmid != RM_XACT_ID && record->xl_rmid != RM_XLOG_ID)
                return false;
        record_info = record->xl_info & ~XLR_INFO_MASK;
        if (record->xl_rmid == RM_XACT_ID && record_info == XLOG_XACT_COMMIT_COMPACT)
        {
                xl_xact_commit_compact *recordXactCommitData;

                recordXactCommitData = (xl_xact_commit_compact *) XLogRecGetData(record);
                recordXtime = recordXactCommitData->xact_time;

                *delayThis = SetRecoveryDelayUntilTime(recordXactCommitData->xact_time);
        }
        else if (record->xl_rmid == RM_XACT_ID && record_info == XLOG_XACT_COMMIT)
        {
                xl_xact_commit *recordXactCommitData;

                recordXactCommitData = (xl_xact_commit *) XLogRecGetData(record);
                recordXtime = recordXactCommitData->xact_time;

                *delayThis = SetRecoveryDelayUntilTime(recordXactCommitData->xact_time);
        }
        else if (record->xl_rmid == RM_XACT_ID && record_info == XLOG_XACT_ABORT)
        {
                xl_xact_abort *recordXactAbortData;

                recordXactAbortData = (xl_xact_abort *) XLogRecGetData(record);
                recordXtime = recordXactAbortData->xact_time;

                /*
                 * We deliberately choose not to delay aborts since they have no
                 * effect on MVCC. We already allow replay of records that don't
                 * have a timestamp, so there is already opportunity for issues
                 * caused by early conflicts on standbys.
                 */
        }
        else if (record->xl_rmid == RM_XLOG_ID && record_info == XLOG_RESTORE_POINT)
        {
                xl_restore_point *recordRestorePointData;

                recordRestorePointData = (xl_restore_point *) XLogRecGetData(record);
                recordXtime = recordRestorePointData->rp_time;
                strncpy(recordRPName, recordRestorePointData->rp_name, MAXFNAMELEN);

                *delayThis = SetRecoveryDelayUntilTime(recordRestorePointData->rp_time);
        }
        else
                return false;

        /* Do we have a PITR target at all? */
        if (recoveryTarget == RECOVERY_TARGET_UNSET)
        {
                /*
                 * Save timestamp of latest transaction commit/abort if this is a
                 * transaction record
                 */
                if (record->xl_rmid == RM_XACT_ID)
                        SetLatestXTime(recordXtime);
                return false;
        }

        if (recoveryTarget == RECOVERY_TARGET_XID)
        {
                /*
                 * There can be only one transaction end record with this exact
                 * transactionid
                 *
                 * when testing for an xid, we MUST test for equality only, since
                 * transactions are numbered in the order they start, not the order
                 * they complete. A higher numbered xid will complete before you about
                 * 50% of the time...
                 */
                stopsHere = (record->xl_xid == recoveryTargetXid);
                if (stopsHere)
                        *includeThis = recoveryTargetInclusive;
        }
        else if (recoveryTarget == RECOVERY_TARGET_NAME)
        {
                /*
                 * There can be many restore points that share the same name, so we
                 * stop at the first one
                 */
                stopsHere = (strcmp(recordRPName, recoveryTargetName) == 0);

                /*
                 * Ignore recoveryTargetInclusive because this is not a transaction
                 * record
                 */
                *includeThis = false;
        }
        else
        {
                /*
                 * There can be many transactions that share the same commit time, so
                 * we stop after the last one, if we are inclusive, or stop at the
                 * first one if we are exclusive
                 */
                if (recoveryTargetInclusive)
                        stopsHere = (recordXtime > recoveryTargetTime);
                else
                        stopsHere = (recordXtime >= recoveryTargetTime);
                if (stopsHere)
                        *includeThis = false;
        }

        if (stopsHere)
        {
                recoveryStopXid = record->xl_xid;
                recoveryStopTime = recordXtime;
                recoveryStopAfter = *includeThis;

                if (record_info == XLOG_XACT_COMMIT_COMPACT || record_info == XLOG_XACT_COMMIT)
                {
                        if (recoveryStopAfter)
                                ereport(LOG,
                                                (errmsg("recovery stopping after commit of transaction %u, time %s",
                                                                recoveryStopXid,
                                                                timestamptz_to_str(recoveryStopTime))));
                        else
                                ereport(LOG,
                                                (errmsg("recovery stopping before commit of transaction %u, time %s",
                                                                recoveryStopXid,
                                                                timestamptz_to_str(recoveryStopTime))));
                }
                else if (record_info == XLOG_XACT_ABORT)
                {
                        if (recoveryStopAfter)
                                ereport(LOG,
                                                (errmsg("recovery stopping after abort of transaction %u, time %s",
                                                                recoveryStopXid,
                                                                timestamptz_to_str(recoveryStopTime))));
                        else
                                ereport(LOG,
                                                (errmsg("recovery stopping before abort of transaction %u, time %s",
                                                                recoveryStopXid,
                                                                timestamptz_to_str(recoveryStopTime))));
                }
                else
                {
                        strncpy(recoveryStopName, recordRPName, MAXFNAMELEN);

                        ereport(LOG,
                                (errmsg("recovery stopping at restore point \"%s\", time %s",
                                                recoveryStopName,
                                                timestamptz_to_str(recoveryStopTime))));
                }

                /*
                 * Note that if we use a RECOVERY_TARGET_TIME then we can stop at a
                 * restore point since they are timestamped, though the latest
                 * transaction time is not updated.
                 */
                if (record->xl_rmid == RM_XACT_ID && recoveryStopAfter)
                        SetLatestXTime(recordXtime);
        }
        else if (record->xl_rmid == RM_XACT_ID)
                SetLatestXTime(recordXtime);

        return stopsHere;
}

/*
 * Wait until shared recoveryPause flag is cleared.
 *
 * XXX Could also be done with shared latch, avoiding the pg_usleep loop.
 * Probably not worth the trouble though.  This state shouldn't be one that
 * anyone cares about server power consumption in.
 */
static void
recoveryPausesHere(void)
{
        /* Don't pause unless users can connect! */
        if (!LocalHotStandbyActive)
                return;

        ereport(LOG,
                        (errmsg("recovery has paused"),
                         errhint("Execute pg_xlog_replay_resume() to continue.")));

        while (RecoveryIsPaused())
        {
                pg_usleep(1000000L);    /* 1000 ms */
                HandleStartupProcInterrupts();
        }
}

bool
RecoveryIsPaused(void)
{
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;
        bool            recoveryPause;

        SpinLockAcquire(&xlogctl->info_lck);
        recoveryPause = xlogctl->recoveryPause;
        SpinLockRelease(&xlogctl->info_lck);

        return recoveryPause;
}

void
SetRecoveryPause(bool recoveryPause)
{
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;

        SpinLockAcquire(&xlogctl->info_lck);
        xlogctl->recoveryPause = recoveryPause;
        SpinLockRelease(&xlogctl->info_lck);
}

static bool
SetRecoveryDelayUntilTime(TimestampTz xtime)
{
        if (min_recovery_apply_delay != 0)
        {
                recoveryDelayUntilTime =
                        TimestampTzPlusMilliseconds(xtime, min_recovery_apply_delay);

                return true;
        }

        return false;
}
/*
 * When min_recovery_apply_delay is set, we wait long enough to make sure
 * certain record types are applied at least that interval behind the master.
 * See recoveryStopsHere().
 *
 * Note that the delay is calculated between the WAL record log time and
 * the current time on standby. We would prefer to keep track of when this
 * standby received each WAL record, which would allow a more consistent
 * approach and one not affected by time synchronisation issues, but that
 * is significantly more effort and complexity for little actual gain in
 * usability.
 */
static void
recoveryApplyDelay(void)
{
        while (true)
        {
                long    secs;
                int             microsecs;

                ResetLatch(&XLogCtl->recoveryWakeupLatch);

                /* might change the trigger file's location */
                HandleStartupProcInterrupts();

                if (CheckForStandbyTrigger())
                        break;

                /*
                 * Wait for difference between GetCurrentTimestamp() and
                 * recoveryDelayUntilTime
                 */
                TimestampDifference(GetCurrentTimestamp(), recoveryDelayUntilTime,
                                                        &secs, &microsecs);

                if (secs <= 0 && microsecs <=0)
                        break;

                elog(DEBUG2, "recovery apply delay %ld seconds, %d milliseconds",
                        secs, microsecs / 1000);

                WaitLatch(&XLogCtl->recoveryWakeupLatch,
                                        WL_LATCH_SET | WL_TIMEOUT | WL_POSTMASTER_DEATH,
                                        secs * 1000L + microsecs / 1000);
        }
}

/*
 * Save timestamp of latest processed commit/abort record.
 *
 * We keep this in XLogCtl, not a simple static variable, so that it can be
 * seen by processes other than the startup process.  Note in particular
 * that CreateRestartPoint is executed in the checkpointer.
 */
static void
SetLatestXTime(TimestampTz xtime)
{
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;

        SpinLockAcquire(&xlogctl->info_lck);
        xlogctl->recoveryLastXTime = xtime;
        SpinLockRelease(&xlogctl->info_lck);
}

/*
 * Fetch timestamp of latest processed commit/abort record.
 */
TimestampTz
GetLatestXTime(void)
{
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;
        TimestampTz xtime;

        SpinLockAcquire(&xlogctl->info_lck);
        xtime = xlogctl->recoveryLastXTime;
        SpinLockRelease(&xlogctl->info_lck);

        return xtime;
}

/*
 * Save timestamp of the next chunk of WAL records to apply.
 *
 * We keep this in XLogCtl, not a simple static variable, so that it can be
 * seen by all backends.
 */
static void
SetCurrentChunkStartTime(TimestampTz xtime)
{
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;

        SpinLockAcquire(&xlogctl->info_lck);
        xlogctl->currentChunkStartTime = xtime;
        SpinLockRelease(&xlogctl->info_lck);
}

/*
 * Fetch timestamp of latest processed commit/abort record.
 * Startup process maintains an accurate local copy in XLogReceiptTime
 */
TimestampTz
GetCurrentChunkReplayStartTime(void)
{
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;
        TimestampTz xtime;

        SpinLockAcquire(&xlogctl->info_lck);
        xtime = xlogctl->currentChunkStartTime;
        SpinLockRelease(&xlogctl->info_lck);

        return xtime;
}

/*
 * Returns time of receipt of current chunk of XLOG data, as well as
 * whether it was received from streaming replication or from archives.
 */
void
GetXLogReceiptTime(TimestampTz *rtime, bool *fromStream)
{
        /*
         * This must be executed in the startup process, since we don't export the
         * relevant state to shared memory.
         */
        Assert(InRecovery);

        *rtime = XLogReceiptTime;
        *fromStream = (XLogReceiptSource == XLOG_FROM_STREAM);
}

/*
 * Note that text field supplied is a parameter name and does not require
 * translation
 */
#define RecoveryRequiresIntParameter(param_name, currValue, minValue) \
do { \
        if ((currValue) < (minValue)) \
                ereport(ERROR, \
                                (errcode(ERRCODE_INVALID_PARAMETER_VALUE), \
                                 errmsg("hot standby is not possible because " \
                                                "%s = %d is a lower setting than on the master server " \
                                                "(its value was %d)", \
                                                param_name, \
                                                currValue, \
                                                minValue))); \
} while(0)

/*
 * Check to see if required parameters are set high enough on this server
 * for various aspects of recovery operation.
 */
static void
CheckRequiredParameterValues(void)
{
        /*
         * For archive recovery, the WAL must be generated with at least 'archive'
         * wal_level.
         */
        if (InArchiveRecovery && ControlFile->wal_level == WAL_LEVEL_MINIMAL)
        {
                ereport(WARNING,
                                (errmsg("WAL was generated with wal_level=minimal, data may be missing"),
                                 errhint("This happens if you temporarily set wal_level=minimal without taking a new base backup.")));
        }

        /*
         * For Hot Standby, the WAL must be generated with 'hot_standby' mode, and
         * we must have at least as many backend slots as the primary.
         */
        if (InArchiveRecovery && EnableHotStandby)
        {
                if (ControlFile->wal_level < WAL_LEVEL_HOT_STANDBY)
                        ereport(ERROR,
                                        (errmsg("hot standby is not possible because wal_level was not set to \"hot_standby\" or higher on the master server"),
                                         errhint("Either set wal_level to \"hot_standby\" on the master, or turn off hot_standby here.")));

                /* We ignore autovacuum_max_workers when we make this test. */
                RecoveryRequiresIntParameter("max_connections",
                                                                         MaxConnections,
                                                                         ControlFile->MaxConnections);
                RecoveryRequiresIntParameter("max_worker_processes",
                                                                         max_worker_processes,
                                                                         ControlFile->max_worker_processes);
                RecoveryRequiresIntParameter("max_prepared_transactions",
                                                                         max_prepared_xacts,
                                                                         ControlFile->max_prepared_xacts);
                RecoveryRequiresIntParameter("max_locks_per_transaction",
                                                                         max_locks_per_xact,
                                                                         ControlFile->max_locks_per_xact);
        }
}

/*
 * This must be called ONCE during postmaster or standalone-backend startup
 */
void
StartupXLOG(void)
{
        XLogCtlInsert *Insert;
        CheckPoint      checkPoint;
        bool            wasShutdown;
        bool            reachedStopPoint = false;
        bool            haveBackupLabel = false;
        XLogRecPtr      RecPtr,
                                checkPointLoc,
                                EndOfLog;
        XLogSegNo       endLogSegNo;
        TimeLineID      PrevTimeLineID;
        XLogRecord *record;
        TransactionId oldestActiveXID;
        bool            backupEndRequired = false;
        bool            backupFromStandby = false;
        DBState         dbstate_at_startup;
        XLogReaderState *xlogreader;
        XLogPageReadPrivate private;
        bool            fast_promoted = false;

        /*
         * Read control file and check XLOG status looks valid.
         *
         * Note: in most control paths, *ControlFile is already valid and we need
         * not do ReadControlFile() here, but might as well do it to be sure.
         */
        ReadControlFile();

        if (ControlFile->state < DB_SHUTDOWNED ||
                ControlFile->state > DB_IN_PRODUCTION ||
                !XRecOffIsValid(ControlFile->checkPoint))
                ereport(FATAL,
                                (errmsg("control file contains invalid data")));

        if (ControlFile->state == DB_SHUTDOWNED)
        {
                /* This is the expected case, so don't be chatty in standalone mode */
                ereport(IsPostmasterEnvironment ? LOG : NOTICE,
                                (errmsg("database system was shut down at %s",
                                                str_time(ControlFile->time))));
        }
        else if (ControlFile->state == DB_SHUTDOWNED_IN_RECOVERY)
                ereport(LOG,
                                (errmsg("database system was shut down in recovery at %s",
                                                str_time(ControlFile->time))));
        else if (ControlFile->state == DB_SHUTDOWNING)
                ereport(LOG,
                                (errmsg("database system shutdown was interrupted; last known up at %s",
                                                str_time(ControlFile->time))));
        else if (ControlFile->state == DB_IN_CRASH_RECOVERY)
                ereport(LOG,
                   (errmsg("database system was interrupted while in recovery at %s",
                                   str_time(ControlFile->time)),
                        errhint("This probably means that some data is corrupted and"
                                        " you will have to use the last backup for recovery.")));
        else if (ControlFile->state == DB_IN_ARCHIVE_RECOVERY)
                ereport(LOG,
                                (errmsg("database system was interrupted while in recovery at log time %s",
                                                str_time(ControlFile->checkPointCopy.time)),
                                 errhint("If this has occurred more than once some data might be corrupted"
                          " and you might need to choose an earlier recovery target.")));
        else if (ControlFile->state == DB_IN_PRODUCTION)
                ereport(LOG,
                          (errmsg("database system was interrupted; last known up at %s",
                                          str_time(ControlFile->time))));

        /* This is just to allow attaching to startup process with a debugger */
#ifdef XLOG_REPLAY_DELAY
        if (ControlFile->state != DB_SHUTDOWNED)
                pg_usleep(60000000L);
#endif

        /*
         * Verify that pg_xlog and pg_xlog/archive_status exist.  In cases where
         * someone has performed a copy for PITR, these directories may have been
         * excluded and need to be re-created.
         */
        ValidateXLOGDirectoryStructure();

        /*
         * Clear out any old relcache cache files.      This is *necessary* if we do
         * any WAL replay, since that would probably result in the cache files
         * being out of sync with database reality.  In theory we could leave them
         * in place if the database had been cleanly shut down, but it seems
         * safest to just remove them always and let them be rebuilt during the
         * first backend startup.
         */
        RelationCacheInitFileRemove();

        /*
         * Initialize on the assumption we want to recover to the latest timeline
         * that's active according to pg_control.
         */
        if (ControlFile->minRecoveryPointTLI >
                ControlFile->checkPointCopy.ThisTimeLineID)
                recoveryTargetTLI = ControlFile->minRecoveryPointTLI;
        else
                recoveryTargetTLI = ControlFile->checkPointCopy.ThisTimeLineID;

        /*
         * Check for recovery control file, and if so set up state for offline
         * recovery
         */
        readRecoveryCommandFile();

        /*
         * Save archive_cleanup_command in shared memory so that other processes
         * can see it.
         */
        strncpy(XLogCtl->archiveCleanupCommand,
                        archiveCleanupCommand ? archiveCleanupCommand : "",
                        sizeof(XLogCtl->archiveCleanupCommand));

        if (ArchiveRecoveryRequested)
        {
                if (StandbyModeRequested)
                        ereport(LOG,
                                        (errmsg("entering standby mode")));
                else if (recoveryTarget == RECOVERY_TARGET_XID)
                        ereport(LOG,
                                        (errmsg("starting point-in-time recovery to XID %u",
                                                        recoveryTargetXid)));
                else if (recoveryTarget == RECOVERY_TARGET_TIME)
                        ereport(LOG,
                                        (errmsg("starting point-in-time recovery to %s",
                                                        timestamptz_to_str(recoveryTargetTime))));
                else if (recoveryTarget == RECOVERY_TARGET_NAME)
                        ereport(LOG,
                                        (errmsg("starting point-in-time recovery to \"%s\"",
                                                        recoveryTargetName)));
                else
                        ereport(LOG,
                                        (errmsg("starting archive recovery")));
        }

        /*
         * Take ownership of the wakeup latch if we're going to sleep during
         * recovery.
         */
        if (StandbyModeRequested)
                OwnLatch(&XLogCtl->recoveryWakeupLatch);

        /* Set up XLOG reader facility */
        MemSet(&private, 0, sizeof(XLogPageReadPrivate));
        xlogreader = XLogReaderAllocate(&XLogPageRead, &private);
        if (!xlogreader)
                ereport(ERROR,
                                (errcode(ERRCODE_OUT_OF_MEMORY),
                                 errmsg("out of memory"),
                        errdetail("Failed while allocating an XLog reading processor.")));
        xlogreader->system_identifier = ControlFile->system_identifier;

        if (read_backup_label(&checkPointLoc, &backupEndRequired,
                                                  &backupFromStandby))
        {
                /*
                 * Archive recovery was requested, and thanks to the backup label
                 * file, we know how far we need to replay to reach consistency. Enter
                 * archive recovery directly.
                 */
                InArchiveRecovery = true;
                if (StandbyModeRequested)
                        StandbyMode = true;

                /*
                 * When a backup_label file is present, we want to roll forward from
                 * the checkpoint it identifies, rather than using pg_control.
                 */
                record = ReadCheckpointRecord(xlogreader, checkPointLoc, 0, true);
                if (record != NULL)
                {
                        memcpy(&checkPoint, XLogRecGetData(record), sizeof(CheckPoint));
                        wasShutdown = (record->xl_info == XLOG_CHECKPOINT_SHUTDOWN);
                        ereport(DEBUG1,
                                        (errmsg("checkpoint record is at %X/%X",
                                   (uint32) (checkPointLoc >> 32), (uint32) checkPointLoc)));
                        InRecovery = true;      /* force recovery even if SHUTDOWNED */

                        /*
                         * Make sure that REDO location exists. This may not be the case
                         * if there was a crash during an online backup, which left a
                         * backup_label around that references a WAL segment that's
                         * already been archived.
                         */
                        if (checkPoint.redo < checkPointLoc)
                        {
                                if (!ReadRecord(xlogreader, checkPoint.redo, LOG, false))
                                        ereport(FATAL,
                                                        (errmsg("could not find redo location referenced by checkpoint record"),
                                                         errhint("If you are not restoring from a backup, try removing the file \"%s/backup_label\".", DataDir)));
                        }
                }
                else
                {
                        ereport(FATAL,
                                        (errmsg("could not locate required checkpoint record"),
                                         errhint("If you are not restoring from a backup, try removing the file \"%s/backup_label\".", DataDir)));
                        wasShutdown = false;    /* keep compiler quiet */
                }
                /* set flag to delete it later */
                haveBackupLabel = true;
        }
        else
        {
                /*
                 * It's possible that archive recovery was requested, but we don't
                 * know how far we need to replay the WAL before we reach consistency.
                 * This can happen for example if a base backup is taken from a
                 * running server using an atomic filesystem snapshot, without calling
                 * pg_start/stop_backup. Or if you just kill a running master server
                 * and put it into archive recovery by creating a recovery.conf file.
                 *
                 * Our strategy in that case is to perform crash recovery first,
                 * replaying all the WAL present in pg_xlog, and only enter archive
                 * recovery after that.
                 *
                 * But usually we already know how far we need to replay the WAL (up
                 * to minRecoveryPoint, up to backupEndPoint, or until we see an
                 * end-of-backup record), and we can enter archive recovery directly.
                 */
                if (ArchiveRecoveryRequested &&
                        (ControlFile->minRecoveryPoint != InvalidXLogRecPtr ||
                         ControlFile->backupEndRequired ||
                         ControlFile->backupEndPoint != InvalidXLogRecPtr ||
                         ControlFile->state == DB_SHUTDOWNED))
                {
                        InArchiveRecovery = true;
                        if (StandbyModeRequested)
                                StandbyMode = true;
                }

                /*
                 * Get the last valid checkpoint record.  If the latest one according
                 * to pg_control is broken, try the next-to-last one.
                 */
                checkPointLoc = ControlFile->checkPoint;
                RedoStartLSN = ControlFile->checkPointCopy.redo;
                record = ReadCheckpointRecord(xlogreader, checkPointLoc, 1, true);
                if (record != NULL)
                {
                        ereport(DEBUG1,
                                        (errmsg("checkpoint record is at %X/%X",
                                   (uint32) (checkPointLoc >> 32), (uint32) checkPointLoc)));
                }
                else if (StandbyMode)
                {
                        /*
                         * The last valid checkpoint record required for a streaming
                         * recovery exists in neither standby nor the primary.
                         */
                        ereport(PANIC,
                                        (errmsg("could not locate a valid checkpoint record")));
                }
                else
                {
                        checkPointLoc = ControlFile->prevCheckPoint;
                        record = ReadCheckpointRecord(xlogreader, checkPointLoc, 2, true);
                        if (record != NULL)
                        {
                                ereport(LOG,
                                                (errmsg("using previous checkpoint record at %X/%X",
                                   (uint32) (checkPointLoc >> 32), (uint32) checkPointLoc)));
                                InRecovery = true;              /* force recovery even if SHUTDOWNED */
                        }
                        else
                                ereport(PANIC,
                                         (errmsg("could not locate a valid checkpoint record")));
                }
                memcpy(&checkPoint, XLogRecGetData(record), sizeof(CheckPoint));
                wasShutdown = (record->xl_info == XLOG_CHECKPOINT_SHUTDOWN);
        }

        /*
         * If the location of the checkpoint record is not on the expected
         * timeline in the history of the requested timeline, we cannot proceed:
         * the backup is not part of the history of the requested timeline.
         */
        Assert(expectedTLEs);           /* was initialized by reading checkpoint
                                                                 * record */
        if (tliOfPointInHistory(checkPointLoc, expectedTLEs) !=
                checkPoint.ThisTimeLineID)
        {
                XLogRecPtr      switchpoint;

                /*
                 * tliSwitchPoint will throw an error if the checkpoint's timeline is
                 * not in expectedTLEs at all.
                 */
                switchpoint = tliSwitchPoint(ControlFile->checkPointCopy.ThisTimeLineID, expectedTLEs, NULL);
                ereport(FATAL,
                                (errmsg("requested timeline %u is not a child of this server's history",
                                                recoveryTargetTLI),
                                 errdetail("Latest checkpoint is at %X/%X on timeline %u, but in the history of the requested timeline, the server forked off from that timeline at %X/%X.",
                                                   (uint32) (ControlFile->checkPoint >> 32),
                                                   (uint32) ControlFile->checkPoint,
                                                   ControlFile->checkPointCopy.ThisTimeLineID,
                                                   (uint32) (switchpoint >> 32),
                                                   (uint32) switchpoint)));
        }

        /*
         * The min recovery point should be part of the requested timeline's
         * history, too.
         */
        if (!XLogRecPtrIsInvalid(ControlFile->minRecoveryPoint) &&
          tliOfPointInHistory(ControlFile->minRecoveryPoint - 1, expectedTLEs) !=
                ControlFile->minRecoveryPointTLI)
                ereport(FATAL,
                                (errmsg("requested timeline %u does not contain minimum recovery point %X/%X on timeline %u",
                                                recoveryTargetTLI,
                                                (uint32) (ControlFile->minRecoveryPoint >> 32),
                                                (uint32) ControlFile->minRecoveryPoint,
                                                ControlFile->minRecoveryPointTLI)));

        LastRec = RecPtr = checkPointLoc;

        ereport(DEBUG1,
                        (errmsg("redo record is at %X/%X; shutdown %s",
                                  (uint32) (checkPoint.redo >> 32), (uint32) checkPoint.redo,
                                        wasShutdown ? "TRUE" : "FALSE")));
        ereport(DEBUG1,
                        (errmsg("next transaction ID: %u/%u; next OID: %u",
                                        checkPoint.nextXidEpoch, checkPoint.nextXid,
                                        checkPoint.nextOid)));
        ereport(DEBUG1,
                        (errmsg("next MultiXactId: %u; next MultiXactOffset: %u",
                                        checkPoint.nextMulti, checkPoint.nextMultiOffset)));
        ereport(DEBUG1,
                        (errmsg("oldest unfrozen transaction ID: %u, in database %u",
                                        checkPoint.oldestXid, checkPoint.oldestXidDB)));
        ereport(DEBUG1,
                        (errmsg("oldest MultiXactId: %u, in database %u",
                                        checkPoint.oldestMulti, checkPoint.oldestMultiDB)));
        if (!TransactionIdIsNormal(checkPoint.nextXid))
                ereport(PANIC,
                                (errmsg("invalid next transaction ID")));

        /* initialize shared memory variables from the checkpoint record */
        ShmemVariableCache->nextXid = checkPoint.nextXid;
        ShmemVariableCache->nextOid = checkPoint.nextOid;
        ShmemVariableCache->oidCount = 0;
        MultiXactSetNextMXact(checkPoint.nextMulti, checkPoint.nextMultiOffset);
        SetTransactionIdLimit(checkPoint.oldestXid, checkPoint.oldestXidDB);
        SetMultiXactIdLimit(checkPoint.oldestMulti, checkPoint.oldestMultiDB);
        XLogCtl->ckptXidEpoch = checkPoint.nextXidEpoch;
        XLogCtl->ckptXid = checkPoint.nextXid;

        /*
         * Startup MultiXact.  We need to do this early for two reasons: one
         * is that we might try to access multixacts when we do tuple freezing,
         * and the other is we need its state initialized because we attempt
         * truncation during restartpoints.
         */
        StartupMultiXact();

        /*
         * Initialize unlogged LSN. On a clean shutdown, it's restored from the
         * control file. On recovery, all unlogged relations are blown away, so
         * the unlogged LSN counter can be reset too.
         */
        if (ControlFile->state == DB_SHUTDOWNED)
                XLogCtl->unloggedLSN = ControlFile->unloggedLSN;
        else
                XLogCtl->unloggedLSN = 1;

        /*
         * We must replay WAL entries using the same TimeLineID they were created
         * under, so temporarily adopt the TLI indicated by the checkpoint (see
         * also xlog_redo()).
         */
        ThisTimeLineID = checkPoint.ThisTimeLineID;

        /*
         * Copy any missing timeline history files between 'now' and the recovery
         * target timeline from archive to pg_xlog. While we don't need those
         * files ourselves - the history file of the recovery target timeline
         * covers all the previous timelines in the history too - a cascading
         * standby server might be interested in them. Or, if you archive the WAL
         * from this server to a different archive than the master, it'd be good
         * for all the history files to get archived there after failover, so that
         * you can use one of the old timelines as a PITR target. Timeline history
         * files are small, so it's better to copy them unnecessarily than not
         * copy them and regret later.
         */
        restoreTimeLineHistoryFiles(ThisTimeLineID, recoveryTargetTLI);

        lastFullPageWrites = checkPoint.fullPageWrites;

        RedoRecPtr = XLogCtl->RedoRecPtr = XLogCtl->Insert.RedoRecPtr = checkPoint.redo;

        if (RecPtr < checkPoint.redo)
                ereport(PANIC,
                                (errmsg("invalid redo in checkpoint record")));

        /*
         * Check whether we need to force recovery from WAL.  If it appears to
         * have been a clean shutdown and we did not have a recovery.conf file,
         * then assume no recovery needed.
         */
        if (checkPoint.redo < RecPtr)
        {
                if (wasShutdown)
                        ereport(PANIC,
                                        (errmsg("invalid redo record in shutdown checkpoint")));
                InRecovery = true;
        }
        else if (ControlFile->state != DB_SHUTDOWNED)
                InRecovery = true;
        else if (ArchiveRecoveryRequested)
        {
                /* force recovery due to presence of recovery.conf */
                InRecovery = true;
        }

        /* REDO */
        if (InRecovery)
        {
                int                     rmid;

                /* use volatile pointer to prevent code rearrangement */
                volatile XLogCtlData *xlogctl = XLogCtl;

                /*
                 * Update pg_control to show that we are recovering and to show the
                 * selected checkpoint as the place we are starting from. We also mark
                 * pg_control with any minimum recovery stop point obtained from a
                 * backup history file.
                 */
                dbstate_at_startup = ControlFile->state;
                if (InArchiveRecovery)
                        ControlFile->state = DB_IN_ARCHIVE_RECOVERY;
                else
                {
                        ereport(LOG,
                                        (errmsg("database system was not properly shut down; "
                                                        "automatic recovery in progress")));
                        if (recoveryTargetTLI > ControlFile->checkPointCopy.ThisTimeLineID)
                                ereport(LOG,
                                                (errmsg("crash recovery starts in timeline %u "
                                                                "and has target timeline %u",
                                                                ControlFile->checkPointCopy.ThisTimeLineID,
                                                                recoveryTargetTLI)));
                        ControlFile->state = DB_IN_CRASH_RECOVERY;
                }
                ControlFile->prevCheckPoint = ControlFile->checkPoint;
                ControlFile->checkPoint = checkPointLoc;
                ControlFile->checkPointCopy = checkPoint;
                if (InArchiveRecovery)
                {
                        /* initialize minRecoveryPoint if not set yet */
                        if (ControlFile->minRecoveryPoint < checkPoint.redo)
                        {
                                ControlFile->minRecoveryPoint = checkPoint.redo;
                                ControlFile->minRecoveryPointTLI = checkPoint.ThisTimeLineID;
                        }
                }

                /*
                 * Set backupStartPoint if we're starting recovery from a base backup.
                 *
                 * Set backupEndPoint and use minRecoveryPoint as the backup end
                 * location if we're starting recovery from a base backup which was
                 * taken from the standby. In this case, the database system status in
                 * pg_control must indicate DB_IN_ARCHIVE_RECOVERY. If not, which
                 * means that backup is corrupted, so we cancel recovery.
                 */
                if (haveBackupLabel)
                {
                        ControlFile->backupStartPoint = checkPoint.redo;
                        ControlFile->backupEndRequired = backupEndRequired;

                        if (backupFromStandby)
                        {
                                if (dbstate_at_startup != DB_IN_ARCHIVE_RECOVERY)
                                        ereport(FATAL,
                                                        (errmsg("backup_label contains data inconsistent with control file"),
                                                         errhint("This means that the backup is corrupted and you will "
                                                           "have to use another backup for recovery.")));
                                ControlFile->backupEndPoint = ControlFile->minRecoveryPoint;
                        }
                }
                ControlFile->time = (pg_time_t) time(NULL);
                /* No need to hold ControlFileLock yet, we aren't up far enough */
                UpdateControlFile();

                /* initialize our local copy of minRecoveryPoint */
                minRecoveryPoint = ControlFile->minRecoveryPoint;
                minRecoveryPointTLI = ControlFile->minRecoveryPointTLI;

                /*
                 * Reset pgstat data, because it may be invalid after recovery.
                 */
                pgstat_reset_all();

                /*
                 * If there was a backup label file, it's done its job and the info
                 * has now been propagated into pg_control.  We must get rid of the
                 * label file so that if we crash during recovery, we'll pick up at
                 * the latest recovery restartpoint instead of going all the way back
                 * to the backup start point.  It seems prudent though to just rename
                 * the file out of the way rather than delete it completely.
                 */
                if (haveBackupLabel)
                {
                        unlink(BACKUP_LABEL_OLD);
                        if (rename(BACKUP_LABEL_FILE, BACKUP_LABEL_OLD) != 0)
                                ereport(FATAL,
                                                (errcode_for_file_access(),
                                                 errmsg("could not rename file \"%s\" to \"%s\": %m",
                                                                BACKUP_LABEL_FILE, BACKUP_LABEL_OLD)));
                }

                /* Check that the GUCs used to generate the WAL allow recovery */
                CheckRequiredParameterValues();

                /*
                 * We're in recovery, so unlogged relations may be trashed and must be
                 * reset.  This should be done BEFORE allowing Hot Standby
                 * connections, so that read-only backends don't try to read whatever
                 * garbage is left over from before.
                 */
                ResetUnloggedRelations(UNLOGGED_RELATION_CLEANUP);

                /*
                 * Likewise, delete any saved transaction snapshot files that got left
                 * behind by crashed backends.
                 */
                DeleteAllExportedSnapshotFiles();

                /*
                 * Initialize for Hot Standby, if enabled. We won't let backends in
                 * yet, not until we've reached the min recovery point specified in
                 * control file and we've established a recovery snapshot from a
                 * running-xacts WAL record.
                 */
                if (ArchiveRecoveryRequested && EnableHotStandby)
                {
                        TransactionId *xids;
                        int                     nxids;

                        ereport(DEBUG1,
                                        (errmsg("initializing for hot standby")));

                        InitRecoveryTransactionEnvironment();

                        if (wasShutdown)
                                oldestActiveXID = PrescanPreparedTransactions(&xids, &nxids);
                        else
                                oldestActiveXID = checkPoint.oldestActiveXid;
                        Assert(TransactionIdIsValid(oldestActiveXID));

                        /* Tell procarray about the range of xids it has to deal with */
                        ProcArrayInitRecovery(ShmemVariableCache->nextXid);

                        /*
                         * Startup commit log and subtrans only. MultiXact has already
                         * been started up and other SLRUs are not maintained during
                         * recovery and need not be started yet.
                         */
                        StartupCLOG();
                        StartupSUBTRANS(oldestActiveXID);

                        /*
                         * If we're beginning at a shutdown checkpoint, we know that
                         * nothing was running on the master at this point. So fake-up an
                         * empty running-xacts record and use that here and now. Recover
                         * additional standby state for prepared transactions.
                         */
                        if (wasShutdown)
                        {
                                RunningTransactionsData running;
                                TransactionId latestCompletedXid;

                                /*
                                 * Construct a RunningTransactions snapshot representing a
                                 * shut down server, with only prepared transactions still
                                 * alive. We're never overflowed at this point because all
                                 * subxids are listed with their parent prepared transactions.
                                 */
                                running.xcnt = nxids;
                                running.subxcnt = 0;
                                running.subxid_overflow = false;
                                running.nextXid = checkPoint.nextXid;
                                running.oldestRunningXid = oldestActiveXID;
                                latestCompletedXid = checkPoint.nextXid;
                                TransactionIdRetreat(latestCompletedXid);
                                Assert(TransactionIdIsNormal(latestCompletedXid));
                                running.latestCompletedXid = latestCompletedXid;
                                running.xids = xids;

                                ProcArrayApplyRecoveryInfo(&running);

                                StandbyRecoverPreparedTransactions(false);
                        }
                }

                /* Initialize resource managers */
                for (rmid = 0; rmid <= RM_MAX_ID; rmid++)
                {
                        if (RmgrTable[rmid].rm_startup != NULL)
                                RmgrTable[rmid].rm_startup();
                }

                /*
                 * Initialize shared replayEndRecPtr, lastReplayedEndRecPtr, and
                 * recoveryLastXTime.
                 *
                 * This is slightly confusing if we're starting from an online
                 * checkpoint; we've just read and replayed the checkpoint record, but
                 * we're going to start replay from its redo pointer, which precedes
                 * the location of the checkpoint record itself. So even though the
                 * last record we've replayed is indeed ReadRecPtr, we haven't
                 * replayed all the preceding records yet. That's OK for the current
                 * use of these variables.
                 */
                SpinLockAcquire(&xlogctl->info_lck);
                xlogctl->replayEndRecPtr = ReadRecPtr;
                xlogctl->replayEndTLI = ThisTimeLineID;
                xlogctl->lastReplayedEndRecPtr = EndRecPtr;
                xlogctl->lastReplayedTLI = ThisTimeLineID;
                xlogctl->recoveryLastXTime = 0;
                xlogctl->currentChunkStartTime = 0;
                xlogctl->recoveryPause = false;
                SpinLockRelease(&xlogctl->info_lck);

                /* Also ensure XLogReceiptTime has a sane value */
                XLogReceiptTime = GetCurrentTimestamp();

                /*
                 * Let postmaster know we've started redo now, so that it can launch
                 * checkpointer to perform restartpoints.  We don't bother during
                 * crash recovery as restartpoints can only be performed during
                 * archive recovery.  And we'd like to keep crash recovery simple, to
                 * avoid introducing bugs that could affect you when recovering after
                 * crash.
                 *
                 * After this point, we can no longer assume that we're the only
                 * process in addition to postmaster!  Also, fsync requests are
                 * subsequently to be handled by the checkpointer, not locally.
                 */
                if (ArchiveRecoveryRequested && IsUnderPostmaster)
                {
                        PublishStartupProcessInformation();
                        SetForwardFsyncRequests();
                        SendPostmasterSignal(PMSIGNAL_RECOVERY_STARTED);
                        bgwriterLaunched = true;
                }

                /*
                 * Allow read-only connections immediately if we're consistent
                 * already.
                 */
                CheckRecoveryConsistency();

                /*
                 * Find the first record that logically follows the checkpoint --- it
                 * might physically precede it, though.
                 */
                if (checkPoint.redo < RecPtr)
                {
                        /* back up to find the record */
                        record = ReadRecord(xlogreader, checkPoint.redo, PANIC, false);
                }
                else
                {
                        /* just have to read next record after CheckPoint */
                        record = ReadRecord(xlogreader, InvalidXLogRecPtr, LOG, false);
                }

                if (record != NULL)
                {
                        bool            recoveryContinue = true;
                        bool            recoveryApply = true;
                        bool            recoveryDelay = false;
                        ErrorContextCallback errcallback;
                        TimestampTz xtime;

                        InRedo = true;

                        ereport(LOG,
                                        (errmsg("redo starts at %X/%X",
                                                 (uint32) (ReadRecPtr >> 32), (uint32) ReadRecPtr)));

                        /*
                         * main redo apply loop
                         */
                        do
                        {
                                bool            switchedTLI = false;

#ifdef WAL_DEBUG
                                if (XLOG_DEBUG ||
                                 (rmid == RM_XACT_ID && trace_recovery_messages <= DEBUG2) ||
                                        (rmid != RM_XACT_ID && trace_recovery_messages <= DEBUG3))
                                {
                                        StringInfoData buf;

                                        initStringInfo(&buf);
                                        appendStringInfo(&buf, "REDO @ %X/%X; LSN %X/%X: ",
                                                        (uint32) (ReadRecPtr >> 32), (uint32) ReadRecPtr,
                                                         (uint32) (EndRecPtr >> 32), (uint32) EndRecPtr);
                                        xlog_outrec(&buf, record);
                                        appendStringInfoString(&buf, " - ");
                                        RmgrTable[record->xl_rmid].rm_desc(&buf,
                                                                                                           record->xl_info,
                                                                                                         XLogRecGetData(record));
                                        elog(LOG, "%s", buf.data);
                                        pfree(buf.data);
                                }
#endif

                                /* Handle interrupt signals of startup process */
                                HandleStartupProcInterrupts();

                                /*
                                 * Pause WAL replay, if requested by a hot-standby session via
                                 * SetRecoveryPause().
                                 *
                                 * Note that we intentionally don't take the info_lck spinlock
                                 * here.  We might therefore read a slightly stale value of
                                 * the recoveryPause flag, but it can't be very stale (no
                                 * worse than the last spinlock we did acquire).  Since a
                                 * pause request is a pretty asynchronous thing anyway,
                                 * possibly responding to it one WAL record later than we
                                 * otherwise would is a minor issue, so it doesn't seem worth
                                 * adding another spinlock cycle to prevent that.
                                 */
                                if (xlogctl->recoveryPause)
                                        recoveryPausesHere();

                                /*
                                 * Have we reached our recovery target?
                                 */
                                if (recoveryStopsHere(record, &recoveryApply, &recoveryDelay))
                                {
                                        if (recoveryPauseAtTarget)
                                        {
                                                SetRecoveryPause(true);
                                                recoveryPausesHere();
                                        }
                                        reachedStopPoint = true;        /* see below */
                                        recoveryContinue = false;

                                        /* Exit loop if we reached non-inclusive recovery target */
                                        if (!recoveryApply)
                                                break;
                                }

                                /*
                                 * If we've been asked to lag the master, wait on
                                 * latch until enough time has passed.
                                 */
                                if (recoveryDelay)
                                {
                                        recoveryApplyDelay();

                                        /*
                                         * We test for paused recovery again here. If
                                         * user sets delayed apply, it may be because
                                         * they expect to pause recovery in case of
                                         * problems, so we must test again here otherwise
                                         * pausing during the delay-wait wouldn't work.
                                         */
                                        if (xlogctl->recoveryPause)
                                                recoveryPausesHere();
                                }

                                /* Setup error traceback support for ereport() */
                                errcallback.callback = rm_redo_error_callback;
                                errcallback.arg = (void *) record;
                                errcallback.previous = error_context_stack;
                                error_context_stack = &errcallback;

                                /*
                                 * ShmemVariableCache->nextXid must be beyond record's xid.
                                 *
                                 * We don't expect anyone else to modify nextXid, hence we
                                 * don't need to hold a lock while examining it.  We still
                                 * acquire the lock to modify it, though.
                                 */
                                if (TransactionIdFollowsOrEquals(record->xl_xid,
                                                                                                 ShmemVariableCache->nextXid))
                                {
                                        LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
                                        ShmemVariableCache->nextXid = record->xl_xid;
                                        TransactionIdAdvance(ShmemVariableCache->nextXid);
                                        LWLockRelease(XidGenLock);
                                }

                                /*
                                 * Before replaying this record, check if this record causes
                                 * the current timeline to change. The record is already
                                 * considered to be part of the new timeline, so we update
                                 * ThisTimeLineID before replaying it. That's important so
                                 * that replayEndTLI, which is recorded as the minimum
                                 * recovery point's TLI if recovery stops after this record,
                                 * is set correctly.
                                 */
                                if (record->xl_rmid == RM_XLOG_ID)
                                {
                                        TimeLineID      newTLI = ThisTimeLineID;
                                        TimeLineID      prevTLI = ThisTimeLineID;
                                        uint8           info = record->xl_info & ~XLR_INFO_MASK;

                                        if (info == XLOG_CHECKPOINT_SHUTDOWN)
                                        {
                                                CheckPoint      checkPoint;

                                                memcpy(&checkPoint, XLogRecGetData(record), sizeof(CheckPoint));
                                                newTLI = checkPoint.ThisTimeLineID;
                                                prevTLI = checkPoint.PrevTimeLineID;
                                        }
                                        else if (info == XLOG_END_OF_RECOVERY)
                                        {
                                                xl_end_of_recovery xlrec;

                                                memcpy(&xlrec, XLogRecGetData(record), sizeof(xl_end_of_recovery));
                                                newTLI = xlrec.ThisTimeLineID;
                                                prevTLI = xlrec.PrevTimeLineID;
                                        }

                                        if (newTLI != ThisTimeLineID)
                                        {
                                                /* Check that it's OK to switch to this TLI */
                                                checkTimeLineSwitch(EndRecPtr, newTLI, prevTLI);

                                                /* Following WAL records should be run with new TLI */
                                                ThisTimeLineID = newTLI;
                                                switchedTLI = true;
                                        }
                                }

                                /*
                                 * Update shared replayEndRecPtr before replaying this record,
                                 * so that XLogFlush will update minRecoveryPoint correctly.
                                 */
                                SpinLockAcquire(&xlogctl->info_lck);
                                xlogctl->replayEndRecPtr = EndRecPtr;
                                xlogctl->replayEndTLI = ThisTimeLineID;
                                SpinLockRelease(&xlogctl->info_lck);

                                /*
                                 * If we are attempting to enter Hot Standby mode, process
                                 * XIDs we see
                                 */
                                if (standbyState >= STANDBY_INITIALIZED &&
                                        TransactionIdIsValid(record->xl_xid))
                                        RecordKnownAssignedTransactionIds(record->xl_xid);

                                /* Now apply the WAL record itself */
                                RmgrTable[record->xl_rmid].rm_redo(EndRecPtr, record);

                                /* Pop the error context stack */
                                error_context_stack = errcallback.previous;

                                /*
                                 * Update lastReplayedEndRecPtr after this record has been
                                 * successfully replayed.
                                 */
                                SpinLockAcquire(&xlogctl->info_lck);
                                xlogctl->lastReplayedEndRecPtr = EndRecPtr;
                                xlogctl->lastReplayedTLI = ThisTimeLineID;
                                SpinLockRelease(&xlogctl->info_lck);

                                /* Remember this record as the last-applied one */
                                LastRec = ReadRecPtr;

                                /* Allow read-only connections if we're consistent now */
                                CheckRecoveryConsistency();

                                /*
                                 * If this record was a timeline switch, wake up any
                                 * walsenders to notice that we are on a new timeline.
                                 */
                                if (switchedTLI && AllowCascadeReplication())
                                        WalSndWakeup();

                                /* Exit loop if we reached inclusive recovery target */
                                if (!recoveryContinue)
                                        break;

                                /* Else, try to fetch the next WAL record */
                                record = ReadRecord(xlogreader, InvalidXLogRecPtr, LOG, false);
                        } while (record != NULL);

                        /*
                         * end of main redo apply loop
                         */

                        ereport(LOG,
                                        (errmsg("redo done at %X/%X",
                                                 (uint32) (ReadRecPtr >> 32), (uint32) ReadRecPtr)));
                        xtime = GetLatestXTime();
                        if (xtime)
                                ereport(LOG,
                                         (errmsg("last completed transaction was at log time %s",
                                                         timestamptz_to_str(xtime))));
                        InRedo = false;
                }
                else
                {
                        /* there are no WAL records following the checkpoint */
                        ereport(LOG,
                                        (errmsg("redo is not required")));
                }
        }

        /*
         * Kill WAL receiver, if it's still running, before we continue to write
         * the startup checkpoint record. It will trump over the checkpoint and
         * subsequent records if it's still alive when we start writing WAL.
         */
        ShutdownWalRcv();

        /*
         * We don't need the latch anymore. It's not strictly necessary to disown
         * it, but let's do it for the sake of tidiness.
         */
        if (StandbyModeRequested)
                DisownLatch(&XLogCtl->recoveryWakeupLatch);

        /*
         * We are now done reading the xlog from stream. Turn off streaming
         * recovery to force fetching the files (which would be required at end of
         * recovery, e.g., timeline history file) from archive or pg_xlog.
         */
        StandbyMode = false;

        /*
         * Re-fetch the last valid or last applied record, so we can identify the
         * exact endpoint of what we consider the valid portion of WAL.
         */
        record = ReadRecord(xlogreader, LastRec, PANIC, false);
        EndOfLog = EndRecPtr;
        XLByteToPrevSeg(EndOfLog, endLogSegNo);

        /*
         * Complain if we did not roll forward far enough to render the backup
         * dump consistent.  Note: it is indeed okay to look at the local variable
         * minRecoveryPoint here, even though ControlFile->minRecoveryPoint might
         * be further ahead --- ControlFile->minRecoveryPoint cannot have been
         * advanced beyond the WAL we processed.
         */
        if (InRecovery &&
                (EndOfLog < minRecoveryPoint ||
                 !XLogRecPtrIsInvalid(ControlFile->backupStartPoint)))
        {
                if (reachedStopPoint)
                {
                        /* stopped because of stop request */
                        ereport(FATAL,
                                        (errmsg("requested recovery stop point is before consistent recovery point")));
                }

                /*
                 * Ran off end of WAL before reaching end-of-backup WAL record, or
                 * minRecoveryPoint. That's usually a bad sign, indicating that you
                 * tried to recover from an online backup but never called
                 * pg_stop_backup(), or you didn't archive all the WAL up to that
                 * point. However, this also happens in crash recovery, if the system
                 * crashes while an online backup is in progress. We must not treat
                 * that as an error, or the database will refuse to start up.
                 */
                if (ArchiveRecoveryRequested || ControlFile->backupEndRequired)
                {
                        if (ControlFile->backupEndRequired)
                                ereport(FATAL,
                                                (errmsg("WAL ends before end of online backup"),
                                                 errhint("All WAL generated while online backup was taken must be available at recovery.")));
                        else if (!XLogRecPtrIsInvalid(ControlFile->backupStartPoint))
                                ereport(FATAL,
                                                (errmsg("WAL ends before end of online backup"),
                                                 errhint("Online backup started with pg_start_backup() must be ended with pg_stop_backup(), and all WAL up to that point must be available at recovery.")));
                        else
                                ereport(FATAL,
                                          (errmsg("WAL ends before consistent recovery point")));
                }
        }

        /*
         * Consider whether we need to assign a new timeline ID.
         *
         * If we are doing an archive recovery, we always assign a new ID.      This
         * handles a couple of issues.  If we stopped short of the end of WAL
         * during recovery, then we are clearly generating a new timeline and must
         * assign it a unique new ID.  Even if we ran to the end, modifying the
         * current last segment is problematic because it may result in trying to
         * overwrite an already-archived copy of that segment, and we encourage
         * DBAs to make their archive_commands reject that.  We can dodge the
         * problem by making the new active segment have a new timeline ID.
         *
         * In a normal crash recovery, we can just extend the timeline we were in.
         */
        PrevTimeLineID = ThisTimeLineID;
        if (ArchiveRecoveryRequested)
        {
                char            reason[200];

                Assert(InArchiveRecovery);

                ThisTimeLineID = findNewestTimeLine(recoveryTargetTLI) + 1;
                ereport(LOG,
                                (errmsg("selected new timeline ID: %u", ThisTimeLineID)));

                /*
                 * Create a comment for the history file to explain why and where
                 * timeline changed.
                 */
                if (recoveryTarget == RECOVERY_TARGET_XID)
                        snprintf(reason, sizeof(reason),
                                         "%s transaction %u",
                                         recoveryStopAfter ? "after" : "before",
                                         recoveryStopXid);
                else if (recoveryTarget == RECOVERY_TARGET_TIME)
                        snprintf(reason, sizeof(reason),
                                         "%s %s\n",
                                         recoveryStopAfter ? "after" : "before",
                                         timestamptz_to_str(recoveryStopTime));
                else if (recoveryTarget == RECOVERY_TARGET_NAME)
                        snprintf(reason, sizeof(reason),
                                         "at restore point \"%s\"",
                                         recoveryStopName);
                else
                        snprintf(reason, sizeof(reason), "no recovery target specified");

                writeTimeLineHistory(ThisTimeLineID, recoveryTargetTLI,
                                                         EndRecPtr, reason);
        }

        /* Save the selected TimeLineID in shared memory, too */
        XLogCtl->ThisTimeLineID = ThisTimeLineID;
        XLogCtl->PrevTimeLineID = PrevTimeLineID;

        /*
         * We are now done reading the old WAL.  Turn off archive fetching if it
         * was active, and make a writable copy of the last WAL segment. (Note
         * that we also have a copy of the last block of the old WAL in readBuf;
         * we will use that below.)
         */
        if (ArchiveRecoveryRequested)
                exitArchiveRecovery(xlogreader->readPageTLI, endLogSegNo);

        /*
         * Prepare to write WAL starting at EndOfLog position, and init xlog
         * buffer cache using the block containing the last record from the
         * previous incarnation.
         */
        openLogSegNo = endLogSegNo;
        openLogFile = XLogFileOpen(openLogSegNo);
        openLogOff = 0;
        Insert = &XLogCtl->Insert;
        Insert->PrevBytePos = XLogRecPtrToBytePos(LastRec);
        Insert->CurrBytePos = XLogRecPtrToBytePos(EndOfLog);

        /*
         * Tricky point here: readBuf contains the *last* block that the LastRec
         * record spans, not the one it starts in.      The last block is indeed the
         * one we want to use.
         */
        if (EndOfLog % XLOG_BLCKSZ != 0)
        {
                char       *page;
                int                     len;
                int                     firstIdx;
                XLogRecPtr      pageBeginPtr;

                pageBeginPtr = EndOfLog - (EndOfLog % XLOG_BLCKSZ);
                Assert(readOff == pageBeginPtr % XLogSegSize);

                firstIdx = XLogRecPtrToBufIdx(EndOfLog);

                /* Copy the valid part of the last block, and zero the rest */
                page = &XLogCtl->pages[firstIdx * XLOG_BLCKSZ];
                len = EndOfLog % XLOG_BLCKSZ;
                memcpy(page, xlogreader->readBuf, len);
                memset(page + len, 0, XLOG_BLCKSZ - len);

                XLogCtl->xlblocks[firstIdx] = pageBeginPtr + XLOG_BLCKSZ;
                XLogCtl->InitializedUpTo = pageBeginPtr + XLOG_BLCKSZ;
        }
        else
        {
                /*
                 * There is no partial block to copy. Just set InitializedUpTo,
                 * and let the first attempt to insert a log record to initialize
                 * the next buffer.
                 */
                XLogCtl->InitializedUpTo = EndOfLog;
        }

        LogwrtResult.Write = LogwrtResult.Flush = EndOfLog;

        XLogCtl->LogwrtResult = LogwrtResult;

        XLogCtl->LogwrtRqst.Write = EndOfLog;
        XLogCtl->LogwrtRqst.Flush = EndOfLog;

        /* Pre-scan prepared transactions to find out the range of XIDs present */
        oldestActiveXID = PrescanPreparedTransactions(NULL, NULL);

        /*
         * Update full_page_writes in shared memory and write an XLOG_FPW_CHANGE
         * record before resource manager writes cleanup WAL records or checkpoint
         * record is written.
         */
        Insert->fullPageWrites = lastFullPageWrites;
        LocalSetXLogInsertAllowed();
        UpdateFullPageWrites();
        LocalXLogInsertAllowed = -1;

        if (InRecovery)
        {
                int                     rmid;

                /*
                 * Resource managers might need to write WAL records, eg, to record
                 * index cleanup actions.  So temporarily enable XLogInsertAllowed in
                 * this process only.
                 */
                LocalSetXLogInsertAllowed();

                /*
                 * Allow resource managers to do any required cleanup.
                 */
                for (rmid = 0; rmid <= RM_MAX_ID; rmid++)
                {
                        if (RmgrTable[rmid].rm_cleanup != NULL)
                                RmgrTable[rmid].rm_cleanup();
                }

                /* Disallow XLogInsert again */
                LocalXLogInsertAllowed = -1;

                /*
                 * Perform a checkpoint to update all our recovery activity to disk.
                 *
                 * Note that we write a shutdown checkpoint rather than an on-line
                 * one. This is not particularly critical, but since we may be
                 * assigning a new TLI, using a shutdown checkpoint allows us to have
                 * the rule that TLI only changes in shutdown checkpoints, which
                 * allows some extra error checking in xlog_redo.
                 *
                 * In fast promotion, only create a lightweight end-of-recovery record
                 * instead of a full checkpoint. A checkpoint is requested later,
                 * after we're fully out of recovery mode and already accepting
                 * queries.
                 */
                if (bgwriterLaunched)
                {
                        if (fast_promote)
                        {
                                checkPointLoc = ControlFile->prevCheckPoint;

                                /*
                                 * Confirm the last checkpoint is available for us to recover
                                 * from if we fail. Note that we don't check for the secondary
                                 * checkpoint since that isn't available in most base backups.
                                 */
                                record = ReadCheckpointRecord(xlogreader, checkPointLoc, 1, false);
                                if (record != NULL)
                                {
                                        fast_promoted = true;

                                        /*
                                         * Insert a special WAL record to mark the end of
                                         * recovery, since we aren't doing a checkpoint. That
                                         * means that the checkpointer process may likely be in
                                         * the middle of a time-smoothed restartpoint and could
                                         * continue to be for minutes after this. That sounds
                                         * strange, but the effect is roughly the same and it
                                         * would be stranger to try to come out of the
                                         * restartpoint and then checkpoint. We request a
                                         * checkpoint later anyway, just for safety.
                                         */
                                        CreateEndOfRecoveryRecord();
                                }
                        }

                        if (!fast_promoted)
                                RequestCheckpoint(CHECKPOINT_END_OF_RECOVERY |
                                                                  CHECKPOINT_IMMEDIATE |
                                                                  CHECKPOINT_WAIT);
                }
                else
                        CreateCheckPoint(CHECKPOINT_END_OF_RECOVERY | CHECKPOINT_IMMEDIATE);

                /*
                 * And finally, execute the recovery_end_command, if any.
                 */
                if (recoveryEndCommand)
                        ExecuteRecoveryCommand(recoveryEndCommand,
                                                                   "recovery_end_command",
                                                                   true);
        }

        /*
         * Preallocate additional log files, if wanted.
         */
        PreallocXlogFiles(EndOfLog);

        /*
         * Reset initial contents of unlogged relations.  This has to be done
         * AFTER recovery is complete so that any unlogged relations created
         * during recovery also get picked up.
         */
        if (InRecovery)
                ResetUnloggedRelations(UNLOGGED_RELATION_INIT);

        /*
         * Okay, we're officially UP.
         */
        InRecovery = false;

        LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
        ControlFile->state = DB_IN_PRODUCTION;
        ControlFile->time = (pg_time_t) time(NULL);
        UpdateControlFile();
        LWLockRelease(ControlFileLock);

        /* start the archive_timeout timer running */
        XLogCtl->lastSegSwitchTime = (pg_time_t) time(NULL);

        /* also initialize latestCompletedXid, to nextXid - 1 */
        LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
        ShmemVariableCache->latestCompletedXid = ShmemVariableCache->nextXid;
        TransactionIdRetreat(ShmemVariableCache->latestCompletedXid);
        LWLockRelease(ProcArrayLock);

        /*
         * Start up the commit log and subtrans, if not already done for hot
         * standby.
         */
        if (standbyState == STANDBY_DISABLED)
        {
                StartupCLOG();
                StartupSUBTRANS(oldestActiveXID);
        }

        /*
         * Perform end of recovery actions for any SLRUs that need it.
         */
        TrimCLOG();
        TrimMultiXact();

        /* Reload shared-memory state for prepared transactions */
        RecoverPreparedTransactions();

        /*
         * Shutdown the recovery environment. This must occur after
         * RecoverPreparedTransactions(), see notes for lock_twophase_recover()
         */
        if (standbyState != STANDBY_DISABLED)
                ShutdownRecoveryTransactionEnvironment();

        /* Shut down xlogreader */
        if (readFile >= 0)
        {
                close(readFile);
                readFile = -1;
        }
        XLogReaderFree(xlogreader);

        /*
         * If any of the critical GUCs have changed, log them before we allow
         * backends to write WAL.
         */
        LocalSetXLogInsertAllowed();
        XLogReportParameters();

        /*
         * All done.  Allow backends to write WAL.      (Although the bool flag is
         * probably atomic in itself, we use the info_lck here to ensure that
         * there are no race conditions concerning visibility of other recent
         * updates to shared memory.)
         */
        {
                /* use volatile pointer to prevent code rearrangement */
                volatile XLogCtlData *xlogctl = XLogCtl;

                SpinLockAcquire(&xlogctl->info_lck);
                xlogctl->SharedRecoveryInProgress = false;
                SpinLockRelease(&xlogctl->info_lck);
        }

        /*
         * If there were cascading standby servers connected to us, nudge any wal
         * sender processes to notice that we've been promoted.
         */
        WalSndWakeup();

        /*
         * If this was a fast promotion, request an (online) checkpoint now. This
         * isn't required for consistency, but the last restartpoint might be far
         * back, and in case of a crash, recovering from it might take a longer
         * than is appropriate now that we're not in standby mode anymore.
         */
        if (fast_promoted)
                RequestCheckpoint(CHECKPOINT_FORCE);
}

/*
 * Checks if recovery has reached a consistent state. When consistency is
 * reached and we have a valid starting standby snapshot, tell postmaster
 * that it can start accepting read-only connections.
 */
static void
CheckRecoveryConsistency(void)
{
        /*
         * During crash recovery, we don't reach a consistent state until we've
         * replayed all the WAL.
         */
        if (XLogRecPtrIsInvalid(minRecoveryPoint))
                return;

        /*
         * Have we reached the point where our base backup was completed?
         */
        if (!XLogRecPtrIsInvalid(ControlFile->backupEndPoint) &&
                ControlFile->backupEndPoint <= EndRecPtr)
        {
                /*
                 * We have reached the end of base backup, as indicated by pg_control.
                 * The data on disk is now consistent. Reset backupStartPoint and
                 * backupEndPoint, and update minRecoveryPoint to make sure we don't
                 * allow starting up at an earlier point even if recovery is stopped
                 * and restarted soon after this.
                 */
                elog(DEBUG1, "end of backup reached");

                LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);

                if (ControlFile->minRecoveryPoint < EndRecPtr)
                        ControlFile->minRecoveryPoint = EndRecPtr;

                ControlFile->backupStartPoint = InvalidXLogRecPtr;
                ControlFile->backupEndPoint = InvalidXLogRecPtr;
                ControlFile->backupEndRequired = false;
                UpdateControlFile();

                LWLockRelease(ControlFileLock);
        }

        /*
         * Have we passed our safe starting point? Note that minRecoveryPoint is
         * known to be incorrectly set if ControlFile->backupEndRequired, until
         * the XLOG_BACKUP_RECORD arrives to advise us of the correct
         * minRecoveryPoint. All we know prior to that is that we're not
         * consistent yet.
         */
        if (!reachedConsistency && !ControlFile->backupEndRequired &&
                minRecoveryPoint <= XLogCtl->lastReplayedEndRecPtr &&
                XLogRecPtrIsInvalid(ControlFile->backupStartPoint))
        {
                /*
                 * Check to see if the XLOG sequence contained any unresolved
                 * references to uninitialized pages.
                 */
                XLogCheckInvalidPages();

                reachedConsistency = true;
                ereport(LOG,
                                (errmsg("consistent recovery state reached at %X/%X",
                                                (uint32) (XLogCtl->lastReplayedEndRecPtr >> 32),
                                                (uint32) XLogCtl->lastReplayedEndRecPtr)));
        }

        /*
         * Have we got a valid starting snapshot that will allow queries to be
         * run? If so, we can tell postmaster that the database is consistent now,
         * enabling connections.
         */
        if (standbyState == STANDBY_SNAPSHOT_READY &&
                !LocalHotStandbyActive &&
                reachedConsistency &&
                IsUnderPostmaster)
        {
                /* use volatile pointer to prevent code rearrangement */
                volatile XLogCtlData *xlogctl = XLogCtl;

                SpinLockAcquire(&xlogctl->info_lck);
                xlogctl->SharedHotStandbyActive = true;
                SpinLockRelease(&xlogctl->info_lck);

                LocalHotStandbyActive = true;

                SendPostmasterSignal(PMSIGNAL_BEGIN_HOT_STANDBY);
        }
}

/*
 * Is the system still in recovery?
 *
 * Unlike testing InRecovery, this works in any process that's connected to
 * shared memory.
 *
 * As a side-effect, we initialize the local TimeLineID and RedoRecPtr
 * variables the first time we see that recovery is finished.
 */
bool
RecoveryInProgress(void)
{
        /*
         * We check shared state each time only until we leave recovery mode. We
         * can't re-enter recovery, so there's no need to keep checking after the
         * shared variable has once been seen false.
         */
        if (!LocalRecoveryInProgress)
                return false;
        else
        {
                /*
                 * use volatile pointer to make sure we make a fresh read of the
                 * shared variable.
                 */
                volatile XLogCtlData *xlogctl = XLogCtl;

                LocalRecoveryInProgress = xlogctl->SharedRecoveryInProgress;

                /*
                 * Initialize TimeLineID and RedoRecPtr when we discover that recovery
                 * is finished. InitPostgres() relies upon this behaviour to ensure
                 * that InitXLOGAccess() is called at backend startup.  (If you change
                 * this, see also LocalSetXLogInsertAllowed.)
                 */
                if (!LocalRecoveryInProgress)
                {
                        /*
                         * If we just exited recovery, make sure we read TimeLineID and
                         * RedoRecPtr after SharedRecoveryInProgress (for machines with
                         * weak memory ordering).
                         */
                        pg_memory_barrier();
                        InitXLOGAccess();
                }
                /*
                 * Note: We don't need a memory barrier when we're still in recovery.
                 * We might exit recovery immediately after return, so the caller
                 * can't rely on 'true' meaning that we're still in recovery anyway.
                 */

                return LocalRecoveryInProgress;
        }
}

/*
 * Is HotStandby active yet? This is only important in special backends
 * since normal backends won't ever be able to connect until this returns
 * true. Postmaster knows this by way of signal, not via shared memory.
 *
 * Unlike testing standbyState, this works in any process that's connected to
 * shared memory.
 */
bool
HotStandbyActive(void)
{
        /*
         * We check shared state each time only until Hot Standby is active. We
         * can't de-activate Hot Standby, so there's no need to keep checking
         * after the shared variable has once been seen true.
         */
        if (LocalHotStandbyActive)
                return true;
        else
        {
                /* use volatile pointer to prevent code rearrangement */
                volatile XLogCtlData *xlogctl = XLogCtl;

                /* spinlock is essential on machines with weak memory ordering! */
                SpinLockAcquire(&xlogctl->info_lck);
                LocalHotStandbyActive = xlogctl->SharedHotStandbyActive;
                SpinLockRelease(&xlogctl->info_lck);

                return LocalHotStandbyActive;
        }
}

/*
 * Is this process allowed to insert new WAL records?
 *
 * Ordinarily this is essentially equivalent to !RecoveryInProgress().
 * But we also have provisions for forcing the result "true" or "false"
 * within specific processes regardless of the global state.
 */
bool
XLogInsertAllowed(void)
{
        /*
         * If value is "unconditionally true" or "unconditionally false", just
         * return it.  This provides the normal fast path once recovery is known
         * done.
         */
        if (LocalXLogInsertAllowed >= 0)
                return (bool) LocalXLogInsertAllowed;

        /*
         * Else, must check to see if we're still in recovery.
         */
        if (RecoveryInProgress())
                return false;

        /*
         * On exit from recovery, reset to "unconditionally true", since there is
         * no need to keep checking.
         */
        LocalXLogInsertAllowed = 1;
        return true;
}

/*
 * Make XLogInsertAllowed() return true in the current process only.
 *
 * Note: it is allowed to switch LocalXLogInsertAllowed back to -1 later,
 * and even call LocalSetXLogInsertAllowed() again after that.
 */
static void
LocalSetXLogInsertAllowed(void)
{
        Assert(LocalXLogInsertAllowed == -1);
        LocalXLogInsertAllowed = 1;

        /* Initialize as RecoveryInProgress() would do when switching state */
        InitXLOGAccess();
}

/*
 * Subroutine to try to fetch and validate a prior checkpoint record.
 *
 * whichChkpt identifies the checkpoint (merely for reporting purposes).
 * 1 for "primary", 2 for "secondary", 0 for "other" (backup_label)
 */
static XLogRecord *
ReadCheckpointRecord(XLogReaderState *xlogreader, XLogRecPtr RecPtr,
                                         int whichChkpt, bool report)
{
        XLogRecord *record;

        if (!XRecOffIsValid(RecPtr))
        {
                if (!report)
                        return NULL;

                switch (whichChkpt)
                {
                        case 1:
                                ereport(LOG,
                                (errmsg("invalid primary checkpoint link in control file")));
                                break;
                        case 2:
                                ereport(LOG,
                                                (errmsg("invalid secondary checkpoint link in control file")));
                                break;
                        default:
                                ereport(LOG,
                                   (errmsg("invalid checkpoint link in backup_label file")));
                                break;
                }
                return NULL;
        }

        record = ReadRecord(xlogreader, RecPtr, LOG, true);

        if (record == NULL)
        {
                if (!report)
                        return NULL;

                switch (whichChkpt)
                {
                        case 1:
                                ereport(LOG,
                                                (errmsg("invalid primary checkpoint record")));
                                break;
                        case 2:
                                ereport(LOG,
                                                (errmsg("invalid secondary checkpoint record")));
                                break;
                        default:
                                ereport(LOG,
                                                (errmsg("invalid checkpoint record")));
                                break;
                }
                return NULL;
        }
        if (record->xl_rmid != RM_XLOG_ID)
        {
                switch (whichChkpt)
                {
                        case 1:
                                ereport(LOG,
                                                (errmsg("invalid resource manager ID in primary checkpoint record")));
                                break;
                        case 2:
                                ereport(LOG,
                                                (errmsg("invalid resource manager ID in secondary checkpoint record")));
                                break;
                        default:
                                ereport(LOG,
                                (errmsg("invalid resource manager ID in checkpoint record")));
                                break;
                }
                return NULL;
        }
        if (record->xl_info != XLOG_CHECKPOINT_SHUTDOWN &&
                record->xl_info != XLOG_CHECKPOINT_ONLINE)
        {
                switch (whichChkpt)
                {
                        case 1:
                                ereport(LOG,
                                   (errmsg("invalid xl_info in primary checkpoint record")));
                                break;
                        case 2:
                                ereport(LOG,
                                 (errmsg("invalid xl_info in secondary checkpoint record")));
                                break;
                        default:
                                ereport(LOG,
                                                (errmsg("invalid xl_info in checkpoint record")));
                                break;
                }
                return NULL;
        }
        if (record->xl_len != sizeof(CheckPoint) ||
                record->xl_tot_len != SizeOfXLogRecord + sizeof(CheckPoint))
        {
                switch (whichChkpt)
                {
                        case 1:
                                ereport(LOG,
                                        (errmsg("invalid length of primary checkpoint record")));
                                break;
                        case 2:
                                ereport(LOG,
                                  (errmsg("invalid length of secondary checkpoint record")));
                                break;
                        default:
                                ereport(LOG,
                                                (errmsg("invalid length of checkpoint record")));
                                break;
                }
                return NULL;
        }
        return record;
}

/*
 * This must be called during startup of a backend process, except that
 * it need not be called in a standalone backend (which does StartupXLOG
 * instead).  We need to initialize the local copies of ThisTimeLineID and
 * RedoRecPtr.
 *
 * Note: before Postgres 8.0, we went to some effort to keep the postmaster
 * process's copies of ThisTimeLineID and RedoRecPtr valid too.  This was
 * unnecessary however, since the postmaster itself never touches XLOG anyway.
 */
void
InitXLOGAccess(void)
{
        /* ThisTimeLineID doesn't change so we need no lock to copy it */
        ThisTimeLineID = XLogCtl->ThisTimeLineID;
        Assert(ThisTimeLineID != 0 || IsBootstrapProcessingMode());

        /* Use GetRedoRecPtr to copy the RedoRecPtr safely */
        (void) GetRedoRecPtr();
}

/*
 * Return the current Redo pointer from shared memory.
 *
 * As a side-effect, the local RedoRecPtr copy is updated.
 */
XLogRecPtr
GetRedoRecPtr(void)
{
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;
        XLogRecPtr ptr;

        /*
         * The possibly not up-to-date copy in XlogCtl is enough. Even if we
         * grabbed a WAL insertion slot to read the master copy, someone might
         * update it just after we've released the lock.
         */
        SpinLockAcquire(&xlogctl->info_lck);
        ptr = xlogctl->RedoRecPtr;
        SpinLockRelease(&xlogctl->info_lck);

        if (RedoRecPtr < ptr)
                RedoRecPtr = ptr;

        return RedoRecPtr;
}

/*
 * GetInsertRecPtr -- Returns the current insert position.
 *
 * NOTE: The value *actually* returned is the position of the last full
 * xlog page. It lags behind the real insert position by at most 1 page.
 * For that, we don't need to scan through WAL insertion slots, and an
 * approximation is enough for the current usage of this function.
 */
XLogRecPtr
GetInsertRecPtr(void)
{
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;
        XLogRecPtr      recptr;

        SpinLockAcquire(&xlogctl->info_lck);
        recptr = xlogctl->LogwrtRqst.Write;
        SpinLockRelease(&xlogctl->info_lck);

        return recptr;
}

/*
 * GetFlushRecPtr -- Returns the current flush position, ie, the last WAL
 * position known to be fsync'd to disk.
 */
XLogRecPtr
GetFlushRecPtr(void)
{
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;
        XLogRecPtr      recptr;

        SpinLockAcquire(&xlogctl->info_lck);
        recptr = xlogctl->LogwrtResult.Flush;
        SpinLockRelease(&xlogctl->info_lck);

        return recptr;
}

/*
 * Get the time of the last xlog segment switch
 */
pg_time_t
GetLastSegSwitchTime(void)
{
        pg_time_t       result;

        /* Need WALWriteLock, but shared lock is sufficient */
        LWLockAcquire(WALWriteLock, LW_SHARED);
        result = XLogCtl->lastSegSwitchTime;
        LWLockRelease(WALWriteLock);

        return result;
}

/*
 * GetNextXidAndEpoch - get the current nextXid value and associated epoch
 *
 * This is exported for use by code that would like to have 64-bit XIDs.
 * We don't really support such things, but all XIDs within the system
 * can be presumed "close to" the result, and thus the epoch associated
 * with them can be determined.
 */
void
GetNextXidAndEpoch(TransactionId *xid, uint32 *epoch)
{
        uint32          ckptXidEpoch;
        TransactionId ckptXid;
        TransactionId nextXid;

        /* Must read checkpoint info first, else have race condition */
        {
                /* use volatile pointer to prevent code rearrangement */
                volatile XLogCtlData *xlogctl = XLogCtl;

                SpinLockAcquire(&xlogctl->info_lck);
                ckptXidEpoch = xlogctl->ckptXidEpoch;
                ckptXid = xlogctl->ckptXid;
                SpinLockRelease(&xlogctl->info_lck);
        }

        /* Now fetch current nextXid */
        nextXid = ReadNewTransactionId();

        /*
         * nextXid is certainly logically later than ckptXid.  So if it's
         * numerically less, it must have wrapped into the next epoch.
         */
        if (nextXid < ckptXid)
                ckptXidEpoch++;

        *xid = nextXid;
        *epoch = ckptXidEpoch;
}

/*
 * This must be called ONCE during postmaster or standalone-backend shutdown
 */
void
ShutdownXLOG(int code, Datum arg)
{
        /* Don't be chatty in standalone mode */
        ereport(IsPostmasterEnvironment ? LOG : NOTICE,
                        (errmsg("shutting down")));

        if (RecoveryInProgress())
                CreateRestartPoint(CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_IMMEDIATE);
        else
        {
                /*
                 * If archiving is enabled, rotate the last XLOG file so that all the
                 * remaining records are archived (postmaster wakes up the archiver
                 * process one more time at the end of shutdown). The checkpoint
                 * record will go to the next XLOG file and won't be archived (yet).
                 */
                if (XLogArchivingActive() && XLogArchiveCommandSet())
                        RequestXLogSwitch();

                CreateCheckPoint(CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_IMMEDIATE);
        }
        ShutdownCLOG();
        ShutdownSUBTRANS();
        ShutdownMultiXact();

        /* Don't be chatty in standalone mode */
        ereport(IsPostmasterEnvironment ? LOG : NOTICE,
                        (errmsg("database system is shut down")));
}

/*
 * Log start of a checkpoint.
 */
static void
LogCheckpointStart(int flags, bool restartpoint)
{
        const char *msg;

        /*
         * XXX: This is hopelessly untranslatable. We could call gettext_noop for
         * the main message, but what about all the flags?
         */
        if (restartpoint)
                msg = "restartpoint starting:%s%s%s%s%s%s%s";
        else
                msg = "checkpoint starting:%s%s%s%s%s%s%s";

        elog(LOG, msg,
                 (flags & CHECKPOINT_IS_SHUTDOWN) ? " shutdown" : "",
                 (flags & CHECKPOINT_END_OF_RECOVERY) ? " end-of-recovery" : "",
                 (flags & CHECKPOINT_IMMEDIATE) ? " immediate" : "",
                 (flags & CHECKPOINT_FORCE) ? " force" : "",
                 (flags & CHECKPOINT_WAIT) ? " wait" : "",
                 (flags & CHECKPOINT_CAUSE_XLOG) ? " xlog" : "",
                 (flags & CHECKPOINT_CAUSE_TIME) ? " time" : "");
}

/*
 * Log end of a checkpoint.
 */
static void
LogCheckpointEnd(bool restartpoint)
{
        long            write_secs,
                                sync_secs,
                                total_secs,
                                longest_secs,
                                average_secs;
        int                     write_usecs,
                                sync_usecs,
                                total_usecs,
                                longest_usecs,
                                average_usecs;
        uint64          average_sync_time;

        CheckpointStats.ckpt_end_t = GetCurrentTimestamp();

        TimestampDifference(CheckpointStats.ckpt_write_t,
                                                CheckpointStats.ckpt_sync_t,
                                                &write_secs, &write_usecs);

        TimestampDifference(CheckpointStats.ckpt_sync_t,
                                                CheckpointStats.ckpt_sync_end_t,
                                                &sync_secs, &sync_usecs);

        /* Accumulate checkpoint timing summary data, in milliseconds. */
        BgWriterStats.m_checkpoint_write_time +=
                write_secs * 1000 + write_usecs / 1000;
        BgWriterStats.m_checkpoint_sync_time +=
                sync_secs * 1000 + sync_usecs / 1000;

        /*
         * All of the published timing statistics are accounted for.  Only
         * continue if a log message is to be written.
         */
        if (!log_checkpoints)
                return;

        TimestampDifference(CheckpointStats.ckpt_start_t,
                                                CheckpointStats.ckpt_end_t,
                                                &total_secs, &total_usecs);

        /*
         * Timing values returned from CheckpointStats are in microseconds.
         * Convert to the second plus microsecond form that TimestampDifference
         * returns for homogeneous printing.
         */
        longest_secs = (long) (CheckpointStats.ckpt_longest_sync / 1000000);
        longest_usecs = CheckpointStats.ckpt_longest_sync -
                (uint64) longest_secs *1000000;

        average_sync_time = 0;
        if (CheckpointStats.ckpt_sync_rels > 0)
                average_sync_time = CheckpointStats.ckpt_agg_sync_time /
                        CheckpointStats.ckpt_sync_rels;
        average_secs = (long) (average_sync_time / 1000000);
        average_usecs = average_sync_time - (uint64) average_secs *1000000;

        if (restartpoint)
                elog(LOG, "restartpoint complete: wrote %d buffers (%.1f%%); "
                         "%d transaction log file(s) added, %d removed, %d recycled; "
                         "write=%ld.%03d s, sync=%ld.%03d s, total=%ld.%03d s; "
                         "sync files=%d, longest=%ld.%03d s, average=%ld.%03d s",
                         CheckpointStats.ckpt_bufs_written,
                         (double) CheckpointStats.ckpt_bufs_written * 100 / NBuffers,
                         CheckpointStats.ckpt_segs_added,
                         CheckpointStats.ckpt_segs_removed,
                         CheckpointStats.ckpt_segs_recycled,
                         write_secs, write_usecs / 1000,
                         sync_secs, sync_usecs / 1000,
                         total_secs, total_usecs / 1000,
                         CheckpointStats.ckpt_sync_rels,
                         longest_secs, longest_usecs / 1000,
                         average_secs, average_usecs / 1000);
        else
                elog(LOG, "checkpoint complete: wrote %d buffers (%.1f%%); "
                         "%d transaction log file(s) added, %d removed, %d recycled; "
                         "write=%ld.%03d s, sync=%ld.%03d s, total=%ld.%03d s; "
                         "sync files=%d, longest=%ld.%03d s, average=%ld.%03d s",
                         CheckpointStats.ckpt_bufs_written,
                         (double) CheckpointStats.ckpt_bufs_written * 100 / NBuffers,
                         CheckpointStats.ckpt_segs_added,
                         CheckpointStats.ckpt_segs_removed,
                         CheckpointStats.ckpt_segs_recycled,
                         write_secs, write_usecs / 1000,
                         sync_secs, sync_usecs / 1000,
                         total_secs, total_usecs / 1000,
                         CheckpointStats.ckpt_sync_rels,
                         longest_secs, longest_usecs / 1000,
                         average_secs, average_usecs / 1000);
}

/*
 * Perform a checkpoint --- either during shutdown, or on-the-fly
 *
 * flags is a bitwise OR of the following:
 *      CHECKPOINT_IS_SHUTDOWN: checkpoint is for database shutdown.
 *      CHECKPOINT_END_OF_RECOVERY: checkpoint is for end of WAL recovery.
 *      CHECKPOINT_IMMEDIATE: finish the checkpoint ASAP,
 *              ignoring checkpoint_completion_target parameter.
 *      CHECKPOINT_FORCE: force a checkpoint even if no XLOG activity has occurred
 *              since the last one (implied by CHECKPOINT_IS_SHUTDOWN or
 *              CHECKPOINT_END_OF_RECOVERY).
 *
 * Note: flags contains other bits, of interest here only for logging purposes.
 * In particular note that this routine is synchronous and does not pay
 * attention to CHECKPOINT_WAIT.
 *
 * If !shutdown then we are writing an online checkpoint. This is a very special
 * kind of operation and WAL record because the checkpoint action occurs over
 * a period of time yet logically occurs at just a single LSN. The logical
 * position of the WAL record (redo ptr) is the same or earlier than the
 * physical position. When we replay WAL we locate the checkpoint via its
 * physical position then read the redo ptr and actually start replay at the
 * earlier logical position. Note that we don't write *anything* to WAL at
 * the logical position, so that location could be any other kind of WAL record.
 * All of this mechanism allows us to continue working while we checkpoint.
 * As a result, timing of actions is critical here and be careful to note that
 * this function will likely take minutes to execute on a busy system.
 */
void
CreateCheckPoint(int flags)
{
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;
        bool            shutdown;
        CheckPoint      checkPoint;
        XLogRecPtr      recptr;
        XLogCtlInsert *Insert = &XLogCtl->Insert;
        XLogRecData rdata;
        uint32          freespace;
        XLogSegNo       _logSegNo;
        XLogRecPtr      curInsert;
        VirtualTransactionId *vxids;
        int                     nvxids;

        /*
         * An end-of-recovery checkpoint is really a shutdown checkpoint, just
         * issued at a different time.
         */
        if (flags & (CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_END_OF_RECOVERY))
                shutdown = true;
        else
                shutdown = false;

        /* sanity check */
        if (RecoveryInProgress() && (flags & CHECKPOINT_END_OF_RECOVERY) == 0)
                elog(ERROR, "can't create a checkpoint during recovery");

        /*
         * Acquire CheckpointLock to ensure only one checkpoint happens at a time.
         * (This is just pro forma, since in the present system structure there is
         * only one process that is allowed to issue checkpoints at any given
         * time.)
         */
        LWLockAcquire(CheckpointLock, LW_EXCLUSIVE);

        /*
         * Prepare to accumulate statistics.
         *
         * Note: because it is possible for log_checkpoints to change while a
         * checkpoint proceeds, we always accumulate stats, even if
         * log_checkpoints is currently off.
         */
        MemSet(&CheckpointStats, 0, sizeof(CheckpointStats));
        CheckpointStats.ckpt_start_t = GetCurrentTimestamp();

        /*
         * Use a critical section to force system panic if we have trouble.
         */
        START_CRIT_SECTION();

        if (shutdown)
        {
                LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
                ControlFile->state = DB_SHUTDOWNING;
                ControlFile->time = (pg_time_t) time(NULL);
                UpdateControlFile();
                LWLockRelease(ControlFileLock);
        }

        /*
         * Let smgr prepare for checkpoint; this has to happen before we determine
         * the REDO pointer.  Note that smgr must not do anything that'd have to
         * be undone if we decide no checkpoint is needed.
         */
        smgrpreckpt();

        /* Begin filling in the checkpoint WAL record */
        MemSet(&checkPoint, 0, sizeof(checkPoint));
        checkPoint.time = (pg_time_t) time(NULL);

        /*
         * For Hot Standby, derive the oldestActiveXid before we fix the redo
         * pointer. This allows us to begin accumulating changes to assemble our
         * starting snapshot of locks and transactions.
         */
        if (!shutdown && XLogStandbyInfoActive())
                checkPoint.oldestActiveXid = GetOldestActiveTransactionId();
        else
                checkPoint.oldestActiveXid = InvalidTransactionId;

        /*
         * We must block concurrent insertions while examining insert state to
         * determine the checkpoint REDO pointer.
         */
        WALInsertSlotAcquire(true);
        curInsert = XLogBytePosToRecPtr(Insert->CurrBytePos);

        /*
         * If this isn't a shutdown or forced checkpoint, and we have not inserted
         * any XLOG records since the start of the last checkpoint, skip the
         * checkpoint.  The idea here is to avoid inserting duplicate checkpoints
         * when the system is idle. That wastes log space, and more importantly it
         * exposes us to possible loss of both current and previous checkpoint
         * records if the machine crashes just as we're writing the update.
         * (Perhaps it'd make even more sense to checkpoint only when the previous
         * checkpoint record is in a different xlog page?)
         *
         * We have to make two tests to determine that nothing has happened since
         * the start of the last checkpoint: current insertion point must match
         * the end of the last checkpoint record, and its redo pointer must point
         * to itself.
         */
        if ((flags & (CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_END_OF_RECOVERY |
                                  CHECKPOINT_FORCE)) == 0)
        {
                if (curInsert == ControlFile->checkPoint +
                        MAXALIGN(SizeOfXLogRecord + sizeof(CheckPoint)) &&
                        ControlFile->checkPoint == ControlFile->checkPointCopy.redo)
                {
                        WALInsertSlotRelease();
                        LWLockRelease(CheckpointLock);
                        END_CRIT_SECTION();
                        return;
                }
        }

        /*
         * An end-of-recovery checkpoint is created before anyone is allowed to
         * write WAL. To allow us to write the checkpoint record, temporarily
         * enable XLogInsertAllowed.  (This also ensures ThisTimeLineID is
         * initialized, which we need here and in AdvanceXLInsertBuffer.)
         */
        if (flags & CHECKPOINT_END_OF_RECOVERY)
                LocalSetXLogInsertAllowed();

        checkPoint.ThisTimeLineID = ThisTimeLineID;
        if (flags & CHECKPOINT_END_OF_RECOVERY)
                checkPoint.PrevTimeLineID = XLogCtl->PrevTimeLineID;
        else
                checkPoint.PrevTimeLineID = ThisTimeLineID;

        checkPoint.fullPageWrites = Insert->fullPageWrites;

        /*
         * Compute new REDO record ptr = location of next XLOG record.
         *
         * NB: this is NOT necessarily where the checkpoint record itself will be,
         * since other backends may insert more XLOG records while we're off doing
         * the buffer flush work.  Those XLOG records are logically after the
         * checkpoint, even though physically before it.  Got that?
         */
        freespace = INSERT_FREESPACE(curInsert);
        if (freespace == 0)
        {
                if (curInsert % XLogSegSize == 0)
                        curInsert += SizeOfXLogLongPHD;
                else
                        curInsert += SizeOfXLogShortPHD;
        }
        checkPoint.redo = curInsert;

        /*
         * Here we update the shared RedoRecPtr for future XLogInsert calls; this
         * must be done while holding the insertion slots.
         *
         * Note: if we fail to complete the checkpoint, RedoRecPtr will be left
         * pointing past where it really needs to point.  This is okay; the only
         * consequence is that XLogInsert might back up whole buffers that it
         * didn't really need to.  We can't postpone advancing RedoRecPtr because
         * XLogInserts that happen while we are dumping buffers must assume that
         * their buffer changes are not included in the checkpoint.
         */
        RedoRecPtr = xlogctl->Insert.RedoRecPtr = checkPoint.redo;

        /*
         * Now we can release the WAL insertion slots, allowing other xacts to
         * proceed while we are flushing disk buffers.
         */
        WALInsertSlotRelease();

        /* Update the info_lck-protected copy of RedoRecPtr as well */
        SpinLockAcquire(&xlogctl->info_lck);
        xlogctl->RedoRecPtr = checkPoint.redo;
        SpinLockRelease(&xlogctl->info_lck);

        /*
         * If enabled, log checkpoint start.  We postpone this until now so as not
         * to log anything if we decided to skip the checkpoint.
         */
        if (log_checkpoints)
                LogCheckpointStart(flags, false);

        TRACE_POSTGRESQL_CHECKPOINT_START(flags);

        /*
         * In some cases there are groups of actions that must all occur on one
         * side or the other of a checkpoint record. Before flushing the
         * checkpoint record we must explicitly wait for any backend currently
         * performing those groups of actions.
         *
         * One example is end of transaction, so we must wait for any transactions
         * that are currently in commit critical sections.      If an xact inserted
         * its commit record into XLOG just before the REDO point, then a crash
         * restart from the REDO point would not replay that record, which means
         * that our flushing had better include the xact's update of pg_clog.  So
         * we wait till he's out of his commit critical section before proceeding.
         * See notes in RecordTransactionCommit().
         *
         * Because we've already released the insertion slots, this test is a bit
         * fuzzy: it is possible that we will wait for xacts we didn't really need
         * to wait for.  But the delay should be short and it seems better to make
         * checkpoint take a bit longer than to hold off insertions longer than
         * necessary.
         * (In fact, the whole reason we have this issue is that xact.c does
         * commit record XLOG insertion and clog update as two separate steps
         * protected by different locks, but again that seems best on grounds of
         * minimizing lock contention.)
         *
         * A transaction that has not yet set delayChkpt when we look cannot be at
         * risk, since he's not inserted his commit record yet; and one that's
         * already cleared it is not at risk either, since he's done fixing clog
         * and we will correctly flush the update below.  So we cannot miss any
         * xacts we need to wait for.
         */
        vxids = GetVirtualXIDsDelayingChkpt(&nvxids);
        if (nvxids > 0)
        {
                do
                {
                        pg_usleep(10000L);      /* wait for 10 msec */
                } while (HaveVirtualXIDsDelayingChkpt(vxids, nvxids));
        }
        pfree(vxids);

        /*
         * Get the other info we need for the checkpoint record.
         */
        LWLockAcquire(XidGenLock, LW_SHARED);
        checkPoint.nextXid = ShmemVariableCache->nextXid;
        checkPoint.oldestXid = ShmemVariableCache->oldestXid;
        checkPoint.oldestXidDB = ShmemVariableCache->oldestXidDB;
        LWLockRelease(XidGenLock);

        /* Increase XID epoch if we've wrapped around since last checkpoint */
        checkPoint.nextXidEpoch = ControlFile->checkPointCopy.nextXidEpoch;
        if (checkPoint.nextXid < ControlFile->checkPointCopy.nextXid)
                checkPoint.nextXidEpoch++;

        LWLockAcquire(OidGenLock, LW_SHARED);
        checkPoint.nextOid = ShmemVariableCache->nextOid;
        if (!shutdown)
                checkPoint.nextOid += ShmemVariableCache->oidCount;
        LWLockRelease(OidGenLock);

        MultiXactGetCheckptMulti(shutdown,
                                                         &checkPoint.nextMulti,
                                                         &checkPoint.nextMultiOffset,
                                                         &checkPoint.oldestMulti,
                                                         &checkPoint.oldestMultiDB);

        /*
         * Having constructed the checkpoint record, ensure all shmem disk buffers
         * and commit-log buffers are flushed to disk.
         *
         * This I/O could fail for various reasons.  If so, we will fail to
         * complete the checkpoint, but there is no reason to force a system
         * panic. Accordingly, exit critical section while doing it.
         */
        END_CRIT_SECTION();

        CheckPointGuts(checkPoint.redo, flags);

        /*
         * Take a snapshot of running transactions and write this to WAL. This
         * allows us to reconstruct the state of running transactions during
         * archive recovery, if required. Skip, if this info disabled.
         *
         * If we are shutting down, or Startup process is completing crash
         * recovery we don't need to write running xact data.
         */
        if (!shutdown && XLogStandbyInfoActive())
                LogStandbySnapshot();

        START_CRIT_SECTION();

        /*
         * Now insert the checkpoint record into XLOG.
         */
        rdata.data = (char *) (&checkPoint);
        rdata.len = sizeof(checkPoint);
        rdata.buffer = InvalidBuffer;
        rdata.next = NULL;

        recptr = XLogInsert(RM_XLOG_ID,
                                                shutdown ? XLOG_CHECKPOINT_SHUTDOWN :
                                                XLOG_CHECKPOINT_ONLINE,
                                                &rdata);

        XLogFlush(recptr);

        /*
         * We mustn't write any new WAL after a shutdown checkpoint, or it will be
         * overwritten at next startup.  No-one should even try, this just allows
         * sanity-checking.  In the case of an end-of-recovery checkpoint, we want
         * to just temporarily disable writing until the system has exited
         * recovery.
         */
        if (shutdown)
        {
                if (flags & CHECKPOINT_END_OF_RECOVERY)
                        LocalXLogInsertAllowed = -1;            /* return to "check" state */
                else
                        LocalXLogInsertAllowed = 0; /* never again write WAL */
        }

        /*
         * We now have ProcLastRecPtr = start of actual checkpoint record, recptr
         * = end of actual checkpoint record.
         */
        if (shutdown && checkPoint.redo != ProcLastRecPtr)
                ereport(PANIC,
                                (errmsg("concurrent transaction log activity while database system is shutting down")));

        /*
         * Select point at which we can truncate the log, which we base on the
         * prior checkpoint's earliest info.
         */
        XLByteToSeg(ControlFile->checkPointCopy.redo, _logSegNo);

        /*
         * Update the control file.
         */
        LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
        if (shutdown)
                ControlFile->state = DB_SHUTDOWNED;
        ControlFile->prevCheckPoint = ControlFile->checkPoint;
        ControlFile->checkPoint = ProcLastRecPtr;
        ControlFile->checkPointCopy = checkPoint;
        ControlFile->time = (pg_time_t) time(NULL);
        /* crash recovery should always recover to the end of WAL */
        ControlFile->minRecoveryPoint = InvalidXLogRecPtr;
        ControlFile->minRecoveryPointTLI = 0;

        /*
         * Persist unloggedLSN value. It's reset on crash recovery, so this goes
         * unused on non-shutdown checkpoints, but seems useful to store it always
         * for debugging purposes.
         */
        SpinLockAcquire(&XLogCtl->ulsn_lck);
        ControlFile->unloggedLSN = XLogCtl->unloggedLSN;
        SpinLockRelease(&XLogCtl->ulsn_lck);

        UpdateControlFile();
        LWLockRelease(ControlFileLock);

        /* Update shared-memory copy of checkpoint XID/epoch */
        {
                /* use volatile pointer to prevent code rearrangement */
                volatile XLogCtlData *xlogctl = XLogCtl;

                SpinLockAcquire(&xlogctl->info_lck);
                xlogctl->ckptXidEpoch = checkPoint.nextXidEpoch;
                xlogctl->ckptXid = checkPoint.nextXid;
                SpinLockRelease(&xlogctl->info_lck);
        }

        /*
         * We are now done with critical updates; no need for system panic if we
         * have trouble while fooling with old log segments.
         */
        END_CRIT_SECTION();

        /*
         * Let smgr do post-checkpoint cleanup (eg, deleting old files).
         */
        smgrpostckpt();

        /*
         * Delete old log files (those no longer needed even for previous
         * checkpoint or the standbys in XLOG streaming).
         */
        if (_logSegNo)
        {
                KeepLogSeg(recptr, &_logSegNo);
                _logSegNo--;
                RemoveOldXlogFiles(_logSegNo, recptr);
        }

        /*
         * Make more log segments if needed.  (Do this after recycling old log
         * segments, since that may supply some of the needed files.)
         */
        if (!shutdown)
                PreallocXlogFiles(recptr);

        /*
         * Truncate pg_subtrans if possible.  We can throw away all data before
         * the oldest XMIN of any running transaction.  No future transaction will
         * attempt to reference any pg_subtrans entry older than that (see Asserts
         * in subtrans.c).      During recovery, though, we mustn't do this because
         * StartupSUBTRANS hasn't been called yet.
         */
        if (!RecoveryInProgress())
                TruncateSUBTRANS(GetOldestXmin(true, false));

        /* Real work is done, but log and update stats before releasing lock. */
        LogCheckpointEnd(false);

        TRACE_POSTGRESQL_CHECKPOINT_DONE(CheckpointStats.ckpt_bufs_written,
                                                                         NBuffers,
                                                                         CheckpointStats.ckpt_segs_added,
                                                                         CheckpointStats.ckpt_segs_removed,
                                                                         CheckpointStats.ckpt_segs_recycled);

        LWLockRelease(CheckpointLock);
}

/*
 * Mark the end of recovery in WAL though without running a full checkpoint.
 * We can expect that a restartpoint is likely to be in progress as we
 * do this, though we are unwilling to wait for it to complete. So be
 * careful to avoid taking the CheckpointLock anywhere here.
 *
 * CreateRestartPoint() allows for the case where recovery may end before
 * the restartpoint completes so there is no concern of concurrent behaviour.
 */
void
CreateEndOfRecoveryRecord(void)
{
        xl_end_of_recovery xlrec;
        XLogRecData rdata;
        XLogRecPtr      recptr;

        /* sanity check */
        if (!RecoveryInProgress())
                elog(ERROR, "can only be used to end recovery");

        xlrec.end_time = time(NULL);

        WALInsertSlotAcquire(true);
        xlrec.ThisTimeLineID = ThisTimeLineID;
        xlrec.PrevTimeLineID = XLogCtl->PrevTimeLineID;
        WALInsertSlotRelease();

        LocalSetXLogInsertAllowed();

        START_CRIT_SECTION();

        rdata.data = (char *) &xlrec;
        rdata.len = sizeof(xl_end_of_recovery);
        rdata.buffer = InvalidBuffer;
        rdata.next = NULL;

        recptr = XLogInsert(RM_XLOG_ID, XLOG_END_OF_RECOVERY, &rdata);

        XLogFlush(recptr);

        /*
         * Update the control file so that crash recovery can follow the timeline
         * changes to this point.
         */
        LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
        ControlFile->time = (pg_time_t) xlrec.end_time;
        ControlFile->minRecoveryPoint = recptr;
        ControlFile->minRecoveryPointTLI = ThisTimeLineID;
        UpdateControlFile();
        LWLockRelease(ControlFileLock);

        END_CRIT_SECTION();

        LocalXLogInsertAllowed = -1;    /* return to "check" state */
}

/*
 * Flush all data in shared memory to disk, and fsync
 *
 * This is the common code shared between regular checkpoints and
 * recovery restartpoints.
 */
static void
CheckPointGuts(XLogRecPtr checkPointRedo, int flags)
{
        CheckPointCLOG();
        CheckPointSUBTRANS();
        CheckPointMultiXact();
        CheckPointPredicate();
        CheckPointRelationMap();
        CheckPointBuffers(flags);       /* performs all required fsyncs */
        /* We deliberately delay 2PC checkpointing as long as possible */
        CheckPointTwoPhase(checkPointRedo);
}

/*
 * Save a checkpoint for recovery restart if appropriate
 *
 * This function is called each time a checkpoint record is read from XLOG.
 * It must determine whether the checkpoint represents a safe restartpoint or
 * not.  If so, the checkpoint record is stashed in shared memory so that
 * CreateRestartPoint can consult it.  (Note that the latter function is
 * executed by the checkpointer, while this one will be executed by the
 * startup process.)
 */
static void
RecoveryRestartPoint(const CheckPoint *checkPoint)
{
        int                     rmid;

        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;

        /*
         * Is it safe to restartpoint?  We must ask each of the resource managers
         * whether they have any partial state information that might prevent a
         * correct restart from this point.  If so, we skip this opportunity, but
         * return at the next checkpoint record for another try.
         */
        for (rmid = 0; rmid <= RM_MAX_ID; rmid++)
        {
                if (RmgrTable[rmid].rm_safe_restartpoint != NULL)
                        if (!(RmgrTable[rmid].rm_safe_restartpoint()))
                        {
                                elog(trace_recovery(DEBUG2),
                                         "RM %d not safe to record restart point at %X/%X",
                                         rmid,
                                         (uint32) (checkPoint->redo >> 32),
                                         (uint32) checkPoint->redo);
                                return;
                        }
        }

        /*
         * Also refrain from creating a restartpoint if we have seen any
         * references to non-existent pages. Restarting recovery from the
         * restartpoint would not see the references, so we would lose the
         * cross-check that the pages belonged to a relation that was dropped
         * later.
         */
        if (XLogHaveInvalidPages())
        {
                elog(trace_recovery(DEBUG2),
                         "could not record restart point at %X/%X because there "
                         "are unresolved references to invalid pages",
                         (uint32) (checkPoint->redo >> 32),
                         (uint32) checkPoint->redo);
                return;
        }

        /*
         * Copy the checkpoint record to shared memory, so that checkpointer can
         * work out the next time it wants to perform a restartpoint.
         */
        SpinLockAcquire(&xlogctl->info_lck);
        xlogctl->lastCheckPointRecPtr = ReadRecPtr;
        xlogctl->lastCheckPoint = *checkPoint;
        SpinLockRelease(&xlogctl->info_lck);
}

/*
 * Establish a restartpoint if possible.
 *
 * This is similar to CreateCheckPoint, but is used during WAL recovery
 * to establish a point from which recovery can roll forward without
 * replaying the entire recovery log.
 *
 * Returns true if a new restartpoint was established. We can only establish
 * a restartpoint if we have replayed a safe checkpoint record since last
 * restartpoint.
 */
bool
CreateRestartPoint(int flags)
{
        XLogRecPtr      lastCheckPointRecPtr;
        CheckPoint      lastCheckPoint;
        XLogSegNo       _logSegNo;
        TimestampTz xtime;

        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;

        /*
         * Acquire CheckpointLock to ensure only one restartpoint or checkpoint
         * happens at a time.
         */
        LWLockAcquire(CheckpointLock, LW_EXCLUSIVE);

        /* Get a local copy of the last safe checkpoint record. */
        SpinLockAcquire(&xlogctl->info_lck);
        lastCheckPointRecPtr = xlogctl->lastCheckPointRecPtr;
        lastCheckPoint = xlogctl->lastCheckPoint;
        SpinLockRelease(&xlogctl->info_lck);

        /*
         * Check that we're still in recovery mode. It's ok if we exit recovery
         * mode after this check, the restart point is valid anyway.
         */
        if (!RecoveryInProgress())
        {
                ereport(DEBUG2,
                          (errmsg("skipping restartpoint, recovery has already ended")));
                LWLockRelease(CheckpointLock);
                return false;
        }

        /*
         * If the last checkpoint record we've replayed is already our last
         * restartpoint, we can't perform a new restart point. We still update
         * minRecoveryPoint in that case, so that if this is a shutdown restart
         * point, we won't start up earlier than before. That's not strictly
         * necessary, but when hot standby is enabled, it would be rather weird if
         * the database opened up for read-only connections at a point-in-time
         * before the last shutdown. Such time travel is still possible in case of
         * immediate shutdown, though.
         *
         * We don't explicitly advance minRecoveryPoint when we do create a
         * restartpoint. It's assumed that flushing the buffers will do that as a
         * side-effect.
         */
        if (XLogRecPtrIsInvalid(lastCheckPointRecPtr) ||
                lastCheckPoint.redo <= ControlFile->checkPointCopy.redo)
        {
                ereport(DEBUG2,
                                (errmsg("skipping restartpoint, already performed at %X/%X",
                                                (uint32) (lastCheckPoint.redo >> 32),
                                                (uint32) lastCheckPoint.redo)));

                UpdateMinRecoveryPoint(InvalidXLogRecPtr, true);
                if (flags & CHECKPOINT_IS_SHUTDOWN)
                {
                        LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
                        ControlFile->state = DB_SHUTDOWNED_IN_RECOVERY;
                        ControlFile->time = (pg_time_t) time(NULL);
                        UpdateControlFile();
                        LWLockRelease(ControlFileLock);
                }
                LWLockRelease(CheckpointLock);
                return false;
        }

        /*
         * Update the shared RedoRecPtr so that the startup process can calculate
         * the number of segments replayed since last restartpoint, and request a
         * restartpoint if it exceeds checkpoint_segments.
         *
         * Like in CreateCheckPoint(), hold off insertions to update it, although
         * during recovery this is just pro forma, because no WAL insertions are
         * happening.
         */
        WALInsertSlotAcquire(true);
        xlogctl->Insert.RedoRecPtr = lastCheckPoint.redo;
        WALInsertSlotRelease();

        /* Also update the info_lck-protected copy */
        SpinLockAcquire(&xlogctl->info_lck);
        xlogctl->RedoRecPtr = lastCheckPoint.redo;
        SpinLockRelease(&xlogctl->info_lck);

        /*
         * Prepare to accumulate statistics.
         *
         * Note: because it is possible for log_checkpoints to change while a
         * checkpoint proceeds, we always accumulate stats, even if
         * log_checkpoints is currently off.
         */
        MemSet(&CheckpointStats, 0, sizeof(CheckpointStats));
        CheckpointStats.ckpt_start_t = GetCurrentTimestamp();

        if (log_checkpoints)
                LogCheckpointStart(flags, true);

        CheckPointGuts(lastCheckPoint.redo, flags);

        /*
         * Select point at which we can truncate the xlog, which we base on the
         * prior checkpoint's earliest info.
         */
        XLByteToSeg(ControlFile->checkPointCopy.redo, _logSegNo);

        /*
         * Update pg_control, using current time.  Check that it still shows
         * IN_ARCHIVE_RECOVERY state and an older checkpoint, else do nothing;
         * this is a quick hack to make sure nothing really bad happens if somehow
         * we get here after the end-of-recovery checkpoint.
         */
        LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
        if (ControlFile->state == DB_IN_ARCHIVE_RECOVERY &&
                ControlFile->checkPointCopy.redo < lastCheckPoint.redo)
        {
                ControlFile->prevCheckPoint = ControlFile->checkPoint;
                ControlFile->checkPoint = lastCheckPointRecPtr;
                ControlFile->checkPointCopy = lastCheckPoint;
                ControlFile->time = (pg_time_t) time(NULL);
                if (flags & CHECKPOINT_IS_SHUTDOWN)
                        ControlFile->state = DB_SHUTDOWNED_IN_RECOVERY;
                UpdateControlFile();
        }
        LWLockRelease(ControlFileLock);

        /*
         * Due to an historical accident multixact truncations are not WAL-logged,
         * but just performed everytime the mxact horizon is increased. So, unless
         * we explicitly execute truncations on a standby it will never clean out
         * /pg_multixact which obviously is bad, both because it uses space and
         * because we can wrap around into pre-existing data...
         *
         * We can only do the truncation here, after the UpdateControlFile()
         * above, because we've now safely established a restart point, that
         * guarantees we will not need need to access those multis.
         *
         * It's probably worth improving this.
         */
        TruncateMultiXact(lastCheckPoint.oldestMulti);

        /*
         * Delete old log files (those no longer needed even for previous
         * checkpoint/restartpoint) to prevent the disk holding the xlog from
         * growing full.
         */
        if (_logSegNo)
        {
                XLogRecPtr      receivePtr;
                XLogRecPtr      replayPtr;
                TimeLineID      replayTLI;
                XLogRecPtr      endptr;

                /*
                 * Get the current end of xlog replayed or received, whichever is
                 * later.
                 */
                receivePtr = GetWalRcvWriteRecPtr(NULL, NULL);
                replayPtr = GetXLogReplayRecPtr(&replayTLI);
                endptr = (receivePtr < replayPtr) ? replayPtr : receivePtr;

                KeepLogSeg(endptr, &_logSegNo);
                _logSegNo--;

                /*
                 * Try to recycle segments on a useful timeline. If we've been promoted
                 * since the beginning of this restartpoint, use the new timeline
                 * chosen at end of recovery (RecoveryInProgress() sets ThisTimeLineID
                 * in that case). If we're still in recovery, use the timeline we're
                 * currently replaying.
                 *
                 * There is no guarantee that the WAL segments will be useful on the
                 * current timeline; if recovery proceeds to a new timeline right
                 * after this, the pre-allocated WAL segments on this timeline will
                 * not be used, and will go wasted until recycled on the next
                 * restartpoint. We'll live with that.
                 */
                if (RecoveryInProgress())
                        ThisTimeLineID = replayTLI;

                RemoveOldXlogFiles(_logSegNo, endptr);

                /*
                 * Make more log segments if needed.  (Do this after recycling old log
                 * segments, since that may supply some of the needed files.)
                 */
                PreallocXlogFiles(endptr);

                /*
                 * ThisTimeLineID is normally not set when we're still in recovery.
                 * However, recycling/preallocating segments above needed
                 * ThisTimeLineID to determine which timeline to install the segments
                 * on. Reset it now, to restore the normal state of affairs for
                 * debugging purposes.
                 */
                if (RecoveryInProgress())
                        ThisTimeLineID = 0;
        }

        /*
         * Truncate pg_subtrans if possible.  We can throw away all data before
         * the oldest XMIN of any running transaction.  No future transaction will
         * attempt to reference any pg_subtrans entry older than that (see Asserts
         * in subtrans.c).      When hot standby is disabled, though, we mustn't do
         * this because StartupSUBTRANS hasn't been called yet.
         */
        if (EnableHotStandby)
                TruncateSUBTRANS(GetOldestXmin(true, false));

        /* Real work is done, but log and update before releasing lock. */
        LogCheckpointEnd(true);

        xtime = GetLatestXTime();
        ereport((log_checkpoints ? LOG : DEBUG2),
                        (errmsg("recovery restart point at %X/%X",
                 (uint32) (lastCheckPoint.redo >> 32), (uint32) lastCheckPoint.redo),
                   xtime ? errdetail("last completed transaction was at log time %s",
                                                         timestamptz_to_str(xtime)) : 0));

        LWLockRelease(CheckpointLock);

        /*
         * Finally, execute archive_cleanup_command, if any.
         */
        if (XLogCtl->archiveCleanupCommand[0])
                ExecuteRecoveryCommand(XLogCtl->archiveCleanupCommand,
                                                           "archive_cleanup_command",
                                                           false);

        return true;
}

/*
 * Retreat *logSegNo to the last segment that we need to retain because of
 * wal_keep_segments. This is calculated by subtracting wal_keep_segments
 * from the given xlog location, recptr.
 */
static void
KeepLogSeg(XLogRecPtr recptr, XLogSegNo *logSegNo)
{
        XLogSegNo       segno;

        if (wal_keep_segments == 0)
                return;

        XLByteToSeg(recptr, segno);

        /* avoid underflow, don't go below 1 */
        if (segno <= wal_keep_segments)
                segno = 1;
        else
                segno = segno - wal_keep_segments;

        /* don't delete WAL segments newer than the calculated segment */
        if (segno < *logSegNo)
                *logSegNo = segno;
}

/*
 * Write a NEXTOID log record
 */
void
XLogPutNextOid(Oid nextOid)
{
        XLogRecData rdata;

        rdata.data = (char *) (&nextOid);
        rdata.len = sizeof(Oid);
        rdata.buffer = InvalidBuffer;
        rdata.next = NULL;
        (void) XLogInsert(RM_XLOG_ID, XLOG_NEXTOID, &rdata);

        /*
         * We need not flush the NEXTOID record immediately, because any of the
         * just-allocated OIDs could only reach disk as part of a tuple insert or
         * update that would have its own XLOG record that must follow the NEXTOID
         * record.      Therefore, the standard buffer LSN interlock applied to those
         * records will ensure no such OID reaches disk before the NEXTOID record
         * does.
         *
         * Note, however, that the above statement only covers state "within" the
         * database.  When we use a generated OID as a file or directory name, we
         * are in a sense violating the basic WAL rule, because that filesystem
         * change may reach disk before the NEXTOID WAL record does.  The impact
         * of this is that if a database crash occurs immediately afterward, we
         * might after restart re-generate the same OID and find that it conflicts
         * with the leftover file or directory.  But since for safety's sake we
         * always loop until finding a nonconflicting filename, this poses no real
         * problem in practice. See pgsql-hackers discussion 27-Sep-2006.
         */
}

/*
 * Write an XLOG SWITCH record.
 *
 * Here we just blindly issue an XLogInsert request for the record.
 * All the magic happens inside XLogInsert.
 *
 * The return value is either the end+1 address of the switch record,
 * or the end+1 address of the prior segment if we did not need to
 * write a switch record because we are already at segment start.
 */
XLogRecPtr
RequestXLogSwitch(void)
{
        XLogRecPtr      RecPtr;
        XLogRecData rdata;

        /* XLOG SWITCH, alone among xlog record types, has no data */
        rdata.buffer = InvalidBuffer;
        rdata.data = NULL;
        rdata.len = 0;
        rdata.next = NULL;

        RecPtr = XLogInsert(RM_XLOG_ID, XLOG_SWITCH, &rdata);

        return RecPtr;
}

/*
 * Write a RESTORE POINT record
 */
XLogRecPtr
XLogRestorePoint(const char *rpName)
{
        XLogRecPtr      RecPtr;
        XLogRecData rdata;
        xl_restore_point xlrec;

        xlrec.rp_time = GetCurrentTimestamp();
        strncpy(xlrec.rp_name, rpName, MAXFNAMELEN);

        rdata.buffer = InvalidBuffer;
        rdata.data = (char *) &xlrec;
        rdata.len = sizeof(xl_restore_point);
        rdata.next = NULL;

        RecPtr = XLogInsert(RM_XLOG_ID, XLOG_RESTORE_POINT, &rdata);

        ereport(LOG,
                        (errmsg("restore point \"%s\" created at %X/%X",
                                        rpName, (uint32) (RecPtr >> 32), (uint32) RecPtr)));

        return RecPtr;
}

/*
 * Write a backup block if needed when we are setting a hint. Note that
 * this may be called for a variety of page types, not just heaps.
 *
 * Callable while holding just share lock on the buffer content.
 *
 * We can't use the plain backup block mechanism since that relies on the
 * Buffer being exclusively locked. Since some modifications (setting LSN, hint
 * bits) are allowed in a sharelocked buffer that can lead to wal checksum
 * failures. So instead we copy the page and insert the copied data as normal
 * record data.
 *
 * We only need to do something if page has not yet been full page written in
 * this checkpoint round. The LSN of the inserted wal record is returned if we
 * had to write, InvalidXLogRecPtr otherwise.
 *
 * It is possible that multiple concurrent backends could attempt to write WAL
 * records. In that case, multiple copies of the same block would be recorded
 * in separate WAL records by different backends, though that is still OK from
 * a correctness perspective.
 */
XLogRecPtr
XLogSaveBufferForHint(Buffer buffer, bool buffer_std)
{
        XLogRecPtr      recptr = InvalidXLogRecPtr;
        XLogRecPtr      lsn;
        XLogRecData rdata[2];
        BkpBlock        bkpb;

        /*
         * Ensure no checkpoint can change our view of RedoRecPtr.
         */
        Assert(MyPgXact->delayChkpt);

        /*
         * Update RedoRecPtr so XLogCheckBuffer can make the right decision
         */
        GetRedoRecPtr();

        /*
         * Setup phony rdata element for use within XLogCheckBuffer only. We reuse
         * and reset rdata for any actual WAL record insert.
         */
        rdata[0].buffer = buffer;
        rdata[0].buffer_std = buffer_std;

        /*
         * Check buffer while not holding an exclusive lock.
         */
        if (XLogCheckBuffer(rdata, false, &lsn, &bkpb))
        {
                char            copied_buffer[BLCKSZ];
                char       *origdata = (char *) BufferGetBlock(buffer);

                /*
                 * Copy buffer so we don't have to worry about concurrent hint bit or
                 * lsn updates. We assume pd_lower/upper cannot be changed without an
                 * exclusive lock, so the contents bkp are not racy.
                 *
                 * With buffer_std set to false, XLogCheckBuffer() sets hole_length and
                 * hole_offset to 0; so the following code is safe for either case.
                 */
                memcpy(copied_buffer, origdata, bkpb.hole_offset);
                memcpy(copied_buffer + bkpb.hole_offset,
                           origdata + bkpb.hole_offset + bkpb.hole_length,
                           BLCKSZ - bkpb.hole_offset - bkpb.hole_length);

                /*
                 * Header for backup block.
                 */
                rdata[0].data = (char *) &bkpb;
                rdata[0].len = sizeof(BkpBlock);
                rdata[0].buffer = InvalidBuffer;
                rdata[0].next = &(rdata[1]);

                /*
                 * Save copy of the buffer.
                 */
                rdata[1].data = copied_buffer;
                rdata[1].len = BLCKSZ - bkpb.hole_length;
                rdata[1].buffer = InvalidBuffer;
                rdata[1].next = NULL;

                recptr = XLogInsert(RM_XLOG_ID, XLOG_FPI, rdata);
        }

        return recptr;
}

/*
 * Check if any of the GUC parameters that are critical for hot standby
 * have changed, and update the value in pg_control file if necessary.
 */
static void
XLogReportParameters(void)
{
        if (wal_level != ControlFile->wal_level ||
                wal_log_hints != ControlFile->wal_log_hints ||
                MaxConnections != ControlFile->MaxConnections ||
                max_worker_processes != ControlFile->max_worker_processes ||
                max_prepared_xacts != ControlFile->max_prepared_xacts ||
                max_locks_per_xact != ControlFile->max_locks_per_xact)
        {
                /*
                 * The change in number of backend slots doesn't need to be WAL-logged
                 * if archiving is not enabled, as you can't start archive recovery
                 * with wal_level=minimal anyway. We don't really care about the
                 * values in pg_control either if wal_level=minimal, but seems better
                 * to keep them up-to-date to avoid confusion.
                 */
                if (wal_level != ControlFile->wal_level || XLogIsNeeded())
                {
                        XLogRecData rdata;
                        xl_parameter_change xlrec;

                        xlrec.MaxConnections = MaxConnections;
                        xlrec.max_worker_processes = max_worker_processes;
                        xlrec.max_prepared_xacts = max_prepared_xacts;
                        xlrec.max_locks_per_xact = max_locks_per_xact;
                        xlrec.wal_level = wal_level;
                        xlrec.wal_log_hints = wal_log_hints;

                        rdata.buffer = InvalidBuffer;
                        rdata.data = (char *) &xlrec;
                        rdata.len = sizeof(xlrec);
                        rdata.next = NULL;

                        XLogInsert(RM_XLOG_ID, XLOG_PARAMETER_CHANGE, &rdata);
                }

                ControlFile->MaxConnections = MaxConnections;
                ControlFile->max_worker_processes = max_worker_processes;
                ControlFile->max_prepared_xacts = max_prepared_xacts;
                ControlFile->max_locks_per_xact = max_locks_per_xact;
                ControlFile->wal_level = wal_level;
                ControlFile->wal_log_hints = wal_log_hints;
                UpdateControlFile();
        }
}

/*
 * Update full_page_writes in shared memory, and write an
 * XLOG_FPW_CHANGE record if necessary.
 *
 * Note: this function assumes there is no other process running
 * concurrently that could update it.
 */
void
UpdateFullPageWrites(void)
{
        XLogCtlInsert *Insert = &XLogCtl->Insert;

        /*
         * Do nothing if full_page_writes has not been changed.
         *
         * It's safe to check the shared full_page_writes without the lock,
         * because we assume that there is no concurrently running process which
         * can update it.
         */
        if (fullPageWrites == Insert->fullPageWrites)
                return;

        START_CRIT_SECTION();

        /*
         * It's always safe to take full page images, even when not strictly
         * required, but not the other round. So if we're setting full_page_writes
         * to true, first set it true and then write the WAL record. If we're
         * setting it to false, first write the WAL record and then set the global
         * flag.
         */
        if (fullPageWrites)
        {
                WALInsertSlotAcquire(true);
                Insert->fullPageWrites = true;
                WALInsertSlotRelease();
        }

        /*
         * Write an XLOG_FPW_CHANGE record. This allows us to keep track of
         * full_page_writes during archive recovery, if required.
         */
        if (XLogStandbyInfoActive() && !RecoveryInProgress())
        {
                XLogRecData rdata;

                rdata.data = (char *) (&fullPageWrites);
                rdata.len = sizeof(bool);
                rdata.buffer = InvalidBuffer;
                rdata.next = NULL;

                XLogInsert(RM_XLOG_ID, XLOG_FPW_CHANGE, &rdata);
        }

        if (!fullPageWrites)
        {
                WALInsertSlotAcquire(true);
                Insert->fullPageWrites = false;
                WALInsertSlotRelease();
        }
        END_CRIT_SECTION();
}

/*
 * Check that it's OK to switch to new timeline during recovery.
 *
 * 'lsn' is the address of the shutdown checkpoint record we're about to
 * replay. (Currently, timeline can only change at a shutdown checkpoint).
 */
static void
checkTimeLineSwitch(XLogRecPtr lsn, TimeLineID newTLI, TimeLineID prevTLI)
{
        /* Check that the record agrees on what the current (old) timeline is */
        if (prevTLI != ThisTimeLineID)
                ereport(PANIC,
                                (errmsg("unexpected previous timeline ID %u (current timeline ID %u) in checkpoint record",
                                                prevTLI, ThisTimeLineID)));

        /*
         * The new timeline better be in the list of timelines we expect to see,
         * according to the timeline history. It should also not decrease.
         */
        if (newTLI < ThisTimeLineID || !tliInHistory(newTLI, expectedTLEs))
                ereport(PANIC,
                 (errmsg("unexpected timeline ID %u (after %u) in checkpoint record",
                                 newTLI, ThisTimeLineID)));

        /*
         * If we have not yet reached min recovery point, and we're about to
         * switch to a timeline greater than the timeline of the min recovery
         * point: trouble. After switching to the new timeline, we could not
         * possibly visit the min recovery point on the correct timeline anymore.
         * This can happen if there is a newer timeline in the archive that
         * branched before the timeline the min recovery point is on, and you
         * attempt to do PITR to the new timeline.
         */
        if (!XLogRecPtrIsInvalid(minRecoveryPoint) &&
                lsn < minRecoveryPoint &&
                newTLI > minRecoveryPointTLI)
                ereport(PANIC,
                                (errmsg("unexpected timeline ID %u in checkpoint record, before reaching minimum recovery point %X/%X on timeline %u",
                                                newTLI,
                                                (uint32) (minRecoveryPoint >> 32),
                                                (uint32) minRecoveryPoint,
                                                minRecoveryPointTLI)));

        /* Looks good */
}

/*
 * XLOG resource manager's routines
 *
 * Definitions of info values are in include/catalog/pg_control.h, though
 * not all record types are related to control file updates.
 */
void
xlog_redo(XLogRecPtr lsn, XLogRecord *record)
{
        uint8           info = record->xl_info & ~XLR_INFO_MASK;

        /* Backup blocks are not used by XLOG rmgr */
        Assert(!(record->xl_info & XLR_BKP_BLOCK_MASK));

        if (info == XLOG_NEXTOID)
        {
                Oid                     nextOid;

                /*
                 * We used to try to take the maximum of ShmemVariableCache->nextOid
                 * and the recorded nextOid, but that fails if the OID counter wraps
                 * around.      Since no OID allocation should be happening during replay
                 * anyway, better to just believe the record exactly.  We still take
                 * OidGenLock while setting the variable, just in case.
                 */
                memcpy(&nextOid, XLogRecGetData(record), sizeof(Oid));
                LWLockAcquire(OidGenLock, LW_EXCLUSIVE);
                ShmemVariableCache->nextOid = nextOid;
                ShmemVariableCache->oidCount = 0;
                LWLockRelease(OidGenLock);
        }
        else if (info == XLOG_CHECKPOINT_SHUTDOWN)
        {
                CheckPoint      checkPoint;

                memcpy(&checkPoint, XLogRecGetData(record), sizeof(CheckPoint));
                /* In a SHUTDOWN checkpoint, believe the counters exactly */
                LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
                ShmemVariableCache->nextXid = checkPoint.nextXid;
                LWLockRelease(XidGenLock);
                LWLockAcquire(OidGenLock, LW_EXCLUSIVE);
                ShmemVariableCache->nextOid = checkPoint.nextOid;
                ShmemVariableCache->oidCount = 0;
                LWLockRelease(OidGenLock);
                MultiXactSetNextMXact(checkPoint.nextMulti,
                                                          checkPoint.nextMultiOffset);
                SetTransactionIdLimit(checkPoint.oldestXid, checkPoint.oldestXidDB);
                SetMultiXactIdLimit(checkPoint.oldestMulti, checkPoint.oldestMultiDB);

                /*
                 * If we see a shutdown checkpoint while waiting for an end-of-backup
                 * record, the backup was canceled and the end-of-backup record will
                 * never arrive.
                 */
                if (ArchiveRecoveryRequested &&
                        !XLogRecPtrIsInvalid(ControlFile->backupStartPoint) &&
                        XLogRecPtrIsInvalid(ControlFile->backupEndPoint))
                        ereport(PANIC,
                        (errmsg("online backup was canceled, recovery cannot continue")));

                /*
                 * If we see a shutdown checkpoint, we know that nothing was running
                 * on the master at this point. So fake-up an empty running-xacts
                 * record and use that here and now. Recover additional standby state
                 * for prepared transactions.
                 */
                if (standbyState >= STANDBY_INITIALIZED)
                {
                        TransactionId *xids;
                        int                     nxids;
                        TransactionId oldestActiveXID;
                        TransactionId latestCompletedXid;
                        RunningTransactionsData running;

                        oldestActiveXID = PrescanPreparedTransactions(&xids, &nxids);

                        /*
                         * Construct a RunningTransactions snapshot representing a shut
                         * down server, with only prepared transactions still alive. We're
                         * never overflowed at this point because all subxids are listed
                         * with their parent prepared transactions.
                         */
                        running.xcnt = nxids;
                        running.subxcnt = 0;
                        running.subxid_overflow = false;
                        running.nextXid = checkPoint.nextXid;
                        running.oldestRunningXid = oldestActiveXID;
                        latestCompletedXid = checkPoint.nextXid;
                        TransactionIdRetreat(latestCompletedXid);
                        Assert(TransactionIdIsNormal(latestCompletedXid));
                        running.latestCompletedXid = latestCompletedXid;
                        running.xids = xids;

                        ProcArrayApplyRecoveryInfo(&running);

                        StandbyRecoverPreparedTransactions(true);
                }

                /* ControlFile->checkPointCopy always tracks the latest ckpt XID */
                ControlFile->checkPointCopy.nextXidEpoch = checkPoint.nextXidEpoch;
                ControlFile->checkPointCopy.nextXid = checkPoint.nextXid;

                /* Update shared-memory copy of checkpoint XID/epoch */
                {
                        /* use volatile pointer to prevent code rearrangement */
                        volatile XLogCtlData *xlogctl = XLogCtl;

                        SpinLockAcquire(&xlogctl->info_lck);
                        xlogctl->ckptXidEpoch = checkPoint.nextXidEpoch;
                        xlogctl->ckptXid = checkPoint.nextXid;
                        SpinLockRelease(&xlogctl->info_lck);
                }

                /*
                 * We should've already switched to the new TLI before replaying this
                 * record.
                 */
                if (checkPoint.ThisTimeLineID != ThisTimeLineID)
                        ereport(PANIC,
                                        (errmsg("unexpected timeline ID %u (should be %u) in checkpoint record",
                                                        checkPoint.ThisTimeLineID, ThisTimeLineID)));

                RecoveryRestartPoint(&checkPoint);
        }
        else if (info == XLOG_CHECKPOINT_ONLINE)
        {
                CheckPoint      checkPoint;

                memcpy(&checkPoint, XLogRecGetData(record), sizeof(CheckPoint));
                /* In an ONLINE checkpoint, treat the XID counter as a minimum */
                LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
                if (TransactionIdPrecedes(ShmemVariableCache->nextXid,
                                                                  checkPoint.nextXid))
                        ShmemVariableCache->nextXid = checkPoint.nextXid;
                LWLockRelease(XidGenLock);
                /* ... but still treat OID counter as exact */
                LWLockAcquire(OidGenLock, LW_EXCLUSIVE);
                ShmemVariableCache->nextOid = checkPoint.nextOid;
                ShmemVariableCache->oidCount = 0;
                LWLockRelease(OidGenLock);
                MultiXactAdvanceNextMXact(checkPoint.nextMulti,
                                                                  checkPoint.nextMultiOffset);
                if (TransactionIdPrecedes(ShmemVariableCache->oldestXid,
                                                                  checkPoint.oldestXid))
                        SetTransactionIdLimit(checkPoint.oldestXid,
                                                                  checkPoint.oldestXidDB);
                MultiXactAdvanceOldest(checkPoint.oldestMulti,
                                                           checkPoint.oldestMultiDB);

                /* ControlFile->checkPointCopy always tracks the latest ckpt XID */
                ControlFile->checkPointCopy.nextXidEpoch = checkPoint.nextXidEpoch;
                ControlFile->checkPointCopy.nextXid = checkPoint.nextXid;

                /* Update shared-memory copy of checkpoint XID/epoch */
                {
                        /* use volatile pointer to prevent code rearrangement */
                        volatile XLogCtlData *xlogctl = XLogCtl;

                        SpinLockAcquire(&xlogctl->info_lck);
                        xlogctl->ckptXidEpoch = checkPoint.nextXidEpoch;
                        xlogctl->ckptXid = checkPoint.nextXid;
                        SpinLockRelease(&xlogctl->info_lck);
                }

                /* TLI should not change in an on-line checkpoint */
                if (checkPoint.ThisTimeLineID != ThisTimeLineID)
                        ereport(PANIC,
                                        (errmsg("unexpected timeline ID %u (should be %u) in checkpoint record",
                                                        checkPoint.ThisTimeLineID, ThisTimeLineID)));

                RecoveryRestartPoint(&checkPoint);
        }
        else if (info == XLOG_END_OF_RECOVERY)
        {
                xl_end_of_recovery xlrec;

                memcpy(&xlrec, XLogRecGetData(record), sizeof(xl_end_of_recovery));

                /*
                 * For Hot Standby, we could treat this like a Shutdown Checkpoint,
                 * but this case is rarer and harder to test, so the benefit doesn't
                 * outweigh the potential extra cost of maintenance.
                 */

                /*
                 * We should've already switched to the new TLI before replaying this
                 * record.
                 */
                if (xlrec.ThisTimeLineID != ThisTimeLineID)
                        ereport(PANIC,
                                        (errmsg("unexpected timeline ID %u (should be %u) in checkpoint record",
                                                        xlrec.ThisTimeLineID, ThisTimeLineID)));
        }
        else if (info == XLOG_NOOP)
        {
                /* nothing to do here */
        }
        else if (info == XLOG_SWITCH)
        {
                /* nothing to do here */
        }
        else if (info == XLOG_RESTORE_POINT)
        {
                /* nothing to do here */
        }
        else if (info == XLOG_FPI)
        {
                char       *data;
                BkpBlock        bkpb;

                /*
                 * Full-page image (FPI) records contain a backup block stored "inline"
                 * in the normal data since the locking when writing hint records isn't
                 * sufficient to use the normal backup block mechanism, which assumes
                 * exclusive lock on the buffer supplied.
                 *
                 * Since the only change in these backup block are hint bits, there
                 * are no recovery conflicts generated.
                 *
                 * This also means there is no corresponding API call for this, so an
                 * smgr implementation has no need to implement anything. Which means
                 * nothing is needed in md.c etc
                 */
                data = XLogRecGetData(record);
                memcpy(&bkpb, data, sizeof(BkpBlock));
                data += sizeof(BkpBlock);

                RestoreBackupBlockContents(lsn, bkpb, data, false, false);
        }
        else if (info == XLOG_BACKUP_END)
        {
                XLogRecPtr      startpoint;

                memcpy(&startpoint, XLogRecGetData(record), sizeof(startpoint));

                if (ControlFile->backupStartPoint == startpoint)
                {
                        /*
                         * We have reached the end of base backup, the point where
                         * pg_stop_backup() was done. The data on disk is now consistent.
                         * Reset backupStartPoint, and update minRecoveryPoint to make
                         * sure we don't allow starting up at an earlier point even if
                         * recovery is stopped and restarted soon after this.
                         */
                        elog(DEBUG1, "end of backup reached");

                        LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);

                        if (ControlFile->minRecoveryPoint < lsn)
                        {
                                ControlFile->minRecoveryPoint = lsn;
                                ControlFile->minRecoveryPointTLI = ThisTimeLineID;
                        }
                        ControlFile->backupStartPoint = InvalidXLogRecPtr;
                        ControlFile->backupEndRequired = false;
                        UpdateControlFile();

                        LWLockRelease(ControlFileLock);
                }
        }
        else if (info == XLOG_PARAMETER_CHANGE)
        {
                xl_parameter_change xlrec;

                /* Update our copy of the parameters in pg_control */
                memcpy(&xlrec, XLogRecGetData(record), sizeof(xl_parameter_change));

                LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
                ControlFile->MaxConnections = xlrec.MaxConnections;
                ControlFile->max_worker_processes = xlrec.max_worker_processes;
                ControlFile->max_prepared_xacts = xlrec.max_prepared_xacts;
                ControlFile->max_locks_per_xact = xlrec.max_locks_per_xact;
                ControlFile->wal_level = xlrec.wal_level;
                ControlFile->wal_log_hints = wal_log_hints;

                /*
                 * Update minRecoveryPoint to ensure that if recovery is aborted, we
                 * recover back up to this point before allowing hot standby again.
                 * This is particularly important if wal_level was set to 'archive'
                 * before, and is now 'hot_standby', to ensure you don't run queries
                 * against the WAL preceding the wal_level change. Same applies to
                 * decreasing max_* settings.
                 */
                minRecoveryPoint = ControlFile->minRecoveryPoint;
                minRecoveryPointTLI = ControlFile->minRecoveryPointTLI;
                if (minRecoveryPoint != 0 && minRecoveryPoint < lsn)
                {
                        ControlFile->minRecoveryPoint = lsn;
                        ControlFile->minRecoveryPointTLI = ThisTimeLineID;
                }

                UpdateControlFile();
                LWLockRelease(ControlFileLock);

                /* Check to see if any changes to max_connections give problems */
                CheckRequiredParameterValues();
        }
        else if (info == XLOG_FPW_CHANGE)
        {
                /* use volatile pointer to prevent code rearrangement */
                volatile XLogCtlData *xlogctl = XLogCtl;
                bool            fpw;

                memcpy(&fpw, XLogRecGetData(record), sizeof(bool));

                /*
                 * Update the LSN of the last replayed XLOG_FPW_CHANGE record so that
                 * do_pg_start_backup() and do_pg_stop_backup() can check whether
                 * full_page_writes has been disabled during online backup.
                 */
                if (!fpw)
                {
                        SpinLockAcquire(&xlogctl->info_lck);
                        if (xlogctl->lastFpwDisableRecPtr < ReadRecPtr)
                                xlogctl->lastFpwDisableRecPtr = ReadRecPtr;
                        SpinLockRelease(&xlogctl->info_lck);
                }

                /* Keep track of full_page_writes */
                lastFullPageWrites = fpw;
        }
}

#ifdef WAL_DEBUG

static void
xlog_outrec(StringInfo buf, XLogRecord *record)
{
        int                     i;

        appendStringInfo(buf, "prev %X/%X; xid %u",
                                         (uint32) (record->xl_prev >> 32),
                                         (uint32) record->xl_prev,
                                         record->xl_xid);

        appendStringInfo(buf, "; len %u",
                                         record->xl_len);

        for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
        {
                if (record->xl_info & XLR_BKP_BLOCK(i))
                        appendStringInfo(buf, "; bkpb%d", i);
        }

        appendStringInfo(buf, ": %s", RmgrTable[record->xl_rmid].rm_name);
}
#endif   /* WAL_DEBUG */


/*
 * Return the (possible) sync flag used for opening a file, depending on the
 * value of the GUC wal_sync_method.
 */
static int
get_sync_bit(int method)
{
        int                     o_direct_flag = 0;

        /* If fsync is disabled, never open in sync mode */
        if (!enableFsync)
                return 0;

        /*
         * Optimize writes by bypassing kernel cache with O_DIRECT when using
         * O_SYNC/O_FSYNC and O_DSYNC.  But only if archiving and streaming are
         * disabled, otherwise the archive command or walsender process will read
         * the WAL soon after writing it, which is guaranteed to cause a physical
         * read if we bypassed the kernel cache. We also skip the
         * posix_fadvise(POSIX_FADV_DONTNEED) call in XLogFileClose() for the same
         * reason.
         *
         * Never use O_DIRECT in walreceiver process for similar reasons; the WAL
         * written by walreceiver is normally read by the startup process soon
         * after its written. Also, walreceiver performs unaligned writes, which
         * don't work with O_DIRECT, so it is required for correctness too.
         */
        if (!XLogIsNeeded() && !AmWalReceiverProcess())
                o_direct_flag = PG_O_DIRECT;

        switch (method)
        {
                        /*
                         * enum values for all sync options are defined even if they are
                         * not supported on the current platform.  But if not, they are
                         * not included in the enum option array, and therefore will never
                         * be seen here.
                         */
                case SYNC_METHOD_FSYNC:
                case SYNC_METHOD_FSYNC_WRITETHROUGH:
                case SYNC_METHOD_FDATASYNC:
                        return 0;
#ifdef OPEN_SYNC_FLAG
                case SYNC_METHOD_OPEN:
                        return OPEN_SYNC_FLAG | o_direct_flag;
#endif
#ifdef OPEN_DATASYNC_FLAG
                case SYNC_METHOD_OPEN_DSYNC:
                        return OPEN_DATASYNC_FLAG | o_direct_flag;
#endif
                default:
                        /* can't happen (unless we are out of sync with option array) */
                        elog(ERROR, "unrecognized wal_sync_method: %d", method);
                        return 0;                       /* silence warning */
        }
}

/*
 * GUC support
 */
void
assign_xlog_sync_method(int new_sync_method, void *extra)
{
        if (sync_method != new_sync_method)
        {
                /*
                 * To ensure that no blocks escape unsynced, force an fsync on the
                 * currently open log segment (if any).  Also, if the open flag is
                 * changing, close the log file so it will be reopened (with new flag
                 * bit) at next use.
                 */
                if (openLogFile >= 0)
                {
                        if (pg_fsync(openLogFile) != 0)
                                ereport(PANIC,
                                                (errcode_for_file_access(),
                                                 errmsg("could not fsync log segment %s: %m",
                                                          XLogFileNameP(ThisTimeLineID, openLogSegNo))));
                        if (get_sync_bit(sync_method) != get_sync_bit(new_sync_method))
                                XLogFileClose();
                }
        }
}


/*
 * Issue appropriate kind of fsync (if any) for an XLOG output file.
 *
 * 'fd' is a file descriptor for the XLOG file to be fsync'd.
 * 'log' and 'seg' are for error reporting purposes.
 */
void
issue_xlog_fsync(int fd, XLogSegNo segno)
{
        switch (sync_method)
        {
                case SYNC_METHOD_FSYNC:
                        if (pg_fsync_no_writethrough(fd) != 0)
                                ereport(PANIC,
                                                (errcode_for_file_access(),
                                                 errmsg("could not fsync log file %s: %m",
                                                                XLogFileNameP(ThisTimeLineID, segno))));
                        break;
#ifdef HAVE_FSYNC_WRITETHROUGH
                case SYNC_METHOD_FSYNC_WRITETHROUGH:
                        if (pg_fsync_writethrough(fd) != 0)
                                ereport(PANIC,
                                                (errcode_for_file_access(),
                                          errmsg("could not fsync write-through log file %s: %m",
                                                         XLogFileNameP(ThisTimeLineID, segno))));
                        break;
#endif
#ifdef HAVE_FDATASYNC
                case SYNC_METHOD_FDATASYNC:
                        if (pg_fdatasync(fd) != 0)
                                ereport(PANIC,
                                                (errcode_for_file_access(),
                                                 errmsg("could not fdatasync log file %s: %m",
                                                                XLogFileNameP(ThisTimeLineID, segno))));
                        break;
#endif
                case SYNC_METHOD_OPEN:
                case SYNC_METHOD_OPEN_DSYNC:
                        /* write synced it already */
                        break;
                default:
                        elog(PANIC, "unrecognized wal_sync_method: %d", sync_method);
                        break;
        }
}

/*
 * Return the filename of given log segment, as a palloc'd string.
 */
char *
XLogFileNameP(TimeLineID tli, XLogSegNo segno)
{
        char       *result = palloc(MAXFNAMELEN);

        XLogFileName(result, tli, segno);
        return result;
}

/*
 * do_pg_start_backup is the workhorse of the user-visible pg_start_backup()
 * function. It creates the necessary starting checkpoint and constructs the
 * backup label file.
 *
 * There are two kind of backups: exclusive and non-exclusive. An exclusive
 * backup is started with pg_start_backup(), and there can be only one active
 * at a time. The backup label file of an exclusive backup is written to
 * $PGDATA/backup_label, and it is removed by pg_stop_backup().
 *
 * A non-exclusive backup is used for the streaming base backups (see
 * src/backend/replication/basebackup.c). The difference to exclusive backups
 * is that the backup label file is not written to disk. Instead, its would-be
 * contents are returned in *labelfile, and the caller is responsible for
 * including it in the backup archive as 'backup_label'. There can be many
 * non-exclusive backups active at the same time, and they don't conflict
 * with an exclusive backup either.
 *
 * Returns the minimum WAL position that must be present to restore from this
 * backup, and the corresponding timeline ID in *starttli_p.
 *
 * Every successfully started non-exclusive backup must be stopped by calling
 * do_pg_stop_backup() or do_pg_abort_backup().
 *
 * It is the responsibility of the caller of this function to verify the
 * permissions of the calling user!
 */
XLogRecPtr
do_pg_start_backup(const char *backupidstr, bool fast, TimeLineID *starttli_p,
                                   char **labelfile)
{
        bool            exclusive = (labelfile == NULL);
        bool            backup_started_in_recovery = false;
        XLogRecPtr      checkpointloc;
        XLogRecPtr      startpoint;
        TimeLineID      starttli;
        pg_time_t       stamp_time;
        char            strfbuf[128];
        char            xlogfilename[MAXFNAMELEN];
        XLogSegNo       _logSegNo;
        struct stat stat_buf;
        FILE       *fp;
        StringInfoData labelfbuf;

        backup_started_in_recovery = RecoveryInProgress();

        /*
         * Currently only non-exclusive backup can be taken during recovery.
         */
        if (backup_started_in_recovery && exclusive)
                ereport(ERROR,
                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                 errmsg("recovery is in progress"),
                                 errhint("WAL control functions cannot be executed during recovery.")));

        /*
         * During recovery, we don't need to check WAL level. Because, if WAL
         * level is not sufficient, it's impossible to get here during recovery.
         */
        if (!backup_started_in_recovery && !XLogIsNeeded())
                ereport(ERROR,
                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                          errmsg("WAL level not sufficient for making an online backup"),
                                 errhint("wal_level must be set to \"archive\", \"hot_standby\" or \"logical\" at server start.")));

        if (strlen(backupidstr) > MAXPGPATH)
                ereport(ERROR,
                                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                                 errmsg("backup label too long (max %d bytes)",
                                                MAXPGPATH)));

        /*
         * Mark backup active in shared memory.  We must do full-page WAL writes
         * during an on-line backup even if not doing so at other times, because
         * it's quite possible for the backup dump to obtain a "torn" (partially
         * written) copy of a database page if it reads the page concurrently with
         * our write to the same page.  This can be fixed as long as the first
         * write to the page in the WAL sequence is a full-page write. Hence, we
         * turn on forcePageWrites and then force a CHECKPOINT, to ensure there
         * are no dirty pages in shared memory that might get dumped while the
         * backup is in progress without having a corresponding WAL record.  (Once
         * the backup is complete, we need not force full-page writes anymore,
         * since we expect that any pages not modified during the backup interval
         * must have been correctly captured by the backup.)
         *
         * Note that forcePageWrites has no effect during an online backup from
         * the standby.
         *
         * We must hold all the insertion slots to change the value of
         * forcePageWrites, to ensure adequate interlocking against XLogInsert().
         */
        WALInsertSlotAcquire(true);
        if (exclusive)
        {
                if (XLogCtl->Insert.exclusiveBackup)
                {
                        WALInsertSlotRelease();
                        ereport(ERROR,
                                        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                         errmsg("a backup is already in progress"),
                                         errhint("Run pg_stop_backup() and try again.")));
                }
                XLogCtl->Insert.exclusiveBackup = true;
        }
        else
                XLogCtl->Insert.nonExclusiveBackups++;
        XLogCtl->Insert.forcePageWrites = true;
        WALInsertSlotRelease();

        /* Ensure we release forcePageWrites if fail below */
        PG_ENSURE_ERROR_CLEANUP(pg_start_backup_callback, (Datum) BoolGetDatum(exclusive));
        {
                bool            gotUniqueStartpoint = false;

                /*
                 * Force an XLOG file switch before the checkpoint, to ensure that the
                 * WAL segment the checkpoint is written to doesn't contain pages with
                 * old timeline IDs.  That would otherwise happen if you called
                 * pg_start_backup() right after restoring from a PITR archive: the
                 * first WAL segment containing the startup checkpoint has pages in
                 * the beginning with the old timeline ID.      That can cause trouble at
                 * recovery: we won't have a history file covering the old timeline if
                 * pg_xlog directory was not included in the base backup and the WAL
                 * archive was cleared too before starting the backup.
                 *
                 * This also ensures that we have emitted a WAL page header that has
                 * XLP_BKP_REMOVABLE off before we emit the checkpoint record.
                 * Therefore, if a WAL archiver (such as pglesslog) is trying to
                 * compress out removable backup blocks, it won't remove any that
                 * occur after this point.
                 *
                 * During recovery, we skip forcing XLOG file switch, which means that
                 * the backup taken during recovery is not available for the special
                 * recovery case described above.
                 */
                if (!backup_started_in_recovery)
                        RequestXLogSwitch();

                do
                {
                        bool            checkpointfpw;

                        /*
                         * Force a CHECKPOINT.  Aside from being necessary to prevent torn
                         * page problems, this guarantees that two successive backup runs
                         * will have different checkpoint positions and hence different
                         * history file names, even if nothing happened in between.
                         *
                         * During recovery, establish a restartpoint if possible. We use
                         * the last restartpoint as the backup starting checkpoint. This
                         * means that two successive backup runs can have same checkpoint
                         * positions.
                         *
                         * Since the fact that we are executing do_pg_start_backup()
                         * during recovery means that checkpointer is running, we can use
                         * RequestCheckpoint() to establish a restartpoint.
                         *
                         * We use CHECKPOINT_IMMEDIATE only if requested by user (via
                         * passing fast = true).  Otherwise this can take awhile.
                         */
                        RequestCheckpoint(CHECKPOINT_FORCE | CHECKPOINT_WAIT |
                                                          (fast ? CHECKPOINT_IMMEDIATE : 0));

                        /*
                         * Now we need to fetch the checkpoint record location, and also
                         * its REDO pointer.  The oldest point in WAL that would be needed
                         * to restore starting from the checkpoint is precisely the REDO
                         * pointer.
                         */
                        LWLockAcquire(ControlFileLock, LW_SHARED);
                        checkpointloc = ControlFile->checkPoint;
                        startpoint = ControlFile->checkPointCopy.redo;
                        starttli = ControlFile->checkPointCopy.ThisTimeLineID;
                        checkpointfpw = ControlFile->checkPointCopy.fullPageWrites;
                        LWLockRelease(ControlFileLock);

                        if (backup_started_in_recovery)
                        {
                                /* use volatile pointer to prevent code rearrangement */
                                volatile XLogCtlData *xlogctl = XLogCtl;
                                XLogRecPtr      recptr;

                                /*
                                 * Check to see if all WAL replayed during online backup
                                 * (i.e., since last restartpoint used as backup starting
                                 * checkpoint) contain full-page writes.
                                 */
                                SpinLockAcquire(&xlogctl->info_lck);
                                recptr = xlogctl->lastFpwDisableRecPtr;
                                SpinLockRelease(&xlogctl->info_lck);

                                if (!checkpointfpw || startpoint <= recptr)
                                        ereport(ERROR,
                                                  (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                                   errmsg("WAL generated with full_page_writes=off was replayed "
                                                                  "since last restartpoint"),
                                                   errhint("This means that the backup being taken on the standby "
                                                                   "is corrupt and should not be used. "
                                                                   "Enable full_page_writes and run CHECKPOINT on the master, "
                                                                   "and then try an online backup again.")));

                                /*
                                 * During recovery, since we don't use the end-of-backup WAL
                                 * record and don't write the backup history file, the
                                 * starting WAL location doesn't need to be unique. This means
                                 * that two base backups started at the same time might use
                                 * the same checkpoint as starting locations.
                                 */
                                gotUniqueStartpoint = true;
                        }

                        /*
                         * If two base backups are started at the same time (in WAL sender
                         * processes), we need to make sure that they use different
                         * checkpoints as starting locations, because we use the starting
                         * WAL location as a unique identifier for the base backup in the
                         * end-of-backup WAL record and when we write the backup history
                         * file. Perhaps it would be better generate a separate unique ID
                         * for each backup instead of forcing another checkpoint, but
                         * taking a checkpoint right after another is not that expensive
                         * either because only few buffers have been dirtied yet.
                         */
                        WALInsertSlotAcquire(true);
                        if (XLogCtl->Insert.lastBackupStart < startpoint)
                        {
                                XLogCtl->Insert.lastBackupStart = startpoint;
                                gotUniqueStartpoint = true;
                        }
                        WALInsertSlotRelease();
                } while (!gotUniqueStartpoint);

                XLByteToSeg(startpoint, _logSegNo);
                XLogFileName(xlogfilename, ThisTimeLineID, _logSegNo);

                /*
                 * Construct backup label file
                 */
                initStringInfo(&labelfbuf);

                /* Use the log timezone here, not the session timezone */
                stamp_time = (pg_time_t) time(NULL);
                pg_strftime(strfbuf, sizeof(strfbuf),
                                        "%Y-%m-%d %H:%M:%S %Z",
                                        pg_localtime(&stamp_time, log_timezone));
                appendStringInfo(&labelfbuf, "START WAL LOCATION: %X/%X (file %s)\n",
                         (uint32) (startpoint >> 32), (uint32) startpoint, xlogfilename);
                appendStringInfo(&labelfbuf, "CHECKPOINT LOCATION: %X/%X\n",
                                         (uint32) (checkpointloc >> 32), (uint32) checkpointloc);
                appendStringInfo(&labelfbuf, "BACKUP METHOD: %s\n",
                                                 exclusive ? "pg_start_backup" : "streamed");
                appendStringInfo(&labelfbuf, "BACKUP FROM: %s\n",
                                                 backup_started_in_recovery ? "standby" : "master");
                appendStringInfo(&labelfbuf, "START TIME: %s\n", strfbuf);
                appendStringInfo(&labelfbuf, "LABEL: %s\n", backupidstr);

                /*
                 * Okay, write the file, or return its contents to caller.
                 */
                if (exclusive)
                {
                        /*
                         * Check for existing backup label --- implies a backup is already
                         * running.  (XXX given that we checked exclusiveBackup above,
                         * maybe it would be OK to just unlink any such label file?)
                         */
                        if (stat(BACKUP_LABEL_FILE, &stat_buf) != 0)
                        {
                                if (errno != ENOENT)
                                        ereport(ERROR,
                                                        (errcode_for_file_access(),
                                                         errmsg("could not stat file \"%s\": %m",
                                                                        BACKUP_LABEL_FILE)));
                        }
                        else
                                ereport(ERROR,
                                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                                 errmsg("a backup is already in progress"),
                                                 errhint("If you're sure there is no backup in progress, remove file \"%s\" and try again.",
                                                                 BACKUP_LABEL_FILE)));

                        fp = AllocateFile(BACKUP_LABEL_FILE, "w");

                        if (!fp)
                                ereport(ERROR,
                                                (errcode_for_file_access(),
                                                 errmsg("could not create file \"%s\": %m",
                                                                BACKUP_LABEL_FILE)));
                        if (fwrite(labelfbuf.data, labelfbuf.len, 1, fp) != 1 ||
                                fflush(fp) != 0 ||
                                pg_fsync(fileno(fp)) != 0 ||
                                ferror(fp) ||
                                FreeFile(fp))
                                ereport(ERROR,
                                                (errcode_for_file_access(),
                                                 errmsg("could not write file \"%s\": %m",
                                                                BACKUP_LABEL_FILE)));
                        pfree(labelfbuf.data);
                }
                else
                        *labelfile = labelfbuf.data;
        }
        PG_END_ENSURE_ERROR_CLEANUP(pg_start_backup_callback, (Datum) BoolGetDatum(exclusive));

        /*
         * We're done.  As a convenience, return the starting WAL location.
         */
        if (starttli_p)
                *starttli_p = starttli;
        return startpoint;
}

/* Error cleanup callback for pg_start_backup */
static void
pg_start_backup_callback(int code, Datum arg)
{
        bool            exclusive = DatumGetBool(arg);

        /* Update backup counters and forcePageWrites on failure */
        WALInsertSlotAcquire(true);
        if (exclusive)
        {
                Assert(XLogCtl->Insert.exclusiveBackup);
                XLogCtl->Insert.exclusiveBackup = false;
        }
        else
        {
                Assert(XLogCtl->Insert.nonExclusiveBackups > 0);
                XLogCtl->Insert.nonExclusiveBackups--;
        }

        if (!XLogCtl->Insert.exclusiveBackup &&
                XLogCtl->Insert.nonExclusiveBackups == 0)
        {
                XLogCtl->Insert.forcePageWrites = false;
        }
        WALInsertSlotRelease();
}

/*
 * do_pg_stop_backup is the workhorse of the user-visible pg_stop_backup()
 * function.

 * If labelfile is NULL, this stops an exclusive backup. Otherwise this stops
 * the non-exclusive backup specified by 'labelfile'.
 *
 * Returns the last WAL position that must be present to restore from this
 * backup, and the corresponding timeline ID in *stoptli_p.
 *
 * It is the responsibility of the caller of this function to verify the
 * permissions of the calling user!
 */
XLogRecPtr
do_pg_stop_backup(char *labelfile, bool waitforarchive, TimeLineID *stoptli_p)
{
        bool            exclusive = (labelfile == NULL);
        bool            backup_started_in_recovery = false;
        XLogRecPtr      startpoint;
        XLogRecPtr      stoppoint;
        TimeLineID      stoptli;
        XLogRecData rdata;
        pg_time_t       stamp_time;
        char            strfbuf[128];
        char            histfilepath[MAXPGPATH];
        char            startxlogfilename[MAXFNAMELEN];
        char            stopxlogfilename[MAXFNAMELEN];
        char            lastxlogfilename[MAXFNAMELEN];
        char            histfilename[MAXFNAMELEN];
        char            backupfrom[20];
        XLogSegNo       _logSegNo;
        FILE       *lfp;
        FILE       *fp;
        char            ch;
        int                     seconds_before_warning;
        int                     waits = 0;
        bool            reported_waiting = false;
        char       *remaining;
        char       *ptr;
        uint32          hi,
                                lo;

        backup_started_in_recovery = RecoveryInProgress();

        /*
         * Currently only non-exclusive backup can be taken during recovery.
         */
        if (backup_started_in_recovery && exclusive)
                ereport(ERROR,
                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                 errmsg("recovery is in progress"),
                                 errhint("WAL control functions cannot be executed during recovery.")));

        /*
         * During recovery, we don't need to check WAL level. Because, if WAL
         * level is not sufficient, it's impossible to get here during recovery.
         */
        if (!backup_started_in_recovery && !XLogIsNeeded())
                ereport(ERROR,
                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                          errmsg("WAL level not sufficient for making an online backup"),
                                 errhint("wal_level must be set to \"archive\", \"hot_standby\" or \"logical\" at server start.")));

        /*
         * OK to update backup counters and forcePageWrites
         */
        WALInsertSlotAcquire(true);
        if (exclusive)
                XLogCtl->Insert.exclusiveBackup = false;
        else
        {
                /*
                 * The user-visible pg_start/stop_backup() functions that operate on
                 * exclusive backups can be called at any time, but for non-exclusive
                 * backups, it is expected that each do_pg_start_backup() call is
                 * matched by exactly one do_pg_stop_backup() call.
                 */
                Assert(XLogCtl->Insert.nonExclusiveBackups > 0);
                XLogCtl->Insert.nonExclusiveBackups--;
        }

        if (!XLogCtl->Insert.exclusiveBackup &&
                XLogCtl->Insert.nonExclusiveBackups == 0)
        {
                XLogCtl->Insert.forcePageWrites = false;
        }
        WALInsertSlotRelease();

        if (exclusive)
        {
                /*
                 * Read the existing label file into memory.
                 */
                struct stat statbuf;
                int                     r;

                if (stat(BACKUP_LABEL_FILE, &statbuf))
                {
                        if (errno != ENOENT)
                                ereport(ERROR,
                                                (errcode_for_file_access(),
                                                 errmsg("could not stat file \"%s\": %m",
                                                                BACKUP_LABEL_FILE)));
                        ereport(ERROR,
                                        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                         errmsg("a backup is not in progress")));
                }

                lfp = AllocateFile(BACKUP_LABEL_FILE, "r");
                if (!lfp)
                {
                        ereport(ERROR,
                                        (errcode_for_file_access(),
                                         errmsg("could not read file \"%s\": %m",
                                                        BACKUP_LABEL_FILE)));
                }
                labelfile = palloc(statbuf.st_size + 1);
                r = fread(labelfile, statbuf.st_size, 1, lfp);
                labelfile[statbuf.st_size] = '\0';

                /*
                 * Close and remove the backup label file
                 */
                if (r != 1 || ferror(lfp) || FreeFile(lfp))
                        ereport(ERROR,
                                        (errcode_for_file_access(),
                                         errmsg("could not read file \"%s\": %m",
                                                        BACKUP_LABEL_FILE)));
                if (unlink(BACKUP_LABEL_FILE) != 0)
                        ereport(ERROR,
                                        (errcode_for_file_access(),
                                         errmsg("could not remove file \"%s\": %m",
                                                        BACKUP_LABEL_FILE)));
        }

        /*
         * Read and parse the START WAL LOCATION line (this code is pretty crude,
         * but we are not expecting any variability in the file format).
         */
        if (sscanf(labelfile, "START WAL LOCATION: %X/%X (file %24s)%c",
                           &hi, &lo, startxlogfilename,
                           &ch) != 4 || ch != '\n')
                ereport(ERROR,
                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                 errmsg("invalid data in file \"%s\"", BACKUP_LABEL_FILE)));
        startpoint = ((uint64) hi) << 32 | lo;
        remaining = strchr(labelfile, '\n') + 1;        /* %n is not portable enough */

        /*
         * Parse the BACKUP FROM line. If we are taking an online backup from the
         * standby, we confirm that the standby has not been promoted during the
         * backup.
         */
        ptr = strstr(remaining, "BACKUP FROM:");
        if (!ptr || sscanf(ptr, "BACKUP FROM: %19s\n", backupfrom) != 1)
                ereport(ERROR,
                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                 errmsg("invalid data in file \"%s\"", BACKUP_LABEL_FILE)));
        if (strcmp(backupfrom, "standby") == 0 && !backup_started_in_recovery)
                ereport(ERROR,
                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                 errmsg("the standby was promoted during online backup"),
                                 errhint("This means that the backup being taken is corrupt "
                                                 "and should not be used. "
                                                 "Try taking another online backup.")));

        /*
         * During recovery, we don't write an end-of-backup record. We assume that
         * pg_control was backed up last and its minimum recovery point can be
         * available as the backup end location. Since we don't have an
         * end-of-backup record, we use the pg_control value to check whether
         * we've reached the end of backup when starting recovery from this
         * backup. We have no way of checking if pg_control wasn't backed up last
         * however.
         *
         * We don't force a switch to new WAL file and wait for all the required
         * files to be archived. This is okay if we use the backup to start the
         * standby. But, if it's for an archive recovery, to ensure all the
         * required files are available, a user should wait for them to be
         * archived, or include them into the backup.
         *
         * We return the current minimum recovery point as the backup end
         * location. Note that it can be greater than the exact backup end
         * location if the minimum recovery point is updated after the backup of
         * pg_control. This is harmless for current uses.
         *
         * XXX currently a backup history file is for informational and debug
         * purposes only. It's not essential for an online backup. Furthermore,
         * even if it's created, it will not be archived during recovery because
         * an archiver is not invoked. So it doesn't seem worthwhile to write a
         * backup history file during recovery.
         */
        if (backup_started_in_recovery)
        {
                /* use volatile pointer to prevent code rearrangement */
                volatile XLogCtlData *xlogctl = XLogCtl;
                XLogRecPtr      recptr;

                /*
                 * Check to see if all WAL replayed during online backup contain
                 * full-page writes.
                 */
                SpinLockAcquire(&xlogctl->info_lck);
                recptr = xlogctl->lastFpwDisableRecPtr;
                SpinLockRelease(&xlogctl->info_lck);

                if (startpoint <= recptr)
                        ereport(ERROR,
                                        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                           errmsg("WAL generated with full_page_writes=off was replayed "
                                          "during online backup"),
                         errhint("This means that the backup being taken on the standby "
                                         "is corrupt and should not be used. "
                                 "Enable full_page_writes and run CHECKPOINT on the master, "
                                         "and then try an online backup again.")));


                LWLockAcquire(ControlFileLock, LW_SHARED);
                stoppoint = ControlFile->minRecoveryPoint;
                stoptli = ControlFile->minRecoveryPointTLI;
                LWLockRelease(ControlFileLock);

                if (stoptli_p)
                        *stoptli_p = stoptli;
                return stoppoint;
        }

        /*
         * Write the backup-end xlog record
         */
        rdata.data = (char *) (&startpoint);
        rdata.len = sizeof(startpoint);
        rdata.buffer = InvalidBuffer;
        rdata.next = NULL;
        stoppoint = XLogInsert(RM_XLOG_ID, XLOG_BACKUP_END, &rdata);
        stoptli = ThisTimeLineID;

        /*
         * Force a switch to a new xlog segment file, so that the backup is valid
         * as soon as archiver moves out the current segment file.
         */
        RequestXLogSwitch();

        XLByteToPrevSeg(stoppoint, _logSegNo);
        XLogFileName(stopxlogfilename, ThisTimeLineID, _logSegNo);

        /* Use the log timezone here, not the session timezone */
        stamp_time = (pg_time_t) time(NULL);
        pg_strftime(strfbuf, sizeof(strfbuf),
                                "%Y-%m-%d %H:%M:%S %Z",
                                pg_localtime(&stamp_time, log_timezone));

        /*
         * Write the backup history file
         */
        XLByteToSeg(startpoint, _logSegNo);
        BackupHistoryFilePath(histfilepath, ThisTimeLineID, _logSegNo,
                                                  (uint32) (startpoint % XLogSegSize));
        fp = AllocateFile(histfilepath, "w");
        if (!fp)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not create file \"%s\": %m",
                                                histfilepath)));
        fprintf(fp, "START WAL LOCATION: %X/%X (file %s)\n",
                (uint32) (startpoint >> 32), (uint32) startpoint, startxlogfilename);
        fprintf(fp, "STOP WAL LOCATION: %X/%X (file %s)\n",
                        (uint32) (stoppoint >> 32), (uint32) stoppoint, stopxlogfilename);
        /* transfer remaining lines from label to history file */
        fprintf(fp, "%s", remaining);
        fprintf(fp, "STOP TIME: %s\n", strfbuf);
        if (fflush(fp) || ferror(fp) || FreeFile(fp))
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not write file \"%s\": %m",
                                                histfilepath)));

        /*
         * Clean out any no-longer-needed history files.  As a side effect, this
         * will post a .ready file for the newly created history file, notifying
         * the archiver that history file may be archived immediately.
         */
        CleanupBackupHistory();

        /*
         * If archiving is enabled, wait for all the required WAL files to be
         * archived before returning. If archiving isn't enabled, the required WAL
         * needs to be transported via streaming replication (hopefully with
         * wal_keep_segments set high enough), or some more exotic mechanism like
         * polling and copying files from pg_xlog with script. We have no
         * knowledge of those mechanisms, so it's up to the user to ensure that he
         * gets all the required WAL.
         *
         * We wait until both the last WAL file filled during backup and the
         * history file have been archived, and assume that the alphabetic sorting
         * property of the WAL files ensures any earlier WAL files are safely
         * archived as well.
         *
         * We wait forever, since archive_command is supposed to work and we
         * assume the admin wanted his backup to work completely. If you don't
         * wish to wait, you can set statement_timeout.  Also, some notices are
         * issued to clue in anyone who might be doing this interactively.
         */
        if (waitforarchive && XLogArchivingActive())
        {
                XLByteToPrevSeg(stoppoint, _logSegNo);
                XLogFileName(lastxlogfilename, ThisTimeLineID, _logSegNo);

                XLByteToSeg(startpoint, _logSegNo);
                BackupHistoryFileName(histfilename, ThisTimeLineID, _logSegNo,
                                                          (uint32) (startpoint % XLogSegSize));

                seconds_before_warning = 60;
                waits = 0;

                while (XLogArchiveIsBusy(lastxlogfilename) ||
                           XLogArchiveIsBusy(histfilename))
                {
                        CHECK_FOR_INTERRUPTS();

                        if (!reported_waiting && waits > 5)
                        {
                                ereport(NOTICE,
                                                (errmsg("pg_stop_backup cleanup done, waiting for required WAL segments to be archived")));
                                reported_waiting = true;
                        }

                        pg_usleep(1000000L);

                        if (++waits >= seconds_before_warning)
                        {
                                seconds_before_warning *= 2;    /* This wraps in >10 years... */
                                ereport(WARNING,
                                                (errmsg("pg_stop_backup still waiting for all required WAL segments to be archived (%d seconds elapsed)",
                                                                waits),
                                                 errhint("Check that your archive_command is executing properly.  "
                                                                 "pg_stop_backup can be canceled safely, "
                                                                 "but the database backup will not be usable without all the WAL segments.")));
                        }
                }

                ereport(NOTICE,
                                (errmsg("pg_stop_backup complete, all required WAL segments have been archived")));
        }
        else if (waitforarchive)
                ereport(NOTICE,
                                (errmsg("WAL archiving is not enabled; you must ensure that all required WAL segments are copied through other means to complete the backup")));

        /*
         * We're done.  As a convenience, return the ending WAL location.
         */
        if (stoptli_p)
                *stoptli_p = stoptli;
        return stoppoint;
}


/*
 * do_pg_abort_backup: abort a running backup
 *
 * This does just the most basic steps of do_pg_stop_backup(), by taking the
 * system out of backup mode, thus making it a lot more safe to call from
 * an error handler.
 *
 * NB: This is only for aborting a non-exclusive backup that doesn't write
 * backup_label. A backup started with pg_stop_backup() needs to be finished
 * with pg_stop_backup().
 */
void
do_pg_abort_backup(void)
{
        WALInsertSlotAcquire(true);
        Assert(XLogCtl->Insert.nonExclusiveBackups > 0);
        XLogCtl->Insert.nonExclusiveBackups--;

        if (!XLogCtl->Insert.exclusiveBackup &&
                XLogCtl->Insert.nonExclusiveBackups == 0)
        {
                XLogCtl->Insert.forcePageWrites = false;
        }
        WALInsertSlotRelease();
}

/*
 * Get latest redo apply position.
 *
 * Exported to allow WALReceiver to read the pointer directly.
 */
XLogRecPtr
GetXLogReplayRecPtr(TimeLineID *replayTLI)
{
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;
        XLogRecPtr      recptr;
        TimeLineID      tli;

        SpinLockAcquire(&xlogctl->info_lck);
        recptr = xlogctl->lastReplayedEndRecPtr;
        tli = xlogctl->lastReplayedTLI;
        SpinLockRelease(&xlogctl->info_lck);

        if (replayTLI)
                *replayTLI = tli;
        return recptr;
}

/*
 * Get latest WAL insert pointer
 */
XLogRecPtr
GetXLogInsertRecPtr(void)
{
        volatile XLogCtlInsert *Insert = &XLogCtl->Insert;
        uint64          current_bytepos;

        SpinLockAcquire(&Insert->insertpos_lck);
        current_bytepos = Insert->CurrBytePos;
        SpinLockRelease(&Insert->insertpos_lck);

        return XLogBytePosToRecPtr(current_bytepos);
}

/*
 * Get latest WAL write pointer
 */
XLogRecPtr
GetXLogWriteRecPtr(void)
{
        {
                /* use volatile pointer to prevent code rearrangement */
                volatile XLogCtlData *xlogctl = XLogCtl;

                SpinLockAcquire(&xlogctl->info_lck);
                LogwrtResult = xlogctl->LogwrtResult;
                SpinLockRelease(&xlogctl->info_lck);
        }

        return LogwrtResult.Write;
}

/*
 * Returns the redo pointer of the last checkpoint or restartpoint. This is
 * the oldest point in WAL that we still need, if we have to restart recovery.
 */
void
GetOldestRestartPoint(XLogRecPtr *oldrecptr, TimeLineID *oldtli)
{
        LWLockAcquire(ControlFileLock, LW_SHARED);
        *oldrecptr = ControlFile->checkPointCopy.redo;
        *oldtli = ControlFile->checkPointCopy.ThisTimeLineID;
        LWLockRelease(ControlFileLock);
}

/*
 * read_backup_label: check to see if a backup_label file is present
 *
 * If we see a backup_label during recovery, we assume that we are recovering
 * from a backup dump file, and we therefore roll forward from the checkpoint
 * identified by the label file, NOT what pg_control says.      This avoids the
 * problem that pg_control might have been archived one or more checkpoints
 * later than the start of the dump, and so if we rely on it as the start
 * point, we will fail to restore a consistent database state.
 *
 * Returns TRUE if a backup_label was found (and fills the checkpoint
 * location and its REDO location into *checkPointLoc and RedoStartLSN,
 * respectively); returns FALSE if not. If this backup_label came from a
 * streamed backup, *backupEndRequired is set to TRUE. If this backup_label
 * was created during recovery, *backupFromStandby is set to TRUE.
 */
static bool
read_backup_label(XLogRecPtr *checkPointLoc, bool *backupEndRequired,
                                  bool *backupFromStandby)
{
        char            startxlogfilename[MAXFNAMELEN];
        TimeLineID      tli;
        FILE       *lfp;
        char            ch;
        char            backuptype[20];
        char            backupfrom[20];
        uint32          hi,
                                lo;

        *backupEndRequired = false;
        *backupFromStandby = false;

        /*
         * See if label file is present
         */
        lfp = AllocateFile(BACKUP_LABEL_FILE, "r");
        if (!lfp)
        {
                if (errno != ENOENT)
                        ereport(FATAL,
                                        (errcode_for_file_access(),
                                         errmsg("could not read file \"%s\": %m",
                                                        BACKUP_LABEL_FILE)));
                return false;                   /* it's not there, all is fine */
        }

        /*
         * Read and parse the START WAL LOCATION and CHECKPOINT lines (this code
         * is pretty crude, but we are not expecting any variability in the file
         * format).
         */
        if (fscanf(lfp, "START WAL LOCATION: %X/%X (file %08X%16s)%c",
                           &hi, &lo, &tli, startxlogfilename, &ch) != 5 || ch != '\n')
                ereport(FATAL,
                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                 errmsg("invalid data in file \"%s\"", BACKUP_LABEL_FILE)));
        RedoStartLSN = ((uint64) hi) << 32 | lo;
        if (fscanf(lfp, "CHECKPOINT LOCATION: %X/%X%c",
                           &hi, &lo, &ch) != 3 || ch != '\n')
                ereport(FATAL,
                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                 errmsg("invalid data in file \"%s\"", BACKUP_LABEL_FILE)));
        *checkPointLoc = ((uint64) hi) << 32 | lo;

        /*
         * BACKUP METHOD and BACKUP FROM lines are new in 9.2. We can't restore
         * from an older backup anyway, but since the information on it is not
         * strictly required, don't error out if it's missing for some reason.
         */
        if (fscanf(lfp, "BACKUP METHOD: %19s\n", backuptype) == 1)
        {
                if (strcmp(backuptype, "streamed") == 0)
                        *backupEndRequired = true;
        }

        if (fscanf(lfp, "BACKUP FROM: %19s\n", backupfrom) == 1)
        {
                if (strcmp(backupfrom, "standby") == 0)
                        *backupFromStandby = true;
        }

        if (ferror(lfp) || FreeFile(lfp))
                ereport(FATAL,
                                (errcode_for_file_access(),
                                 errmsg("could not read file \"%s\": %m",
                                                BACKUP_LABEL_FILE)));

        return true;
}

/*
 * Error context callback for errors occurring during rm_redo().
 */
static void
rm_redo_error_callback(void *arg)
{
        XLogRecord *record = (XLogRecord *) arg;
        StringInfoData buf;

        initStringInfo(&buf);
        RmgrTable[record->xl_rmid].rm_desc(&buf,
                                                                           record->xl_info,
                                                                           XLogRecGetData(record));

        /* don't bother emitting empty description */
        if (buf.len > 0)
                errcontext("xlog redo %s", buf.data);

        pfree(buf.data);
}

/*
 * BackupInProgress: check if online backup mode is active
 *
 * This is done by checking for existence of the "backup_label" file.
 */
bool
BackupInProgress(void)
{
        struct stat stat_buf;

        return (stat(BACKUP_LABEL_FILE, &stat_buf) == 0);
}

/*
 * CancelBackup: rename the "backup_label" file to cancel backup mode
 *
 * If the "backup_label" file exists, it will be renamed to "backup_label.old".
 * Note that this will render an online backup in progress useless.
 * To correctly finish an online backup, pg_stop_backup must be called.
 */
void
CancelBackup(void)
{
        struct stat stat_buf;

        /* if the file is not there, return */
        if (stat(BACKUP_LABEL_FILE, &stat_buf) < 0)
                return;

        /* remove leftover file from previously canceled backup if it exists */
        unlink(BACKUP_LABEL_OLD);

        if (rename(BACKUP_LABEL_FILE, BACKUP_LABEL_OLD) == 0)
        {
                ereport(LOG,
                                (errmsg("online backup mode canceled"),
                                 errdetail("\"%s\" was renamed to \"%s\".",
                                                   BACKUP_LABEL_FILE, BACKUP_LABEL_OLD)));
        }
        else
        {
                ereport(WARNING,
                                (errcode_for_file_access(),
                                 errmsg("online backup mode was not canceled"),
                                 errdetail("Could not rename \"%s\" to \"%s\": %m.",
                                                   BACKUP_LABEL_FILE, BACKUP_LABEL_OLD)));
        }
}

/*
 * Read the XLOG page containing RecPtr into readBuf (if not read already).
 * Returns number of bytes read, if the page is read successfully, or -1
 * in case of errors.  When errors occur, they are ereport'ed, but only
 * if they have not been previously reported.
 *
 * This is responsible for restoring files from archive as needed, as well
 * as for waiting for the requested WAL record to arrive in standby mode.
 *
 * 'emode' specifies the log level used for reporting "file not found" or
 * "end of WAL" situations in archive recovery, or in standby mode when a
 * trigger file is found. If set to WARNING or below, XLogPageRead() returns
 * false in those situations, on higher log levels the ereport() won't
 * return.
 *
 * In standby mode, if after a successful return of XLogPageRead() the
 * caller finds the record it's interested in to be broken, it should
 * ereport the error with the level determined by
 * emode_for_corrupt_record(), and then set lastSourceFailed
 * and call XLogPageRead() again with the same arguments. This lets
 * XLogPageRead() to try fetching the record from another source, or to
 * sleep and retry.
 */
static int
XLogPageRead(XLogReaderState *xlogreader, XLogRecPtr targetPagePtr, int reqLen,
                         XLogRecPtr targetRecPtr, char *readBuf, TimeLineID *readTLI)
{
        XLogPageReadPrivate *private =
        (XLogPageReadPrivate *) xlogreader->private_data;
        int                     emode = private->emode;
        uint32          targetPageOff;
        XLogSegNo targetSegNo PG_USED_FOR_ASSERTS_ONLY;

        XLByteToSeg(targetPagePtr, targetSegNo);
        targetPageOff = targetPagePtr % XLogSegSize;

        /*
         * See if we need to switch to a new segment because the requested record
         * is not in the currently open one.
         */
        if (readFile >= 0 && !XLByteInSeg(targetPagePtr, readSegNo))
        {
                /*
                 * Request a restartpoint if we've replayed too much xlog since the
                 * last one.
                 */
                if (StandbyModeRequested && bgwriterLaunched)
                {
                        if (XLogCheckpointNeeded(readSegNo))
                        {
                                (void) GetRedoRecPtr();
                                if (XLogCheckpointNeeded(readSegNo))
                                        RequestCheckpoint(CHECKPOINT_CAUSE_XLOG);
                        }
                }

                close(readFile);
                readFile = -1;
                readSource = 0;
        }

        XLByteToSeg(targetPagePtr, readSegNo);

retry:
        /* See if we need to retrieve more data */
        if (readFile < 0 ||
                (readSource == XLOG_FROM_STREAM &&
                 receivedUpto < targetPagePtr + reqLen))
        {
                if (!WaitForWALToBecomeAvailable(targetPagePtr + reqLen,
                                                                                 private->randAccess,
                                                                                 private->fetching_ckpt,
                                                                                 targetRecPtr))
                {
                        if (readFile >= 0)
                                close(readFile);
                        readFile = -1;
                        readLen = 0;
                        readSource = 0;

                        return -1;
                }
        }

        /*
         * At this point, we have the right segment open and if we're streaming we
         * know the requested record is in it.
         */
        Assert(readFile != -1);

        /*
         * If the current segment is being streamed from master, calculate how
         * much of the current page we have received already. We know the
         * requested record has been received, but this is for the benefit of
         * future calls, to allow quick exit at the top of this function.
         */
        if (readSource == XLOG_FROM_STREAM)
        {
                if (((targetPagePtr) / XLOG_BLCKSZ) != (receivedUpto / XLOG_BLCKSZ))
                        readLen = XLOG_BLCKSZ;
                else
                        readLen = receivedUpto % XLogSegSize - targetPageOff;
        }
        else
                readLen = XLOG_BLCKSZ;

        /* Read the requested page */
        readOff = targetPageOff;
        if (lseek(readFile, (off_t) readOff, SEEK_SET) < 0)
        {
                char            fname[MAXFNAMELEN];

                XLogFileName(fname, curFileTLI, readSegNo);
                ereport(emode_for_corrupt_record(emode, targetPagePtr + reqLen),
                                (errcode_for_file_access(),
                                 errmsg("could not seek in log segment %s to offset %u: %m",
                                                fname, readOff)));
                goto next_record_is_invalid;
        }

        if (read(readFile, readBuf, XLOG_BLCKSZ) != XLOG_BLCKSZ)
        {
                char            fname[MAXFNAMELEN];

                XLogFileName(fname, curFileTLI, readSegNo);
                ereport(emode_for_corrupt_record(emode, targetPagePtr + reqLen),
                                (errcode_for_file_access(),
                                 errmsg("could not read from log segment %s, offset %u: %m",
                                                fname, readOff)));
                goto next_record_is_invalid;
        }

        Assert(targetSegNo == readSegNo);
        Assert(targetPageOff == readOff);
        Assert(reqLen <= readLen);

        *readTLI = curFileTLI;
        return readLen;

next_record_is_invalid:
        lastSourceFailed = true;

        if (readFile >= 0)
                close(readFile);
        readFile = -1;
        readLen = 0;
        readSource = 0;

        /* In standby-mode, keep trying */
        if (StandbyMode)
                goto retry;
        else
                return -1;
}

/*
 * Open the WAL segment containing WAL position 'RecPtr'.
 *
 * The segment can be fetched via restore_command, or via walreceiver having
 * streamed the record, or it can already be present in pg_xlog. Checking
 * pg_xlog is mainly for crash recovery, but it will be polled in standby mode
 * too, in case someone copies a new segment directly to pg_xlog. That is not
 * documented or recommended, though.
 *
 * If 'fetching_ckpt' is true, we're fetching a checkpoint record, and should
 * prepare to read WAL starting from RedoStartLSN after this.
 *
 * 'RecPtr' might not point to the beginning of the record we're interested
 * in, it might also point to the page or segment header. In that case,
 * 'tliRecPtr' is the position of the WAL record we're interested in. It is
 * used to decide which timeline to stream the requested WAL from.
 *
 * If the record is not immediately available, the function returns false
 * if we're not in standby mode. In standby mode, waits for it to become
 * available.
 *
 * When the requested record becomes available, the function opens the file
 * containing it (if not open already), and returns true. When end of standby
 * mode is triggered by the user, and there is no more WAL available, returns
 * false.
 */
static bool
WaitForWALToBecomeAvailable(XLogRecPtr RecPtr, bool randAccess,
                                                        bool fetching_ckpt, XLogRecPtr tliRecPtr)
{
        static pg_time_t last_fail_time = 0;
        pg_time_t       now;

        /*-------
         * Standby mode is implemented by a state machine:
         *
         * 1. Read from archive (XLOG_FROM_ARCHIVE)
         * 2. Read from pg_xlog (XLOG_FROM_PG_XLOG)
         * 3. Check trigger file
         * 4. Read from primary server via walreceiver (XLOG_FROM_STREAM)
         * 5. Rescan timelines
         * 6. Sleep 5 seconds, and loop back to 1.
         *
         * Failure to read from the current source advances the state machine to
         * the next state. In addition, successfully reading a file from pg_xlog
         * moves the state machine from state 2 back to state 1 (we always prefer
         * files in the archive over files in pg_xlog).
         *
         * 'currentSource' indicates the current state. There are no currentSource
         * values for "check trigger", "rescan timelines", and "sleep" states,
         * those actions are taken when reading from the previous source fails, as
         * part of advancing to the next state.
         *-------
         */
        if (!InArchiveRecovery)
                currentSource = XLOG_FROM_PG_XLOG;
        else if (currentSource == 0)
                currentSource = XLOG_FROM_ARCHIVE;

        for (;;)
        {
                int                     oldSource = currentSource;

                /*
                 * First check if we failed to read from the current source, and
                 * advance the state machine if so. The failure to read might've
                 * happened outside this function, e.g when a CRC check fails on a
                 * record, or within this loop.
                 */
                if (lastSourceFailed)
                {
                        switch (currentSource)
                        {
                                case XLOG_FROM_ARCHIVE:
                                        currentSource = XLOG_FROM_PG_XLOG;
                                        break;

                                case XLOG_FROM_PG_XLOG:

                                        /*
                                         * Check to see if the trigger file exists. Note that we
                                         * do this only after failure, so when you create the
                                         * trigger file, we still finish replaying as much as we
                                         * can from archive and pg_xlog before failover.
                                         */
                                        if (StandbyMode && CheckForStandbyTrigger())
                                        {
                                                ShutdownWalRcv();
                                                return false;
                                        }

                                        /*
                                         * Not in standby mode, and we've now tried the archive
                                         * and pg_xlog.
                                         */
                                        if (!StandbyMode)
                                                return false;

                                        /*
                                         * If primary_conninfo is set, launch walreceiver to try
                                         * to stream the missing WAL.
                                         *
                                         * If fetching_ckpt is TRUE, RecPtr points to the initial
                                         * checkpoint location. In that case, we use RedoStartLSN
                                         * as the streaming start position instead of RecPtr, so
                                         * that when we later jump backwards to start redo at
                                         * RedoStartLSN, we will have the logs streamed already.
                                         */
                                        if (PrimaryConnInfo)
                                        {
                                                XLogRecPtr      ptr;
                                                TimeLineID      tli;

                                                if (fetching_ckpt)
                                                {
                                                        ptr = RedoStartLSN;
                                                        tli = ControlFile->checkPointCopy.ThisTimeLineID;
                                                }
                                                else
                                                {
                                                        ptr = tliRecPtr;
                                                        tli = tliOfPointInHistory(tliRecPtr, expectedTLEs);

                                                        if (curFileTLI > 0 && tli < curFileTLI)
                                                                elog(ERROR, "according to history file, WAL location %X/%X belongs to timeline %u, but previous recovered WAL file came from timeline %u",
                                                                         (uint32) (ptr >> 32), (uint32) ptr,
                                                                         tli, curFileTLI);
                                                }
                                                curFileTLI = tli;
                                                RequestXLogStreaming(tli, ptr, PrimaryConnInfo);
                                                receivedUpto = 0;
                                        }

                                        /*
                                         * Move to XLOG_FROM_STREAM state in either case. We'll
                                         * get immediate failure if we didn't launch walreceiver,
                                         * and move on to the next state.
                                         */
                                        currentSource = XLOG_FROM_STREAM;
                                        break;

                                case XLOG_FROM_STREAM:

                                        /*
                                         * Failure while streaming. Most likely, we got here
                                         * because streaming replication was terminated, or
                                         * promotion was triggered. But we also get here if we
                                         * find an invalid record in the WAL streamed from master,
                                         * in which case something is seriously wrong. There's
                                         * little chance that the problem will just go away, but
                                         * PANIC is not good for availability either, especially
                                         * in hot standby mode. So, we treat that the same as
                                         * disconnection, and retry from archive/pg_xlog again.
                                         * The WAL in the archive should be identical to what was
                                         * streamed, so it's unlikely that it helps, but one can
                                         * hope...
                                         */

                                        /*
                                         * Before we leave XLOG_FROM_STREAM state, make sure that
                                         * walreceiver is not active, so that it won't overwrite
                                         * WAL that we restore from archive.
                                         */
                                        if (WalRcvStreaming())
                                                ShutdownWalRcv();

                                        /*
                                         * Before we sleep, re-scan for possible new timelines if
                                         * we were requested to recover to the latest timeline.
                                         */
                                        if (recoveryTargetIsLatest)
                                        {
                                                if (rescanLatestTimeLine())
                                                {
                                                        currentSource = XLOG_FROM_ARCHIVE;
                                                        break;
                                                }
                                        }

                                        /*
                                         * XLOG_FROM_STREAM is the last state in our state
                                         * machine, so we've exhausted all the options for
                                         * obtaining the requested WAL. We're going to loop back
                                         * and retry from the archive, but if it hasn't been long
                                         * since last attempt, sleep 5 seconds to avoid
                                         * busy-waiting.
                                         */
                                        now = (pg_time_t) time(NULL);
                                        if ((now - last_fail_time) < 5)
                                        {
                                                pg_usleep(1000000L * (5 - (now - last_fail_time)));
                                                now = (pg_time_t) time(NULL);
                                        }
                                        last_fail_time = now;
                                        currentSource = XLOG_FROM_ARCHIVE;
                                        break;

                                default:
                                        elog(ERROR, "unexpected WAL source %d", currentSource);
                        }
                }
                else if (currentSource == XLOG_FROM_PG_XLOG)
                {
                        /*
                         * We just successfully read a file in pg_xlog. We prefer files in
                         * the archive over ones in pg_xlog, so try the next file again
                         * from the archive first.
                         */
                        if (InArchiveRecovery)
                                currentSource = XLOG_FROM_ARCHIVE;
                }

                if (currentSource != oldSource)
                        elog(DEBUG2, "switched WAL source from %s to %s after %s",
                                 xlogSourceNames[oldSource], xlogSourceNames[currentSource],
                                 lastSourceFailed ? "failure" : "success");

                /*
                 * We've now handled possible failure. Try to read from the chosen
                 * source.
                 */
                lastSourceFailed = false;

                switch (currentSource)
                {
                        case XLOG_FROM_ARCHIVE:
                        case XLOG_FROM_PG_XLOG:
                                /* Close any old file we might have open. */
                                if (readFile >= 0)
                                {
                                        close(readFile);
                                        readFile = -1;
                                }
                                /* Reset curFileTLI if random fetch. */
                                if (randAccess)
                                        curFileTLI = 0;

                                /*
                                 * Try to restore the file from archive, or read an existing
                                 * file from pg_xlog.
                                 */
                                readFile = XLogFileReadAnyTLI(readSegNo, DEBUG2, currentSource);
                                if (readFile >= 0)
                                        return true;    /* success! */

                                /*
                                 * Nope, not found in archive or pg_xlog.
                                 */
                                lastSourceFailed = true;
                                break;

                        case XLOG_FROM_STREAM:
                                {
                                        bool            havedata;

                                        /*
                                         * Check if WAL receiver is still active.
                                         */
                                        if (!WalRcvStreaming())
                                        {
                                                lastSourceFailed = true;
                                                break;
                                        }

                                        /*
                                         * Walreceiver is active, so see if new data has arrived.
                                         *
                                         * We only advance XLogReceiptTime when we obtain fresh
                                         * WAL from walreceiver and observe that we had already
                                         * processed everything before the most recent "chunk"
                                         * that it flushed to disk.  In steady state where we are
                                         * keeping up with the incoming data, XLogReceiptTime will
                                         * be updated on each cycle. When we are behind,
                                         * XLogReceiptTime will not advance, so the grace time
                                         * allotted to conflicting queries will decrease.
                                         */
                                        if (RecPtr < receivedUpto)
                                                havedata = true;
                                        else
                                        {
                                                XLogRecPtr      latestChunkStart;

                                                receivedUpto = GetWalRcvWriteRecPtr(&latestChunkStart, &receiveTLI);
                                                if (RecPtr < receivedUpto && receiveTLI == curFileTLI)
                                                {
                                                        havedata = true;
                                                        if (latestChunkStart <= RecPtr)
                                                        {
                                                                XLogReceiptTime = GetCurrentTimestamp();
                                                                SetCurrentChunkStartTime(XLogReceiptTime);
                                                        }
                                                }
                                                else
                                                        havedata = false;
                                        }
                                        if (havedata)
                                        {
                                                /*
                                                 * Great, streamed far enough.  Open the file if it's
                                                 * not open already.  Also read the timeline history
                                                 * file if we haven't initialized timeline history
                                                 * yet; it should be streamed over and present in
                                                 * pg_xlog by now.      Use XLOG_FROM_STREAM so that
                                                 * source info is set correctly and XLogReceiptTime
                                                 * isn't changed.
                                                 */
                                                if (readFile < 0)
                                                {
                                                        if (!expectedTLEs)
                                                                expectedTLEs = readTimeLineHistory(receiveTLI);
                                                        readFile = XLogFileRead(readSegNo, PANIC,
                                                                                                        receiveTLI,
                                                                                                        XLOG_FROM_STREAM, false);
                                                        Assert(readFile >= 0);
                                                }
                                                else
                                                {
                                                        /* just make sure source info is correct... */
                                                        readSource = XLOG_FROM_STREAM;
                                                        XLogReceiptSource = XLOG_FROM_STREAM;
                                                        return true;
                                                }
                                                break;
                                        }

                                        /*
                                         * Data not here yet. Check for trigger, then wait for
                                         * walreceiver to wake us up when new WAL arrives.
                                         */
                                        if (CheckForStandbyTrigger())
                                        {
                                                /*
                                                 * Note that we don't "return false" immediately here.
                                                 * After being triggered, we still want to replay all
                                                 * the WAL that was already streamed. It's in pg_xlog
                                                 * now, so we just treat this as a failure, and the
                                                 * state machine will move on to replay the streamed
                                                 * WAL from pg_xlog, and then recheck the trigger and
                                                 * exit replay.
                                                 */
                                                lastSourceFailed = true;
                                                break;
                                        }

                                        /*
                                         * Wait for more WAL to arrive. Time out after 5 seconds,
                                         * like when polling the archive, to react to a trigger
                                         * file promptly.
                                         */
                                        WaitLatch(&XLogCtl->recoveryWakeupLatch,
                                                          WL_LATCH_SET | WL_TIMEOUT,
                                                          5000L);
                                        ResetLatch(&XLogCtl->recoveryWakeupLatch);
                                        break;
                                }

                        default:
                                elog(ERROR, "unexpected WAL source %d", currentSource);
                }

                /*
                 * This possibly-long loop needs to handle interrupts of startup
                 * process.
                 */
                HandleStartupProcInterrupts();
        } while (StandbyMode);

        return false;
}

/*
 * Determine what log level should be used to report a corrupt WAL record
 * in the current WAL page, previously read by XLogPageRead().
 *
 * 'emode' is the error mode that would be used to report a file-not-found
 * or legitimate end-of-WAL situation.   Generally, we use it as-is, but if
 * we're retrying the exact same record that we've tried previously, only
 * complain the first time to keep the noise down.      However, we only do when
 * reading from pg_xlog, because we don't expect any invalid records in archive
 * or in records streamed from master. Files in the archive should be complete,
 * and we should never hit the end of WAL because we stop and wait for more WAL
 * to arrive before replaying it.
 *
 * NOTE: This function remembers the RecPtr value it was last called with,
 * to suppress repeated messages about the same record. Only call this when
 * you are about to ereport(), or you might cause a later message to be
 * erroneously suppressed.
 */
static int
emode_for_corrupt_record(int emode, XLogRecPtr RecPtr)
{
        static XLogRecPtr lastComplaint = 0;

        if (readSource == XLOG_FROM_PG_XLOG && emode == LOG)
        {
                if (RecPtr == lastComplaint)
                        emode = DEBUG1;
                else
                        lastComplaint = RecPtr;
        }
        return emode;
}

/*
 * Check to see whether the user-specified trigger file exists and whether a
 * promote request has arrived.  If either condition holds, return true.
 */
static bool
CheckForStandbyTrigger(void)
{
        struct stat stat_buf;
        static bool triggered = false;

        if (triggered)
                return true;

        if (IsPromoteTriggered())
        {
                /*
                 * In 9.1 and 9.2 the postmaster unlinked the promote file inside the
                 * signal handler. It now leaves the file in place and lets the
                 * Startup process do the unlink. This allows Startup to know whether
                 * it should create a full checkpoint before starting up (fallback
                 * mode). Fast promotion takes precedence.
                 */
                if (stat(PROMOTE_SIGNAL_FILE, &stat_buf) == 0)
                {
                        unlink(PROMOTE_SIGNAL_FILE);
                        unlink(FALLBACK_PROMOTE_SIGNAL_FILE);
                        fast_promote = true;
                }
                else if (stat(FALLBACK_PROMOTE_SIGNAL_FILE, &stat_buf) == 0)
                {
                        unlink(FALLBACK_PROMOTE_SIGNAL_FILE);
                        fast_promote = false;
                }

                ereport(LOG, (errmsg("received promote request")));

                ResetPromoteTriggered();
                triggered = true;
                return true;
        }

        if (TriggerFile == NULL)
                return false;

        if (stat(TriggerFile, &stat_buf) == 0)
        {
                ereport(LOG,
                                (errmsg("trigger file found: %s", TriggerFile)));
                unlink(TriggerFile);
                triggered = true;
                fast_promote = true;
                return true;
        }
        return false;
}

/*
 * Check to see if a promote request has arrived. Should be
 * called by postmaster after receiving SIGUSR1.
 */
bool
CheckPromoteSignal(void)
{
        struct stat stat_buf;

        if (stat(PROMOTE_SIGNAL_FILE, &stat_buf) == 0 ||
                stat(FALLBACK_PROMOTE_SIGNAL_FILE, &stat_buf) == 0)
                return true;

        return false;
}

/*
 * Wake up startup process to replay newly arrived WAL, or to notice that
 * failover has been requested.
 */
void
WakeupRecovery(void)
{
        SetLatch(&XLogCtl->recoveryWakeupLatch);
}

/*
 * Update the WalWriterSleeping flag.
 */
void
SetWalWriterSleeping(bool sleeping)
{
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;

        SpinLockAcquire(&xlogctl->info_lck);
        xlogctl->WalWriterSleeping = sleeping;
        SpinLockRelease(&xlogctl->info_lck);
}