walsummarizer.c

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/*-------------------------------------------------------------------------
 *
 * walsummarizer.c
 *
 * Background process to perform WAL summarization, if it is enabled.
 * It continuously scans the write-ahead log and periodically emits a
 * summary file which indicates which blocks in which relation forks
 * were modified by WAL records in the LSN range covered by the summary
 * file. See walsummary.c and blkreftable.c for more details on the
 * naming and contents of WAL summary files.
 *
 * If configured to do, this background process will also remove WAL
 * summary files when the file timestamp is older than a configurable
 * threshold (but only if the WAL has been removed first).
 *
 * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    src/backend/postmaster/walsummarizer.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "access/timeline.h"
#include "access/xlog.h"
#include "access/xlog_internal.h"
#include "access/xlogrecovery.h"
#include "access/xlogutils.h"
#include "backup/walsummary.h"
#include "catalog/storage_xlog.h"
#include "commands/dbcommands_xlog.h"
#include "common/blkreftable.h"
#include "libpq/pqsignal.h"
#include "miscadmin.h"
#include "postmaster/auxprocess.h"
#include "postmaster/interrupt.h"
#include "postmaster/walsummarizer.h"
#include "replication/walreceiver.h"
#include "storage/fd.h"
#include "storage/ipc.h"
#include "storage/latch.h"
#include "storage/lwlock.h"
#include "storage/proc.h"
#include "storage/procsignal.h"
#include "storage/shmem.h"
#include "utils/guc.h"
#include "utils/memutils.h"
#include "utils/wait_event.h"
 
/*
 * Data in shared memory related to WAL summarization.
 */
typedef struct
{
    /*
     * These fields are protected by WALSummarizerLock.
     *
     * Until we've discovered what summary files already exist on disk and
     * stored that information in shared memory, initialized is false and the
     * other fields here contain no meaningful information. After that has
     * been done, initialized is true.
     *
     * summarized_tli and summarized_lsn indicate the last LSN and TLI at
     * which the next summary file will start. Normally, these are the LSN and
     * TLI at which the last file ended; in such case, lsn_is_exact is true.
     * If, however, the LSN is just an approximation, then lsn_is_exact is
     * false. This can happen if, for example, there are no existing WAL
     * summary files at startup. In that case, we have to derive the position
     * at which to start summarizing from the WAL files that exist on disk,
     * and so the LSN might point to the start of the next file even though
     * that might happen to be in the middle of a WAL record.
     *
     * summarizer_pgprocno is the proc number of the summarizer process, if
     * one is running, or else INVALID_PROC_NUMBER.
     *
     * pending_lsn is used by the summarizer to advertise the ending LSN of a
     * record it has recently read. It shouldn't ever be less than
     * summarized_lsn, but might be greater, because the summarizer buffers
     * data for a range of LSNs in memory before writing out a new file.
     */
    bool        initialized;
    TimeLineID  summarized_tli;
    XLogRecPtr  summarized_lsn;
    bool        lsn_is_exact;
    ProcNumber  summarizer_pgprocno;
    XLogRecPtr  pending_lsn;
 
    /*
     * This field handles its own synchronization.
     */
    ConditionVariable summary_file_cv;
} WalSummarizerData;
 
/*
 * Private data for our xlogreader's page read callback.
 */
typedef struct
{
    TimeLineID  tli;
    bool        historic;
    XLogRecPtr  read_upto;
    bool        end_of_wal;
} SummarizerReadLocalXLogPrivate;
 
/* Pointer to shared memory state. */
static WalSummarizerData *WalSummarizerCtl;
 
/*
 * When we reach end of WAL and need to read more, we sleep for a number of
 * milliseconds that is an integer multiple of MS_PER_SLEEP_QUANTUM. This is
 * the multiplier. It should vary between 1 and MAX_SLEEP_QUANTA, depending
 * on system activity. See summarizer_wait_for_wal() for how we adjust this.
 */
static long sleep_quanta = 1;
 
/*
 * The sleep time will always be a multiple of 200ms and will not exceed
 * thirty seconds (150 * 200 = 30 * 1000). Note that the timeout here needs
 * to be substantially less than the maximum amount of time for which an
 * incremental backup will wait for this process to catch up. Otherwise, an
 * incremental backup might time out on an idle system just because we sleep
 * for too long.
 */
#define MAX_SLEEP_QUANTA        150
#define MS_PER_SLEEP_QUANTUM    200
 
/*
 * This is a count of the number of pages of WAL that we've read since the
 * last time we waited for more WAL to appear.
 */
static long pages_read_since_last_sleep = 0;
 
/*
 * Most recent RedoRecPtr value observed by MaybeRemoveOldWalSummaries.
 */
static XLogRecPtr redo_pointer_at_last_summary_removal = InvalidXLogRecPtr;
 
/*
 * GUC parameters
 */
bool        summarize_wal = false;
int         wal_summary_keep_time = 10 * HOURS_PER_DAY * MINS_PER_HOUR;
 
static void WalSummarizerShutdown(int code, Datum arg);
static XLogRecPtr GetLatestLSN(TimeLineID *tli);
static void HandleWalSummarizerInterrupts(void);
static XLogRecPtr SummarizeWAL(TimeLineID tli, XLogRecPtr start_lsn,
                               bool exact, XLogRecPtr switch_lsn,
                               XLogRecPtr maximum_lsn);
static void SummarizeDbaseRecord(XLogReaderState *xlogreader,
                                 BlockRefTable *brtab);
static void SummarizeSmgrRecord(XLogReaderState *xlogreader,
                                BlockRefTable *brtab);
static void SummarizeXactRecord(XLogReaderState *xlogreader,
                                BlockRefTable *brtab);
static bool SummarizeXlogRecord(XLogReaderState *xlogreader);
static int  summarizer_read_local_xlog_page(XLogReaderState *state,
                                            XLogRecPtr targetPagePtr,
                                            int reqLen,
                                            XLogRecPtr targetRecPtr,
                                            char *cur_page);
static void summarizer_wait_for_wal(void);
static void MaybeRemoveOldWalSummaries(void);
 
/*
 * Amount of shared memory required for this module.
 */
Size
WalSummarizerShmemSize(void)
{
    return sizeof(WalSummarizerData);
}
 
/*
 * Create or attach to shared memory segment for this module.
 */
void
WalSummarizerShmemInit(void)
{
    bool        found;
 
    WalSummarizerCtl = (WalSummarizerData *)
        ShmemInitStruct("Wal Summarizer Ctl", WalSummarizerShmemSize(),
                        &found);
 
    if (!found)
    {
        /*
         * First time through, so initialize.
         *
         * We're just filling in dummy values here -- the real initialization
         * will happen when GetOldestUnsummarizedLSN() is called for the first
         * time.
         */
        WalSummarizerCtl->initialized = false;
        WalSummarizerCtl->summarized_tli = 0;
        WalSummarizerCtl->summarized_lsn = InvalidXLogRecPtr;
        WalSummarizerCtl->lsn_is_exact = false;
        WalSummarizerCtl->summarizer_pgprocno = INVALID_PROC_NUMBER;
        WalSummarizerCtl->pending_lsn = InvalidXLogRecPtr;
        ConditionVariableInit(&WalSummarizerCtl->summary_file_cv);
    }
}
 
/*
 * Entry point for walsummarizer process.
 */
void
WalSummarizerMain(char *startup_data, size_t startup_data_len)
{
    sigjmp_buf  local_sigjmp_buf;
    MemoryContext context;
 
    /*
     * Within this function, 'current_lsn' and 'current_tli' refer to the
     * point from which the next WAL summary file should start. 'exact' is
     * true if 'current_lsn' is known to be the start of a WAL record or WAL
     * segment, and false if it might be in the middle of a record someplace.
     *
     * 'switch_lsn' and 'switch_tli', if set, are the LSN at which we need to
     * switch to a new timeline and the timeline to which we need to switch.
     * If not set, we either haven't figured out the answers yet or we're
     * already on the latest timeline.
     */
    XLogRecPtr  current_lsn;
    TimeLineID  current_tli;
    bool        exact;
    XLogRecPtr  switch_lsn = InvalidXLogRecPtr;
    TimeLineID  switch_tli = 0;
 
    Assert(startup_data_len == 0);
 
    MyBackendType = B_WAL_SUMMARIZER;
    AuxiliaryProcessMainCommon();
 
    ereport(DEBUG1,
            (errmsg_internal("WAL summarizer started")));
 
    /*
     * Properly accept or ignore signals the postmaster might send us
     *
     * We have no particular use for SIGINT at the moment, but seems
     * reasonable to treat like SIGTERM.
     */
    pqsignal(SIGHUP, SignalHandlerForConfigReload);
    pqsignal(SIGINT, SignalHandlerForShutdownRequest);
    pqsignal(SIGTERM, SignalHandlerForShutdownRequest);
    /* SIGQUIT handler was already set up by InitPostmasterChild */
    pqsignal(SIGALRM, SIG_IGN);
    pqsignal(SIGPIPE, SIG_IGN);
    pqsignal(SIGUSR1, procsignal_sigusr1_handler);
    pqsignal(SIGUSR2, SIG_IGN); /* not used */
 
    /* Advertise ourselves. */
    on_shmem_exit(WalSummarizerShutdown, (Datum) 0);
    LWLockAcquire(WALSummarizerLock, LW_EXCLUSIVE);
    WalSummarizerCtl->summarizer_pgprocno = MyProcNumber;
    LWLockRelease(WALSummarizerLock);
 
    /* Create and switch to a memory context that we can reset on error. */
    context = AllocSetContextCreate(TopMemoryContext,
                                    "Wal Summarizer",
                                    ALLOCSET_DEFAULT_SIZES);
    MemoryContextSwitchTo(context);
 
    /*
     * Reset some signals that are accepted by postmaster but not here
     */
    pqsignal(SIGCHLD, SIG_DFL);
 
    /*
     * If an exception is encountered, processing resumes here.
     */
    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();
 
        /* Release resources we might have acquired. */
        LWLockReleaseAll();
        ConditionVariableCancelSleep();
        pgstat_report_wait_end();
        ReleaseAuxProcessResources(false);
        AtEOXact_Files(false);
        AtEOXact_HashTables(false);
 
        /*
         * Now return to normal top-level context and clear ErrorContext for
         * next time.
         */
        MemoryContextSwitchTo(context);
        FlushErrorState();
 
        /* Flush any leaked data in the top-level context */
        MemoryContextReset(context);
 
        /* Now we can allow interrupts again */
        RESUME_INTERRUPTS();
 
        /*
         * Sleep for 10 seconds before attempting to resume operations in
         * order to avoid excessive logging.
         *
         * Many of the likely error conditions are things that will repeat
         * every time. For example, if the WAL can't be read or the summary
         * can't be written, only administrator action will cure the problem.
         * So a really fast retry time doesn't seem to be especially
         * beneficial, and it will clutter the logs.
         */
        (void) WaitLatch(MyLatch,
                         WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
                         10000,
                         WAIT_EVENT_WAL_SUMMARIZER_ERROR);
    }
 
    /* We can now handle ereport(ERROR) */
    PG_exception_stack = &local_sigjmp_buf;
 
    /*
     * Unblock signals (they were blocked when the postmaster forked us)
     */
    sigprocmask(SIG_SETMASK, &UnBlockSig, NULL);
 
    /*
     * Fetch information about previous progress from shared memory, and ask
     * GetOldestUnsummarizedLSN to reset pending_lsn to summarized_lsn. We
     * might be recovering from an error, and if so, pending_lsn might have
     * advanced past summarized_lsn, but any WAL we read previously has been
     * lost and will need to be reread.
     *
     * If we discover that WAL summarization is not enabled, just exit.
     */
    current_lsn = GetOldestUnsummarizedLSN(&current_tli, &exact, true);
    if (XLogRecPtrIsInvalid(current_lsn))
        proc_exit(0);
 
    /*
     * Loop forever
     */
    for (;;)
    {
        XLogRecPtr  latest_lsn;
        TimeLineID  latest_tli;
        XLogRecPtr  end_of_summary_lsn;
 
        /* Flush any leaked data in the top-level context */
        MemoryContextReset(context);
 
        /* Process any signals received recently. */
        HandleWalSummarizerInterrupts();
 
        /* If it's time to remove any old WAL summaries, do that now. */
        MaybeRemoveOldWalSummaries();
 
        /* Find the LSN and TLI up to which we can safely summarize. */
        latest_lsn = GetLatestLSN(&latest_tli);
 
        /*
         * If we're summarizing a historic timeline and we haven't yet
         * computed the point at which to switch to the next timeline, do that
         * now.
         *
         * Note that if this is a standby, what was previously the current
         * timeline could become historic at any time.
         *
         * We could try to make this more efficient by caching the results of
         * readTimeLineHistory when latest_tli has not changed, but since we
         * only have to do this once per timeline switch, we probably wouldn't
         * save any significant amount of work in practice.
         */
        if (current_tli != latest_tli && XLogRecPtrIsInvalid(switch_lsn))
        {
            List       *tles = readTimeLineHistory(latest_tli);
 
            switch_lsn = tliSwitchPoint(current_tli, tles, &switch_tli);
            ereport(DEBUG1,
                    errmsg("switch point from TLI %u to TLI %u is at %X/%X",
                           current_tli, switch_tli, LSN_FORMAT_ARGS(switch_lsn)));
        }
 
        /*
         * If we've reached the switch LSN, we can't summarize anything else
         * on this timeline. Switch to the next timeline and go around again.
         */
        if (!XLogRecPtrIsInvalid(switch_lsn) && current_lsn >= switch_lsn)
        {
            current_tli = switch_tli;
            switch_lsn = InvalidXLogRecPtr;
            switch_tli = 0;
            continue;
        }
 
        /* Summarize WAL. */
        end_of_summary_lsn = SummarizeWAL(current_tli,
                                          current_lsn, exact,
                                          switch_lsn, latest_lsn);
        Assert(!XLogRecPtrIsInvalid(end_of_summary_lsn));
        Assert(end_of_summary_lsn >= current_lsn);
 
        /*
         * Update state for next loop iteration.
         *
         * Next summary file should start from exactly where this one ended.
         */
        current_lsn = end_of_summary_lsn;
        exact = true;
 
        /* Update state in shared memory. */
        LWLockAcquire(WALSummarizerLock, LW_EXCLUSIVE);
        Assert(WalSummarizerCtl->pending_lsn <= end_of_summary_lsn);
        WalSummarizerCtl->summarized_lsn = end_of_summary_lsn;
        WalSummarizerCtl->summarized_tli = current_tli;
        WalSummarizerCtl->lsn_is_exact = true;
        WalSummarizerCtl->pending_lsn = end_of_summary_lsn;
        LWLockRelease(WALSummarizerLock);
 
        /* Wake up anyone waiting for more summary files to be written. */
        ConditionVariableBroadcast(&WalSummarizerCtl->summary_file_cv);
    }
}
 
/*
 * Get information about the state of the WAL summarizer.
 */
void
GetWalSummarizerState(TimeLineID *summarized_tli, XLogRecPtr *summarized_lsn,
                      XLogRecPtr *pending_lsn, int *summarizer_pid)
{
    LWLockAcquire(WALSummarizerLock, LW_SHARED);
    if (!WalSummarizerCtl->initialized)
    {
        /*
         * If initialized is false, the rest of the structure contents are
         * undefined.
         */
        *summarized_tli = 0;
        *summarized_lsn = InvalidXLogRecPtr;
        *pending_lsn = InvalidXLogRecPtr;
        *summarizer_pid = -1;
    }
    else
    {
        int         summarizer_pgprocno = WalSummarizerCtl->summarizer_pgprocno;
 
        *summarized_tli = WalSummarizerCtl->summarized_tli;
        *summarized_lsn = WalSummarizerCtl->summarized_lsn;
        if (summarizer_pgprocno == INVALID_PROC_NUMBER)
        {
            /*
             * If the summarizer has exited, the fact that it had processed
             * beyond summarized_lsn is irrelevant now.
             */
            *pending_lsn = WalSummarizerCtl->summarized_lsn;
            *summarizer_pid = -1;
        }
        else
        {
            *pending_lsn = WalSummarizerCtl->pending_lsn;
 
            /*
             * We're not fussed about inexact answers here, since they could
             * become stale instantly, so we don't bother taking the lock, but
             * make sure that invalid PID values are normalized to -1.
             */
            *summarizer_pid = GetPGProcByNumber(summarizer_pgprocno)->pid;
            if (*summarizer_pid <= 0)
                *summarizer_pid = -1;
        }
    }
    LWLockRelease(WALSummarizerLock);
}
 
/*
 * Get the oldest LSN in this server's timeline history that has not yet been
 * summarized.
 *
 * If *tli != NULL, it will be set to the TLI for the LSN that is returned.
 *
 * If *lsn_is_exact != NULL, it will be set to true if the returned LSN is
 * necessarily the start of a WAL record and false if it's just the beginning
 * of a WAL segment.
 *
 * If reset_pending_lsn is true, resets the pending_lsn in shared memory to
 * be equal to the summarized_lsn.
 */
XLogRecPtr
GetOldestUnsummarizedLSN(TimeLineID *tli, bool *lsn_is_exact,
                         bool reset_pending_lsn)
{
    TimeLineID  latest_tli;
    LWLockMode  mode = reset_pending_lsn ? LW_EXCLUSIVE : LW_SHARED;
    int         n;
    List       *tles;
    XLogRecPtr  unsummarized_lsn = InvalidXLogRecPtr;
    TimeLineID  unsummarized_tli = 0;
    bool        should_make_exact = false;
    List       *existing_summaries;
    ListCell   *lc;
 
    /* If not summarizing WAL, do nothing. */
    if (!summarize_wal)
        return InvalidXLogRecPtr;
 
    /*
     * Unless we need to reset the pending_lsn, we initially acquire the lock
     * in shared mode and try to fetch the required information. If we acquire
     * in shared mode and find that the data structure hasn't been
     * initialized, we reacquire the lock in exclusive mode so that we can
     * initialize it. However, if someone else does that first before we get
     * the lock, then we can just return the requested information after all.
     */
    while (1)
    {
        LWLockAcquire(WALSummarizerLock, mode);
 
        if (WalSummarizerCtl->initialized)
        {
            unsummarized_lsn = WalSummarizerCtl->summarized_lsn;
            if (tli != NULL)
                *tli = WalSummarizerCtl->summarized_tli;
            if (lsn_is_exact != NULL)
                *lsn_is_exact = WalSummarizerCtl->lsn_is_exact;
            if (reset_pending_lsn)
                WalSummarizerCtl->pending_lsn =
                    WalSummarizerCtl->summarized_lsn;
            LWLockRelease(WALSummarizerLock);
            return unsummarized_lsn;
        }
 
        if (mode == LW_EXCLUSIVE)
            break;
 
        LWLockRelease(WALSummarizerLock);
        mode = LW_EXCLUSIVE;
    }
 
    /*
     * The data structure needs to be initialized, and we are the first to
     * obtain the lock in exclusive mode, so it's our job to do that
     * initialization.
     *
     * So, find the oldest timeline on which WAL still exists, and the
     * earliest segment for which it exists.
     */
    (void) GetLatestLSN(&latest_tli);
    tles = readTimeLineHistory(latest_tli);
    for (n = list_length(tles) - 1; n >= 0; --n)
    {
        TimeLineHistoryEntry *tle = list_nth(tles, n);
        XLogSegNo   oldest_segno;
 
        oldest_segno = XLogGetOldestSegno(tle->tli);
        if (oldest_segno != 0)
        {
            /* Compute oldest LSN that still exists on disk. */
            XLogSegNoOffsetToRecPtr(oldest_segno, 0, wal_segment_size,
                                    unsummarized_lsn);
 
            unsummarized_tli = tle->tli;
            break;
        }
    }
 
    /* It really should not be possible for us to find no WAL. */
    if (unsummarized_tli == 0)
        ereport(ERROR,
                errcode(ERRCODE_INTERNAL_ERROR),
                errmsg_internal("no WAL found on timeline %u", latest_tli));
 
    /*
     * Don't try to summarize anything older than the end LSN of the newest
     * summary file that exists for this timeline.
     */
    existing_summaries =
        GetWalSummaries(unsummarized_tli,
                        InvalidXLogRecPtr, InvalidXLogRecPtr);
    foreach(lc, existing_summaries)
    {
        WalSummaryFile *ws = lfirst(lc);
 
        if (ws->end_lsn > unsummarized_lsn)
        {
            unsummarized_lsn = ws->end_lsn;
            should_make_exact = true;
        }
    }
 
    /* Update shared memory with the discovered values. */
    WalSummarizerCtl->initialized = true;
    WalSummarizerCtl->summarized_lsn = unsummarized_lsn;
    WalSummarizerCtl->summarized_tli = unsummarized_tli;
    WalSummarizerCtl->lsn_is_exact = should_make_exact;
    WalSummarizerCtl->pending_lsn = unsummarized_lsn;
 
    /* Also return the to the caller as required. */
    if (tli != NULL)
        *tli = WalSummarizerCtl->summarized_tli;
    if (lsn_is_exact != NULL)
        *lsn_is_exact = WalSummarizerCtl->lsn_is_exact;
    LWLockRelease(WALSummarizerLock);
 
    return unsummarized_lsn;
}
 
/*
 * Attempt to set the WAL summarizer's latch.
 *
 * This might not work, because there's no guarantee that the WAL summarizer
 * process was successfully started, and it also might have started but
 * subsequently terminated. So, under normal circumstances, this will get the
 * latch set, but there's no guarantee.
 */
void
SetWalSummarizerLatch(void)
{
    ProcNumber  pgprocno;
 
    if (WalSummarizerCtl == NULL)
        return;
 
    LWLockAcquire(WALSummarizerLock, LW_EXCLUSIVE);
    pgprocno = WalSummarizerCtl->summarizer_pgprocno;
    LWLockRelease(WALSummarizerLock);
 
    if (pgprocno != INVALID_PROC_NUMBER)
        SetLatch(&ProcGlobal->allProcs[pgprocno].procLatch);
}
 
/*
 * Wait until WAL summarization reaches the given LSN, but not longer than
 * the given timeout.
 *
 * The return value is the first still-unsummarized LSN. If it's greater than
 * or equal to the passed LSN, then that LSN was reached. If not, we timed out.
 *
 * Either way, *pending_lsn is set to the value taken from WalSummarizerCtl.
 */
XLogRecPtr
WaitForWalSummarization(XLogRecPtr lsn, long timeout, XLogRecPtr *pending_lsn)
{
    TimestampTz start_time = GetCurrentTimestamp();
    TimestampTz deadline = TimestampTzPlusMilliseconds(start_time, timeout);
    XLogRecPtr  summarized_lsn;
 
    Assert(!XLogRecPtrIsInvalid(lsn));
    Assert(timeout > 0);
 
    while (1)
    {
        TimestampTz now;
        long        remaining_timeout;
 
        /*
         * If the LSN summarized on disk has reached the target value, stop.
         */
        LWLockAcquire(WALSummarizerLock, LW_EXCLUSIVE);
        summarized_lsn = WalSummarizerCtl->summarized_lsn;
        *pending_lsn = WalSummarizerCtl->pending_lsn;
        LWLockRelease(WALSummarizerLock);
        if (summarized_lsn >= lsn)
            break;
 
        /* Timeout reached? If yes, stop. */
        now = GetCurrentTimestamp();
        remaining_timeout = TimestampDifferenceMilliseconds(now, deadline);
        if (remaining_timeout <= 0)
            break;
 
        /* Wait and see. */
        ConditionVariableTimedSleep(&WalSummarizerCtl->summary_file_cv,
                                    remaining_timeout,
                                    WAIT_EVENT_WAL_SUMMARY_READY);
    }
 
    return summarized_lsn;
}
 
/*
 * On exit, update shared memory to make it clear that we're no longer
 * running.
 */
static void
WalSummarizerShutdown(int code, Datum arg)
{
    LWLockAcquire(WALSummarizerLock, LW_EXCLUSIVE);
    WalSummarizerCtl->summarizer_pgprocno = INVALID_PROC_NUMBER;
    LWLockRelease(WALSummarizerLock);
}
 
/*
 * Get the latest LSN that is eligible to be summarized, and set *tli to the
 * corresponding timeline.
 */
static XLogRecPtr
GetLatestLSN(TimeLineID *tli)
{
    if (!RecoveryInProgress())
    {
        /* Don't summarize WAL before it's flushed. */
        return GetFlushRecPtr(tli);
    }
    else
    {
        XLogRecPtr  flush_lsn;
        TimeLineID  flush_tli;
        XLogRecPtr  replay_lsn;
        TimeLineID  replay_tli;
 
        /*
         * What we really want to know is how much WAL has been flushed to
         * disk, but the only flush position available is the one provided by
         * the walreceiver, which may not be running, because this could be
         * crash recovery or recovery via restore_command. So use either the
         * WAL receiver's flush position or the replay position, whichever is
         * further ahead, on the theory that if the WAL has been replayed then
         * it must also have been flushed to disk.
         */
        flush_lsn = GetWalRcvFlushRecPtr(NULL, &flush_tli);
        replay_lsn = GetXLogReplayRecPtr(&replay_tli);
        if (flush_lsn > replay_lsn)
        {
            *tli = flush_tli;
            return flush_lsn;
        }
        else
        {
            *tli = replay_tli;
            return replay_lsn;
        }
    }
}
 
/*
 * Interrupt handler for main loop of WAL summarizer process.
 */
static void
HandleWalSummarizerInterrupts(void)
{
    if (ProcSignalBarrierPending)
        ProcessProcSignalBarrier();
 
    if (ConfigReloadPending)
    {
        ConfigReloadPending = false;
        ProcessConfigFile(PGC_SIGHUP);
    }
 
    if (ShutdownRequestPending || !summarize_wal)
    {
        ereport(DEBUG1,
                errmsg_internal("WAL summarizer shutting down"));
        proc_exit(0);
    }
 
    /* Perform logging of memory contexts of this process */
    if (LogMemoryContextPending)
        ProcessLogMemoryContextInterrupt();
}
 
/*
 * Summarize a range of WAL records on a single timeline.
 *
 * 'tli' is the timeline to be summarized.
 *
 * 'start_lsn' is the point at which we should start summarizing. If this
 * value comes from the end LSN of the previous record as returned by the
 * xlogreader machinery, 'exact' should be true; otherwise, 'exact' should
 * be false, and this function will search forward for the start of a valid
 * WAL record.
 *
 * 'switch_lsn' is the point at which we should switch to a later timeline,
 * if we're summarizing a historic timeline.
 *
 * 'maximum_lsn' identifies the point beyond which we can't count on being
 * able to read any more WAL. It should be the switch point when reading a
 * historic timeline, or the most-recently-measured end of WAL when reading
 * the current timeline.
 *
 * The return value is the LSN at which the WAL summary actually ends. Most
 * often, a summary file ends because we notice that a checkpoint has
 * occurred and reach the redo pointer of that checkpoint, but sometimes
 * we stop for other reasons, such as a timeline switch.
 */
static XLogRecPtr
SummarizeWAL(TimeLineID tli, XLogRecPtr start_lsn, bool exact,
             XLogRecPtr switch_lsn, XLogRecPtr maximum_lsn)
{
    SummarizerReadLocalXLogPrivate *private_data;
    XLogReaderState *xlogreader;
    XLogRecPtr  summary_start_lsn;
    XLogRecPtr  summary_end_lsn = switch_lsn;
    char        temp_path[MAXPGPATH];
    char        final_path[MAXPGPATH];
    WalSummaryIO io;
    BlockRefTable *brtab = CreateEmptyBlockRefTable();
 
    /* Initialize private data for xlogreader. */
    private_data = (SummarizerReadLocalXLogPrivate *)
        palloc0(sizeof(SummarizerReadLocalXLogPrivate));
    private_data->tli = tli;
    private_data->historic = !XLogRecPtrIsInvalid(switch_lsn);
    private_data->read_upto = maximum_lsn;
 
    /* Create xlogreader. */
    xlogreader = XLogReaderAllocate(wal_segment_size, NULL,
                                    XL_ROUTINE(.page_read = &summarizer_read_local_xlog_page,
                                               .segment_open = &wal_segment_open,
                                               .segment_close = &wal_segment_close),
                                    private_data);
    if (xlogreader == NULL)
        ereport(ERROR,
                (errcode(ERRCODE_OUT_OF_MEMORY),
                 errmsg("out of memory"),
                 errdetail("Failed while allocating a WAL reading processor.")));
 
    /*
     * When exact = false, we're starting from an arbitrary point in the WAL
     * and must search forward for the start of the next record.
     *
     * When exact = true, start_lsn should be either the LSN where a record
     * begins, or the LSN of a page where the page header is immediately
     * followed by the start of a new record. XLogBeginRead should tolerate
     * either case.
     *
     * We need to allow for both cases because the behavior of xlogreader
     * varies. When a record spans two or more xlog pages, the ending LSN
     * reported by xlogreader will be the starting LSN of the following
     * record, but when an xlog page boundary falls between two records, the
     * end LSN for the first will be reported as the first byte of the
     * following page. We can't know until we read that page how large the
     * header will be, but we'll have to skip over it to find the next record.
     */
    if (exact)
    {
        /*
         * Even if start_lsn is the beginning of a page rather than the
         * beginning of the first record on that page, we should still use it
         * as the start LSN for the summary file. That's because we detect
         * missing summary files by looking for cases where the end LSN of one
         * file is less than the start LSN of the next file. When only a page
         * header is skipped, nothing has been missed.
         */
        XLogBeginRead(xlogreader, start_lsn);
        summary_start_lsn = start_lsn;
    }
    else
    {
        summary_start_lsn = XLogFindNextRecord(xlogreader, start_lsn);
        if (XLogRecPtrIsInvalid(summary_start_lsn))
        {
            /*
             * If we hit end-of-WAL while trying to find the next valid
             * record, we must be on a historic timeline that has no valid
             * records that begin after start_lsn and before end of WAL.
             */
            if (private_data->end_of_wal)
            {
                ereport(DEBUG1,
                        errmsg_internal("could not read WAL from timeline %u at %X/%X: end of WAL at %X/%X",
                                        tli,
                                        LSN_FORMAT_ARGS(start_lsn),
                                        LSN_FORMAT_ARGS(private_data->read_upto)));
 
                /*
                 * The timeline ends at or after start_lsn, without containing
                 * any records. Thus, we must make sure the main loop does not
                 * iterate. If start_lsn is the end of the timeline, then we
                 * won't actually emit an empty summary file, but otherwise,
                 * we must, to capture the fact that the LSN range in question
                 * contains no interesting WAL records.
                 */
                summary_start_lsn = start_lsn;
                summary_end_lsn = private_data->read_upto;
                switch_lsn = xlogreader->EndRecPtr;
            }
            else
                ereport(ERROR,
                        (errmsg("could not find a valid record after %X/%X",
                                LSN_FORMAT_ARGS(start_lsn))));
        }
 
        /* We shouldn't go backward. */
        Assert(summary_start_lsn >= start_lsn);
    }
 
    /*
     * Main loop: read xlog records one by one.
     */
    while (1)
    {
        int         block_id;
        char       *errormsg;
        XLogRecord *record;
        bool        stop_requested = false;
 
        HandleWalSummarizerInterrupts();
 
        /* We shouldn't go backward. */
        Assert(summary_start_lsn <= xlogreader->EndRecPtr);
 
        /* Now read the next record. */
        record = XLogReadRecord(xlogreader, &errormsg);
        if (record == NULL)
        {
            if (private_data->end_of_wal)
            {
                /*
                 * This timeline must be historic and must end before we were
                 * able to read a complete record.
                 */
                ereport(DEBUG1,
                        errmsg_internal("could not read WAL from timeline %u at %X/%X: end of WAL at %X/%X",
                                        tli,
                                        LSN_FORMAT_ARGS(xlogreader->EndRecPtr),
                                        LSN_FORMAT_ARGS(private_data->read_upto)));
                /* Summary ends at end of WAL. */
                summary_end_lsn = private_data->read_upto;
                break;
            }
            if (errormsg)
                ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not read WAL from timeline %u at %X/%X: %s",
                                tli, LSN_FORMAT_ARGS(xlogreader->EndRecPtr),
                                errormsg)));
            else
                ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not read WAL from timeline %u at %X/%X",
                                tli, LSN_FORMAT_ARGS(xlogreader->EndRecPtr))));
        }
 
        /* We shouldn't go backward. */
        Assert(summary_start_lsn <= xlogreader->EndRecPtr);
 
        if (!XLogRecPtrIsInvalid(switch_lsn) &&
            xlogreader->ReadRecPtr >= switch_lsn)
        {
            /*
             * Whoops! We've read a record that *starts* after the switch LSN,
             * contrary to our goal of reading only until we hit the first
             * record that ends at or after the switch LSN. Pretend we didn't
             * read it after all by bailing out of this loop right here,
             * before we do anything with this record.
             *
             * This can happen because the last record before the switch LSN
             * might be continued across multiple pages, and then we might
             * come to a page with XLP_FIRST_IS_OVERWRITE_CONTRECORD set. In
             * that case, the record that was continued across multiple pages
             * is incomplete and will be disregarded, and the read will
             * restart from the beginning of the page that is flagged
             * XLP_FIRST_IS_OVERWRITE_CONTRECORD.
             *
             * If this case occurs, we can fairly say that the current summary
             * file ends at the switch LSN exactly. The first record on the
             * page marked XLP_FIRST_IS_OVERWRITE_CONTRECORD will be
             * discovered when generating the next summary file.
             */
            summary_end_lsn = switch_lsn;
            break;
        }
 
        /* Special handling for particular types of WAL records. */
        switch (XLogRecGetRmid(xlogreader))
        {
            case RM_DBASE_ID:
                SummarizeDbaseRecord(xlogreader, brtab);
                break;
            case RM_SMGR_ID:
                SummarizeSmgrRecord(xlogreader, brtab);
                break;
            case RM_XACT_ID:
                SummarizeXactRecord(xlogreader, brtab);
                break;
            case RM_XLOG_ID:
                stop_requested = SummarizeXlogRecord(xlogreader);
                break;
            default:
                break;
        }
 
        /*
         * If we've been told that it's time to end this WAL summary file, do
         * so. As an exception, if there's nothing included in this WAL
         * summary file yet, then stopping doesn't make any sense, and we
         * should wait until the next stop point instead.
         */
        if (stop_requested && xlogreader->ReadRecPtr > summary_start_lsn)
        {
            summary_end_lsn = xlogreader->ReadRecPtr;
            break;
        }
 
        /* Feed block references from xlog record to block reference table. */
        for (block_id = 0; block_id <= XLogRecMaxBlockId(xlogreader);
             block_id++)
        {
            RelFileLocator rlocator;
            ForkNumber  forknum;
            BlockNumber blocknum;
 
            if (!XLogRecGetBlockTagExtended(xlogreader, block_id, &rlocator,
                                            &forknum, &blocknum, NULL))
                continue;
 
            /*
             * As we do elsewhere, ignore the FSM fork, because it's not fully
             * WAL-logged.
             */
            if (forknum != FSM_FORKNUM)
                BlockRefTableMarkBlockModified(brtab, &rlocator, forknum,
                                               blocknum);
        }
 
        /* Update our notion of where this summary file ends. */
        summary_end_lsn = xlogreader->EndRecPtr;
 
        /* Also update shared memory. */
        LWLockAcquire(WALSummarizerLock, LW_EXCLUSIVE);
        Assert(summary_end_lsn >= WalSummarizerCtl->pending_lsn);
        Assert(summary_end_lsn >= WalSummarizerCtl->summarized_lsn);
        WalSummarizerCtl->pending_lsn = summary_end_lsn;
        LWLockRelease(WALSummarizerLock);
 
        /*
         * If we have a switch LSN and have reached it, stop before reading
         * the next record.
         */
        if (!XLogRecPtrIsInvalid(switch_lsn) &&
            xlogreader->EndRecPtr >= switch_lsn)
            break;
    }
 
    /* Destroy xlogreader. */
    pfree(xlogreader->private_data);
    XLogReaderFree(xlogreader);
 
    /*
     * If a timeline switch occurs, we may fail to make any progress at all
     * before exiting the loop above. If that happens, we don't write a WAL
     * summary file at all.
     */
    if (summary_end_lsn > summary_start_lsn)
    {
        /* Generate temporary and final path name. */
        snprintf(temp_path, MAXPGPATH,
                 XLOGDIR "/summaries/temp.summary");
        snprintf(final_path, MAXPGPATH,
                 XLOGDIR "/summaries/%08X%08X%08X%08X%08X.summary",
                 tli,
                 LSN_FORMAT_ARGS(summary_start_lsn),
                 LSN_FORMAT_ARGS(summary_end_lsn));
 
        /* Open the temporary file for writing. */
        io.filepos = 0;
        io.file = PathNameOpenFile(temp_path, O_WRONLY | O_CREAT | O_TRUNC);
        if (io.file < 0)
            ereport(ERROR,
                    (errcode_for_file_access(),
                     errmsg("could not create file \"%s\": %m", temp_path)));
 
        /* Write the data. */
        WriteBlockRefTable(brtab, WriteWalSummary, &io);
 
        /* Close temporary file and shut down xlogreader. */
        FileClose(io.file);
 
        /* Tell the user what we did. */
        ereport(DEBUG1,
                errmsg("summarized WAL on TLI %u from %X/%X to %X/%X",
                       tli,
                       LSN_FORMAT_ARGS(summary_start_lsn),
                       LSN_FORMAT_ARGS(summary_end_lsn)));
 
        /* Durably rename the new summary into place. */
        durable_rename(temp_path, final_path, ERROR);
    }
 
    return summary_end_lsn;
}
 
/*
 * Special handling for WAL records with RM_DBASE_ID.
 */
static void
SummarizeDbaseRecord(XLogReaderState *xlogreader, BlockRefTable *brtab)
{
    uint8       info = XLogRecGetInfo(xlogreader) & ~XLR_INFO_MASK;
 
    /*
     * We use relfilenode zero for a given database OID and tablespace OID to
     * indicate that all relations with that pair of IDs have been recreated
     * if they exist at all. Effectively, we're setting a limit block of 0 for
     * all such relfilenodes.
     *
     * Technically, this special handling is only needed in the case of
     * XLOG_DBASE_CREATE_FILE_COPY, because that can create a whole bunch of
     * relation files in a directory without logging anything specific to each
     * one. If we didn't mark the whole DB OID/TS OID combination in some way,
     * then a tablespace that was dropped after the reference backup and
     * recreated using the FILE_COPY method prior to the incremental backup
     * would look just like one that was never touched at all, which would be
     * catastrophic.
     *
     * But it seems best to adopt this treatment for all records that drop or
     * create a DB OID/TS OID combination. That's similar to how we treat the
     * limit block for individual relations, and it's an extra layer of safety
     * here. We can never lose data by marking more stuff as needing to be
     * backed up in full.
     */
    if (info == XLOG_DBASE_CREATE_FILE_COPY)
    {
        xl_dbase_create_file_copy_rec *xlrec;
        RelFileLocator rlocator;
 
        xlrec =
            (xl_dbase_create_file_copy_rec *) XLogRecGetData(xlogreader);
        rlocator.spcOid = xlrec->tablespace_id;
        rlocator.dbOid = xlrec->db_id;
        rlocator.relNumber = 0;
        BlockRefTableSetLimitBlock(brtab, &rlocator, MAIN_FORKNUM, 0);
    }
    else if (info == XLOG_DBASE_CREATE_WAL_LOG)
    {
        xl_dbase_create_wal_log_rec *xlrec;
        RelFileLocator rlocator;
 
        xlrec = (xl_dbase_create_wal_log_rec *) XLogRecGetData(xlogreader);
        rlocator.spcOid = xlrec->tablespace_id;
        rlocator.dbOid = xlrec->db_id;
        rlocator.relNumber = 0;
        BlockRefTableSetLimitBlock(brtab, &rlocator, MAIN_FORKNUM, 0);
    }
    else if (info == XLOG_DBASE_DROP)
    {
        xl_dbase_drop_rec *xlrec;
        RelFileLocator rlocator;
        int         i;
 
        xlrec = (xl_dbase_drop_rec *) XLogRecGetData(xlogreader);
        rlocator.dbOid = xlrec->db_id;
        rlocator.relNumber = 0;
        for (i = 0; i < xlrec->ntablespaces; ++i)
        {
            rlocator.spcOid = xlrec->tablespace_ids[i];
            BlockRefTableSetLimitBlock(brtab, &rlocator, MAIN_FORKNUM, 0);
        }
    }
}
 
/*
 * Special handling for WAL records with RM_SMGR_ID.
 */
static void
SummarizeSmgrRecord(XLogReaderState *xlogreader, BlockRefTable *brtab)
{
    uint8       info = XLogRecGetInfo(xlogreader) & ~XLR_INFO_MASK;
 
    if (info == XLOG_SMGR_CREATE)
    {
        xl_smgr_create *xlrec;
 
        /*
         * If a new relation fork is created on disk, there is no point
         * tracking anything about which blocks have been modified, because
         * the whole thing will be new. Hence, set the limit block for this
         * fork to 0.
         *
         * Ignore the FSM fork, which is not fully WAL-logged.
         */
        xlrec = (xl_smgr_create *) XLogRecGetData(xlogreader);
 
        if (xlrec->forkNum != FSM_FORKNUM)
            BlockRefTableSetLimitBlock(brtab, &xlrec->rlocator,
                                       xlrec->forkNum, 0);
    }
    else if (info == XLOG_SMGR_TRUNCATE)
    {
        xl_smgr_truncate *xlrec;
 
        xlrec = (xl_smgr_truncate *) XLogRecGetData(xlogreader);
 
        /*
         * If a relation fork is truncated on disk, there is no point in
         * tracking anything about block modifications beyond the truncation
         * point.
         *
         * We ignore SMGR_TRUNCATE_FSM here because the FSM isn't fully
         * WAL-logged and thus we can't track modified blocks for it anyway.
         */
        if ((xlrec->flags & SMGR_TRUNCATE_HEAP) != 0)
            BlockRefTableSetLimitBlock(brtab, &xlrec->rlocator,
                                       MAIN_FORKNUM, xlrec->blkno);
        if ((xlrec->flags & SMGR_TRUNCATE_VM) != 0)
            BlockRefTableSetLimitBlock(brtab, &xlrec->rlocator,
                                       VISIBILITYMAP_FORKNUM, xlrec->blkno);
    }
}
 
/*
 * Special handling for WAL records with RM_XACT_ID.
 */
static void
SummarizeXactRecord(XLogReaderState *xlogreader, BlockRefTable *brtab)
{
    uint8       info = XLogRecGetInfo(xlogreader) & ~XLR_INFO_MASK;
    uint8       xact_info = info & XLOG_XACT_OPMASK;
 
    if (xact_info == XLOG_XACT_COMMIT ||
        xact_info == XLOG_XACT_COMMIT_PREPARED)
    {
        xl_xact_commit *xlrec = (xl_xact_commit *) XLogRecGetData(xlogreader);
        xl_xact_parsed_commit parsed;
        int         i;
 
        /*
         * Don't track modified blocks for any relations that were removed on
         * commit.
         */
        ParseCommitRecord(XLogRecGetInfo(xlogreader), xlrec, &parsed);
        for (i = 0; i < parsed.nrels; ++i)
        {
            ForkNumber  forknum;
 
            for (forknum = 0; forknum <= MAX_FORKNUM; ++forknum)
                if (forknum != FSM_FORKNUM)
                    BlockRefTableSetLimitBlock(brtab, &parsed.xlocators[i],
                                               forknum, 0);
        }
    }
    else if (xact_info == XLOG_XACT_ABORT ||
             xact_info == XLOG_XACT_ABORT_PREPARED)
    {
        xl_xact_abort *xlrec = (xl_xact_abort *) XLogRecGetData(xlogreader);
        xl_xact_parsed_abort parsed;
        int         i;
 
        /*
         * Don't track modified blocks for any relations that were removed on
         * abort.
         */
        ParseAbortRecord(XLogRecGetInfo(xlogreader), xlrec, &parsed);
        for (i = 0; i < parsed.nrels; ++i)
        {
            ForkNumber  forknum;
 
            for (forknum = 0; forknum <= MAX_FORKNUM; ++forknum)
                if (forknum != FSM_FORKNUM)
                    BlockRefTableSetLimitBlock(brtab, &parsed.xlocators[i],
                                               forknum, 0);
        }
    }
}
 
/*
 * Special handling for WAL records with RM_XLOG_ID.
 */
static bool
SummarizeXlogRecord(XLogReaderState *xlogreader)
{
    uint8       info = XLogRecGetInfo(xlogreader) & ~XLR_INFO_MASK;
 
    if (info == XLOG_CHECKPOINT_REDO || info == XLOG_CHECKPOINT_SHUTDOWN)
    {
        /*
         * This is an LSN at which redo might begin, so we'd like
         * summarization to stop just before this WAL record.
         */
        return true;
    }
 
    return false;
}
 
/*
 * Similar to read_local_xlog_page, but limited to read from one particular
 * timeline. If the end of WAL is reached, it will wait for more if reading
 * from the current timeline, or give up if reading from a historic timeline.
 * In the latter case, it will also set private_data->end_of_wal = true.
 *
 * Caller must set private_data->tli to the TLI of interest,
 * private_data->read_upto to the lowest LSN that is not known to be safe
 * to read on that timeline, and private_data->historic to true if and only
 * if the timeline is not the current timeline. This function will update
 * private_data->read_upto and private_data->historic if more WAL appears
 * on the current timeline or if the current timeline becomes historic.
 */
static int
summarizer_read_local_xlog_page(XLogReaderState *state,
                                XLogRecPtr targetPagePtr, int reqLen,
                                XLogRecPtr targetRecPtr, char *cur_page)
{
    int         count;
    WALReadError errinfo;
    SummarizerReadLocalXLogPrivate *private_data;
 
    HandleWalSummarizerInterrupts();
 
    private_data = (SummarizerReadLocalXLogPrivate *)
        state->private_data;
 
    while (1)
    {
        if (targetPagePtr + XLOG_BLCKSZ <= private_data->read_upto)
        {
            /*
             * more than one block available; read only that block, have
             * caller come back if they need more.
             */
            count = XLOG_BLCKSZ;
            break;
        }
        else if (targetPagePtr + reqLen > private_data->read_upto)
        {
            /* We don't seem to have enough data. */
            if (private_data->historic)
            {
                /*
                 * This is a historic timeline, so there will never be any
                 * more data than we have currently.
                 */
                private_data->end_of_wal = true;
                return -1;
            }
            else
            {
                XLogRecPtr  latest_lsn;
                TimeLineID  latest_tli;
 
                /*
                 * This is - or at least was up until very recently - the
                 * current timeline, so more data might show up.  Delay here
                 * so we don't tight-loop.
                 */
                HandleWalSummarizerInterrupts();
                summarizer_wait_for_wal();
 
                /* Recheck end-of-WAL. */
                latest_lsn = GetLatestLSN(&latest_tli);
                if (private_data->tli == latest_tli)
                {
                    /* Still the current timeline, update max LSN. */
                    Assert(latest_lsn >= private_data->read_upto);
                    private_data->read_upto = latest_lsn;
                }
                else
                {
                    List       *tles = readTimeLineHistory(latest_tli);
                    XLogRecPtr  switchpoint;
 
                    /*
                     * The timeline we're scanning is no longer the latest
                     * one. Figure out when it ended.
                     */
                    private_data->historic = true;
                    switchpoint = tliSwitchPoint(private_data->tli, tles,
                                                 NULL);
 
                    /*
                     * Allow reads up to exactly the switch point.
                     *
                     * It's possible that this will cause read_upto to move
                     * backwards, because walreceiver might have read a
                     * partial record and flushed it to disk, and we'd view
                     * that data as safe to read. However, the
                     * XLOG_END_OF_RECOVERY record will be written at the end
                     * of the last complete WAL record, not at the end of the
                     * WAL that we've flushed to disk.
                     *
                     * So switchpoint < private->read_upto is possible here,
                     * but switchpoint < state->EndRecPtr should not be.
                     */
                    Assert(switchpoint >= state->EndRecPtr);
                    private_data->read_upto = switchpoint;
 
                    /* Debugging output. */
                    ereport(DEBUG1,
                            errmsg("timeline %u became historic, can read up to %X/%X",
                                   private_data->tli, LSN_FORMAT_ARGS(private_data->read_upto)));
                }
 
                /* Go around and try again. */
            }
        }
        else
        {
            /* enough bytes available to satisfy the request */
            count = private_data->read_upto - targetPagePtr;
            break;
        }
    }
 
    if (!WALRead(state, cur_page, targetPagePtr, count,
                 private_data->tli, &errinfo))
        WALReadRaiseError(&errinfo);
 
    /* Track that we read a page, for sleep time calculation. */
    ++pages_read_since_last_sleep;
 
    /* number of valid bytes in the buffer */
    return count;
}
 
/*
 * Sleep for long enough that we believe it's likely that more WAL will
 * be available afterwards.
 */
static void
summarizer_wait_for_wal(void)
{
    if (pages_read_since_last_sleep == 0)
    {
        /*
         * No pages were read since the last sleep, so double the sleep time,
         * but not beyond the maximum allowable value.
         */
        sleep_quanta = Min(sleep_quanta * 2, MAX_SLEEP_QUANTA);
    }
    else if (pages_read_since_last_sleep > 1)
    {
        /*
         * Multiple pages were read since the last sleep, so reduce the sleep
         * time.
         *
         * A large burst of activity should be able to quickly reduce the
         * sleep time to the minimum, but we don't want a handful of extra WAL
         * records to provoke a strong reaction. We choose to reduce the sleep
         * time by 1 quantum for each page read beyond the first, which is a
         * fairly arbitrary way of trying to be reactive without overreacting.
         */
        if (pages_read_since_last_sleep > sleep_quanta - 1)
            sleep_quanta = 1;
        else
            sleep_quanta -= pages_read_since_last_sleep;
    }
 
    /* OK, now sleep. */
    (void) WaitLatch(MyLatch,
                     WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
                     sleep_quanta * MS_PER_SLEEP_QUANTUM,
                     WAIT_EVENT_WAL_SUMMARIZER_WAL);
    ResetLatch(MyLatch);
 
    /* Reset count of pages read. */
    pages_read_since_last_sleep = 0;
}
 
/*
 * Remove WAL summaries whose mtimes are older than wal_summary_keep_time.
 */
static void
MaybeRemoveOldWalSummaries(void)
{
    XLogRecPtr  redo_pointer = GetRedoRecPtr();
    List       *wslist;
    time_t      cutoff_time;
 
    /* If WAL summary removal is disabled, don't do anything. */
    if (wal_summary_keep_time == 0)
        return;
 
    /*
     * If the redo pointer has not advanced, don't do anything.
     *
     * This has the effect that we only try to remove old WAL summary files
     * once per checkpoint cycle.
     */
    if (redo_pointer == redo_pointer_at_last_summary_removal)
        return;
    redo_pointer_at_last_summary_removal = redo_pointer;
 
    /*
     * Files should only be removed if the last modification time precedes the
     * cutoff time we compute here.
     */
    cutoff_time = time(NULL) - wal_summary_keep_time * SECS_PER_MINUTE;
 
    /* Get all the summaries that currently exist. */
    wslist = GetWalSummaries(0, InvalidXLogRecPtr, InvalidXLogRecPtr);
 
    /* Loop until all summaries have been considered for removal. */
    while (wslist != NIL)
    {
        ListCell   *lc;
        XLogSegNo   oldest_segno;
        XLogRecPtr  oldest_lsn = InvalidXLogRecPtr;
        TimeLineID  selected_tli;
 
        HandleWalSummarizerInterrupts();
 
        /*
         * Pick a timeline for which some summary files still exist on disk,
         * and find the oldest LSN that still exists on disk for that
         * timeline.
         */
        selected_tli = ((WalSummaryFile *) linitial(wslist))->tli;
        oldest_segno = XLogGetOldestSegno(selected_tli);
        if (oldest_segno != 0)
            XLogSegNoOffsetToRecPtr(oldest_segno, 0, wal_segment_size,
                                    oldest_lsn);
 
 
        /* Consider each WAL file on the selected timeline in turn. */
        foreach(lc, wslist)
        {
            WalSummaryFile *ws = lfirst(lc);
 
            HandleWalSummarizerInterrupts();
 
            /* If it's not on this timeline, it's not time to consider it. */
            if (selected_tli != ws->tli)
                continue;
 
            /*
             * If the WAL doesn't exist any more, we can remove it if the file
             * modification time is old enough.
             */
            if (XLogRecPtrIsInvalid(oldest_lsn) || ws->end_lsn <= oldest_lsn)
                RemoveWalSummaryIfOlderThan(ws, cutoff_time);
 
            /*
             * Whether we removed the file or not, we need not consider it
             * again.
             */
            wslist = foreach_delete_current(wslist, lc);
            pfree(ws);
        }
    }
}