[postgresql] / src / backend / postmaster / checkpointer.c

/*-------------------------------------------------------------------------
 *
 * checkpointer.c
 *
 * The checkpointer is new as of Postgres 9.2.  It handles all checkpoints.
 * Checkpoints are automatically dispatched after a certain amount of time has
 * elapsed since the last one, and it can be signaled to perform requested
 * checkpoints as well.  (The GUC parameter that mandates a checkpoint every
 * so many WAL segments is implemented by having backends signal when they
 * fill WAL segments; the checkpointer itself doesn't watch for the
 * condition.)
 *
 * The checkpointer is started by the postmaster as soon as the startup
 * subprocess finishes, or as soon as recovery begins if we are doing archive
 * recovery.  It remains alive until the postmaster commands it to terminate.
 * Normal termination is by SIGUSR2, which instructs the checkpointer to
 * execute a shutdown checkpoint and then exit(0).      (All backends must be
 * stopped before SIGUSR2 is issued!)  Emergency termination is by SIGQUIT;
 * like any backend, the checkpointer will simply abort and exit on SIGQUIT.
 *
 * If the checkpointer exits unexpectedly, the postmaster treats that the same
 * as a backend crash: shared memory may be corrupted, so remaining backends
 * should be killed by SIGQUIT and then a recovery cycle started.  (Even if
 * shared memory isn't corrupted, we have lost information about which
 * files need to be fsync'd for the next checkpoint, and so a system
 * restart needs to be forced.)
 *
 *
 * Portions Copyright (c) 1996-2012, PostgreSQL Global Development Group
 *
 *
 * IDENTIFICATION
 *        src/backend/postmaster/checkpointer.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <signal.h>
#include <sys/time.h>
#include <time.h>
#include <unistd.h>

#include "access/xlog_internal.h"
#include "libpq/pqsignal.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "postmaster/bgwriter.h"
#include "replication/syncrep.h"
#include "storage/bufmgr.h"
#include "storage/ipc.h"
#include "storage/lwlock.h"
#include "storage/proc.h"
#include "storage/shmem.h"
#include "storage/smgr.h"
#include "storage/spin.h"
#include "utils/guc.h"
#include "utils/memutils.h"
#include "utils/resowner.h"


/*----------
 * Shared memory area for communication between checkpointer and backends
 *
 * The ckpt counters allow backends to watch for completion of a checkpoint
 * request they send.  Here's how it works:
 *      * At start of a checkpoint, checkpointer reads (and clears) the request
 *        flags and increments ckpt_started, while holding ckpt_lck.
 *      * On completion of a checkpoint, checkpointer sets ckpt_done to
 *        equal ckpt_started.
 *      * On failure of a checkpoint, checkpointer increments ckpt_failed
 *        and sets ckpt_done to equal ckpt_started.
 *
 * The algorithm for backends is:
 *      1. Record current values of ckpt_failed and ckpt_started, and
 *         set request flags, while holding ckpt_lck.
 *      2. Send signal to request checkpoint.
 *      3. Sleep until ckpt_started changes.  Now you know a checkpoint has
 *         begun since you started this algorithm (although *not* that it was
 *         specifically initiated by your signal), and that it is using your flags.
 *      4. Record new value of ckpt_started.
 *      5. Sleep until ckpt_done >= saved value of ckpt_started.  (Use modulo
 *         arithmetic here in case counters wrap around.)  Now you know a
 *         checkpoint has started and completed, but not whether it was
 *         successful.
 *      6. If ckpt_failed is different from the originally saved value,
 *         assume request failed; otherwise it was definitely successful.
 *
 * ckpt_flags holds the OR of the checkpoint request flags sent by all
 * requesting backends since the last checkpoint start.  The flags are
 * chosen so that OR'ing is the correct way to combine multiple requests.
 *
 * num_backend_writes is used to count the number of buffer writes performed
 * by user backend processes.  This counter should be wide enough that it
 * can't overflow during a single processing cycle.  num_backend_fsync
 * counts the subset of those writes that also had to do their own fsync,
 * because the checkpointer failed to absorb their request.
 *
 * The requests array holds fsync requests sent by backends and not yet
 * absorbed by the checkpointer.
 *
 * Unlike the checkpoint fields, num_backend_writes, num_backend_fsync, and
 * the requests fields are protected by CheckpointerCommLock.
 *----------
 */
typedef struct
{
        RelFileNodeBackend rnode;
        ForkNumber      forknum;
        BlockNumber segno;                      /* see md.c for special values */
        /* might add a real request-type field later; not needed yet */
} CheckpointerRequest;

typedef struct
{
        pid_t           checkpointer_pid;               /* PID (0 if not started) */

        slock_t         ckpt_lck;               /* protects all the ckpt_* fields */

        int                     ckpt_started;   /* advances when checkpoint starts */
        int                     ckpt_done;              /* advances when checkpoint done */
        int                     ckpt_failed;    /* advances when checkpoint fails */

        int                     ckpt_flags;             /* checkpoint flags, as defined in xlog.h */

        uint32          num_backend_writes;             /* counts user backend buffer writes */
        uint32          num_backend_fsync;              /* counts user backend fsync calls */

        int                     num_requests;   /* current # of requests */
        int                     max_requests;   /* allocated array size */
        CheckpointerRequest requests[1];        /* VARIABLE LENGTH ARRAY */
} CheckpointerShmemStruct;

static CheckpointerShmemStruct *CheckpointerShmem;

/* interval for calling AbsorbFsyncRequests in CheckpointWriteDelay */
#define WRITES_PER_ABSORB               1000

/*
 * GUC parameters
 */
int                     CheckPointTimeout = 300;
int                     CheckPointWarning = 30;
double          CheckPointCompletionTarget = 0.5;

/*
 * Flags set by interrupt handlers for later service in the main loop.
 */
static volatile sig_atomic_t got_SIGHUP = false;
static volatile sig_atomic_t checkpoint_requested = false;
static volatile sig_atomic_t shutdown_requested = false;

/*
 * Private state
 */
static bool am_checkpointer = false;

static bool ckpt_active = false;

/* these values are valid when ckpt_active is true: */
static pg_time_t ckpt_start_time;
static XLogRecPtr ckpt_start_recptr;
static double ckpt_cached_elapsed;

static pg_time_t last_checkpoint_time;
static pg_time_t last_xlog_switch_time;

/* Prototypes for private functions */

static void CheckArchiveTimeout(void);
static bool IsCheckpointOnSchedule(double progress);
static bool ImmediateCheckpointRequested(void);
static bool CompactCheckpointerRequestQueue(void);
static void UpdateSharedMemoryConfig(void);

/* Signal handlers */

static void chkpt_quickdie(SIGNAL_ARGS);
static void ChkptSigHupHandler(SIGNAL_ARGS);
static void ReqCheckpointHandler(SIGNAL_ARGS);
static void chkpt_sigusr1_handler(SIGNAL_ARGS);
static void ReqShutdownHandler(SIGNAL_ARGS);


/*
 * Main entry point for checkpointer process
 *
 * This is invoked from BootstrapMain, which has already created the basic
 * execution environment, but not enabled signals yet.
 */
void
CheckpointerMain(void)
{
        sigjmp_buf      local_sigjmp_buf;
        MemoryContext checkpointer_context;

        CheckpointerShmem->checkpointer_pid = MyProcPid;
        am_checkpointer = true;

        /*
         * If possible, make this process a group leader, so that the postmaster
         * can signal any child processes too.  (checkpointer probably never has
         * any child processes, but for consistency we make all postmaster child
         * processes do this.)
         */
#ifdef HAVE_SETSID
        if (setsid() < 0)
                elog(FATAL, "setsid() failed: %m");
#endif

        /*
         * Properly accept or ignore signals the postmaster might send us
         *
         * Note: we deliberately ignore SIGTERM, because during a standard Unix
         * system shutdown cycle, init will SIGTERM all processes at once.      We
         * want to wait for the backends to exit, whereupon the postmaster will
         * tell us it's okay to shut down (via SIGUSR2).
         */
        pqsignal(SIGHUP, ChkptSigHupHandler);           /* set flag to read config
                                                                                                 * file */
        pqsignal(SIGINT, ReqCheckpointHandler);         /* request checkpoint */
        pqsignal(SIGTERM, SIG_IGN); /* ignore SIGTERM */
        pqsignal(SIGQUIT, chkpt_quickdie);      /* hard crash time */
        pqsignal(SIGALRM, SIG_IGN);
        pqsignal(SIGPIPE, SIG_IGN);
        pqsignal(SIGUSR1, chkpt_sigusr1_handler);
        pqsignal(SIGUSR2, ReqShutdownHandler);          /* request shutdown */

        /*
         * Reset some signals that are accepted by postmaster but not here
         */
        pqsignal(SIGCHLD, SIG_DFL);
        pqsignal(SIGTTIN, SIG_DFL);
        pqsignal(SIGTTOU, SIG_DFL);
        pqsignal(SIGCONT, SIG_DFL);
        pqsignal(SIGWINCH, SIG_DFL);

        /* We allow SIGQUIT (quickdie) at all times */
        sigdelset(&BlockSig, SIGQUIT);

        /*
         * Initialize so that first time-driven event happens at the correct time.
         */
        last_checkpoint_time = last_xlog_switch_time = (pg_time_t) time(NULL);

        /*
         * Create a resource owner to keep track of our resources (currently only
         * buffer pins).
         */
        CurrentResourceOwner = ResourceOwnerCreate(NULL, "Checkpointer");

        /*
         * Create a memory context that we will do all our work in.  We do this so
         * that we can reset the context during error recovery and thereby avoid
         * possible memory leaks.  Formerly this code just ran in
         * TopMemoryContext, but resetting that would be a really bad idea.
         */
        checkpointer_context = AllocSetContextCreate(TopMemoryContext,
                                                                                                 "Checkpointer",
                                                                                                 ALLOCSET_DEFAULT_MINSIZE,
                                                                                                 ALLOCSET_DEFAULT_INITSIZE,
                                                                                                 ALLOCSET_DEFAULT_MAXSIZE);
        MemoryContextSwitchTo(checkpointer_context);

        /*
         * If an exception is encountered, processing resumes here.
         *
         * See notes in postgres.c about the design of this coding.
         */
        if (sigsetjmp(local_sigjmp_buf, 1) != 0)
        {
                /* Since not using PG_TRY, must reset error stack by hand */
                error_context_stack = NULL;

                /* Prevent interrupts while cleaning up */
                HOLD_INTERRUPTS();

                /* Report the error to the server log */
                EmitErrorReport();

                /*
                 * These operations are really just a minimal subset of
                 * AbortTransaction().  We don't have very many resources to worry
                 * about in checkpointer, but we do have LWLocks, buffers, and temp
                 * files.
                 */
                LWLockReleaseAll();
                AbortBufferIO();
                UnlockBuffers();
                /* buffer pins are released here: */
                ResourceOwnerRelease(CurrentResourceOwner,
                                                         RESOURCE_RELEASE_BEFORE_LOCKS,
                                                         false, true);
                /* we needn't bother with the other ResourceOwnerRelease phases */
                AtEOXact_Buffers(false);
                AtEOXact_Files();
                AtEOXact_HashTables(false);

                /* Warn any waiting backends that the checkpoint failed. */
                if (ckpt_active)
                {
                        /* use volatile pointer to prevent code rearrangement */
                        volatile CheckpointerShmemStruct *cps = CheckpointerShmem;

                        SpinLockAcquire(&cps->ckpt_lck);
                        cps->ckpt_failed++;
                        cps->ckpt_done = cps->ckpt_started;
                        SpinLockRelease(&cps->ckpt_lck);

                        ckpt_active = false;
                }

                /*
                 * Now return to normal top-level context and clear ErrorContext for
                 * next time.
                 */
                MemoryContextSwitchTo(checkpointer_context);
                FlushErrorState();

                /* Flush any leaked data in the top-level context */
                MemoryContextResetAndDeleteChildren(checkpointer_context);

                /* Now we can allow interrupts again */
                RESUME_INTERRUPTS();

                /*
                 * Sleep at least 1 second after any error.  A write error is likely
                 * to be repeated, and we don't want to be filling the error logs as
                 * fast as we can.
                 */
                pg_usleep(1000000L);

                /*
                 * Close all open files after any error.  This is helpful on Windows,
                 * where holding deleted files open causes various strange errors.
                 * It's not clear we need it elsewhere, but shouldn't hurt.
                 */
                smgrcloseall();
        }

        /* We can now handle ereport(ERROR) */
        PG_exception_stack = &local_sigjmp_buf;

        /*
         * Unblock signals (they were blocked when the postmaster forked us)
         */
        PG_SETMASK(&UnBlockSig);

        /*
         * Use the recovery target timeline ID during recovery
         */
        if (RecoveryInProgress())
                ThisTimeLineID = GetRecoveryTargetTLI();

        /*
         * Ensure all shared memory values are set correctly for the config. Doing
         * this here ensures no race conditions from other concurrent updaters.
         */
        UpdateSharedMemoryConfig();

        /*
         * Advertise our latch that backends can use to wake us up while we're
         * sleeping.
         */
        ProcGlobal->checkpointerLatch = &MyProc->procLatch;

        /*
         * Loop forever
         */
        for (;;)
        {
                bool            do_checkpoint = false;
                int                     flags = 0;
                pg_time_t       now;
                int                     elapsed_secs;
                int                     cur_timeout;
                int                     rc;

                /* Clear any already-pending wakeups */
                ResetLatch(&MyProc->procLatch);

                /*
                 * Process any requests or signals received recently.
                 */
                AbsorbFsyncRequests();

                if (got_SIGHUP)
                {
                        got_SIGHUP = false;
                        ProcessConfigFile(PGC_SIGHUP);

                        /*
                         * Checkpointer is the last process to shut down, so we ask it to
                         * hold the keys for a range of other tasks required most of which
                         * have nothing to do with checkpointing at all.
                         *
                         * For various reasons, some config values can change dynamically
                         * so the primary copy of them is held in shared memory to make
                         * sure all backends see the same value.  We make Checkpointer
                         * responsible for updating the shared memory copy if the
                         * parameter setting changes because of SIGHUP.
                         */
                        UpdateSharedMemoryConfig();
                }
                if (checkpoint_requested)
                {
                        checkpoint_requested = false;
                        do_checkpoint = true;
                        BgWriterStats.m_requested_checkpoints++;
                }
                if (shutdown_requested)
                {
                        /*
                         * From here on, elog(ERROR) should end with exit(1), not send
                         * control back to the sigsetjmp block above
                         */
                        ExitOnAnyError = true;
                        /* Close down the database */
                        ShutdownXLOG(0, 0);
                        /* Normal exit from the checkpointer is here */
                        proc_exit(0);           /* done */
                }

                /*
                 * Force a checkpoint if too much time has elapsed since the last one.
                 * Note that we count a timed checkpoint in stats only when this
                 * occurs without an external request, but we set the CAUSE_TIME flag
                 * bit even if there is also an external request.
                 */
                now = (pg_time_t) time(NULL);
                elapsed_secs = now - last_checkpoint_time;
                if (elapsed_secs >= CheckPointTimeout)
                {
                        if (!do_checkpoint)
                                BgWriterStats.m_timed_checkpoints++;
                        do_checkpoint = true;
                        flags |= CHECKPOINT_CAUSE_TIME;
                }

                /*
                 * Do a checkpoint if requested.
                 */
                if (do_checkpoint)
                {
                        bool            ckpt_performed = false;
                        bool            do_restartpoint;

                        /* use volatile pointer to prevent code rearrangement */
                        volatile CheckpointerShmemStruct *cps = CheckpointerShmem;

                        /*
                         * Check if we should perform a checkpoint or a restartpoint. As a
                         * side-effect, RecoveryInProgress() initializes TimeLineID if
                         * it's not set yet.
                         */
                        do_restartpoint = RecoveryInProgress();

                        /*
                         * Atomically fetch the request flags to figure out what kind of a
                         * checkpoint we should perform, and increase the started-counter
                         * to acknowledge that we've started a new checkpoint.
                         */
                        SpinLockAcquire(&cps->ckpt_lck);
                        flags |= cps->ckpt_flags;
                        cps->ckpt_flags = 0;
                        cps->ckpt_started++;
                        SpinLockRelease(&cps->ckpt_lck);

                        /*
                         * The end-of-recovery checkpoint is a real checkpoint that's
                         * performed while we're still in recovery.
                         */
                        if (flags & CHECKPOINT_END_OF_RECOVERY)
                                do_restartpoint = false;

                        /*
                         * We will warn if (a) too soon since last checkpoint (whatever
                         * caused it) and (b) somebody set the CHECKPOINT_CAUSE_XLOG flag
                         * since the last checkpoint start.  Note in particular that this
                         * implementation will not generate warnings caused by
                         * CheckPointTimeout < CheckPointWarning.
                         */
                        if (!do_restartpoint &&
                                (flags & CHECKPOINT_CAUSE_XLOG) &&
                                elapsed_secs < CheckPointWarning)
                                ereport(LOG,
                                                (errmsg_plural("checkpoints are occurring too frequently (%d second apart)",
                                "checkpoints are occurring too frequently (%d seconds apart)",
                                                                           elapsed_secs,
                                                                           elapsed_secs),
                                                 errhint("Consider increasing the configuration parameter \"checkpoint_segments\".")));

                        /*
                         * Initialize checkpointer-private variables used during
                         * checkpoint
                         */
                        ckpt_active = true;
                        if (!do_restartpoint)
                                ckpt_start_recptr = GetInsertRecPtr();
                        ckpt_start_time = now;
                        ckpt_cached_elapsed = 0;

                        /*
                         * Do the checkpoint.
                         */
                        if (!do_restartpoint)
                        {
                                CreateCheckPoint(flags);
                                ckpt_performed = true;
                        }
                        else
                                ckpt_performed = CreateRestartPoint(flags);

                        /*
                         * After any checkpoint, close all smgr files.  This is so we
                         * won't hang onto smgr references to deleted files indefinitely.
                         */
                        smgrcloseall();

                        /*
                         * Indicate checkpoint completion to any waiting backends.
                         */
                        SpinLockAcquire(&cps->ckpt_lck);
                        cps->ckpt_done = cps->ckpt_started;
                        SpinLockRelease(&cps->ckpt_lck);

                        if (ckpt_performed)
                        {
                                /*
                                 * Note we record the checkpoint start time not end time as
                                 * last_checkpoint_time.  This is so that time-driven
                                 * checkpoints happen at a predictable spacing.
                                 */
                                last_checkpoint_time = now;
                        }
                        else
                        {
                                /*
                                 * We were not able to perform the restartpoint (checkpoints
                                 * throw an ERROR in case of error).  Most likely because we
                                 * have not received any new checkpoint WAL records since the
                                 * last restartpoint. Try again in 15 s.
                                 */
                                last_checkpoint_time = now - CheckPointTimeout + 15;
                        }

                        ckpt_active = false;
                }

                /* Check for archive_timeout and switch xlog files if necessary. */
                CheckArchiveTimeout();

                /*
                 * Send off activity statistics to the stats collector.  (The reason
                 * why we re-use bgwriter-related code for this is that the bgwriter
                 * and checkpointer used to be just one process.  It's probably not
                 * worth the trouble to split the stats support into two independent
                 * stats message types.)
                 */
                pgstat_send_bgwriter();

                /*
                 * Sleep until we are signaled or it's time for another checkpoint or
                 * xlog file switch.
                 */
                now = (pg_time_t) time(NULL);
                elapsed_secs = now - last_checkpoint_time;
                if (elapsed_secs >= CheckPointTimeout)
                        continue;                       /* no sleep for us ... */
                cur_timeout = CheckPointTimeout - elapsed_secs;
                if (XLogArchiveTimeout > 0 && !RecoveryInProgress())
                {
                        elapsed_secs = now - last_xlog_switch_time;
                        if (elapsed_secs >= XLogArchiveTimeout)
                                continue;               /* no sleep for us ... */
                        cur_timeout = Min(cur_timeout, XLogArchiveTimeout - elapsed_secs);
                }

                rc = WaitLatch(&MyProc->procLatch,
                                           WL_LATCH_SET | WL_TIMEOUT | WL_POSTMASTER_DEATH,
                                           cur_timeout * 1000L /* convert to ms */ );

                /*
                 * Emergency bailout if postmaster has died.  This is to avoid the
                 * necessity for manual cleanup of all postmaster children.
                 */
                if (rc & WL_POSTMASTER_DEATH)
                        exit(1);
        }
}

/*
 * CheckArchiveTimeout -- check for archive_timeout and switch xlog files
 *
 * This will switch to a new WAL file and force an archive file write
 * if any activity is recorded in the current WAL file, including just
 * a single checkpoint record.
 */
static void
CheckArchiveTimeout(void)
{
        pg_time_t       now;
        pg_time_t       last_time;

        if (XLogArchiveTimeout <= 0 || RecoveryInProgress())
                return;

        now = (pg_time_t) time(NULL);

        /* First we do a quick check using possibly-stale local state. */
        if ((int) (now - last_xlog_switch_time) < XLogArchiveTimeout)
                return;

        /*
         * Update local state ... note that last_xlog_switch_time is the last time
         * a switch was performed *or requested*.
         */
        last_time = GetLastSegSwitchTime();

        last_xlog_switch_time = Max(last_xlog_switch_time, last_time);

        /* Now we can do the real check */
        if ((int) (now - last_xlog_switch_time) >= XLogArchiveTimeout)
        {
                XLogRecPtr      switchpoint;

                /* OK, it's time to switch */
                switchpoint = RequestXLogSwitch();

                /*
                 * If the returned pointer points exactly to a segment boundary,
                 * assume nothing happened.
                 */
                if ((switchpoint % XLogSegSize) != 0)
                        ereport(DEBUG1,
                                (errmsg("transaction log switch forced (archive_timeout=%d)",
                                                XLogArchiveTimeout)));

                /*
                 * Update state in any case, so we don't retry constantly when the
                 * system is idle.
                 */
                last_xlog_switch_time = now;
        }
}

/*
 * Returns true if an immediate checkpoint request is pending.  (Note that
 * this does not check the *current* checkpoint's IMMEDIATE flag, but whether
 * there is one pending behind it.)
 */
static bool
ImmediateCheckpointRequested(void)
{
        if (checkpoint_requested)
        {
                volatile CheckpointerShmemStruct *cps = CheckpointerShmem;

                /*
                 * We don't need to acquire the ckpt_lck in this case because we're
                 * only looking at a single flag bit.
                 */
                if (cps->ckpt_flags & CHECKPOINT_IMMEDIATE)
                        return true;
        }
        return false;
}

/*
 * CheckpointWriteDelay -- control rate of checkpoint
 *
 * This function is called after each page write performed by BufferSync().
 * It is responsible for throttling BufferSync()'s write rate to hit
 * checkpoint_completion_target.
 *
 * The checkpoint request flags should be passed in; currently the only one
 * examined is CHECKPOINT_IMMEDIATE, which disables delays between writes.
 *
 * 'progress' is an estimate of how much of the work has been done, as a
 * fraction between 0.0 meaning none, and 1.0 meaning all done.
 */
void
CheckpointWriteDelay(int flags, double progress)
{
        static int      absorb_counter = WRITES_PER_ABSORB;

        /* Do nothing if checkpoint is being executed by non-checkpointer process */
        if (!am_checkpointer)
                return;

        /*
         * Perform the usual duties and take a nap, unless we're behind schedule,
         * in which case we just try to catch up as quickly as possible.
         */
        if (!(flags & CHECKPOINT_IMMEDIATE) &&
                !shutdown_requested &&
                !ImmediateCheckpointRequested() &&
                IsCheckpointOnSchedule(progress))
        {
                if (got_SIGHUP)
                {
                        got_SIGHUP = false;
                        ProcessConfigFile(PGC_SIGHUP);
                        /* update shmem copies of config variables */
                        UpdateSharedMemoryConfig();
                }

                AbsorbFsyncRequests();
                absorb_counter = WRITES_PER_ABSORB;

                CheckArchiveTimeout();

                /*
                 * Report interim activity statistics to the stats collector.
                 */
                pgstat_send_bgwriter();

                /*
                 * This sleep used to be connected to bgwriter_delay, typically 200ms.
                 * That resulted in more frequent wakeups if not much work to do.
                 * Checkpointer and bgwriter are no longer related so take the Big
                 * Sleep.
                 */
                pg_usleep(100000L);
        }
        else if (--absorb_counter <= 0)
        {
                /*
                 * Absorb pending fsync requests after each WRITES_PER_ABSORB write
                 * operations even when we don't sleep, to prevent overflow of the
                 * fsync request queue.
                 */
                AbsorbFsyncRequests();
                absorb_counter = WRITES_PER_ABSORB;
        }
}

/*
 * IsCheckpointOnSchedule -- are we on schedule to finish this checkpoint
 *               in time?
 *
 * Compares the current progress against the time/segments elapsed since last
 * checkpoint, and returns true if the progress we've made this far is greater
 * than the elapsed time/segments.
 */
static bool
IsCheckpointOnSchedule(double progress)
{
        XLogRecPtr      recptr;
        struct timeval now;
        double          elapsed_xlogs,
                                elapsed_time;

        Assert(ckpt_active);

        /* Scale progress according to checkpoint_completion_target. */
        progress *= CheckPointCompletionTarget;

        /*
         * Check against the cached value first. Only do the more expensive
         * calculations once we reach the target previously calculated. Since
         * neither time or WAL insert pointer moves backwards, a freshly
         * calculated value can only be greater than or equal to the cached value.
         */
        if (progress < ckpt_cached_elapsed)
                return false;

        /*
         * Check progress against WAL segments written and checkpoint_segments.
         *
         * We compare the current WAL insert location against the location
         * computed before calling CreateCheckPoint. The code in XLogInsert that
         * actually triggers a checkpoint when checkpoint_segments is exceeded
         * compares against RedoRecptr, so this is not completely accurate.
         * However, it's good enough for our purposes, we're only calculating an
         * estimate anyway.
         */
        if (!RecoveryInProgress())
        {
                recptr = GetInsertRecPtr();
                elapsed_xlogs = (((double) (recptr - ckpt_start_recptr)) / XLogSegSize) / CheckPointSegments;

                if (progress < elapsed_xlogs)
                {
                        ckpt_cached_elapsed = elapsed_xlogs;
                        return false;
                }
        }

        /*
         * Check progress against time elapsed and checkpoint_timeout.
         */
        gettimeofday(&now, NULL);
        elapsed_time = ((double) ((pg_time_t) now.tv_sec - ckpt_start_time) +
                                        now.tv_usec / 1000000.0) / CheckPointTimeout;

        if (progress < elapsed_time)
        {
                ckpt_cached_elapsed = elapsed_time;
                return false;
        }

        /* It looks like we're on schedule. */
        return true;
}


/* --------------------------------
 *              signal handler routines
 * --------------------------------
 */

/*
 * chkpt_quickdie() occurs when signalled SIGQUIT by the postmaster.
 *
 * Some backend has bought the farm,
 * so we need to stop what we're doing and exit.
 */
static void
chkpt_quickdie(SIGNAL_ARGS)
{
        PG_SETMASK(&BlockSig);

        /*
         * We DO NOT want to run proc_exit() callbacks -- we're here because
         * shared memory may be corrupted, so we don't want to try to clean up our
         * transaction.  Just nail the windows shut and get out of town.  Now that
         * there's an atexit callback to prevent third-party code from breaking
         * things by calling exit() directly, we have to reset the callbacks
         * explicitly to make this work as intended.
         */
        on_exit_reset();

        /*
         * Note we do exit(2) not exit(0).      This is to force the postmaster into a
         * system reset cycle if some idiot DBA sends a manual SIGQUIT to a random
         * backend.  This is necessary precisely because we don't clean up our
         * shared memory state.  (The "dead man switch" mechanism in pmsignal.c
         * should ensure the postmaster sees this as a crash, too, but no harm in
         * being doubly sure.)
         */
        exit(2);
}

/* SIGHUP: set flag to re-read config file at next convenient time */
static void
ChkptSigHupHandler(SIGNAL_ARGS)
{
        int                     save_errno = errno;

        got_SIGHUP = true;
        if (MyProc)
                SetLatch(&MyProc->procLatch);

        errno = save_errno;
}

/* SIGINT: set flag to run a normal checkpoint right away */
static void
ReqCheckpointHandler(SIGNAL_ARGS)
{
        int                     save_errno = errno;

        checkpoint_requested = true;
        if (MyProc)
                SetLatch(&MyProc->procLatch);

        errno = save_errno;
}

/* SIGUSR1: used for latch wakeups */
static void
chkpt_sigusr1_handler(SIGNAL_ARGS)
{
        int                     save_errno = errno;

        latch_sigusr1_handler();

        errno = save_errno;
}

/* SIGUSR2: set flag to run a shutdown checkpoint and exit */
static void
ReqShutdownHandler(SIGNAL_ARGS)
{
        int                     save_errno = errno;

        shutdown_requested = true;
        if (MyProc)
                SetLatch(&MyProc->procLatch);

        errno = save_errno;
}


/* --------------------------------
 *              communication with backends
 * --------------------------------
 */

/*
 * CheckpointerShmemSize
 *              Compute space needed for checkpointer-related shared memory
 */
Size
CheckpointerShmemSize(void)
{
        Size            size;

        /*
         * Currently, the size of the requests[] array is arbitrarily set equal to
         * NBuffers.  This may prove too large or small ...
         */
        size = offsetof(CheckpointerShmemStruct, requests);
        size = add_size(size, mul_size(NBuffers, sizeof(CheckpointerRequest)));

        return size;
}

/*
 * CheckpointerShmemInit
 *              Allocate and initialize checkpointer-related shared memory
 */
void
CheckpointerShmemInit(void)
{
        bool            found;

        CheckpointerShmem = (CheckpointerShmemStruct *)
                ShmemInitStruct("Checkpointer Data",
                                                CheckpointerShmemSize(),
                                                &found);

        if (!found)
        {
                /* First time through, so initialize */
                MemSet(CheckpointerShmem, 0, sizeof(CheckpointerShmemStruct));
                SpinLockInit(&CheckpointerShmem->ckpt_lck);
                CheckpointerShmem->max_requests = NBuffers;
        }
}

/*
 * RequestCheckpoint
 *              Called in backend processes to request a checkpoint
 *
 * 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).
 *      CHECKPOINT_WAIT: wait for completion before returning (otherwise,
 *              just signal checkpointer to do it, and return).
 *      CHECKPOINT_CAUSE_XLOG: checkpoint is requested due to xlog filling.
 *              (This affects logging, and in particular enables CheckPointWarning.)
 */
void
RequestCheckpoint(int flags)
{
        /* use volatile pointer to prevent code rearrangement */
        volatile CheckpointerShmemStruct *cps = CheckpointerShmem;
        int                     ntries;
        int                     old_failed,
                                old_started;

        /*
         * If in a standalone backend, just do it ourselves.
         */
        if (!IsPostmasterEnvironment)
        {
                /*
                 * There's no point in doing slow checkpoints in a standalone backend,
                 * because there's no other backends the checkpoint could disrupt.
                 */
                CreateCheckPoint(flags | CHECKPOINT_IMMEDIATE);

                /*
                 * After any checkpoint, close all smgr files.  This is so we won't
                 * hang onto smgr references to deleted files indefinitely.
                 */
                smgrcloseall();

                return;
        }

        /*
         * Atomically set the request flags, and take a snapshot of the counters.
         * When we see ckpt_started > old_started, we know the flags we set here
         * have been seen by checkpointer.
         *
         * Note that we OR the flags with any existing flags, to avoid overriding
         * a "stronger" request by another backend.  The flag senses must be
         * chosen to make this work!
         */
        SpinLockAcquire(&cps->ckpt_lck);

        old_failed = cps->ckpt_failed;
        old_started = cps->ckpt_started;
        cps->ckpt_flags |= flags;

        SpinLockRelease(&cps->ckpt_lck);

        /*
         * Send signal to request checkpoint.  It's possible that the checkpointer
         * hasn't started yet, or is in process of restarting, so we will retry a
         * few times if needed.  Also, if not told to wait for the checkpoint to
         * occur, we consider failure to send the signal to be nonfatal and merely
         * LOG it.
         */
        for (ntries = 0;; ntries++)
        {
                if (CheckpointerShmem->checkpointer_pid == 0)
                {
                        if (ntries >= 20)       /* max wait 2.0 sec */
                        {
                                elog((flags & CHECKPOINT_WAIT) ? ERROR : LOG,
                                         "could not request checkpoint because checkpointer not running");
                                break;
                        }
                }
                else if (kill(CheckpointerShmem->checkpointer_pid, SIGINT) != 0)
                {
                        if (ntries >= 20)       /* max wait 2.0 sec */
                        {
                                elog((flags & CHECKPOINT_WAIT) ? ERROR : LOG,
                                         "could not signal for checkpoint: %m");
                                break;
                        }
                }
                else
                        break;                          /* signal sent successfully */

                CHECK_FOR_INTERRUPTS();
                pg_usleep(100000L);             /* wait 0.1 sec, then retry */
        }

        /*
         * If requested, wait for completion.  We detect completion according to
         * the algorithm given above.
         */
        if (flags & CHECKPOINT_WAIT)
        {
                int                     new_started,
                                        new_failed;

                /* Wait for a new checkpoint to start. */
                for (;;)
                {
                        SpinLockAcquire(&cps->ckpt_lck);
                        new_started = cps->ckpt_started;
                        SpinLockRelease(&cps->ckpt_lck);

                        if (new_started != old_started)
                                break;

                        CHECK_FOR_INTERRUPTS();
                        pg_usleep(100000L);
                }

                /*
                 * We are waiting for ckpt_done >= new_started, in a modulo sense.
                 */
                for (;;)
                {
                        int                     new_done;

                        SpinLockAcquire(&cps->ckpt_lck);
                        new_done = cps->ckpt_done;
                        new_failed = cps->ckpt_failed;
                        SpinLockRelease(&cps->ckpt_lck);

                        if (new_done - new_started >= 0)
                                break;

                        CHECK_FOR_INTERRUPTS();
                        pg_usleep(100000L);
                }

                if (new_failed != old_failed)
                        ereport(ERROR,
                                        (errmsg("checkpoint request failed"),
                                         errhint("Consult recent messages in the server log for details.")));
        }
}

/*
 * ForwardFsyncRequest
 *              Forward a file-fsync request from a backend to the checkpointer
 *
 * Whenever a backend is compelled to write directly to a relation
 * (which should be seldom, if the checkpointer is getting its job done),
 * the backend calls this routine to pass over knowledge that the relation
 * is dirty and must be fsync'd before next checkpoint.  We also use this
 * opportunity to count such writes for statistical purposes.
 *
 * segno specifies which segment (not block!) of the relation needs to be
 * fsync'd.  (Since the valid range is much less than BlockNumber, we can
 * use high values for special flags; that's all internal to md.c, which
 * see for details.)
 *
 * To avoid holding the lock for longer than necessary, we normally write
 * to the requests[] queue without checking for duplicates.  The checkpointer
 * will have to eliminate dups internally anyway.  However, if we discover
 * that the queue is full, we make a pass over the entire queue to compact
 * it.  This is somewhat expensive, but the alternative is for the backend
 * to perform its own fsync, which is far more expensive in practice.  It
 * is theoretically possible a backend fsync might still be necessary, if
 * the queue is full and contains no duplicate entries.  In that case, we
 * let the backend know by returning false.
 */
bool
ForwardFsyncRequest(RelFileNodeBackend rnode, ForkNumber forknum,
                                        BlockNumber segno)
{
        CheckpointerRequest *request;
        bool            too_full;

        if (!IsUnderPostmaster)
                return false;                   /* probably shouldn't even get here */

        if (am_checkpointer)
                elog(ERROR, "ForwardFsyncRequest must not be called in checkpointer");

        LWLockAcquire(CheckpointerCommLock, LW_EXCLUSIVE);

        /* Count all backend writes regardless of if they fit in the queue */
        CheckpointerShmem->num_backend_writes++;

        /*
         * If the checkpointer isn't running or the request queue is full, the
         * backend will have to perform its own fsync request.  But before forcing
         * that to happen, we can try to compact the request queue.
         */
        if (CheckpointerShmem->checkpointer_pid == 0 ||
                (CheckpointerShmem->num_requests >= CheckpointerShmem->max_requests &&
                 !CompactCheckpointerRequestQueue()))
        {
                /*
                 * Count the subset of writes where backends have to do their own
                 * fsync
                 */
                CheckpointerShmem->num_backend_fsync++;
                LWLockRelease(CheckpointerCommLock);
                return false;
        }

        /* OK, insert request */
        request = &CheckpointerShmem->requests[CheckpointerShmem->num_requests++];
        request->rnode = rnode;
        request->forknum = forknum;
        request->segno = segno;

        /* If queue is more than half full, nudge the checkpointer to empty it */
        too_full = (CheckpointerShmem->num_requests >=
                                CheckpointerShmem->max_requests / 2);

        LWLockRelease(CheckpointerCommLock);

        /* ... but not till after we release the lock */
        if (too_full && ProcGlobal->checkpointerLatch)
                SetLatch(ProcGlobal->checkpointerLatch);

        return true;
}

/*
 * CompactCheckpointerRequestQueue
 *              Remove duplicates from the request queue to avoid backend fsyncs.
 *
 * Although a full fsync request queue is not common, it can lead to severe
 * performance problems when it does happen.  So far, this situation has
 * only been observed to occur when the system is under heavy write load,
 * and especially during the "sync" phase of a checkpoint.      Without this
 * logic, each backend begins doing an fsync for every block written, which
 * gets very expensive and can slow down the whole system.
 *
 * Trying to do this every time the queue is full could lose if there
 * aren't any removable entries.  But should be vanishingly rare in
 * practice: there's one queue entry per shared buffer.
 */
static bool
CompactCheckpointerRequestQueue(void)
{
        struct CheckpointerSlotMapping
        {
                CheckpointerRequest request;
                int                     slot;
        };

        int                     n,
                                preserve_count;
        int                     num_skipped = 0;
        HASHCTL         ctl;
        HTAB       *htab;
        bool       *skip_slot;

        /* must hold CheckpointerCommLock in exclusive mode */
        Assert(LWLockHeldByMe(CheckpointerCommLock));

        /* Initialize temporary hash table */
        MemSet(&ctl, 0, sizeof(ctl));
        ctl.keysize = sizeof(CheckpointerRequest);
        ctl.entrysize = sizeof(struct CheckpointerSlotMapping);
        ctl.hash = tag_hash;
        htab = hash_create("CompactCheckpointerRequestQueue",
                                           CheckpointerShmem->num_requests,
                                           &ctl,
                                           HASH_ELEM | HASH_FUNCTION);

        /* Initialize skip_slot array */
        skip_slot = palloc0(sizeof(bool) * CheckpointerShmem->num_requests);

        /*
         * The basic idea here is that a request can be skipped if it's followed
         * by a later, identical request.  It might seem more sensible to work
         * backwards from the end of the queue and check whether a request is
         * *preceded* by an earlier, identical request, in the hopes of doing less
         * copying.  But that might change the semantics, if there's an
         * intervening FORGET_RELATION_FSYNC or FORGET_DATABASE_FSYNC request, so
         * we do it this way.  It would be possible to be even smarter if we made
         * the code below understand the specific semantics of such requests (it
         * could blow away preceding entries that would end up being canceled
         * anyhow), but it's not clear that the extra complexity would buy us
         * anything.
         */
        for (n = 0; n < CheckpointerShmem->num_requests; ++n)
        {
                CheckpointerRequest *request;
                struct CheckpointerSlotMapping *slotmap;
                bool            found;

                request = &CheckpointerShmem->requests[n];
                slotmap = hash_search(htab, request, HASH_ENTER, &found);
                if (found)
                {
                        skip_slot[slotmap->slot] = true;
                        ++num_skipped;
                }
                slotmap->slot = n;
        }

        /* Done with the hash table. */
        hash_destroy(htab);

        /* If no duplicates, we're out of luck. */
        if (!num_skipped)
        {
                pfree(skip_slot);
                return false;
        }

        /* We found some duplicates; remove them. */
        for (n = 0, preserve_count = 0; n < CheckpointerShmem->num_requests; ++n)
        {
                if (skip_slot[n])
                        continue;
                CheckpointerShmem->requests[preserve_count++] = CheckpointerShmem->requests[n];
        }
        ereport(DEBUG1,
           (errmsg("compacted fsync request queue from %d entries to %d entries",
                           CheckpointerShmem->num_requests, preserve_count)));
        CheckpointerShmem->num_requests = preserve_count;

        /* Cleanup. */
        pfree(skip_slot);
        return true;
}

/*
 * AbsorbFsyncRequests
 *              Retrieve queued fsync requests and pass them to local smgr.
 *
 * This is exported because it must be called during CreateCheckPoint;
 * we have to be sure we have accepted all pending requests just before
 * we start fsync'ing.  Since CreateCheckPoint sometimes runs in
 * non-checkpointer processes, do nothing if not checkpointer.
 */
void
AbsorbFsyncRequests(void)
{
        CheckpointerRequest *requests = NULL;
        CheckpointerRequest *request;
        int                     n;

        if (!am_checkpointer)
                return;

        /*
         * We have to PANIC if we fail to absorb all the pending requests (eg,
         * because our hashtable runs out of memory).  This is because the system
         * cannot run safely if we are unable to fsync what we have been told to
         * fsync.  Fortunately, the hashtable is so small that the problem is
         * quite unlikely to arise in practice.
         */
        START_CRIT_SECTION();

        /*
         * We try to avoid holding the lock for a long time by copying the request
         * array.
         */
        LWLockAcquire(CheckpointerCommLock, LW_EXCLUSIVE);

        /* Transfer stats counts into pending pgstats message */
        BgWriterStats.m_buf_written_backend += CheckpointerShmem->num_backend_writes;
        BgWriterStats.m_buf_fsync_backend += CheckpointerShmem->num_backend_fsync;

        CheckpointerShmem->num_backend_writes = 0;
        CheckpointerShmem->num_backend_fsync = 0;

        n = CheckpointerShmem->num_requests;
        if (n > 0)
        {
                requests = (CheckpointerRequest *) palloc(n * sizeof(CheckpointerRequest));
                memcpy(requests, CheckpointerShmem->requests, n * sizeof(CheckpointerRequest));
        }
        CheckpointerShmem->num_requests = 0;

        LWLockRelease(CheckpointerCommLock);

        for (request = requests; n > 0; request++, n--)
                RememberFsyncRequest(request->rnode, request->forknum, request->segno);

        if (requests)
                pfree(requests);

        END_CRIT_SECTION();
}

/*
 * Update any shared memory configurations based on config parameters
 */
static void
UpdateSharedMemoryConfig(void)
{
        /* update global shmem state for sync rep */
        SyncRepUpdateSyncStandbysDefined();

        /*
         * If full_page_writes has been changed by SIGHUP, we update it in shared
         * memory and write an XLOG_FPW_CHANGE record.
         */
        UpdateFullPageWrites();

        elog(DEBUG2, "checkpointer updated shared memory configuration values");
}

/*
 * FirstCallSinceLastCheckpoint allows a process to take an action once
 * per checkpoint cycle by asynchronously checking for checkpoint completion.
 */
bool
FirstCallSinceLastCheckpoint(void)
{
        /* use volatile pointer to prevent code rearrangement */
        volatile CheckpointerShmemStruct *cps = CheckpointerShmem;
        static int      ckpt_done = 0;
        int                     new_done;
        bool            FirstCall = false;

        SpinLockAcquire(&cps->ckpt_lck);
        new_done = cps->ckpt_done;
        SpinLockRelease(&cps->ckpt_lck);

        if (new_done != ckpt_done)
                FirstCall = true;

        ckpt_done = new_done;

        return FirstCall;
}