pgstat.c

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/* ----------
 * 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, 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().
 *
 * The behavior of different kinds of statistics is determined by the kind's
 * entry in pgstat_kind_infos, 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_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) 2001-2023, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    src/backend/utils/activity/pgstat.c
 * ----------
 */
#include "postgres.h"
 
#include <unistd.h>
 
#include "access/transam.h"
#include "access/xact.h"
#include "lib/dshash.h"
#include "pgstat.h"
#include "port/atomics.h"
#include "storage/fd.h"
#include "storage/ipc.h"
#include "storage/lwlock.h"
#include "storage/pg_shmem.h"
#include "storage/shmem.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
 
 
/* 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_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(int ikind);
 
 
/* ----------
 * GUC parameters
 * ----------
 */
 
bool        pgstat_track_counts = false;
int         pgstat_fetch_consistency = PGSTAT_FETCH_CONSISTENCY_CACHE;
 
 
/* ----------
 * state shared with pgstat_*.c
 * ----------
 */
 
PgStat_LocalState pgStatLocal;
 
 
/* ----------
 * 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 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_infos[PGSTAT_NUM_KINDS] = {
 
    /* stats kinds for variable-numbered objects */
 
    [PGSTAT_KIND_DATABASE] = {
        .name = "database",
 
        .fixed_amount = false,
        /* 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,
 
        .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,
 
        .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,
 
        .accessed_across_databases = true,
        .named_on_disk = 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,
        /* 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,
    },
 
 
    /* stats for fixed-numbered (mostly 1) objects */
 
    [PGSTAT_KIND_ARCHIVER] = {
        .name = "archiver",
 
        .fixed_amount = true,
 
        .reset_all_cb = pgstat_archiver_reset_all_cb,
        .snapshot_cb = pgstat_archiver_snapshot_cb,
    },
 
    [PGSTAT_KIND_BGWRITER] = {
        .name = "bgwriter",
 
        .fixed_amount = true,
 
        .reset_all_cb = pgstat_bgwriter_reset_all_cb,
        .snapshot_cb = pgstat_bgwriter_snapshot_cb,
    },
 
    [PGSTAT_KIND_CHECKPOINTER] = {
        .name = "checkpointer",
 
        .fixed_amount = true,
 
        .reset_all_cb = pgstat_checkpointer_reset_all_cb,
        .snapshot_cb = pgstat_checkpointer_snapshot_cb,
    },
 
    [PGSTAT_KIND_IO] = {
        .name = "io",
 
        .fixed_amount = true,
 
        .reset_all_cb = pgstat_io_reset_all_cb,
        .snapshot_cb = pgstat_io_snapshot_cb,
    },
 
    [PGSTAT_KIND_SLRU] = {
        .name = "slru",
 
        .fixed_amount = true,
 
        .reset_all_cb = pgstat_slru_reset_all_cb,
        .snapshot_cb = pgstat_slru_snapshot_cb,
    },
 
    [PGSTAT_KIND_WAL] = {
        .name = "wal",
 
        .fixed_amount = true,
 
        .reset_all_cb = pgstat_wal_reset_all_cb,
        .snapshot_cb = pgstat_wal_snapshot_cb,
    },
};
 
 
/* ------------------------------------------------------------
 * 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);
 
    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_wal();
 
    /* 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) &&
        !have_iostats &&
        !have_slrustats &&
        !pgstat_have_pending_wal())
    {
        Assert(pending_since == 0);
        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 database / relation / function / ... stats */
    partial_flush |= pgstat_flush_pending_entries(nowait);
 
    /* flush IO stats */
    partial_flush |= pgstat_flush_io(nowait);
 
    /* flush wal stats */
    partial_flush |= pgstat_flush_wal(nowait);
 
    /* flush SLRU stats */
    partial_flush |= pgstat_slru_flush(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;
 
    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 == DatumGetObjectId(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, Oid objoid)
{
    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, objoid, 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));
    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, Oid objoid)
{
    PgStat_HashKey key;
    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.objoid = objoid;
 
    /* 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, objoid, 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);
 
    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, Oid objoid)
{
    /* fixed-numbered stats always exist */
    if (pgstat_get_kind_info(kind)->fixed_amount)
        return true;
 
    return pgstat_get_entry_ref(kind, dboid, objoid, 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 (pgstat_fetch_consistency == PGSTAT_FETCH_CONSISTENCY_SNAPSHOT)
        pgstat_build_snapshot();
    else
        pgstat_build_snapshot_fixed(kind);
 
    Assert(pgStatLocal.snapshot.fixed_valid[kind]);
}
 
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,
                                         kind_info->shared_size);
 
        /*
         * 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),
               kind_info->shared_size);
        LWLockRelease(&stats_data->lock);
    }
    dshash_seq_term(&hstat);
 
    /*
     * Build snapshot of all fixed-numbered stats.
     */
    for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
    {
        const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
 
        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);
 
    Assert(kind_info->fixed_amount);
    Assert(kind_info->snapshot_cb != NULL);
 
    if (pgstat_fetch_consistency == PGSTAT_FETCH_CONSISTENCY_NONE)
    {
        /* rebuild every time */
        pgStatLocal.snapshot.fixed_valid[kind] = false;
    }
    else if (pgStatLocal.snapshot.fixed_valid[kind])
    {
        /* in snapshot mode we shouldn't get called again */
        Assert(pgstat_fetch_consistency == PGSTAT_FETCH_CONSISTENCY_CACHE);
        return;
    }
 
    Assert(!pgStatLocal.snapshot.fixed_valid[kind]);
 
    kind_info->snapshot_cb();
 
    Assert(!pgStatLocal.snapshot.fixed_valid[kind]);
    pgStatLocal.snapshot.fixed_valid[kind] = 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, Oid objoid, 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, objoid,
                                     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, Oid objoid)
{
    PgStat_EntryRef *entry_ref;
 
    entry_ref = pgstat_get_entry_ref(kind, dboid, objoid, 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 stats for database objects (databases, relations,
 * functions).
 */
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 (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
    {
        if (pg_strcasecmp(kind_str, pgstat_kind_infos[kind].name) == 0)
            return kind;
    }
 
    ereport(ERROR,
            (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
             errmsg("invalid statistics kind: \"%s\"", kind_str)));
    return PGSTAT_KIND_DATABASE;    /* avoid compiler warnings */
}
 
static inline bool
pgstat_is_kind_valid(int ikind)
{
    return ikind >= PGSTAT_KIND_FIRST_VALID && ikind <= PGSTAT_KIND_LAST;
}
 
const PgStat_KindInfo *
pgstat_get_kind_info(PgStat_Kind kind)
{
    Assert(pgstat_is_kind_valid(kind));
 
    return &pgstat_kind_infos[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();
 
    /* 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);
 
    /*
     * XXX: The following could now be generalized to just iterate over
     * pgstat_kind_infos instead of knowing about the different kinds of
     * stats.
     */
 
    /*
     * Write archiver stats struct
     */
    pgstat_build_snapshot_fixed(PGSTAT_KIND_ARCHIVER);
    write_chunk_s(fpout, &pgStatLocal.snapshot.archiver);
 
    /*
     * Write bgwriter stats struct
     */
    pgstat_build_snapshot_fixed(PGSTAT_KIND_BGWRITER);
    write_chunk_s(fpout, &pgStatLocal.snapshot.bgwriter);
 
    /*
     * Write checkpointer stats struct
     */
    pgstat_build_snapshot_fixed(PGSTAT_KIND_CHECKPOINTER);
    write_chunk_s(fpout, &pgStatLocal.snapshot.checkpointer);
 
    /*
     * Write IO stats struct
     */
    pgstat_build_snapshot_fixed(PGSTAT_KIND_IO);
    write_chunk_s(fpout, &pgStatLocal.snapshot.io);
 
    /*
     * Write SLRU stats struct
     */
    pgstat_build_snapshot_fixed(PGSTAT_KIND_SLRU);
    write_chunk_s(fpout, &pgStatLocal.snapshot.slru);
 
    /*
     * Write WAL stats struct
     */
    pgstat_build_snapshot_fixed(PGSTAT_KIND_WAL);
    write_chunk_s(fpout, &pgStatLocal.snapshot.wal);
 
    /*
     * 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 may have some "dropped" entries not yet removed, skip them */
        Assert(!ps->dropped);
        if (ps->dropped)
            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);
 
        if (!kind_info->to_serialized_name)
        {
            /* normal stats entry, identified by PgStat_HashKey */
            fputc('S', 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('N', 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('E', 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 (rename(tmpfile, statfile) < 0)
    {
        ereport(LOG,
                (errcode_for_file_access(),
                 errmsg("could not rename temporary statistics file \"%s\" to \"%s\": %m",
                        tmpfile, statfile)));
        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 the shared stats hash.
 *
 * 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) ||
        format_id != PGSTAT_FILE_FORMAT_ID)
        goto error;
 
    /*
     * XXX: The following could now be generalized to just iterate over
     * pgstat_kind_infos instead of knowing about the different kinds of
     * stats.
     */
 
    /*
     * Read archiver stats struct
     */
    if (!read_chunk_s(fpin, &shmem->archiver.stats))
        goto error;
 
    /*
     * Read bgwriter stats struct
     */
    if (!read_chunk_s(fpin, &shmem->bgwriter.stats))
        goto error;
 
    /*
     * Read checkpointer stats struct
     */
    if (!read_chunk_s(fpin, &shmem->checkpointer.stats))
        goto error;
 
    /*
     * Read IO stats struct
     */
    if (!read_chunk_s(fpin, &shmem->io.stats))
        goto error;
 
    /*
     * Read SLRU stats struct
     */
    if (!read_chunk_s(fpin, &shmem->slru.stats))
        goto error;
 
    /*
     * Read WAL stats struct
     */
    if (!read_chunk_s(fpin, &shmem->wal.stats))
        goto error;
 
    /*
     * We found an existing statistics file. Read it and put all the hash
     * table entries into place.
     */
    for (;;)
    {
        int         t = fgetc(fpin);
 
        switch (t)
        {
            case 'S':
            case 'N':
                {
                    PgStat_HashKey key;
                    PgStatShared_HashEntry *p;
                    PgStatShared_Common *header;
 
                    CHECK_FOR_INTERRUPTS();
 
                    if (t == 'S')
                    {
                        /* normal stats entry, identified by PgStat_HashKey */
                        if (!read_chunk_s(fpin, &key))
                            goto error;
 
                        if (!pgstat_is_kind_valid(key.kind))
                            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))
                            goto error;
                        if (!read_chunk_s(fpin, &name))
                            goto error;
                        if (!pgstat_is_kind_valid(kind))
                            goto error;
 
                        kind_info = pgstat_get_kind_info(kind);
 
                        if (!kind_info->from_serialized_name)
                            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)
                                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 %d/%u/%u",
                             key.kind, key.dboid, key.objoid);
                        goto error;
                    }
 
                    header = pgstat_init_entry(key.kind, p);
                    dshash_release_lock(pgStatLocal.shared_hash, p);
 
                    if (!read_chunk(fpin,
                                    pgstat_get_entry_data(key.kind, header),
                                    pgstat_get_entry_len(key.kind)))
                        goto error;
 
                    break;
                }
            case 'E':
                /* check that 'E' actually signals end of file */
                if (fgetc(fpin) != EOF)
                    goto error;
 
                goto done;
 
            default:
                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 (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
    {
        const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
 
        if (!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;
}