/src/postgres/src/backend/utils/activity/pgstat.c

Source
/* ----------
 * pgstat.c
 *    Infrastructure for the cumulative statistics system.
 *
 * The cumulative statistics system accumulates statistics for different kinds
 * of objects. Some kinds of statistics are collected for a fixed number of
 * objects (most commonly 1), e.g., checkpointer statistics. Other kinds of
 * statistics are collected for a varying number of objects
 * (e.g. relations). See PgStat_KindInfo for a list of currently handled
 * statistics.
 *
 * Statistics are loaded from the filesystem during startup (by the startup
 * process), unless preceded by a crash, in which case all stats are
 * discarded. They are written out by the checkpointer process just before
 * shutting down (if the stats kind allows it), except when shutting down in
 * immediate mode.
 *
 * Fixed-numbered stats are stored in plain (non-dynamic) shared memory.
 *
 * Statistics for variable-numbered objects are stored in dynamic shared
 * memory and can be found via a dshash hashtable. The statistics counters are
 * not part of the dshash entry (PgStatShared_HashEntry) directly, but are
 * separately allocated (PgStatShared_HashEntry->body). The separate
 * allocation allows different kinds of statistics to be stored in the same
 * hashtable without wasting space in PgStatShared_HashEntry.
 *
 * Variable-numbered stats are addressed by PgStat_HashKey while running.  It
 * is not possible to have statistics for an object that cannot be addressed
 * that way at runtime. A wider identifier can be used when serializing to
 * disk (used for replication slot stats).
 *
 * To avoid contention on the shared hashtable, each backend has a
 * backend-local hashtable (pgStatEntryRefHash) in front of the shared
 * hashtable, containing references (PgStat_EntryRef) to shared hashtable
 * entries. The shared hashtable only needs to be accessed when no prior
 * reference is found in the local hashtable. Besides pointing to the
 * shared hashtable entry (PgStatShared_HashEntry) PgStat_EntryRef also
 * contains a pointer to the shared statistics data, as a process-local
 * address, to reduce access costs.
 *
 * The names for structs stored in shared memory are prefixed with
 * PgStatShared instead of PgStat. Each stats entry in shared memory is
 * protected by a dedicated lwlock.
 *
 * Most stats updates are first accumulated locally in each process as pending
 * entries, then later flushed to shared memory (just after commit, or by
 * idle-timeout). This practically eliminates contention on individual stats
 * entries. For most kinds of variable-numbered pending stats data is stored
 * in PgStat_EntryRef->pending. All entries with pending data are in the
 * pgStatPending list. Pending statistics updates are flushed out by
 * pgstat_report_stat().
 *
 * It is possible for external modules to define custom statistics kinds,
 * that can use the same properties as any built-in stats kinds.  Each custom
 * stats kind needs to assign a unique ID to ensure that it does not overlap
 * with other extensions.  In order to reserve a unique stats kind ID, refer
 * to https://wiki.postgresql.org/wiki/CustomCumulativeStats.
 *
 * The behavior of different kinds of statistics is determined by the kind's
 * entry in pgstat_kind_builtin_infos for all the built-in statistics kinds
 * defined, and pgstat_kind_custom_infos for custom kinds registered at
 * startup by pgstat_register_kind().  See PgStat_KindInfo for details.
 *
 * The consistency of read accesses to statistics can be configured using the
 * stats_fetch_consistency GUC (see config.sgml and monitoring.sgml for the
 * settings). When using PGSTAT_FETCH_CONSISTENCY_CACHE or
 * PGSTAT_FETCH_CONSISTENCY_SNAPSHOT statistics are stored in
 * pgStatLocal.snapshot.
 *
 * To keep things manageable, stats handling is split across several
 * files. Infrastructure pieces are in:
 * - pgstat.c - this file, to tie it all together
 * - pgstat_shmem.c - nearly everything dealing with shared memory, including
 *   the maintenance of hashtable entries
 * - pgstat_xact.c - transactional integration, including the transactional
 *   creation and dropping of stats entries
 *
 * Each statistics kind is handled in a dedicated file:
 * - pgstat_archiver.c
 * - pgstat_backend.c
 * - pgstat_bgwriter.c
 * - pgstat_checkpointer.c
 * - pgstat_database.c
 * - pgstat_function.c
 * - pgstat_io.c
 * - pgstat_relation.c
 * - pgstat_replslot.c
 * - pgstat_slru.c
 * - pgstat_subscription.c
 * - pgstat_wal.c
 *
 * Whenever possible infrastructure files should not contain code related to
 * specific kinds of stats.
 *
 *
 * Copyright (c) 2001-2025, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    src/backend/utils/activity/pgstat.c
 * ----------
 */
#include "postgres.h"

#include <unistd.h>

#include "access/xact.h"
#include "lib/dshash.h"
#include "pgstat.h"
#include "storage/fd.h"
#include "storage/ipc.h"
#include "storage/lwlock.h"
#include "utils/guc_hooks.h"
#include "utils/memutils.h"
#include "utils/pgstat_internal.h"
#include "utils/timestamp.h"


/* ----------
 * Timer definitions.
 *
 * In milliseconds.
 * ----------
 */

/* minimum interval non-forced stats flushes.*/
#define PGSTAT_MIN_INTERVAL     1000
/* how long until to block flushing pending stats updates */
#define PGSTAT_MAX_INTERVAL     60000
/* when to call pgstat_report_stat() again, even when idle */
#define PGSTAT_IDLE_INTERVAL    10000

/* ----------
 * Initial size hints for the hash tables used in statistics.
 * ----------
 */

#define PGSTAT_SNAPSHOT_HASH_SIZE 512

/* ---------
 * Identifiers in stats file.
 * ---------
 */
#define PGSTAT_FILE_ENTRY_END 'E' /* end of file */
#define PGSTAT_FILE_ENTRY_FIXED 'F' /* fixed-numbered stats entry */
#define PGSTAT_FILE_ENTRY_NAME  'N' /* stats entry identified by name */
#define PGSTAT_FILE_ENTRY_HASH  'S' /* stats entry identified by
                   * PgStat_HashKey */

/* hash table for statistics snapshots entry */
typedef struct PgStat_SnapshotEntry
{
  PgStat_HashKey key;
  char    status;     /* for simplehash use */
  void     *data;     /* the stats data itself */
} PgStat_SnapshotEntry;


/* ----------
 * Backend-local Hash Table Definitions
 * ----------
 */

/* for stats snapshot entries */
#define SH_PREFIX pgstat_snapshot
#define SH_ELEMENT_TYPE PgStat_SnapshotEntry
#define SH_KEY_TYPE PgStat_HashKey
#define SH_KEY key
#define SH_HASH_KEY(tb, key) \
  pgstat_hash_hash_key(&key, sizeof(PgStat_HashKey), NULL)
#define SH_EQUAL(tb, a, b) \
  pgstat_cmp_hash_key(&a, &b, sizeof(PgStat_HashKey), NULL) == 0
#define SH_SCOPE static inline
#define SH_DEFINE
#define SH_DECLARE
#include "lib/simplehash.h"


/* ----------
 * Local function forward declarations
 * ----------
 */

static void pgstat_write_statsfile(void);
static void pgstat_read_statsfile(void);

static void pgstat_init_snapshot_fixed(void);

static void pgstat_reset_after_failure(void);

static bool pgstat_flush_pending_entries(bool nowait);

static void pgstat_prep_snapshot(void);
static void pgstat_build_snapshot(void);
static void pgstat_build_snapshot_fixed(PgStat_Kind kind);

static inline bool pgstat_is_kind_valid(PgStat_Kind kind);


/* ----------
 * GUC parameters
 * ----------
 */

bool    pgstat_track_counts = false;
int     pgstat_fetch_consistency = PGSTAT_FETCH_CONSISTENCY_CACHE;


/* ----------
 * state shared with pgstat_*.c
 * ----------
 */

PgStat_LocalState pgStatLocal;

/*
 * Track pending reports for fixed-numbered stats, used by
 * pgstat_report_stat().
 */
bool    pgstat_report_fixed = false;

/* ----------
 * Local data
 *
 * NB: There should be only variables related to stats infrastructure here,
 * not for specific kinds of stats.
 * ----------
 */

/*
 * Memory contexts containing the pgStatEntryRefHash table, the
 * pgStatSharedRef entries, and pending data respectively. Mostly to make it
 * easier to track / attribute memory usage.
 */

static MemoryContext pgStatPendingContext = NULL;

/*
 * Backend local list of PgStat_EntryRef with unflushed pending stats.
 *
 * Newly pending entries should only ever be added to the end of the list,
 * otherwise pgstat_flush_pending_entries() might not see them immediately.
 */
static dlist_head pgStatPending = DLIST_STATIC_INIT(pgStatPending);


/*
 * Force the next stats flush to happen regardless of
 * PGSTAT_MIN_INTERVAL. Useful in test scripts.
 */
static bool pgStatForceNextFlush = false;

/*
 * Force-clear existing snapshot before next use when stats_fetch_consistency
 * is changed.
 */
static bool force_stats_snapshot_clear = false;


/*
 * For assertions that check pgstat is not used before initialization / after
 * shutdown.
 */
#ifdef USE_ASSERT_CHECKING
static bool pgstat_is_initialized = false;
static bool pgstat_is_shutdown = false;
#endif


/*
 * The different kinds of built-in statistics.
 *
 * If reasonably possible, handling specific to one kind of stats should go
 * through this abstraction, rather than making more of pgstat.c aware.
 *
 * See comments for struct PgStat_KindInfo for details about the individual
 * fields.
 *
 * XXX: It'd be nicer to define this outside of this file. But there doesn't
 * seem to be a great way of doing that, given the split across multiple
 * files.
 */
static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_BUILTIN_SIZE] = {

  /* stats kinds for variable-numbered objects */

  [PGSTAT_KIND_DATABASE] = {
    .name = "database",

    .fixed_amount = false,
    .write_to_file = true,
    /* so pg_stat_database entries can be seen in all databases */
    .accessed_across_databases = true,

    .shared_size = sizeof(PgStatShared_Database),
    .shared_data_off = offsetof(PgStatShared_Database, stats),
    .shared_data_len = sizeof(((PgStatShared_Database *) 0)->stats),
    .pending_size = sizeof(PgStat_StatDBEntry),

    .flush_pending_cb = pgstat_database_flush_cb,
    .reset_timestamp_cb = pgstat_database_reset_timestamp_cb,
  },

  [PGSTAT_KIND_RELATION] = {
    .name = "relation",

    .fixed_amount = false,
    .write_to_file = true,

    .shared_size = sizeof(PgStatShared_Relation),
    .shared_data_off = offsetof(PgStatShared_Relation, stats),
    .shared_data_len = sizeof(((PgStatShared_Relation *) 0)->stats),
    .pending_size = sizeof(PgStat_TableStatus),

    .flush_pending_cb = pgstat_relation_flush_cb,
    .delete_pending_cb = pgstat_relation_delete_pending_cb,
  },

  [PGSTAT_KIND_FUNCTION] = {
    .name = "function",

    .fixed_amount = false,
    .write_to_file = true,

    .shared_size = sizeof(PgStatShared_Function),
    .shared_data_off = offsetof(PgStatShared_Function, stats),
    .shared_data_len = sizeof(((PgStatShared_Function *) 0)->stats),
    .pending_size = sizeof(PgStat_FunctionCounts),

    .flush_pending_cb = pgstat_function_flush_cb,
  },

  [PGSTAT_KIND_REPLSLOT] = {
    .name = "replslot",

    .fixed_amount = false,
    .write_to_file = true,

    .accessed_across_databases = true,

    .shared_size = sizeof(PgStatShared_ReplSlot),
    .shared_data_off = offsetof(PgStatShared_ReplSlot, stats),
    .shared_data_len = sizeof(((PgStatShared_ReplSlot *) 0)->stats),

    .reset_timestamp_cb = pgstat_replslot_reset_timestamp_cb,
    .to_serialized_name = pgstat_replslot_to_serialized_name_cb,
    .from_serialized_name = pgstat_replslot_from_serialized_name_cb,
  },

  [PGSTAT_KIND_SUBSCRIPTION] = {
    .name = "subscription",

    .fixed_amount = false,
    .write_to_file = true,
    /* so pg_stat_subscription_stats entries can be seen in all databases */
    .accessed_across_databases = true,

    .shared_size = sizeof(PgStatShared_Subscription),
    .shared_data_off = offsetof(PgStatShared_Subscription, stats),
    .shared_data_len = sizeof(((PgStatShared_Subscription *) 0)->stats),
    .pending_size = sizeof(PgStat_BackendSubEntry),

    .flush_pending_cb = pgstat_subscription_flush_cb,
    .reset_timestamp_cb = pgstat_subscription_reset_timestamp_cb,
  },

  [PGSTAT_KIND_BACKEND] = {
    .name = "backend",

    .fixed_amount = false,
    .write_to_file = false,

    .accessed_across_databases = true,

    .shared_size = sizeof(PgStatShared_Backend),
    .shared_data_off = offsetof(PgStatShared_Backend, stats),
    .shared_data_len = sizeof(((PgStatShared_Backend *) 0)->stats),

    .flush_static_cb = pgstat_backend_flush_cb,
    .reset_timestamp_cb = pgstat_backend_reset_timestamp_cb,
  },

  /* stats for fixed-numbered (mostly 1) objects */

  [PGSTAT_KIND_ARCHIVER] = {
    .name = "archiver",

    .fixed_amount = true,
    .write_to_file = true,

    .snapshot_ctl_off = offsetof(PgStat_Snapshot, archiver),
    .shared_ctl_off = offsetof(PgStat_ShmemControl, archiver),
    .shared_data_off = offsetof(PgStatShared_Archiver, stats),
    .shared_data_len = sizeof(((PgStatShared_Archiver *) 0)->stats),

    .init_shmem_cb = pgstat_archiver_init_shmem_cb,
    .reset_all_cb = pgstat_archiver_reset_all_cb,
    .snapshot_cb = pgstat_archiver_snapshot_cb,
  },

  [PGSTAT_KIND_BGWRITER] = {
    .name = "bgwriter",

    .fixed_amount = true,
    .write_to_file = true,

    .snapshot_ctl_off = offsetof(PgStat_Snapshot, bgwriter),
    .shared_ctl_off = offsetof(PgStat_ShmemControl, bgwriter),
    .shared_data_off = offsetof(PgStatShared_BgWriter, stats),
    .shared_data_len = sizeof(((PgStatShared_BgWriter *) 0)->stats),

    .init_shmem_cb = pgstat_bgwriter_init_shmem_cb,
    .reset_all_cb = pgstat_bgwriter_reset_all_cb,
    .snapshot_cb = pgstat_bgwriter_snapshot_cb,
  },

  [PGSTAT_KIND_CHECKPOINTER] = {
    .name = "checkpointer",

    .fixed_amount = true,
    .write_to_file = true,

    .snapshot_ctl_off = offsetof(PgStat_Snapshot, checkpointer),
    .shared_ctl_off = offsetof(PgStat_ShmemControl, checkpointer),
    .shared_data_off = offsetof(PgStatShared_Checkpointer, stats),
    .shared_data_len = sizeof(((PgStatShared_Checkpointer *) 0)->stats),

    .init_shmem_cb = pgstat_checkpointer_init_shmem_cb,
    .reset_all_cb = pgstat_checkpointer_reset_all_cb,
    .snapshot_cb = pgstat_checkpointer_snapshot_cb,
  },

  [PGSTAT_KIND_IO] = {
    .name = "io",

    .fixed_amount = true,
    .write_to_file = true,

    .snapshot_ctl_off = offsetof(PgStat_Snapshot, io),
    .shared_ctl_off = offsetof(PgStat_ShmemControl, io),
    .shared_data_off = offsetof(PgStatShared_IO, stats),
    .shared_data_len = sizeof(((PgStatShared_IO *) 0)->stats),

    .flush_static_cb = pgstat_io_flush_cb,
    .init_shmem_cb = pgstat_io_init_shmem_cb,
    .reset_all_cb = pgstat_io_reset_all_cb,
    .snapshot_cb = pgstat_io_snapshot_cb,
  },

  [PGSTAT_KIND_SLRU] = {
    .name = "slru",

    .fixed_amount = true,
    .write_to_file = true,

    .snapshot_ctl_off = offsetof(PgStat_Snapshot, slru),
    .shared_ctl_off = offsetof(PgStat_ShmemControl, slru),
    .shared_data_off = offsetof(PgStatShared_SLRU, stats),
    .shared_data_len = sizeof(((PgStatShared_SLRU *) 0)->stats),

    .flush_static_cb = pgstat_slru_flush_cb,
    .init_shmem_cb = pgstat_slru_init_shmem_cb,
    .reset_all_cb = pgstat_slru_reset_all_cb,
    .snapshot_cb = pgstat_slru_snapshot_cb,
  },

  [PGSTAT_KIND_WAL] = {
    .name = "wal",

    .fixed_amount = true,
    .write_to_file = true,

    .snapshot_ctl_off = offsetof(PgStat_Snapshot, wal),
    .shared_ctl_off = offsetof(PgStat_ShmemControl, wal),
    .shared_data_off = offsetof(PgStatShared_Wal, stats),
    .shared_data_len = sizeof(((PgStatShared_Wal *) 0)->stats),

    .init_backend_cb = pgstat_wal_init_backend_cb,
    .flush_static_cb = pgstat_wal_flush_cb,
    .init_shmem_cb = pgstat_wal_init_shmem_cb,
    .reset_all_cb = pgstat_wal_reset_all_cb,
    .snapshot_cb = pgstat_wal_snapshot_cb,
  },
};

/*
 * Information about custom statistics kinds.
 *
 * These are saved in a different array than the built-in kinds to save
 * in clarity with the initializations.
 *
 * Indexed by PGSTAT_KIND_CUSTOM_MIN, of size PGSTAT_KIND_CUSTOM_SIZE.
 */
static const PgStat_KindInfo **pgstat_kind_custom_infos = NULL;

/* ------------------------------------------------------------
 * Functions managing the state of the stats system for all backends.
 * ------------------------------------------------------------
 */

/*
 * Read on-disk stats into memory at server start.
 *
 * Should only be called by the startup process or in single user mode.
 */
void
pgstat_restore_stats(void)
{
  pgstat_read_statsfile();
}

/*
 * Remove the stats file.  This is currently used only if WAL recovery is
 * needed after a crash.
 *
 * Should only be called by the startup process or in single user mode.
 */
void
pgstat_discard_stats(void)
{
  int     ret;

  /* NB: this needs to be done even in single user mode */

  ret = unlink(PGSTAT_STAT_PERMANENT_FILENAME);
  if (ret != 0)
  {
    if (errno == ENOENT)
      elog(DEBUG2,
         "didn't need to unlink permanent stats file \"%s\" - didn't exist",
         PGSTAT_STAT_PERMANENT_FILENAME);
    else
      ereport(LOG,
          (errcode_for_file_access(),
           errmsg("could not unlink permanent statistics file \"%s\": %m",
              PGSTAT_STAT_PERMANENT_FILENAME)));
  }
  else
  {
    ereport(DEBUG2,
        (errcode_for_file_access(),
         errmsg_internal("unlinked permanent statistics file \"%s\"",
                 PGSTAT_STAT_PERMANENT_FILENAME)));
  }

  /*
   * Reset stats contents. This will set reset timestamps of fixed-numbered
   * stats to the current time (no variable stats exist).
   */
  pgstat_reset_after_failure();
}

/*
 * pgstat_before_server_shutdown() needs to be called by exactly one process
 * during regular server shutdowns. Otherwise all stats will be lost.
 *
 * We currently only write out stats for proc_exit(0). We might want to change
 * that at some point... But right now pgstat_discard_stats() would be called
 * during the start after a disorderly shutdown, anyway.
 */
void
pgstat_before_server_shutdown(int code, Datum arg)
{
  Assert(pgStatLocal.shmem != NULL);
  Assert(!pgStatLocal.shmem->is_shutdown);

  /*
   * Stats should only be reported after pgstat_initialize() and before
   * pgstat_shutdown(). This is a convenient point to catch most violations
   * of this rule.
   */
  Assert(pgstat_is_initialized && !pgstat_is_shutdown);

  /* flush out our own pending changes before writing out */
  pgstat_report_stat(true);

  /*
   * Only write out file during normal shutdown. Don't even signal that
   * we've shutdown during irregular shutdowns, because the shutdown
   * sequence isn't coordinated to ensure this backend shuts down last.
   */
  if (code == 0)
  {
    pgStatLocal.shmem->is_shutdown = true;
    pgstat_write_statsfile();
  }
}


/* ------------------------------------------------------------
 * Backend initialization / shutdown functions
 * ------------------------------------------------------------
 */

/*
 * Shut down a single backend's statistics reporting at process exit.
 *
 * Flush out any remaining statistics counts.  Without this, operations
 * triggered during backend exit (such as temp table deletions) won't be
 * counted.
 */
static void
pgstat_shutdown_hook(int code, Datum arg)
{
  Assert(!pgstat_is_shutdown);
  Assert(IsUnderPostmaster || !IsPostmasterEnvironment);

  /*
   * If we got as far as discovering our own database ID, we can flush out
   * what we did so far.  Otherwise, we'd be reporting an invalid database
   * ID, so forget it.  (This means that accesses to pg_database during
   * failed backend starts might never get counted.)
   */
  if (OidIsValid(MyDatabaseId))
    pgstat_report_disconnect(MyDatabaseId);

  pgstat_report_stat(true);

  /* there shouldn't be any pending changes left */
  Assert(dlist_is_empty(&pgStatPending));
  dlist_init(&pgStatPending);

  /* drop the backend stats entry */
  if (!pgstat_drop_entry(PGSTAT_KIND_BACKEND, InvalidOid, MyProcNumber))
    pgstat_request_entry_refs_gc();

  pgstat_detach_shmem();

#ifdef USE_ASSERT_CHECKING
  pgstat_is_shutdown = true;
#endif
}

/*
 * Initialize pgstats state, and set up our on-proc-exit hook. Called from
 * BaseInit().
 *
 * NOTE: MyDatabaseId isn't set yet; so the shutdown hook has to be careful.
 */
void
pgstat_initialize(void)
{
  Assert(!pgstat_is_initialized);

  pgstat_attach_shmem();

  pgstat_init_snapshot_fixed();

  /* Backend initialization callbacks */
  for (PgStat_Kind kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
  {
    const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);

    if (kind_info == NULL || kind_info->init_backend_cb == NULL)
      continue;

    kind_info->init_backend_cb();
  }

  /* Set up a process-exit hook to clean up */
  before_shmem_exit(pgstat_shutdown_hook, 0);

#ifdef USE_ASSERT_CHECKING
  pgstat_is_initialized = true;
#endif
}


/* ------------------------------------------------------------
 * Public functions used by backends follow
 * ------------------------------------------------------------
 */

/*
 * Must be called by processes that performs DML: tcop/postgres.c, logical
 * receiver processes, SPI worker, etc. to flush pending statistics updates to
 * shared memory.
 *
 * Unless called with 'force', pending stats updates are flushed happen once
 * per PGSTAT_MIN_INTERVAL (1000ms). When not forced, stats flushes do not
 * block on lock acquisition, except if stats updates have been pending for
 * longer than PGSTAT_MAX_INTERVAL (60000ms).
 *
 * Whenever pending stats updates remain at the end of pgstat_report_stat() a
 * suggested idle timeout is returned. Currently this is always
 * PGSTAT_IDLE_INTERVAL (10000ms). Callers can use the returned time to set up
 * a timeout after which to call pgstat_report_stat(true), but are not
 * required to do so.
 *
 * Note that this is called only when not within a transaction, so it is fair
 * to use transaction stop time as an approximation of current time.
 */
long
pgstat_report_stat(bool force)
{
  static TimestampTz pending_since = 0;
  static TimestampTz last_flush = 0;
  bool    partial_flush;
  TimestampTz now;
  bool    nowait;

  pgstat_assert_is_up();
  Assert(!IsTransactionOrTransactionBlock());

  /* "absorb" the forced flush even if there's nothing to flush */
  if (pgStatForceNextFlush)
  {
    force = true;
    pgStatForceNextFlush = false;
  }

  /* Don't expend a clock check if nothing to do */
  if (dlist_is_empty(&pgStatPending) &&
    !pgstat_report_fixed)
  {
    return 0;
  }

  /*
   * There should never be stats to report once stats are shut down. Can't
   * assert that before the checks above, as there is an unconditional
   * pgstat_report_stat() call in pgstat_shutdown_hook() - which at least
   * the process that ran pgstat_before_server_shutdown() will still call.
   */
  Assert(!pgStatLocal.shmem->is_shutdown);

  if (force)
  {
    /*
     * Stats reports are forced either when it's been too long since stats
     * have been reported or in processes that force stats reporting to
     * happen at specific points (including shutdown). In the former case
     * the transaction stop time might be quite old, in the latter it
     * would never get cleared.
     */
    now = GetCurrentTimestamp();
  }
  else
  {
    now = GetCurrentTransactionStopTimestamp();

    if (pending_since > 0 &&
      TimestampDifferenceExceeds(pending_since, now, PGSTAT_MAX_INTERVAL))
    {
      /* don't keep pending updates longer than PGSTAT_MAX_INTERVAL */
      force = true;
    }
    else if (last_flush > 0 &&
         !TimestampDifferenceExceeds(last_flush, now, PGSTAT_MIN_INTERVAL))
    {
      /* don't flush too frequently */
      if (pending_since == 0)
        pending_since = now;

      return PGSTAT_IDLE_INTERVAL;
    }
  }

  pgstat_update_dbstats(now);

  /* don't wait for lock acquisition when !force */
  nowait = !force;

  partial_flush = false;

  /* flush of variable-numbered stats tracked in pending entries list */
  partial_flush |= pgstat_flush_pending_entries(nowait);

  /* flush of other stats kinds */
  if (pgstat_report_fixed)
  {
    for (PgStat_Kind kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
    {
      const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);

      if (!kind_info)
        continue;
      if (!kind_info->flush_static_cb)
        continue;

      partial_flush |= kind_info->flush_static_cb(nowait);
    }
  }

  last_flush = now;

  /*
   * If some of the pending stats could not be flushed due to lock
   * contention, let the caller know when to retry.
   */
  if (partial_flush)
  {
    /* force should have prevented us from getting here */
    Assert(!force);

    /* remember since when stats have been pending */
    if (pending_since == 0)
      pending_since = now;

    return PGSTAT_IDLE_INTERVAL;
  }

  pending_since = 0;
  pgstat_report_fixed = false;

  return 0;
}

/*
 * Force locally pending stats to be flushed during the next
 * pgstat_report_stat() call. This is useful for writing tests.
 */
void
pgstat_force_next_flush(void)
{
  pgStatForceNextFlush = true;
}

/*
 * Only for use by pgstat_reset_counters()
 */
static bool
match_db_entries(PgStatShared_HashEntry *entry, Datum match_data)
{
  return entry->key.dboid == MyDatabaseId;
}

/*
 * Reset counters for our database.
 *
 * Permission checking for this function is managed through the normal
 * GRANT system.
 */
void
pgstat_reset_counters(void)
{
  TimestampTz ts = GetCurrentTimestamp();

  pgstat_reset_matching_entries(match_db_entries,
                  ObjectIdGetDatum(MyDatabaseId),
                  ts);
}

/*
 * Reset a single variable-numbered entry.
 *
 * If the stats kind is within a database, also reset the database's
 * stat_reset_timestamp.
 *
 * Permission checking for this function is managed through the normal
 * GRANT system.
 */
void
pgstat_reset(PgStat_Kind kind, Oid dboid, uint64 objid)
{
  const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
  TimestampTz ts = GetCurrentTimestamp();

  /* not needed atm, and doesn't make sense with the current signature */
  Assert(!pgstat_get_kind_info(kind)->fixed_amount);

  /* reset the "single counter" */
  pgstat_reset_entry(kind, dboid, objid, ts);

  if (!kind_info->accessed_across_databases)
    pgstat_reset_database_timestamp(dboid, ts);
}

/*
 * Reset stats for all entries of a kind.
 *
 * Permission checking for this function is managed through the normal
 * GRANT system.
 */
void
pgstat_reset_of_kind(PgStat_Kind kind)
{
  const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
  TimestampTz ts = GetCurrentTimestamp();

  if (kind_info->fixed_amount)
    kind_info->reset_all_cb(ts);
  else
    pgstat_reset_entries_of_kind(kind, ts);
}


/* ------------------------------------------------------------
 * Fetching of stats
 * ------------------------------------------------------------
 */

/*
 * Discard any data collected in the current transaction.  Any subsequent
 * request will cause new snapshots to be read.
 *
 * This is also invoked during transaction commit or abort to discard
 * the no-longer-wanted snapshot.  Updates of stats_fetch_consistency can
 * cause this routine to be called.
 */
void
pgstat_clear_snapshot(void)
{
  pgstat_assert_is_up();

  memset(&pgStatLocal.snapshot.fixed_valid, 0,
       sizeof(pgStatLocal.snapshot.fixed_valid));
  memset(&pgStatLocal.snapshot.custom_valid, 0,
       sizeof(pgStatLocal.snapshot.custom_valid));
  pgStatLocal.snapshot.stats = NULL;
  pgStatLocal.snapshot.mode = PGSTAT_FETCH_CONSISTENCY_NONE;

  /* Release memory, if any was allocated */
  if (pgStatLocal.snapshot.context)
  {
    MemoryContextDelete(pgStatLocal.snapshot.context);

    /* Reset variables */
    pgStatLocal.snapshot.context = NULL;
  }

  /*
   * Historically the backend_status.c facilities lived in this file, and
   * were reset with the same function. For now keep it that way, and
   * forward the reset request.
   */
  pgstat_clear_backend_activity_snapshot();

  /* Reset this flag, as it may be possible that a cleanup was forced. */
  force_stats_snapshot_clear = false;
}

void *
pgstat_fetch_entry(PgStat_Kind kind, Oid dboid, uint64 objid)
{
  PgStat_HashKey key = {0};
  PgStat_EntryRef *entry_ref;
  void     *stats_data;
  const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);

  /* should be called from backends */
  Assert(IsUnderPostmaster || !IsPostmasterEnvironment);
  Assert(!kind_info->fixed_amount);

  pgstat_prep_snapshot();

  key.kind = kind;
  key.dboid = dboid;
  key.objid = objid;

  /* if we need to build a full snapshot, do so */
  if (pgstat_fetch_consistency == PGSTAT_FETCH_CONSISTENCY_SNAPSHOT)
    pgstat_build_snapshot();

  /* if caching is desired, look up in cache */
  if (pgstat_fetch_consistency > PGSTAT_FETCH_CONSISTENCY_NONE)
  {
    PgStat_SnapshotEntry *entry = NULL;

    entry = pgstat_snapshot_lookup(pgStatLocal.snapshot.stats, key);

    if (entry)
      return entry->data;

    /*
     * If we built a full snapshot and the key is not in
     * pgStatLocal.snapshot.stats, there are no matching stats.
     */
    if (pgstat_fetch_consistency == PGSTAT_FETCH_CONSISTENCY_SNAPSHOT)
      return NULL;
  }

  pgStatLocal.snapshot.mode = pgstat_fetch_consistency;

  entry_ref = pgstat_get_entry_ref(kind, dboid, objid, false, NULL);

  if (entry_ref == NULL || entry_ref->shared_entry->dropped)
  {
    /* create empty entry when using PGSTAT_FETCH_CONSISTENCY_CACHE */
    if (pgstat_fetch_consistency == PGSTAT_FETCH_CONSISTENCY_CACHE)
    {
      PgStat_SnapshotEntry *entry = NULL;
      bool    found;

      entry = pgstat_snapshot_insert(pgStatLocal.snapshot.stats, key, &found);
      Assert(!found);
      entry->data = NULL;
    }
    return NULL;
  }

  /*
   * Allocate in caller's context for PGSTAT_FETCH_CONSISTENCY_NONE,
   * otherwise we could quickly end up with a fair bit of memory used due to
   * repeated accesses.
   */
  if (pgstat_fetch_consistency == PGSTAT_FETCH_CONSISTENCY_NONE)
    stats_data = palloc(kind_info->shared_data_len);
  else
    stats_data = MemoryContextAlloc(pgStatLocal.snapshot.context,
                    kind_info->shared_data_len);

  (void) pgstat_lock_entry_shared(entry_ref, false);
  memcpy(stats_data,
       pgstat_get_entry_data(kind, entry_ref->shared_stats),
       kind_info->shared_data_len);
  pgstat_unlock_entry(entry_ref);

  if (pgstat_fetch_consistency > PGSTAT_FETCH_CONSISTENCY_NONE)
  {
    PgStat_SnapshotEntry *entry = NULL;
    bool    found;

    entry = pgstat_snapshot_insert(pgStatLocal.snapshot.stats, key, &found);
    entry->data = stats_data;
  }

  return stats_data;
}

/*
 * If a stats snapshot has been taken, return the timestamp at which that was
 * done, and set *have_snapshot to true. Otherwise *have_snapshot is set to
 * false.
 */
TimestampTz
pgstat_get_stat_snapshot_timestamp(bool *have_snapshot)
{
  if (force_stats_snapshot_clear)
    pgstat_clear_snapshot();

  if (pgStatLocal.snapshot.mode == PGSTAT_FETCH_CONSISTENCY_SNAPSHOT)
  {
    *have_snapshot = true;
    return pgStatLocal.snapshot.snapshot_timestamp;
  }

  *have_snapshot = false;

  return 0;
}

bool
pgstat_have_entry(PgStat_Kind kind, Oid dboid, uint64 objid)
{
  /* fixed-numbered stats always exist */
  if (pgstat_get_kind_info(kind)->fixed_amount)
    return true;

  return pgstat_get_entry_ref(kind, dboid, objid, false, NULL) != NULL;
}

/*
 * Ensure snapshot for fixed-numbered 'kind' exists.
 *
 * Typically used by the pgstat_fetch_* functions for a kind of stats, before
 * massaging the data into the desired format.
 */
void
pgstat_snapshot_fixed(PgStat_Kind kind)
{
  Assert(pgstat_is_kind_valid(kind));
  Assert(pgstat_get_kind_info(kind)->fixed_amount);

  if (force_stats_snapshot_clear)
    pgstat_clear_snapshot();

  if (pgstat_fetch_consistency == PGSTAT_FETCH_CONSISTENCY_SNAPSHOT)
    pgstat_build_snapshot();
  else
    pgstat_build_snapshot_fixed(kind);

  if (pgstat_is_kind_builtin(kind))
    Assert(pgStatLocal.snapshot.fixed_valid[kind]);
  else if (pgstat_is_kind_custom(kind))
    Assert(pgStatLocal.snapshot.custom_valid[kind - PGSTAT_KIND_CUSTOM_MIN]);
}

static void
pgstat_init_snapshot_fixed(void)
{
  /*
   * Initialize fixed-numbered statistics data in snapshots, only for custom
   * stats kinds.
   */
  for (PgStat_Kind kind = PGSTAT_KIND_CUSTOM_MIN; kind <= PGSTAT_KIND_CUSTOM_MAX; kind++)
  {
    const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);

    if (!kind_info || !kind_info->fixed_amount)
      continue;

    pgStatLocal.snapshot.custom_data[kind - PGSTAT_KIND_CUSTOM_MIN] =
      MemoryContextAlloc(TopMemoryContext, kind_info->shared_data_len);
  }
}

static void
pgstat_prep_snapshot(void)
{
  if (force_stats_snapshot_clear)
    pgstat_clear_snapshot();

  if (pgstat_fetch_consistency == PGSTAT_FETCH_CONSISTENCY_NONE ||
    pgStatLocal.snapshot.stats != NULL)
    return;

  if (!pgStatLocal.snapshot.context)
    pgStatLocal.snapshot.context = AllocSetContextCreate(TopMemoryContext,
                               "PgStat Snapshot",
                               ALLOCSET_SMALL_SIZES);

  pgStatLocal.snapshot.stats =
    pgstat_snapshot_create(pgStatLocal.snapshot.context,
                 PGSTAT_SNAPSHOT_HASH_SIZE,
                 NULL);
}

static void
pgstat_build_snapshot(void)
{
  dshash_seq_status hstat;
  PgStatShared_HashEntry *p;

  /* should only be called when we need a snapshot */
  Assert(pgstat_fetch_consistency == PGSTAT_FETCH_CONSISTENCY_SNAPSHOT);

  /* snapshot already built */
  if (pgStatLocal.snapshot.mode == PGSTAT_FETCH_CONSISTENCY_SNAPSHOT)
    return;

  pgstat_prep_snapshot();

  Assert(pgStatLocal.snapshot.stats->members == 0);

  pgStatLocal.snapshot.snapshot_timestamp = GetCurrentTimestamp();

  /*
   * Snapshot all variable stats.
   */
  dshash_seq_init(&hstat, pgStatLocal.shared_hash, false);
  while ((p = dshash_seq_next(&hstat)) != NULL)
  {
    PgStat_Kind kind = p->key.kind;
    const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
    bool    found;
    PgStat_SnapshotEntry *entry;
    PgStatShared_Common *stats_data;

    /*
     * Check if the stats object should be included in the snapshot.
     * Unless the stats kind can be accessed from all databases (e.g.,
     * database stats themselves), we only include stats for the current
     * database or objects not associated with a database (e.g. shared
     * relations).
     */
    if (p->key.dboid != MyDatabaseId &&
      p->key.dboid != InvalidOid &&
      !kind_info->accessed_across_databases)
      continue;

    if (p->dropped)
      continue;

    Assert(pg_atomic_read_u32(&p->refcount) > 0);

    stats_data = dsa_get_address(pgStatLocal.dsa, p->body);
    Assert(stats_data);

    entry = pgstat_snapshot_insert(pgStatLocal.snapshot.stats, p->key, &found);
    Assert(!found);

    entry->data = MemoryContextAlloc(pgStatLocal.snapshot.context,
                     pgstat_get_entry_len(kind));

    /*
     * Acquire the LWLock directly instead of using
     * pg_stat_lock_entry_shared() which requires a reference.
     */
    LWLockAcquire(&stats_data->lock, LW_SHARED);
    memcpy(entry->data,
         pgstat_get_entry_data(kind, stats_data),
         pgstat_get_entry_len(kind));
    LWLockRelease(&stats_data->lock);
  }
  dshash_seq_term(&hstat);

  /*
   * Build snapshot of all fixed-numbered stats.
   */
  for (PgStat_Kind kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
  {
    const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);

    if (!kind_info)
      continue;
    if (!kind_info->fixed_amount)
    {
      Assert(kind_info->snapshot_cb == NULL);
      continue;
    }

    pgstat_build_snapshot_fixed(kind);
  }

  pgStatLocal.snapshot.mode = PGSTAT_FETCH_CONSISTENCY_SNAPSHOT;
}

static void
pgstat_build_snapshot_fixed(PgStat_Kind kind)
{
  const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
  int     idx;
  bool     *valid;

  /* Position in fixed_valid or custom_valid */
  if (pgstat_is_kind_builtin(kind))
  {
    idx = kind;
    valid = pgStatLocal.snapshot.fixed_valid;
  }
  else
  {
    idx = kind - PGSTAT_KIND_CUSTOM_MIN;
    valid = pgStatLocal.snapshot.custom_valid;
  }

  Assert(kind_info->fixed_amount);
  Assert(kind_info->snapshot_cb != NULL);

  if (pgstat_fetch_consistency == PGSTAT_FETCH_CONSISTENCY_NONE)
  {
    /* rebuild every time */
    valid[idx] = false;
  }
  else if (valid[idx])
  {
    /* in snapshot mode we shouldn't get called again */
    Assert(pgstat_fetch_consistency == PGSTAT_FETCH_CONSISTENCY_CACHE);
    return;
  }

  Assert(!valid[idx]);

  kind_info->snapshot_cb();

  Assert(!valid[idx]);
  valid[idx] = true;
}


/* ------------------------------------------------------------
 * Backend-local pending stats infrastructure
 * ------------------------------------------------------------
 */

/*
 * Returns the appropriate PgStat_EntryRef, preparing it to receive pending
 * stats if not already done.
 *
 * If created_entry is non-NULL, it'll be set to true if the entry is newly
 * created, false otherwise.
 */
PgStat_EntryRef *
pgstat_prep_pending_entry(PgStat_Kind kind, Oid dboid, uint64 objid, bool *created_entry)
{
  PgStat_EntryRef *entry_ref;

  /* need to be able to flush out */
  Assert(pgstat_get_kind_info(kind)->flush_pending_cb != NULL);

  if (unlikely(!pgStatPendingContext))
  {
    pgStatPendingContext =
      AllocSetContextCreate(TopMemoryContext,
                  "PgStat Pending",
                  ALLOCSET_SMALL_SIZES);
  }

  entry_ref = pgstat_get_entry_ref(kind, dboid, objid,
                   true, created_entry);

  if (entry_ref->pending == NULL)
  {
    size_t    entrysize = pgstat_get_kind_info(kind)->pending_size;

    Assert(entrysize != (size_t) -1);

    entry_ref->pending = MemoryContextAllocZero(pgStatPendingContext, entrysize);
    dlist_push_tail(&pgStatPending, &entry_ref->pending_node);
  }

  return entry_ref;
}

/*
 * Return an existing stats entry, or NULL.
 *
 * This should only be used for helper function for pgstatfuncs.c - outside of
 * that it shouldn't be needed.
 */
PgStat_EntryRef *
pgstat_fetch_pending_entry(PgStat_Kind kind, Oid dboid, uint64 objid)
{
  PgStat_EntryRef *entry_ref;

  entry_ref = pgstat_get_entry_ref(kind, dboid, objid, false, NULL);

  if (entry_ref == NULL || entry_ref->pending == NULL)
    return NULL;

  return entry_ref;
}

void
pgstat_delete_pending_entry(PgStat_EntryRef *entry_ref)
{
  PgStat_Kind kind = entry_ref->shared_entry->key.kind;
  const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
  void     *pending_data = entry_ref->pending;

  Assert(pending_data != NULL);
  /* !fixed_amount stats should be handled explicitly */
  Assert(!pgstat_get_kind_info(kind)->fixed_amount);

  if (kind_info->delete_pending_cb)
    kind_info->delete_pending_cb(entry_ref);

  pfree(pending_data);
  entry_ref->pending = NULL;

  dlist_delete(&entry_ref->pending_node);
}

/*
 * Flush out pending variable-numbered stats.
 */
static bool
pgstat_flush_pending_entries(bool nowait)
{
  bool    have_pending = false;
  dlist_node *cur = NULL;

  /*
   * Need to be a bit careful iterating over the list of pending entries.
   * Processing a pending entry may queue further pending entries to the end
   * of the list that we want to process, so a simple iteration won't do.
   * Further complicating matters is that we want to delete the current
   * entry in each iteration from the list if we flushed successfully.
   *
   * So we just keep track of the next pointer in each loop iteration.
   */
  if (!dlist_is_empty(&pgStatPending))
    cur = dlist_head_node(&pgStatPending);

  while (cur)
  {
    PgStat_EntryRef *entry_ref =
      dlist_container(PgStat_EntryRef, pending_node, cur);
    PgStat_HashKey key = entry_ref->shared_entry->key;
    PgStat_Kind kind = key.kind;
    const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
    bool    did_flush;
    dlist_node *next;

    Assert(!kind_info->fixed_amount);
    Assert(kind_info->flush_pending_cb != NULL);

    /* flush the stats, if possible */
    did_flush = kind_info->flush_pending_cb(entry_ref, nowait);

    Assert(did_flush || nowait);

    /* determine next entry, before deleting the pending entry */
    if (dlist_has_next(&pgStatPending, cur))
      next = dlist_next_node(&pgStatPending, cur);
    else
      next = NULL;

    /* if successfully flushed, remove entry */
    if (did_flush)
      pgstat_delete_pending_entry(entry_ref);
    else
      have_pending = true;

    cur = next;
  }

  Assert(dlist_is_empty(&pgStatPending) == !have_pending);

  return have_pending;
}


/* ------------------------------------------------------------
 * Helper / infrastructure functions
 * ------------------------------------------------------------
 */

PgStat_Kind
pgstat_get_kind_from_str(char *kind_str)
{
  for (PgStat_Kind kind = PGSTAT_KIND_BUILTIN_MIN; kind <= PGSTAT_KIND_BUILTIN_MAX; kind++)
  {
    if (pg_strcasecmp(kind_str, pgstat_kind_builtin_infos[kind].name) == 0)
      return kind;
  }

  /* Check the custom set of cumulative stats */
  if (pgstat_kind_custom_infos)
  {
    for (PgStat_Kind kind = PGSTAT_KIND_CUSTOM_MIN; kind <= PGSTAT_KIND_CUSTOM_MAX; kind++)
    {
      uint32    idx = kind - PGSTAT_KIND_CUSTOM_MIN;

      if (pgstat_kind_custom_infos[idx] &&
        pg_strcasecmp(kind_str, pgstat_kind_custom_infos[idx]->name) == 0)
        return kind;
    }
  }

  ereport(ERROR,
      (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
       errmsg("invalid statistics kind: \"%s\"", kind_str)));
  return PGSTAT_KIND_INVALID; /* avoid compiler warnings */
}

static inline bool
pgstat_is_kind_valid(PgStat_Kind kind)
{
  return pgstat_is_kind_builtin(kind) || pgstat_is_kind_custom(kind);
}

const PgStat_KindInfo *
pgstat_get_kind_info(PgStat_Kind kind)
{
  if (pgstat_is_kind_builtin(kind))
    return &pgstat_kind_builtin_infos[kind];

  if (pgstat_is_kind_custom(kind))
  {
    uint32    idx = kind - PGSTAT_KIND_CUSTOM_MIN;

    if (pgstat_kind_custom_infos == NULL ||
      pgstat_kind_custom_infos[idx] == NULL)
      return NULL;
    return pgstat_kind_custom_infos[idx];
  }

  return NULL;
}

/*
 * Register a new stats kind.
 *
 * PgStat_Kinds must be globally unique across all extensions. Refer
 * to https://wiki.postgresql.org/wiki/CustomCumulativeStats to reserve a
 * unique ID for your extension, to avoid conflicts with other extension
 * developers. During development, use PGSTAT_KIND_EXPERIMENTAL to avoid
 * needlessly reserving a new ID.
 */
void
pgstat_register_kind(PgStat_Kind kind, const PgStat_KindInfo *kind_info)
{
  uint32    idx = kind - PGSTAT_KIND_CUSTOM_MIN;

  if (kind_info->name == NULL || strlen(kind_info->name) == 0)
    ereport(ERROR,
        (errmsg("custom cumulative statistics name is invalid"),
         errhint("Provide a non-empty name for the custom cumulative statistics.")));

  if (!pgstat_is_kind_custom(kind))
    ereport(ERROR, (errmsg("custom cumulative statistics ID %u is out of range", kind),
            errhint("Provide a custom cumulative statistics ID between %u and %u.",
                PGSTAT_KIND_CUSTOM_MIN, PGSTAT_KIND_CUSTOM_MAX)));

  if (!process_shared_preload_libraries_in_progress)
    ereport(ERROR,
        (errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
         errdetail("Custom cumulative statistics must be registered while initializing modules in \"shared_preload_libraries\".")));

  /*
   * Check some data for fixed-numbered stats.
   */
  if (kind_info->fixed_amount)
  {
    if (kind_info->shared_size == 0)
      ereport(ERROR,
          (errmsg("custom cumulative statistics property is invalid"),
           errhint("Custom cumulative statistics require a shared memory size for fixed-numbered objects.")));
  }

  /*
   * If pgstat_kind_custom_infos is not available yet, allocate it.
   */
  if (pgstat_kind_custom_infos == NULL)
  {
    pgstat_kind_custom_infos = (const PgStat_KindInfo **)
      MemoryContextAllocZero(TopMemoryContext,
                   sizeof(PgStat_KindInfo *) * PGSTAT_KIND_CUSTOM_SIZE);
  }

  if (pgstat_kind_custom_infos[idx] != NULL &&
    pgstat_kind_custom_infos[idx]->name != NULL)
    ereport(ERROR,
        (errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
         errdetail("Custom cumulative statistics \"%s\" already registered with the same ID.",
               pgstat_kind_custom_infos[idx]->name)));

  /* check for existing custom stats with the same name */
  for (PgStat_Kind existing_kind = PGSTAT_KIND_CUSTOM_MIN; existing_kind <= PGSTAT_KIND_CUSTOM_MAX; existing_kind++)
  {
    uint32    existing_idx = existing_kind - PGSTAT_KIND_CUSTOM_MIN;

    if (pgstat_kind_custom_infos[existing_idx] == NULL)
      continue;
    if (!pg_strcasecmp(pgstat_kind_custom_infos[existing_idx]->name, kind_info->name))
      ereport(ERROR,
          (errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
           errdetail("Existing cumulative statistics with ID %u has the same name.", existing_kind)));
  }

  /* Register it */
  pgstat_kind_custom_infos[idx] = kind_info;
  ereport(LOG,
      (errmsg("registered custom cumulative statistics \"%s\" with ID %u",
          kind_info->name, kind)));
}

/*
 * Stats should only be reported after pgstat_initialize() and before
 * pgstat_shutdown(). This check is put in a few central places to catch
 * violations of this rule more easily.
 */
#ifdef USE_ASSERT_CHECKING
void
pgstat_assert_is_up(void)
{
  Assert(pgstat_is_initialized && !pgstat_is_shutdown);
}
#endif


/* ------------------------------------------------------------
 * reading and writing of on-disk stats file
 * ------------------------------------------------------------
 */

/* helpers for pgstat_write_statsfile() */
static void
write_chunk(FILE *fpout, void *ptr, size_t len)
{
  int     rc;

  rc = fwrite(ptr, len, 1, fpout);

  /* we'll check for errors with ferror once at the end */
  (void) rc;
}

#define write_chunk_s(fpout, ptr) write_chunk(fpout, ptr, sizeof(*ptr))

/*
 * This function is called in the last process that is accessing the shared
 * stats so locking is not required.
 */
static void
pgstat_write_statsfile(void)
{
  FILE     *fpout;
  int32   format_id;
  const char *tmpfile = PGSTAT_STAT_PERMANENT_TMPFILE;
  const char *statfile = PGSTAT_STAT_PERMANENT_FILENAME;
  dshash_seq_status hstat;
  PgStatShared_HashEntry *ps;

  pgstat_assert_is_up();

  /* should be called only by the checkpointer or single user mode */
  Assert(!IsUnderPostmaster || MyBackendType == B_CHECKPOINTER);

  /* we're shutting down, so it's ok to just override this */
  pgstat_fetch_consistency = PGSTAT_FETCH_CONSISTENCY_NONE;

  elog(DEBUG2, "writing stats file \"%s\"", statfile);

  /*
   * Open the statistics temp file to write out the current values.
   */
  fpout = AllocateFile(tmpfile, PG_BINARY_W);
  if (fpout == NULL)
  {
    ereport(LOG,
        (errcode_for_file_access(),
         errmsg("could not open temporary statistics file \"%s\": %m",
            tmpfile)));
    return;
  }

  /*
   * Write the file header --- currently just a format ID.
   */
  format_id = PGSTAT_FILE_FORMAT_ID;
  write_chunk_s(fpout, &format_id);

  /* Write various stats structs for fixed number of objects */
  for (PgStat_Kind kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
  {
    char     *ptr;
    const PgStat_KindInfo *info = pgstat_get_kind_info(kind);

    if (!info || !info->fixed_amount)
      continue;

    if (pgstat_is_kind_builtin(kind))
      Assert(info->snapshot_ctl_off != 0);

    /* skip if no need to write to file */
    if (!info->write_to_file)
      continue;

    pgstat_build_snapshot_fixed(kind);
    if (pgstat_is_kind_builtin(kind))
      ptr = ((char *) &pgStatLocal.snapshot) + info->snapshot_ctl_off;
    else
      ptr = pgStatLocal.snapshot.custom_data[kind - PGSTAT_KIND_CUSTOM_MIN];

    fputc(PGSTAT_FILE_ENTRY_FIXED, fpout);
    write_chunk_s(fpout, &kind);
    write_chunk(fpout, ptr, info->shared_data_len);
  }

  /*
   * Walk through the stats entries
   */
  dshash_seq_init(&hstat, pgStatLocal.shared_hash, false);
  while ((ps = dshash_seq_next(&hstat)) != NULL)
  {
    PgStatShared_Common *shstats;
    const PgStat_KindInfo *kind_info = NULL;

    CHECK_FOR_INTERRUPTS();

    /*
     * We should not see any "dropped" entries when writing the stats
     * file, as all backends and auxiliary processes should have cleaned
     * up their references before they terminated.
     *
     * However, since we are already shutting down, it is not worth
     * crashing the server over any potential cleanup issues, so we simply
     * skip such entries if encountered.
     */
    Assert(!ps->dropped);
    if (ps->dropped)
      continue;

    /*
     * This discards data related to custom stats kinds that are unknown
     * to this process.
     */
    if (!pgstat_is_kind_valid(ps->key.kind))
    {
      elog(WARNING, "found unknown stats entry %u/%u/%" PRIu64,
         ps->key.kind, ps->key.dboid,
         ps->key.objid);
      continue;
    }

    shstats = (PgStatShared_Common *) dsa_get_address(pgStatLocal.dsa, ps->body);

    kind_info = pgstat_get_kind_info(ps->key.kind);

    /* if not dropped the valid-entry refcount should exist */
    Assert(pg_atomic_read_u32(&ps->refcount) > 0);

    /* skip if no need to write to file */
    if (!kind_info->write_to_file)
      continue;

    if (!kind_info->to_serialized_name)
    {
      /* normal stats entry, identified by PgStat_HashKey */
      fputc(PGSTAT_FILE_ENTRY_HASH, fpout);
      write_chunk_s(fpout, &ps->key);
    }
    else
    {
      /* stats entry identified by name on disk (e.g. slots) */
      NameData  name;

      kind_info->to_serialized_name(&ps->key, shstats, &name);

      fputc(PGSTAT_FILE_ENTRY_NAME, fpout);
      write_chunk_s(fpout, &ps->key.kind);
      write_chunk_s(fpout, &name);
    }

    /* Write except the header part of the entry */
    write_chunk(fpout,
          pgstat_get_entry_data(ps->key.kind, shstats),
          pgstat_get_entry_len(ps->key.kind));
  }
  dshash_seq_term(&hstat);

  /*
   * No more output to be done. Close the temp file and replace the old
   * pgstat.stat with it.  The ferror() check replaces testing for error
   * after each individual fputc or fwrite (in write_chunk()) above.
   */
  fputc(PGSTAT_FILE_ENTRY_END, fpout);

  if (ferror(fpout))
  {
    ereport(LOG,
        (errcode_for_file_access(),
         errmsg("could not write temporary statistics file \"%s\": %m",
            tmpfile)));
    FreeFile(fpout);
    unlink(tmpfile);
  }
  else if (FreeFile(fpout) < 0)
  {
    ereport(LOG,
        (errcode_for_file_access(),
         errmsg("could not close temporary statistics file \"%s\": %m",
            tmpfile)));
    unlink(tmpfile);
  }
  else if (durable_rename(tmpfile, statfile, LOG) < 0)
  {
    /* durable_rename already emitted log message */
    unlink(tmpfile);
  }
}

/* helpers for pgstat_read_statsfile() */
static bool
read_chunk(FILE *fpin, void *ptr, size_t len)
{
  return fread(ptr, 1, len, fpin) == len;
}

#define read_chunk_s(fpin, ptr) read_chunk(fpin, ptr, sizeof(*ptr))

/*
 * Reads in existing statistics file into memory.
 *
 * This function is called in the only process that is accessing the shared
 * stats so locking is not required.
 */
static void
pgstat_read_statsfile(void)
{
  FILE     *fpin;
  int32   format_id;
  bool    found;
  const char *statfile = PGSTAT_STAT_PERMANENT_FILENAME;
  PgStat_ShmemControl *shmem = pgStatLocal.shmem;

  /* shouldn't be called from postmaster */
  Assert(IsUnderPostmaster || !IsPostmasterEnvironment);

  elog(DEBUG2, "reading stats file \"%s\"", statfile);

  /*
   * Try to open the stats file. If it doesn't exist, the backends simply
   * returns zero for anything and statistics simply starts from scratch
   * with empty counters.
   *
   * ENOENT is a possibility if stats collection was previously disabled or
   * has not yet written the stats file for the first time.  Any other
   * failure condition is suspicious.
   */
  if ((fpin = AllocateFile(statfile, PG_BINARY_R)) == NULL)
  {
    if (errno != ENOENT)
      ereport(LOG,
          (errcode_for_file_access(),
           errmsg("could not open statistics file \"%s\": %m",
              statfile)));
    pgstat_reset_after_failure();
    return;
  }

  /*
   * Verify it's of the expected format.
   */
  if (!read_chunk_s(fpin, &format_id))
  {
    elog(WARNING, "could not read format ID");
    goto error;
  }

  if (format_id != PGSTAT_FILE_FORMAT_ID)
  {
    elog(WARNING, "found incorrect format ID %d (expected %d)",
       format_id, PGSTAT_FILE_FORMAT_ID);
    goto error;
  }

  /*
   * We found an existing statistics file. Read it and put all the stats
   * data into place.
   */
  for (;;)
  {
    int     t = fgetc(fpin);

    switch (t)
    {
      case PGSTAT_FILE_ENTRY_FIXED:
        {
          PgStat_Kind kind;
          const PgStat_KindInfo *info;
          char     *ptr;

          /* entry for fixed-numbered stats */
          if (!read_chunk_s(fpin, &kind))
          {
            elog(WARNING, "could not read stats kind for entry of type %c", t);
            goto error;
          }

          if (!pgstat_is_kind_valid(kind))
          {
            elog(WARNING, "invalid stats kind %u for entry of type %c",
               kind, t);
            goto error;
          }

          info = pgstat_get_kind_info(kind);
          if (!info)
          {
            elog(WARNING, "could not find information of kind %u for entry of type %c",
               kind, t);
            goto error;
          }

          if (!info->fixed_amount)
          {
            elog(WARNING, "invalid fixed_amount in stats kind %u for entry of type %c",
               kind, t);
            goto error;
          }

          /* Load back stats into shared memory */
          if (pgstat_is_kind_builtin(kind))
            ptr = ((char *) shmem) + info->shared_ctl_off +
              info->shared_data_off;
          else
          {
            int     idx = kind - PGSTAT_KIND_CUSTOM_MIN;

            ptr = ((char *) shmem->custom_data[idx]) +
              info->shared_data_off;
          }

          if (!read_chunk(fpin, ptr, info->shared_data_len))
          {
            elog(WARNING, "could not read data of stats kind %u for entry of type %c with size %u",
               kind, t, info->shared_data_len);
            goto error;
          }

          break;
        }
      case PGSTAT_FILE_ENTRY_HASH:
      case PGSTAT_FILE_ENTRY_NAME:
        {
          PgStat_HashKey key;
          PgStatShared_HashEntry *p;
          PgStatShared_Common *header;

          CHECK_FOR_INTERRUPTS();

          if (t == PGSTAT_FILE_ENTRY_HASH)
          {
            /* normal stats entry, identified by PgStat_HashKey */
            if (!read_chunk_s(fpin, &key))
            {
              elog(WARNING, "could not read key for entry of type %c", t);
              goto error;
            }

            if (!pgstat_is_kind_valid(key.kind))
            {
              elog(WARNING, "invalid stats kind for entry %u/%u/%" PRIu64 " of type %c",
                 key.kind, key.dboid,
                 key.objid, t);
              goto error;
            }

            if (!pgstat_get_kind_info(key.kind))
            {
              elog(WARNING, "could not find information of kind for entry %u/%u/%" PRIu64 " of type %c",
                 key.kind, key.dboid,
                 key.objid, t);
              goto error;
            }
          }
          else
          {
            /* stats entry identified by name on disk (e.g. slots) */
            const PgStat_KindInfo *kind_info = NULL;
            PgStat_Kind kind;
            NameData  name;

            if (!read_chunk_s(fpin, &kind))
            {
              elog(WARNING, "could not read stats kind for entry of type %c", t);
              goto error;
            }
            if (!read_chunk_s(fpin, &name))
            {
              elog(WARNING, "could not read name of stats kind %u for entry of type %c",
                 kind, t);
              goto error;
            }
            if (!pgstat_is_kind_valid(kind))
            {
              elog(WARNING, "invalid stats kind %u for entry of type %c",
                 kind, t);
              goto error;
            }

            kind_info = pgstat_get_kind_info(kind);
            if (!kind_info)
            {
              elog(WARNING, "could not find information of kind %u for entry of type %c",
                 kind, t);
              goto error;
            }

            if (!kind_info->from_serialized_name)
            {
              elog(WARNING, "invalid from_serialized_name in stats kind %u for entry of type %c",
                 kind, t);
              goto error;
            }

            if (!kind_info->from_serialized_name(&name, &key))
            {
              /* skip over data for entry we don't care about */
              if (fseek(fpin, pgstat_get_entry_len(kind), SEEK_CUR) != 0)
              {
                elog(WARNING, "could not seek \"%s\" of stats kind %u for entry of type %c",
                   NameStr(name), kind, t);
                goto error;
              }

              continue;
            }

            Assert(key.kind == kind);
          }

          /*
           * This intentionally doesn't use pgstat_get_entry_ref() -
           * putting all stats into checkpointer's
           * pgStatEntryRefHash would be wasted effort and memory.
           */
          p = dshash_find_or_insert(pgStatLocal.shared_hash, &key, &found);

          /* don't allow duplicate entries */
          if (found)
          {
            dshash_release_lock(pgStatLocal.shared_hash, p);
            elog(WARNING, "found duplicate stats entry %u/%u/%" PRIu64 " of type %c",
               key.kind, key.dboid,
               key.objid, t);
            goto error;
          }

          header = pgstat_init_entry(key.kind, p);
          dshash_release_lock(pgStatLocal.shared_hash, p);
          if (header == NULL)
          {
            /*
             * It would be tempting to switch this ERROR to a
             * WARNING, but it would mean that all the statistics
             * are discarded when the environment fails on OOM.
             */
            elog(ERROR, "could not allocate entry %u/%u/%" PRIu64 " of type %c",
               key.kind, key.dboid,
               key.objid, t);
          }

          if (!read_chunk(fpin,
                  pgstat_get_entry_data(key.kind, header),
                  pgstat_get_entry_len(key.kind)))
          {
            elog(WARNING, "could not read data for entry %u/%u/%" PRIu64 " of type %c",
               key.kind, key.dboid,
               key.objid, t);
            goto error;
          }

          break;
        }
      case PGSTAT_FILE_ENTRY_END:

        /*
         * check that PGSTAT_FILE_ENTRY_END actually signals end of
         * file
         */
        if (fgetc(fpin) != EOF)
        {
          elog(WARNING, "could not read end-of-file");
          goto error;
        }

        goto done;

      default:
        elog(WARNING, "could not read entry of type %c", t);
        goto error;
    }
  }

done:
  FreeFile(fpin);

  elog(DEBUG2, "removing permanent stats file \"%s\"", statfile);
  unlink(statfile);

  return;

error:
  ereport(LOG,
      (errmsg("corrupted statistics file \"%s\"", statfile)));

  pgstat_reset_after_failure();

  goto done;
}

/*
 * Helper to reset / drop stats after a crash or after restoring stats from
 * disk failed, potentially after already loading parts.
 */
static void
pgstat_reset_after_failure(void)
{
  TimestampTz ts = GetCurrentTimestamp();

  /* reset fixed-numbered stats */
  for (PgStat_Kind kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
  {
    const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);

    if (!kind_info || !kind_info->fixed_amount)
      continue;

    kind_info->reset_all_cb(ts);
  }

  /* and drop variable-numbered ones */
  pgstat_drop_all_entries();
}

/*
 * GUC assign_hook for stats_fetch_consistency.
 */
void
assign_stats_fetch_consistency(int newval, void *extra)
{
  /*
   * Changing this value in a transaction may cause snapshot state
   * inconsistencies, so force a clear of the current snapshot on the next
   * snapshot build attempt.
   */
  if (pgstat_fetch_consistency != newval)
    force_stats_snapshot_clear = true;
}

Coverage Report

Created: 2025-09-27 06:52