/src/postgres/src/backend/postmaster/walsummarizer.c

Source
/*-------------------------------------------------------------------------
 *
 * 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) 1996-2026, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    src/backend/postmaster/walsummarizer.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/timeline.h"
#include "access/visibilitymap.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 "pgstat.h"
#include "postmaster/auxprocess.h"
#include "postmaster/interrupt.h"
#include "postmaster/walsummarizer.h"
#include "replication/walreceiver.h"
#include "storage/aio_subsys.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 "storage/subsystems.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;
  int     num_descendant_tlis;
  TimeLineID *descendant_tlis;
} SummarizerReadLocalXLogPrivate;

/* Pointer to shared memory state. */
static WalSummarizerData *WalSummarizerCtl;

static void WalSummarizerShmemRequest(void *arg);
static void WalSummarizerShmemInit(void *arg);

const ShmemCallbacks WalSummarizerShmemCallbacks = {
  .request_fn = WalSummarizerShmemRequest,
  .init_fn = WalSummarizerShmemInit,
};

/*
 * 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 XLogRecPtr WalSummarizerSwitchPoint(TimeLineID current_tli, List *tles,
                       int *num_descendant_tlis,
                       TimeLineID **descendant_tlis);
static void ProcessWalSummarizerInterrupts(void);
static XLogRecPtr SummarizeWAL(TimeLineID tli, XLogRecPtr start_lsn,
                 bool exact, XLogRecPtr switch_lsn,
                 XLogRecPtr maximum_lsn,
                 int num_descendant_tlis, TimeLineID *descendant_tlis);
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,
                bool *new_fast_forward);
static void summarizer_wal_segment_open(XLogReaderState *state,
                    XLogSegNo nextSegNo,
                    TimeLineID *tli_p);
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);

/*
 * Register shared memory space needed by this module.
 */
static void
WalSummarizerShmemRequest(void *arg)
{
  ShmemRequestStruct(.name = "Wal Summarizer Ctl",
             .size = sizeof(WalSummarizerData),
             .ptr = (void **) &WalSummarizerCtl,
    );
}

/*
 * Initialize shared memory for this module.
 */
static void
WalSummarizerShmemInit(void *arg)
{
  /*
   * 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(const void *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', is the LSN at which we need to switch to a new timeline.
   * If not set, we either haven't figured out the answer yet or we're
   * already on the latest timeline. 'descendant_tlis' stores an array of
   * future timeline IDs to which we know we'll need to switch, and
   * 'num_descendant_tlis' is the length of that array. The first element of
   * the array is the first timeline to which we will need to switch.
   */
  XLogRecPtr  current_lsn;
  TimeLineID  current_tli;
  bool    exact;
  XLogRecPtr  switch_lsn = InvalidXLogRecPtr;
  int     num_descendant_tlis = 0;
  TimeLineID *descendant_tlis = NULL;

  Assert(startup_data_len == 0);

  AuxiliaryProcessMainCommon();

  ereport(DEBUG1,
      (errmsg_internal("WAL summarizer started")));

  /*
   * Properly accept or ignore signals the postmaster might send us
   */
  pqsignal(SIGHUP, SignalHandlerForConfigReload);
  pqsignal(SIGINT, PG_SIG_IGN);  /* no query to cancel */
  pqsignal(SIGTERM, SignalHandlerForShutdownRequest);
  /* SIGQUIT handler was already set up by InitPostmasterChild */
  pqsignal(SIGALRM, PG_SIG_IGN);
  pqsignal(SIGPIPE, PG_SIG_IGN);
  pqsignal(SIGUSR1, procsignal_sigusr1_handler);
  pqsignal(SIGUSR2, PG_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);

  /*
   * 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();
    pgaio_error_cleanup();
    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(NULL,
             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);
  if (!XLogRecPtrIsValid(current_lsn))
    proc_exit(0);

  /*
   * Loop forever
   */
  for (;;)
  {
    XLogRecPtr  latest_lsn;
    TimeLineID  latest_tli;
    XLogRecPtr  maximum_lsn;
    XLogRecPtr  end_of_summary_lsn;

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

    /* Process any signals received recently. */
    ProcessWalSummarizerInterrupts();

    /* 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 && !XLogRecPtrIsValid(switch_lsn))
    {
      List     *tles = readTimeLineHistory(latest_tli);
      int     new_num_descendant_tlis;
      TimeLineID *new_descendant_tlis;

      /*
       * Make sure that the array of descendant TLIs get stored into
       * TopMemoryContext.
       */
      MemoryContextSwitchTo(TopMemoryContext);
      switch_lsn = WalSummarizerSwitchPoint(current_tli, tles,
                          &new_num_descendant_tlis,
                          &new_descendant_tlis);
      MemoryContextSwitchTo(context);

      /*
       * Free any old array of descendant TLIs and install the new
       * values.
       */
      if (descendant_tlis != NULL)
        pfree(descendant_tlis);
      num_descendant_tlis = new_num_descendant_tlis;
      descendant_tlis = new_descendant_tlis;

      /* Debug message. */
      ereport(DEBUG1,
          errmsg_internal("switch point from TLI %u to TLI %u is at %X/%08X",
                  current_tli, descendant_tlis[0],
                  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,
     * backing up to the exact switch point if we passed it.
     */
    if (XLogRecPtrIsValid(switch_lsn) && current_lsn >= switch_lsn)
    {
      /* Restart summarization from switch point. */
      Assert(num_descendant_tlis > 0);
      current_tli = descendant_tlis[0];
      current_lsn = switch_lsn;

      /* Switch point, if any, and future TLIs, not yet known. */
      switch_lsn = InvalidXLogRecPtr;
      num_descendant_tlis = 0;
      pfree(descendant_tlis);
      descendant_tlis = NULL;

      /* Update (really, rewind, if needed) state in shared memory. */
      LWLockAcquire(WALSummarizerLock, LW_EXCLUSIVE);
      WalSummarizerCtl->summarized_lsn = current_lsn;
      WalSummarizerCtl->summarized_tli = current_tli;
      WalSummarizerCtl->lsn_is_exact = true;
      WalSummarizerCtl->pending_lsn = current_lsn;
      LWLockRelease(WALSummarizerLock);

      continue;
    }

    /* Summarize WAL. */
    maximum_lsn = XLogRecPtrIsValid(switch_lsn) ? switch_lsn : latest_lsn;
    end_of_summary_lsn = SummarizeWAL(current_tli,
                      current_lsn, exact,
                      switch_lsn, maximum_lsn,
                      num_descendant_tlis, descendant_tlis);
    Assert(XLogRecPtrIsValid(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);
    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, and update shared memory state as appropriate.
 *
 * 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.
 */
XLogRecPtr
GetOldestUnsummarizedLSN(TimeLineID *tli, bool *lsn_is_exact)
{
  TimeLineID  latest_tli;
  int     n;
  List     *tles;
  XLogRecPtr  unsummarized_lsn = InvalidXLogRecPtr;
  TimeLineID  unsummarized_tli = 0;
  bool    should_make_exact = false;
  List     *existing_summaries;
  ListCell   *lc;
  bool    am_wal_summarizer = AmWalSummarizerProcess();

  /* If not summarizing WAL, do nothing. */
  if (!summarize_wal)
    return InvalidXLogRecPtr;

  /*
   * If we are not the WAL summarizer process, then we normally just want to
   * read the values from shared memory. However, as an exception, if shared
   * memory hasn't been initialized yet, then we need to do that so that we
   * can read legal values and not remove any WAL too early.
   */
  if (!am_wal_summarizer)
  {
    LWLockAcquire(WALSummarizerLock, LW_SHARED);

    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;
      LWLockRelease(WALSummarizerLock);
      return unsummarized_lsn;
    }

    LWLockRelease(WALSummarizerLock);
  }

  /*
   * Find the oldest timeline on which WAL still exists, and the earliest
   * segment for which it exists.
   *
   * Note that we do this every time the WAL summarizer process restarts or
   * recovers from an error, in case the contents of pg_wal have changed
   * under us e.g. if some files were removed, either manually - which
   * shouldn't really happen, but might - or by postgres itself, if
   * summarize_wal was turned off and then back on again.
   */
  (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;
    }
  }

  /*
   * 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;
    }
  }

  /* 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));

  /*
   * If we're the WAL summarizer, we always want to store the values we just
   * computed into shared memory, because those are the values we're going
   * to use to drive our operation, and so they are the authoritative
   * values. Otherwise, we only store values into shared memory if shared
   * memory is uninitialized. Our values are not canonical in such a case,
   * but it's better to have something than nothing, to guide WAL retention.
   */
  LWLockAcquire(WALSummarizerLock, LW_EXCLUSIVE);
  if (am_wal_summarizer || !WalSummarizerCtl->initialized)
  {
    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;
  }
  else
    unsummarized_lsn = WalSummarizerCtl->summarized_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;
}

/*
 * Wake up the WAL summarizer process.
 *
 * 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
WakeupWalSummarizer(void)
{
  ProcNumber  pgprocno;

  if (WalSummarizerCtl == NULL)
    return;

  LWLockAcquire(WALSummarizerLock, LW_SHARED);
  pgprocno = WalSummarizerCtl->summarizer_pgprocno;
  LWLockRelease(WALSummarizerLock);

  if (pgprocno != INVALID_PROC_NUMBER)
    SetLatch(&GetPGProcByNumber(pgprocno)->procLatch);
}

/*
 * Wait until WAL summarization reaches the given LSN, but time out with an
 * error if the summarizer seems to be stick.
 *
 * Returns immediately if summarize_wal is turned off while we wait. Caller
 * is expected to handle this case, if necessary.
 */
void
WaitForWalSummarization(XLogRecPtr lsn)
{
  TimestampTz initial_time,
        cycle_time,
        current_time;
  XLogRecPtr  prior_pending_lsn = InvalidXLogRecPtr;
  int     deadcycles = 0;

  initial_time = cycle_time = GetCurrentTimestamp();

  while (1)
  {
    long    timeout_in_ms = 10000;
    XLogRecPtr  summarized_lsn;
    XLogRecPtr  pending_lsn;

    CHECK_FOR_INTERRUPTS();

    /* If WAL summarization is disabled while we're waiting, give up. */
    if (!summarize_wal)
      return;

    /*
     * If the LSN summarized on disk has reached the target value, stop.
     */
    LWLockAcquire(WALSummarizerLock, LW_SHARED);
    summarized_lsn = WalSummarizerCtl->summarized_lsn;
    pending_lsn = WalSummarizerCtl->pending_lsn;
    LWLockRelease(WALSummarizerLock);

    /* If WAL summarization has progressed sufficiently, stop waiting. */
    if (summarized_lsn >= lsn)
      break;

    /* Recheck current time. */
    current_time = GetCurrentTimestamp();

    /* Have we finished the current cycle of waiting? */
    if (TimestampDifferenceMilliseconds(cycle_time,
                      current_time) >= timeout_in_ms)
    {
      long    elapsed_seconds;

      /* Begin new wait cycle. */
      cycle_time = TimestampTzPlusMilliseconds(cycle_time,
                           timeout_in_ms);

      /*
       * Keep track of the number of cycles during which there has been
       * no progression of pending_lsn. If pending_lsn is not advancing,
       * that means that not only are no new files appearing on disk,
       * but we're not even incorporating new records into the in-memory
       * state.
       */
      if (pending_lsn > prior_pending_lsn)
      {
        prior_pending_lsn = pending_lsn;
        deadcycles = 0;
      }
      else
        ++deadcycles;

      /*
       * If we've managed to wait for an entire minute without the WAL
       * summarizer absorbing a single WAL record, error out; probably
       * something is wrong.
       *
       * We could consider also erroring out if the summarizer is taking
       * too long to catch up, but it's not clear what rate of progress
       * would be acceptable and what would be too slow. So instead, we
       * just try to error out in the case where there's no progress at
       * all. That seems likely to catch a reasonable number of the
       * things that can go wrong in practice (e.g. the summarizer
       * process is completely hung, say because somebody hooked up a
       * debugger to it or something) without giving up too quickly when
       * the system is just slow.
       */
      if (deadcycles >= 6)
        ereport(ERROR,
            (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
             errmsg("WAL summarization is not progressing"),
             errdetail("Summarization is needed through %X/%08X, but is stuck at %X/%08X on disk and %X/%08X in memory.",
                   LSN_FORMAT_ARGS(lsn),
                   LSN_FORMAT_ARGS(summarized_lsn),
                   LSN_FORMAT_ARGS(pending_lsn))));


      /*
       * Otherwise, just let the user know what's happening.
       */
      elapsed_seconds =
        TimestampDifferenceMilliseconds(initial_time,
                        current_time) / 1000;
      ereport(WARNING,
          (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
           errmsg_plural("still waiting for WAL summarization through %X/%08X after %ld second",
                   "still waiting for WAL summarization through %X/%08X after %ld seconds",
                   elapsed_seconds,
                   LSN_FORMAT_ARGS(lsn),
                   elapsed_seconds),
           errdetail("Summarization has reached %X/%08X on disk and %X/%08X in memory.",
                 LSN_FORMAT_ARGS(summarized_lsn),
                 LSN_FORMAT_ARGS(pending_lsn))));
    }

    /*
     * Align the wait time to prevent drift. This doesn't really matter,
     * but we'd like the warnings about how long we've been waiting to say
     * 10 seconds, 20 seconds, 30 seconds, 40 seconds ... without ever
     * drifting to something that is not a multiple of ten.
     */
    timeout_in_ms -=
      TimestampDifferenceMilliseconds(cycle_time, current_time);

    /* Wait and see. */
    ConditionVariableTimedSleep(&WalSummarizerCtl->summary_file_cv,
                  timeout_in_ms,
                  WAIT_EVENT_WAL_SUMMARY_READY);
  }

  ConditionVariableCancelSleep();
}

/*
 * 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;
    TimeLineID  insert_tli;

    /*
     * After the insert TLI has been set and before the control file has
     * been updated to show the DB in production, RecoveryInProgress()
     * will return true, because it's not yet safe for all backends to
     * begin writing WAL. However, replay has already ceased, so from our
     * point of view, recovery is already over. We should summarize up to
     * where replay stopped and then prepare to resume at the start of the
     * insert timeline.
     */
    if ((insert_tli = GetWALInsertionTimeLineIfSet()) != 0)
    {
      *tli = insert_tli;
      return GetXLogReplayRecPtr(NULL);
    }

    /*
     * 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;
    }
  }
}

/*
 * Compute the LSN at which we switched from current_tli to some later timeline.
 * 'tles' must be the timeline history of the latest timeline.
 *
 * As a side effect, we set *num_descendant_tlis to the number of later TLIs that
 * appear in the timeline history, and *descendant_tlis to an array of those TLIs,
 * starting with immediate successor of current_tli.
 */
static XLogRecPtr
WalSummarizerSwitchPoint(TimeLineID current_tli, List *tles,
             int *num_descendant_tlis, TimeLineID **descendant_tlis)
{
  XLogRecPtr  switch_lsn = InvalidXLogRecPtr;
  int     count = 0;

  /*
   * Find the switch point and, at the same time, count the number of TLIs
   * in this history that are descendants of that TLI.
   */
  foreach_ptr(TimeLineHistoryEntry, tle, tles)
  {
    if (tle->tli == current_tli)
    {
      switch_lsn = tle->end;
      break;
    }
    ++count;
  }

  /* Sanity checks. */
  if (!XLogRecPtrIsValid(switch_lsn))
    ereport(ERROR,
        (errmsg("requested timeline %u is not in this server's history",
            current_tli)));
  if (count == 0)
    elog(ERROR, "cannot compute switch point for current TLI %u", current_tli);

  /*
   * Generate an array of TLIs that are part of this history and descendants
   * of current_tli. The TLE list starts with the newest timeline and works
   * backward toward older timelines; we want the opposite ordering.
   */
  *num_descendant_tlis = count;
  *descendant_tlis = palloc_array(TimeLineID, count);
  for (int i = 0; i < count; ++i)
  {
    TimeLineHistoryEntry *tle;

    tle = (TimeLineHistoryEntry *) list_nth(tles, count - i - 1);
    (*descendant_tlis)[i] = tle->tli;
  }

  /* Return value is the switchpoint. */
  return switch_lsn;
}

/*
 * Interrupt handler for main loop of WAL summarizer process.
 */
static void
ProcessWalSummarizerInterrupts(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,
       int num_descendant_tlis, TimeLineID *descendant_tlis)
{
  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();
  bool    fast_forward = true;
  char     *errormsg;

  /* Initialize private data for xlogreader. */
  private_data = palloc0_object(SummarizerReadLocalXLogPrivate);
  private_data->tli = tli;
  private_data->historic = XLogRecPtrIsValid(switch_lsn);
  private_data->read_upto = maximum_lsn;
  private_data->num_descendant_tlis = num_descendant_tlis;
  private_data->descendant_tlis = descendant_tlis;

  /* Create xlogreader. */
  xlogreader = XLogReaderAllocate(wal_segment_size, NULL,
                  XL_ROUTINE(.page_read = &summarizer_read_local_xlog_page,
                         .segment_open = &summarizer_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, &errormsg);
    if (!XLogRecPtrIsValid(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/%08X: end of WAL at %X/%08X",
                    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
      {
        if (errormsg)
          ereport(ERROR,
              errmsg("could not find a valid record after %X/%08X: %s",
                   LSN_FORMAT_ARGS(start_lsn), errormsg));
        else
          ereport(ERROR,
              errmsg("could not find a valid record after %X/%08X",
                   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;
    XLogRecord *record;
    uint8   rmid;

    ProcessWalSummarizerInterrupts();

    /* 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/%08X: end of WAL at %X/%08X",
                    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/%08X: %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/%08X",
                tli, LSN_FORMAT_ARGS(xlogreader->EndRecPtr))));
    }

    /* We shouldn't go backward. */
    Assert(summary_start_lsn <= xlogreader->EndRecPtr);

    if (XLogRecPtrIsValid(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;
    }

    /*
     * Certain types of records require special handling. Redo points and
     * shutdown checkpoints trigger creation of new summary files and can
     * also cause us to enter or exit "fast forward" mode. Other types of
     * records can require special updates to the block reference table.
     */
    rmid = XLogRecGetRmid(xlogreader);
    if (rmid == RM_XLOG_ID)
    {
      bool    new_fast_forward;

      /*
       * If we've already processed some WAL records when we hit a redo
       * point or shutdown checkpoint, then we stop summarization before
       * including this record in the current file, so that it will be
       * the first record in the next file.
       *
       * When we hit one of those record types as the first record in a
       * file, we adjust our notion of whether we're fast-forwarding.
       * Any WAL generated with wal_level=minimal must be skipped
       * without actually generating any summary file, because an
       * incremental backup that crosses such WAL would be unsafe.
       */
      if (SummarizeXlogRecord(xlogreader, &new_fast_forward))
      {
        if (xlogreader->ReadRecPtr > summary_start_lsn)
        {
          summary_end_lsn = xlogreader->ReadRecPtr;
          break;
        }
        else
          fast_forward = new_fast_forward;
      }
    }
    else if (!fast_forward)
    {
      /*
       * This switch handles record types that require extra updates to
       * the contents of the block reference table.
       */
      switch (rmid)
      {
        case RM_DBASE_ID:
          SummarizeDbaseRecord(xlogreader, brtab);
          break;
        case RM_SMGR_ID:
          SummarizeSmgrRecord(xlogreader, brtab);
          break;
        case RM_XACT_ID:
          SummarizeXactRecord(xlogreader, brtab);
          break;
      }
    }

    /*
     * If we're in fast-forward mode, we don't really need to do anything.
     * Otherwise, feed block references from xlog record to block
     * reference table.
     */
    if (!fast_forward)
    {
      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->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 (XLogRecPtrIsValid(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. We can also skip writing a file if we're in
   * fast-forward mode.
   */
  if (summary_end_lsn > summary_start_lsn && !fast_forward)
  {
    /* 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_internal("summarized WAL on TLI %u from %X/%08X to %X/%08X",
                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);
  }

  /* If we skipped a non-zero amount of WAL, log a debug message. */
  if (summary_end_lsn > summary_start_lsn && fast_forward)
    ereport(DEBUG1,
        errmsg_internal("skipped summarizing WAL on TLI %u from %X/%08X to %X/%08X",
                tli,
                LSN_FORMAT_ARGS(summary_start_lsn),
                LSN_FORMAT_ARGS(summary_end_lsn)));

  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)
    {
      BlockNumber vmlimit = visibilitymap_truncation_length(xlrec->blkno);

      BlockRefTableSetLimitBlock(brtab, &xlrec->rlocator,
                     VISIBILITYMAP_FORKNUM, vmlimit);

      /*
       * visibilitymap_prepare_truncate() may clear trailing bits in the
       * last remaining VM page without WAL-logging the page.  Mark it
       * as modified so that incremental backups include its current
       * contents. Do this even at a VM page boundary, where no bits
       * need clearing.
       */
      if (vmlimit > 0)
        BlockRefTableMarkBlockModified(brtab, &xlrec->rlocator,
                         VISIBILITYMAP_FORKNUM,
                         vmlimit - 1);
    }
  }
}

/*
 * 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.
 *
 * The return value is true if WAL summarization should stop before this
 * record and false otherwise. When the return value is true,
 * *new_fast_forward indicates whether future processing should be done
 * in fast forward mode (i.e. read WAL without emitting summaries) or not.
 */
static bool
SummarizeXlogRecord(XLogReaderState *xlogreader, bool *new_fast_forward)
{
  uint8   info = XLogRecGetInfo(xlogreader) & ~XLR_INFO_MASK;
  int     record_wal_level;

  if (info == XLOG_CHECKPOINT_REDO)
  {
    xl_checkpoint_redo xlrec;

    /* Payload is wal_level at the time record was written. */
    memcpy(&xlrec, XLogRecGetData(xlogreader), sizeof(xl_checkpoint_redo));
    record_wal_level = xlrec.wal_level;
  }
  else if (info == XLOG_CHECKPOINT_SHUTDOWN)
  {
    CheckPoint  rec_ckpt;

    /* Extract wal_level at time record was written from payload. */
    memcpy(&rec_ckpt, XLogRecGetData(xlogreader), sizeof(CheckPoint));
    record_wal_level = rec_ckpt.wal_level;
  }
  else if (info == XLOG_PARAMETER_CHANGE)
  {
    xl_parameter_change xlrec;

    /* Extract wal_level at time record was written from payload. */
    memcpy(&xlrec, XLogRecGetData(xlogreader),
         sizeof(xl_parameter_change));
    record_wal_level = xlrec.wal_level;
  }
  else if (info == XLOG_END_OF_RECOVERY)
  {
    xl_end_of_recovery xlrec;

    /* Extract wal_level at time record was written from payload. */
    memcpy(&xlrec, XLogRecGetData(xlogreader), sizeof(xl_end_of_recovery));
    record_wal_level = xlrec.wal_level;
  }
  else
  {
    /* No special handling required. Return false. */
    return false;
  }

  /*
   * Redo can only begin at an XLOG_CHECKPOINT_REDO or
   * XLOG_CHECKPOINT_SHUTDOWN record, so we want WAL summarization to begin
   * at those points. Hence, when those records are encountered, return
   * true, so that we stop just before summarizing either of those records.
   *
   * We also reach here if we just saw XLOG_END_OF_RECOVERY or
   * XLOG_PARAMETER_CHANGE. These are not places where recovery can start,
   * but they're still relevant here. A new timeline can begin with
   * XLOG_END_OF_RECOVERY, so we need to confirm the WAL level at that
   * point; and a restart can provoke XLOG_PARAMETER_CHANGE after an
   * intervening change to postgresql.conf, which might force us to stop
   * summarizing.
   */
  *new_fast_forward = (record_wal_level == WAL_LEVEL_MINIMAL);
  return true;
}

/*
 * Similar to wal_segment_open, but checks for a file on any descendant timelines
 * known to us if no file is found on the requested timeline.
 */
static void
summarizer_wal_segment_open(XLogReaderState *state, XLogSegNo nextSegNo,
              TimeLineID *tli_p)
{
  SummarizerReadLocalXLogPrivate *private_data = state->private_data;
  int     count = 0;
  TimeLineID  tli = *tli_p;
  char    path[MAXPGPATH];

  for (;;)
  {
    XLogFilePath(path, tli, nextSegNo, state->segcxt.ws_segsize);
    state->seg.ws_file = BasicOpenFile(path, O_RDONLY | PG_BINARY);
    if (state->seg.ws_file >= 0)
    {
      XLogReaderRegisterResetCallback(state);
      *tli_p = tli;
      return;
    }

    /*
     * If the error is anything other than file-not-found, complain at
     * once.
     */
    if (errno != ENOENT)
      ereport(ERROR,
          (errcode_for_file_access(),
           errmsg("could not open file \"%s\": %m",
              path)));

    /* Try other timelines, if any remain. */
    if (count >= private_data->num_descendant_tlis)
      break;
    tli = private_data->descendant_tlis[count];
    ++count;
  }

  /* Complain about the originally requested filename. */
  XLogFilePath(path, *tli_p, nextSegNo, state->segcxt.ws_segsize);
  ereport(ERROR,
      (errcode_for_file_access(),
       errmsg("requested WAL segment %s has already been removed",
          path)));
}

/*
 * 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;

  ProcessWalSummarizerInterrupts();

  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.
         */
        ProcessWalSummarizerInterrupts();
        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 we might have been promoted before
           * reaching the end of the previous timeline. In that
           * case, the next loop iteration will likely conclude that
           * we've reached end of WAL.
           */
          private_data->read_upto = switchpoint;

          /* Debugging output. */
          ereport(DEBUG1,
              errmsg_internal("timeline %u became historic, can read up to %X/%08X",
                      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;
  }

  /* Report pending statistics to the cumulative stats system. */
  pgstat_report_wal(false);

  /* 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;

    ProcessWalSummarizerInterrupts();

    /*
     * 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);

      ProcessWalSummarizerInterrupts();

      /* 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 (!XLogRecPtrIsValid(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);
    }
  }
}

Coverage Report

Created: 2026-11-12 07:20