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

/*-------------------------------------------------------------------------
 *
 * bgwriter.c
 *
 * The background writer (bgwriter) is new as of Postgres 8.0.  It attempts
 * to keep regular backends from having to write out dirty shared buffers
 * (which they would only do when needing to free a shared buffer to read in
 * another page).  In the best scenario all writes from shared buffers will
 * be issued by the background writer process.  However, regular backends are
 * still empowered to issue writes if the bgwriter fails to maintain enough
 * clean shared buffers.
 *
 * The bgwriter is also charged with handling all checkpoints.  It will
 * automatically dispatch a checkpoint 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 the bgwriter
 * when they fill WAL segments; the bgwriter itself doesn't watch for the
 * condition.)
 *
 * The bgwriter is started by the postmaster as soon as the startup subprocess
 * finishes.  It remains alive until the postmaster commands it to terminate.
 * Normal termination is by SIGUSR2, which instructs the bgwriter 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 bgwriter will simply abort and exit on SIGQUIT.
 *
 * If the bgwriter 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) 1996-2007, PostgreSQL Global Development Group
 *
 *
 * IDENTIFICATION
 *        $PostgreSQL: pgsql/src/backend/postmaster/bgwriter.c,v 1.41 2007/07/03 14:51:24 tgl Exp $
 *
 *-------------------------------------------------------------------------
 */
#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 "storage/fd.h"
#include "storage/freespace.h"
#include "storage/ipc.h"
#include "storage/lwlock.h"
#include "storage/pmsignal.h"
#include "storage/shmem.h"
#include "storage/smgr.h"
#include "storage/spin.h"
#include "tcop/tcopprot.h"
#include "utils/guc.h"
#include "utils/memutils.h"
#include "utils/resowner.h"


/*----------
 * Shared memory area for communication between bgwriter 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, bgwriter reads (and clears) the request flags
 *        and increments ckpt_started, while holding ckpt_lck.
 *      * On completion of a checkpoint, bgwriter sets ckpt_done to
 *        equal ckpt_started.
 *      * On failure of a checkpoint, bgwriter 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.
 *
 * The requests array holds fsync requests sent by backends and not yet
 * absorbed by the bgwriter.  Unlike the checkpoint fields, the requests
 * fields are protected by BgWriterCommLock.
 *----------
 */
typedef struct
{
        RelFileNode rnode;
        BlockNumber segno;                      /* see md.c for special values */
        /* might add a real request-type field later; not needed yet */
} BgWriterRequest;

typedef struct
{
        pid_t           bgwriter_pid;   /* PID of bgwriter (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 */

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

static BgWriterShmemStruct *BgWriterShmem;

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

/*
 * GUC parameters
 */
int                     BgWriterDelay = 200;
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_bg_writer = false;

static bool ckpt_active = false;

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

static time_t last_checkpoint_time;
static time_t last_xlog_switch_time;

/* Prototypes for private functions */

static void CheckArchiveTimeout(void);
static void BgWriterNap(void);
static bool IsCheckpointOnSchedule(double progress);
static bool ImmediateCheckpointRequested(void);

/* Signal handlers */

static void bg_quickdie(SIGNAL_ARGS);
static void BgSigHupHandler(SIGNAL_ARGS);
static void ReqCheckpointHandler(SIGNAL_ARGS);
static void ReqShutdownHandler(SIGNAL_ARGS);


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

        Assert(BgWriterShmem != NULL);
        BgWriterShmem->bgwriter_pid = MyProcPid;
        am_bg_writer = true;

        /*
         * If possible, make this process a group leader, so that the postmaster
         * can signal any child processes too.  (bgwriter 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).
         *
         * SIGUSR1 is presently unused; keep it spare in case someday we want this
         * process to participate in sinval messaging.
         */
        pqsignal(SIGHUP, BgSigHupHandler);      /* set flag to read config file */
        pqsignal(SIGINT, ReqCheckpointHandler);         /* request checkpoint */
        pqsignal(SIGTERM, SIG_IGN); /* ignore SIGTERM */
        pqsignal(SIGQUIT, bg_quickdie);         /* hard crash time */
        pqsignal(SIGALRM, SIG_IGN);
        pqsignal(SIGPIPE, SIG_IGN);
        pqsignal(SIGUSR1, SIG_IGN); /* reserve for sinval */
        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 */
#ifdef HAVE_SIGPROCMASK
        sigdelset(&BlockSig, SIGQUIT);
#else
        BlockSig &= ~(sigmask(SIGQUIT));
#endif

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

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

        /*
         * 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.
         */
        bgwriter_context = AllocSetContextCreate(TopMemoryContext,
                                                                                         "Background Writer",
                                                                                         ALLOCSET_DEFAULT_MINSIZE,
                                                                                         ALLOCSET_DEFAULT_INITSIZE,
                                                                                         ALLOCSET_DEFAULT_MAXSIZE);
        MemoryContextSwitchTo(bgwriter_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 bgwriter, 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();

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

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

                        ckpt_active = false;
                }

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

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

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

                /*
                 * Emergency bailout if postmaster has died.  This is to avoid the
                 * necessity for manual cleanup of all postmaster children.
                 */
                if (!PostmasterIsAlive(true))
                        exit(1);

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

                if (got_SIGHUP)
                {
                        got_SIGHUP = false;
                        ProcessConfigFile(PGC_SIGHUP);
                }
                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);
                        DumpFreeSpaceMap(0, 0);
                        /* Normal exit from the bgwriter 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 = 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, otherwise do one cycle of
                 * dirty-buffer writing.
                 */
                if (do_checkpoint)
                {
                        /* use volatile pointer to prevent code rearrangement */
                        volatile BgWriterShmemStruct *bgs = BgWriterShmem;

                        /*
                         * 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(&bgs->ckpt_lck);
                        flags |= bgs->ckpt_flags;
                        bgs->ckpt_flags = 0;
                        bgs->ckpt_started++;
                        SpinLockRelease(&bgs->ckpt_lck);

                        /*
                         * 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 ((flags & CHECKPOINT_CAUSE_XLOG) &&
                                elapsed_secs < CheckPointWarning)
                                ereport(LOG,
                                                (errmsg("checkpoints are occurring too frequently (%d seconds apart)",
                                                                elapsed_secs),
                                                 errhint("Consider increasing the configuration parameter \"checkpoint_segments\".")));

                        /*
                         * Initialize bgwriter-private variables used during checkpoint.
                         */
                        ckpt_active = true;
                        ckpt_start_recptr = GetInsertRecPtr();
                        ckpt_start_time = now;
                        ckpt_cached_elapsed = 0;

                        /*
                         * Do the checkpoint.
                         */
                        CreateCheckPoint(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(&bgs->ckpt_lck);
                        bgs->ckpt_done = bgs->ckpt_started;
                        SpinLockRelease(&bgs->ckpt_lck);

                        ckpt_active = false;

                        /*
                         * 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
                        BgBufferSync();

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

                /* Nap for the configured time. */
                BgWriterNap();
        }
}

/*
 * CheckArchiveTimeout -- check for archive_timeout and switch xlog files
 *              if needed
 */
static void
CheckArchiveTimeout(void)
{
        time_t          now;
        time_t          last_time;

        if (XLogArchiveTimeout <= 0)
                return;

        now = 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.xrecoff % 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;
        }
}

/*
 * BgWriterNap -- Nap for the configured time or until a signal is received.
 */
static void
BgWriterNap(void)
{
        long            udelay;

        /*
         * Send off activity statistics to the stats collector
         */
        pgstat_send_bgwriter();

        /*
         * Nap for the configured time, or sleep for 10 seconds if there is no
         * bgwriter activity configured.
         *
         * On some platforms, signals won't interrupt the sleep.  To ensure we
         * respond reasonably promptly when someone signals us, break down the
         * sleep into 1-second increments, and check for interrupts after each
         * nap.
         *
         * We absorb pending requests after each short sleep.
         */
        if ((bgwriter_lru_percent > 0.0 && bgwriter_lru_maxpages > 0) ||
                ckpt_active)
                udelay = BgWriterDelay * 1000L;
        else if (XLogArchiveTimeout > 0)
                udelay = 1000000L;      /* One second */
        else
                udelay = 10000000L; /* Ten seconds */

        while (udelay > 999999L)
        {
                if (got_SIGHUP || shutdown_requested ||
                        (ckpt_active ? ImmediateCheckpointRequested() : checkpoint_requested))
                        break;
                pg_usleep(1000000L);
                AbsorbFsyncRequests();
                udelay -= 1000000L;
        }

        if (!(got_SIGHUP || shutdown_requested ||
                  (ckpt_active ? ImmediateCheckpointRequested() : checkpoint_requested)))
                pg_usleep(udelay);
}

/*
 * 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 BgWriterShmemStruct *bgs = BgWriterShmem;

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

/*
 * CheckpointWriteDelay -- yield control to bgwriter during a checkpoint
 *
 * This function is called after each page write performed by BufferSync().
 * It is responsible for keeping the bgwriter's normal activities in
 * progress during a long checkpoint, and 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-bgwriter process */
        if (!am_bg_writer)
                return;

        /*
         * Perform the usual bgwriter 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);
                }
                BgBufferSync();
                CheckArchiveTimeout();
                BgWriterNap();

                AbsorbFsyncRequests();
                absorb_counter = WRITES_PER_ABSORB;
        }
        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.
         */
        recptr = GetInsertRecPtr();
        elapsed_xlogs =
                (((double) (int32) (recptr.xlogid - ckpt_start_recptr.xlogid)) * XLogSegsPerFile +
                 ((double) (int32) (recptr.xrecoff - ckpt_start_recptr.xrecoff)) / 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) (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
 * --------------------------------
 */

/*
 * bg_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
bg_quickdie(SIGNAL_ARGS)
{
        PG_SETMASK(&BlockSig);

        /*
         * DO NOT proc_exit() -- 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.
         *
         * 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.
         */
        exit(2);
}

/* SIGHUP: set flag to re-read config file at next convenient time */
static void
BgSigHupHandler(SIGNAL_ARGS)
{
        got_SIGHUP = true;
}

/* SIGINT: set flag to run a normal checkpoint right away */
static void
ReqCheckpointHandler(SIGNAL_ARGS)
{
        checkpoint_requested = true;
}

/* SIGUSR2: set flag to run a shutdown checkpoint and exit */
static void
ReqShutdownHandler(SIGNAL_ARGS)
{
        shutdown_requested = true;
}


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

/*
 * BgWriterShmemSize
 *              Compute space needed for bgwriter-related shared memory
 */
Size
BgWriterShmemSize(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(BgWriterShmemStruct, requests);
        size = add_size(size, mul_size(NBuffers, sizeof(BgWriterRequest)));

        return size;
}

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

        BgWriterShmem = (BgWriterShmemStruct *)
                ShmemInitStruct("Background Writer Data",
                                                BgWriterShmemSize(),
                                                &found);
        if (BgWriterShmem == NULL)
                ereport(FATAL,
                                (errcode(ERRCODE_OUT_OF_MEMORY),
                                 errmsg("not enough shared memory for background writer")));
        if (found)
                return;                                 /* already initialized */

        MemSet(BgWriterShmem, 0, sizeof(BgWriterShmemStruct));
        SpinLockInit(&BgWriterShmem->ckpt_lck);
        BgWriterShmem->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_IMMEDIATE: finish the checkpoint ASAP,
 *              ignoring checkpoint_completion_target parameter. 
 *      CHECKPOINT_FORCE: force a checkpoint even if no XLOG activity has occured
 *              since the last one (implied by CHECKPOINT_IS_SHUTDOWN).
 *      CHECKPOINT_WAIT: wait for completion before returning (otherwise,
 *              just signal bgwriter 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 BgWriterShmemStruct *bgs = BgWriterShmem;
        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 bgwriter.
         *
         * 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(&bgs->ckpt_lck);

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

        SpinLockRelease(&bgs->ckpt_lck);

        /*
         * Send signal to request checkpoint.  When not waiting, we
         * consider failure to send the signal to be nonfatal.
         */
        if (BgWriterShmem->bgwriter_pid == 0)
                elog((flags & CHECKPOINT_WAIT) ? ERROR : LOG,
                         "could not request checkpoint because bgwriter not running");
        if (kill(BgWriterShmem->bgwriter_pid, SIGINT) != 0)
                elog((flags & CHECKPOINT_WAIT) ? ERROR : LOG,
                         "could not signal for checkpoint: %m");

        /*
         * 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(&bgs->ckpt_lck);
                        new_started = bgs->ckpt_started;
                        SpinLockRelease(&bgs->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(&bgs->ckpt_lck);
                        new_done = bgs->ckpt_done;
                        new_failed = bgs->ckpt_failed;
                        SpinLockRelease(&bgs->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 bgwriter
 *
 * Whenever a backend is compelled to write directly to a relation
 * (which should be seldom, if the bgwriter 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.
 *
 * 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.)
 *
 * If we are unable to pass over the request (at present, this can happen
 * if the shared memory queue is full), we return false.  That forces
 * the backend to do its own fsync.  We hope that will be even more seldom.
 *
 * Note: we presently make no attempt to eliminate duplicate requests
 * in the requests[] queue.  The bgwriter will have to eliminate dups
 * internally anyway, so we may as well avoid holding the lock longer
 * than we have to here.
 */
bool
ForwardFsyncRequest(RelFileNode rnode, BlockNumber segno)
{
        BgWriterRequest *request;

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

        LWLockAcquire(BgWriterCommLock, LW_EXCLUSIVE);
        if (BgWriterShmem->bgwriter_pid == 0 ||
                BgWriterShmem->num_requests >= BgWriterShmem->max_requests)
        {
                LWLockRelease(BgWriterCommLock);
                return false;
        }
        request = &BgWriterShmem->requests[BgWriterShmem->num_requests++];
        request->rnode = rnode;
        request->segno = segno;
        LWLockRelease(BgWriterCommLock);
        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-bgwriter processes, do nothing if not bgwriter.
 */
void
AbsorbFsyncRequests(void)
{
        BgWriterRequest *requests = NULL;
        BgWriterRequest *request;
        int                     n;

        if (!am_bg_writer)
                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(BgWriterCommLock, LW_EXCLUSIVE);

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

        LWLockRelease(BgWriterCommLock);

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

        if (requests)
                pfree(requests);

        END_CRIT_SECTION();
}