pgstat_shmem.c

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/* -------------------------------------------------------------------------
 *
 * pgstat_shmem.c
 *    Storage of stats entries in shared memory
 *
 * Copyright (c) 2001-2026, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    src/backend/utils/activity/pgstat_shmem.c
 * -------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include "pgstat.h"
#include "storage/shmem.h"
#include "storage/subsystems.h"
#include "utils/memutils.h"
#include "utils/pgstat_internal.h"
 
 
#define PGSTAT_ENTRY_REF_HASH_SIZE  128
 
/* hash table entry for finding the PgStat_EntryRef for a key */
typedef struct PgStat_EntryRefHashEntry
{
    PgStat_HashKey key;         /* hash key */
    char        status;         /* for simplehash use */
    PgStat_EntryRef *entry_ref;
} PgStat_EntryRefHashEntry;
 
 
/* for references to shared statistics entries */
#define SH_PREFIX pgstat_entry_ref_hash
#define SH_ELEMENT_TYPE PgStat_EntryRefHashEntry
#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"
 
 
static void pgstat_drop_database_and_contents(Oid dboid);
 
static void pgstat_free_entry(PgStatShared_HashEntry *shent, dshash_seq_status *hstat);
 
static void pgstat_release_entry_ref(PgStat_HashKey key, PgStat_EntryRef *entry_ref, bool discard_pending);
static bool pgstat_need_entry_refs_gc(void);
static void pgstat_gc_entry_refs(void);
static void pgstat_release_all_entry_refs(bool discard_pending);
typedef bool (*ReleaseMatchCB) (PgStat_EntryRefHashEntry *, Datum data);
static void pgstat_release_matching_entry_refs(bool discard_pending, ReleaseMatchCB match, Datum match_data);
 
static void pgstat_setup_memcxt(void);
 
static void StatsShmemRequest(void *arg);
static void StatsShmemInit(void *arg);
 
const ShmemCallbacks StatsShmemCallbacks = {
    .request_fn = StatsShmemRequest,
    .init_fn = StatsShmemInit,
};
 
/* parameter for the shared hash */
static const dshash_parameters dsh_params = {
    sizeof(PgStat_HashKey),
    sizeof(PgStatShared_HashEntry),
    pgstat_cmp_hash_key,
    pgstat_hash_hash_key,
    dshash_memcpy,
    LWTRANCHE_PGSTATS_HASH
};
 
 
/*
 * Backend local references to shared stats entries. If there are pending
 * updates to a stats entry, the PgStat_EntryRef is added to the pgStatPending
 * list.
 *
 * When a stats entry is dropped each backend needs to release its reference
 * to it before the memory can be released. To trigger that
 * pgStatLocal.shmem->gc_request_count is incremented - which each backend
 * compares to their copy of pgStatSharedRefAge on a regular basis.
 */
static pgstat_entry_ref_hash_hash *pgStatEntryRefHash = NULL;
static int  pgStatSharedRefAge = 0; /* cache age of pgStatLocal.shmem */
 
/*
 * Memory contexts containing the pgStatEntryRefHash table and the
 * pgStatSharedRef entries respectively. Kept separate to make it easier to
 * track / attribute memory usage.
 */
static MemoryContext pgStatSharedRefContext = NULL;
static MemoryContext pgStatEntryRefHashContext = NULL;
 
 
/* ------------------------------------------------------------
 * Public functions called from postmaster follow
 * ------------------------------------------------------------
 */
 
/*
 * The size of the shared memory allocation for stats stored in the shared
 * stats hash table. This allocation will be done as part of the main shared
 * memory, rather than dynamic shared memory, allowing it to be initialized in
 * postmaster.
 */
static Size
pgstat_dsa_init_size(void)
{
    Size        sz;
 
    /*
     * The dshash header / initial buckets array needs to fit into "plain"
     * shared memory, but it's beneficial to not need dsm segments
     * immediately. A size of 256kB seems works well and is not
     * disproportional compared to other constant sized shared memory
     * allocations. NB: To avoid DSMs further, the user can configure
     * min_dynamic_shared_memory.
     */
    sz = 256 * 1024;
    Assert(dsa_minimum_size() <= sz);
    return MAXALIGN(sz);
}
 
/*
 * Compute shared memory space needed for cumulative statistics
 */
static Size
StatsShmemSize(void)
{
    Size        sz;
 
    sz = MAXALIGN(sizeof(PgStat_ShmemControl));
    sz = add_size(sz, pgstat_dsa_init_size());
 
    /* Add shared memory for all the custom fixed-numbered statistics */
    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)
            continue;
        if (!kind_info->fixed_amount)
            continue;
 
        Assert(kind_info->shared_size != 0);
        sz = add_size(sz, MAXALIGN(kind_info->shared_size));
    }
 
    return sz;
}
 
/*
 * Register shared memory area for cumulative statistics
 */
static void
StatsShmemRequest(void *arg)
{
    ShmemRequestStruct(.name = "Shared Memory Stats",
                       .size = StatsShmemSize(),
                       .ptr = (void **) &pgStatLocal.shmem,
        );
}
 
/*
 * Initialize cumulative statistics system during startup
 */
static void
StatsShmemInit(void *arg)
{
    dsa_area   *dsa;
    dshash_table *dsh;
    PgStat_ShmemControl *ctl = pgStatLocal.shmem;
    char       *p = (char *) ctl;
 
    /* the allocation of pgStatLocal.shmem itself */
    p += MAXALIGN(sizeof(PgStat_ShmemControl));
 
    /*
     * Create a small dsa allocation in plain shared memory. This is required
     * because postmaster cannot use dsm segments. It also provides a small
     * efficiency win.
     */
    ctl->raw_dsa_area = p;
    p += pgstat_dsa_init_size();
    dsa = dsa_create_in_place(ctl->raw_dsa_area,
                              pgstat_dsa_init_size(),
                              LWTRANCHE_PGSTATS_DSA, NULL);
    dsa_pin(dsa);
 
    /*
     * To ensure dshash is created in "plain" shared memory, temporarily limit
     * size of dsa to the initial size of the dsa.
     */
    dsa_set_size_limit(dsa, pgstat_dsa_init_size());
 
    /*
     * With the limit in place, create the dshash table. XXX: It'd be nice if
     * there were dshash_create_in_place().
     */
    dsh = dshash_create(dsa, &dsh_params, NULL);
    ctl->hash_handle = dshash_get_hash_table_handle(dsh);
 
    /* lift limit set above */
    dsa_set_size_limit(dsa, -1);
 
    /*
     * Postmaster will never access these again, thus free the local
     * dsa/dshash references.
     */
    dshash_detach(dsh);
    dsa_detach(dsa);
 
    pg_atomic_init_u64(&ctl->gc_request_count, 1);
 
    /* Do the per-kind initialization */
    for (PgStat_Kind kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
    {
        const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
        char       *ptr;
 
        if (!kind_info)
            continue;
 
        /* initialize entry count tracking */
        if (kind_info->track_entry_count)
            pg_atomic_init_u64(&ctl->entry_counts[kind - 1], 0);
 
        /* initialize fixed-numbered stats */
        if (kind_info->fixed_amount)
        {
            if (pgstat_is_kind_builtin(kind))
                ptr = ((char *) ctl) + kind_info->shared_ctl_off;
            else
            {
                int         idx = kind - PGSTAT_KIND_CUSTOM_MIN;
 
                Assert(kind_info->shared_size != 0);
                ctl->custom_data[idx] = p;
                p += MAXALIGN(kind_info->shared_size);
                ptr = ctl->custom_data[idx];
            }
 
            kind_info->init_shmem_cb(ptr);
        }
    }
}
 
void
pgstat_attach_shmem(void)
{
    MemoryContext oldcontext;
 
    Assert(pgStatLocal.dsa == NULL);
 
    /* stats shared memory persists for the backend lifetime */
    oldcontext = MemoryContextSwitchTo(TopMemoryContext);
 
    pgStatLocal.dsa = dsa_attach_in_place(pgStatLocal.shmem->raw_dsa_area,
                                          NULL);
    dsa_pin_mapping(pgStatLocal.dsa);
 
    pgStatLocal.shared_hash = dshash_attach(pgStatLocal.dsa, &dsh_params,
                                            pgStatLocal.shmem->hash_handle,
                                            NULL);
 
    MemoryContextSwitchTo(oldcontext);
}
 
void
pgstat_detach_shmem(void)
{
    Assert(pgStatLocal.dsa);
 
    /* we shouldn't leave references to shared stats */
    pgstat_release_all_entry_refs(false);
 
    dshash_detach(pgStatLocal.shared_hash);
    pgStatLocal.shared_hash = NULL;
 
    dsa_detach(pgStatLocal.dsa);
 
    /*
     * dsa_detach() does not decrement the DSA reference count as no segment
     * was provided to dsa_attach_in_place(), causing no cleanup callbacks to
     * be registered.  Hence, release it manually now.
     */
    dsa_release_in_place(pgStatLocal.shmem->raw_dsa_area);
 
    pgStatLocal.dsa = NULL;
}
 
 
/* ------------------------------------------------------------
 * Maintenance of shared memory stats entries
 * ------------------------------------------------------------
 */
 
/*
 * Initialize entry newly-created.
 *
 * Returns NULL in the event of an allocation failure, so as callers can
 * take cleanup actions as the entry initialized is already inserted in the
 * shared hashtable.
 */
PgStatShared_Common *
pgstat_init_entry(PgStat_Kind kind,
                  PgStatShared_HashEntry *shhashent)
{
    /* Create new stats entry. */
    dsa_pointer chunk;
    PgStatShared_Common *shheader;
    const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
 
    /*
     * Initialize refcount to 1, marking it as valid / not dropped. The entry
     * can't be freed before the initialization because it can't be found as
     * long as we hold the dshash partition lock. Caller needs to increase
     * further if a longer lived reference is needed.
     */
    pg_atomic_init_u32(&shhashent->refcount, 1);
 
    /*
     * Initialize "generation" to 0, as freshly created.
     */
    pg_atomic_init_u32(&shhashent->generation, 0);
    shhashent->dropped = false;
 
    chunk = dsa_allocate_extended(pgStatLocal.dsa,
                                  kind_info->shared_size,
                                  DSA_ALLOC_ZERO | DSA_ALLOC_NO_OOM);
    if (chunk == InvalidDsaPointer)
        return NULL;
 
    shheader = dsa_get_address(pgStatLocal.dsa, chunk);
    shheader->magic = 0xdeadbeef;
 
    /* Link the new entry from the hash entry. */
    shhashent->body = chunk;
 
    /* Increment entry count, if required. */
    if (kind_info->track_entry_count)
        pg_atomic_fetch_add_u64(&pgStatLocal.shmem->entry_counts[kind - 1], 1);
 
    LWLockInitialize(&shheader->lock, LWTRANCHE_PGSTATS_DATA);
 
    return shheader;
}
 
static PgStatShared_Common *
pgstat_reinit_entry(PgStat_Kind kind, PgStatShared_HashEntry *shhashent)
{
    PgStatShared_Common *shheader;
 
    shheader = dsa_get_address(pgStatLocal.dsa, shhashent->body);
 
    /* mark as not dropped anymore */
    pg_atomic_fetch_add_u32(&shhashent->refcount, 1);
 
    /*
     * Increment "generation", to let any backend with local references know
     * that what they point to is outdated.
     */
    pg_atomic_fetch_add_u32(&shhashent->generation, 1);
    shhashent->dropped = false;
 
    /* reinitialize content */
    Assert(shheader->magic == 0xdeadbeef);
    memset(pgstat_get_entry_data(kind, shheader), 0,
           pgstat_get_entry_len(kind));
 
    return shheader;
}
 
static void
pgstat_setup_shared_refs(void)
{
    if (likely(pgStatEntryRefHash != NULL))
        return;
 
    pgStatEntryRefHash =
        pgstat_entry_ref_hash_create(pgStatEntryRefHashContext,
                                     PGSTAT_ENTRY_REF_HASH_SIZE, NULL);
    pgStatSharedRefAge = pg_atomic_read_u64(&pgStatLocal.shmem->gc_request_count);
    Assert(pgStatSharedRefAge != 0);
}
 
/*
 * Helper function for pgstat_get_entry_ref().
 */
static void
pgstat_acquire_entry_ref(PgStat_EntryRef *entry_ref,
                         PgStatShared_HashEntry *shhashent,
                         PgStatShared_Common *shheader)
{
    Assert(shheader->magic == 0xdeadbeef);
    Assert(pg_atomic_read_u32(&shhashent->refcount) > 0);
 
    pg_atomic_fetch_add_u32(&shhashent->refcount, 1);
 
    dshash_release_lock(pgStatLocal.shared_hash, shhashent);
 
    entry_ref->shared_stats = shheader;
    entry_ref->shared_entry = shhashent;
    entry_ref->generation = pg_atomic_read_u32(&shhashent->generation);
}
 
/*
 * Helper function for pgstat_get_entry_ref().
 */
static bool
pgstat_get_entry_ref_cached(PgStat_HashKey key, PgStat_EntryRef **entry_ref_p)
{
    bool        found;
    PgStat_EntryRefHashEntry *cache_entry;
 
    /*
     * We immediately insert a cache entry, because it avoids 1) multiple
     * hashtable lookups in case of a cache miss 2) having to deal with
     * out-of-memory errors after incrementing PgStatShared_Common->refcount.
     */
 
    cache_entry = pgstat_entry_ref_hash_insert(pgStatEntryRefHash, key, &found);
 
    if (!found || !cache_entry->entry_ref)
    {
        PgStat_EntryRef *entry_ref;
 
        cache_entry->entry_ref = entry_ref =
            MemoryContextAlloc(pgStatSharedRefContext,
                               sizeof(PgStat_EntryRef));
        entry_ref->shared_stats = NULL;
        entry_ref->shared_entry = NULL;
        entry_ref->pending = NULL;
 
        found = false;
    }
    else if (cache_entry->entry_ref->shared_stats == NULL)
    {
        Assert(cache_entry->entry_ref->pending == NULL);
        found = false;
    }
    else
    {
        PgStat_EntryRef *entry_ref PG_USED_FOR_ASSERTS_ONLY;
 
        entry_ref = cache_entry->entry_ref;
        Assert(entry_ref->shared_entry != NULL);
        Assert(entry_ref->shared_stats != NULL);
 
        Assert(entry_ref->shared_stats->magic == 0xdeadbeef);
        /* should have at least our reference */
        Assert(pg_atomic_read_u32(&entry_ref->shared_entry->refcount) > 0);
    }
 
    *entry_ref_p = cache_entry->entry_ref;
    return found;
}
 
/*
 * Get a shared stats reference. If create is true, the shared stats object is
 * created if it does not exist.
 *
 * When create is true, and created_entry is non-NULL, it'll be set to true
 * if the entry is newly created, false otherwise.
 */
PgStat_EntryRef *
pgstat_get_entry_ref(PgStat_Kind kind, Oid dboid, uint64 objid, bool create,
                     bool *created_entry)
{
    PgStat_HashKey key = {0};
    PgStatShared_HashEntry *shhashent;
    PgStatShared_Common *shheader = NULL;
    PgStat_EntryRef *entry_ref;
 
    key.kind = kind;
    key.dboid = dboid;
    key.objid = objid;
 
    /*
     * passing in created_entry only makes sense if we possibly could create
     * entry.
     */
    Assert(create || created_entry == NULL);
    pgstat_assert_is_up();
    Assert(pgStatLocal.shared_hash != NULL);
    Assert(!pgStatLocal.shmem->is_shutdown);
 
    pgstat_setup_memcxt();
    pgstat_setup_shared_refs();
 
    if (created_entry != NULL)
        *created_entry = false;
 
    /*
     * Check if other backends dropped stats that could not be deleted because
     * somebody held references to it. If so, check this backend's references.
     * This is not expected to happen often. The location of the check is a
     * bit random, but this is a relatively frequently called path, so better
     * than most.
     */
    if (pgstat_need_entry_refs_gc())
        pgstat_gc_entry_refs();
 
    /*
     * First check the lookup cache hashtable in local memory. If we find a
     * match here we can avoid taking locks / causing contention.
     */
    if (pgstat_get_entry_ref_cached(key, &entry_ref))
        return entry_ref;
 
    Assert(entry_ref != NULL);
 
    /*
     * Do a lookup in the hash table first - it's quite likely that the entry
     * already exists, and that way we only need a shared lock.
     */
    shhashent = dshash_find(pgStatLocal.shared_hash, &key, false);
 
    if (create && !shhashent)
    {
        bool        shfound;
 
        /*
         * It's possible that somebody created the entry since the above
         * lookup. If so, fall through to the same path as if we'd have if it
         * already had been created before the dshash_find() calls.
         */
        shhashent = dshash_find_or_insert(pgStatLocal.shared_hash, &key, &shfound);
        if (!shfound)
        {
            shheader = pgstat_init_entry(kind, shhashent);
            if (shheader == NULL)
            {
                /*
                 * Failed the allocation of a new entry, so clean up the
                 * shared hashtable before giving up.
                 */
                dshash_delete_entry(pgStatLocal.shared_hash, shhashent);
 
                ereport(ERROR,
                        (errcode(ERRCODE_OUT_OF_MEMORY),
                         errmsg("out of memory"),
                         errdetail("Failed while allocating entry %u/%u/%" PRIu64 ".",
                                   key.kind, key.dboid, key.objid)));
            }
            pgstat_acquire_entry_ref(entry_ref, shhashent, shheader);
 
            if (created_entry != NULL)
                *created_entry = true;
 
            return entry_ref;
        }
    }
 
    if (!shhashent)
    {
        /*
         * If we're not creating, delete the reference again. In all
         * likelihood it's just a stats lookup - no point wasting memory for a
         * shared ref to nothing...
         */
        pgstat_release_entry_ref(key, entry_ref, false);
 
        return NULL;
    }
    else
    {
        /*
         * Can get here either because dshash_find() found a match, or if
         * dshash_find_or_insert() found a concurrently inserted entry.
         */
 
        if (shhashent->dropped && create)
        {
            /*
             * There are legitimate cases where the old stats entry might not
             * yet have been dropped by the time it's reused. The most obvious
             * case are replication slot stats, where a new slot can be
             * created with the same index just after dropping. But oid
             * wraparound can lead to other cases as well. We just reset the
             * stats to their plain state, while incrementing its "generation"
             * in the shared entry for any remaining local references.
             */
            shheader = pgstat_reinit_entry(kind, shhashent);
            pgstat_acquire_entry_ref(entry_ref, shhashent, shheader);
 
            if (created_entry != NULL)
                *created_entry = true;
 
            return entry_ref;
        }
        else if (shhashent->dropped)
        {
            dshash_release_lock(pgStatLocal.shared_hash, shhashent);
            pgstat_release_entry_ref(key, entry_ref, false);
 
            return NULL;
        }
        else
        {
            shheader = dsa_get_address(pgStatLocal.dsa, shhashent->body);
            pgstat_acquire_entry_ref(entry_ref, shhashent, shheader);
 
            return entry_ref;
        }
    }
}
 
static void
pgstat_release_entry_ref(PgStat_HashKey key, PgStat_EntryRef *entry_ref,
                         bool discard_pending)
{
    if (entry_ref && entry_ref->pending)
    {
        if (discard_pending)
            pgstat_delete_pending_entry(entry_ref);
        else
            elog(ERROR, "releasing ref with pending data");
    }
 
    if (entry_ref && entry_ref->shared_stats)
    {
        Assert(entry_ref->shared_stats->magic == 0xdeadbeef);
        Assert(entry_ref->pending == NULL);
 
        /*
         * This can't race with another backend looking up the stats entry and
         * increasing the refcount because it is not "legal" to create
         * additional references to dropped entries.
         */
        if (pg_atomic_fetch_sub_u32(&entry_ref->shared_entry->refcount, 1) == 1)
        {
            PgStatShared_HashEntry *shent;
 
            /*
             * We're the last referrer to this entry, try to drop the shared
             * entry.
             */
 
            /* only dropped entries can reach a 0 refcount */
            Assert(entry_ref->shared_entry->dropped);
 
            shent = dshash_find(pgStatLocal.shared_hash,
                                &entry_ref->shared_entry->key,
                                true);
            if (!shent)
                elog(ERROR, "could not find just referenced shared stats entry");
 
            /*
             * This entry may have been reinitialized while trying to release
             * it, so double-check that it has not been reused while holding a
             * lock on its shared entry.
             */
            if (pg_atomic_read_u32(&entry_ref->shared_entry->generation) ==
                entry_ref->generation)
            {
                /* Same "generation", so we're OK with the removal */
                Assert(pg_atomic_read_u32(&entry_ref->shared_entry->refcount) == 0);
                Assert(entry_ref->shared_entry == shent);
                pgstat_free_entry(shent, NULL);
            }
            else
            {
                /*
                 * Shared stats entry has been reinitialized, so do not drop
                 * its shared entry, only release its lock.
                 */
                dshash_release_lock(pgStatLocal.shared_hash, shent);
            }
        }
    }
 
    if (!pgstat_entry_ref_hash_delete(pgStatEntryRefHash, key))
        elog(ERROR, "entry ref vanished before deletion");
 
    if (entry_ref)
        pfree(entry_ref);
}
 
/*
 * Acquire exclusive lock on the entry.
 *
 * If nowait is true, it's just a conditional acquire, and the result
 * *must* be checked to verify success.
 * If nowait is false, waits as necessary, always returning true.
 */
bool
pgstat_lock_entry(PgStat_EntryRef *entry_ref, bool nowait)
{
    LWLock     *lock = &entry_ref->shared_stats->lock;
 
    if (nowait)
        return LWLockConditionalAcquire(lock, LW_EXCLUSIVE);
 
    LWLockAcquire(lock, LW_EXCLUSIVE);
    return true;
}
 
/*
 * Acquire shared lock on the entry.
 *
 * Separate from pgstat_lock_entry() as most callers will need to lock
 * exclusively.  The wait semantics are identical.
 */
bool
pgstat_lock_entry_shared(PgStat_EntryRef *entry_ref, bool nowait)
{
    LWLock     *lock = &entry_ref->shared_stats->lock;
 
    if (nowait)
        return LWLockConditionalAcquire(lock, LW_SHARED);
 
    LWLockAcquire(lock, LW_SHARED);
    return true;
}
 
void
pgstat_unlock_entry(PgStat_EntryRef *entry_ref)
{
    LWLockRelease(&entry_ref->shared_stats->lock);
}
 
/*
 * Helper function to fetch and lock shared stats.
 */
PgStat_EntryRef *
pgstat_get_entry_ref_locked(PgStat_Kind kind, Oid dboid, uint64 objid,
                            bool nowait)
{
    PgStat_EntryRef *entry_ref;
 
    /* find shared table stats entry corresponding to the local entry */
    entry_ref = pgstat_get_entry_ref(kind, dboid, objid, true, NULL);
 
    /* lock the shared entry to protect the content, skip if failed */
    if (!pgstat_lock_entry(entry_ref, nowait))
        return NULL;
 
    return entry_ref;
}
 
void
pgstat_request_entry_refs_gc(void)
{
    pg_atomic_fetch_add_u64(&pgStatLocal.shmem->gc_request_count, 1);
}
 
static bool
pgstat_need_entry_refs_gc(void)
{
    uint64      curage;
 
    if (!pgStatEntryRefHash)
        return false;
 
    /* should have been initialized when creating pgStatEntryRefHash */
    Assert(pgStatSharedRefAge != 0);
 
    curage = pg_atomic_read_u64(&pgStatLocal.shmem->gc_request_count);
 
    return pgStatSharedRefAge != curage;
}
 
static void
pgstat_gc_entry_refs(void)
{
    pgstat_entry_ref_hash_iterator i;
    PgStat_EntryRefHashEntry *ent;
    uint64      curage;
 
    curage = pg_atomic_read_u64(&pgStatLocal.shmem->gc_request_count);
    Assert(curage != 0);
 
    /*
     * Some entries have been dropped or reinitialized.  Invalidate cache
     * pointer to them.
     */
    pgstat_entry_ref_hash_start_iterate(pgStatEntryRefHash, &i);
    while ((ent = pgstat_entry_ref_hash_iterate(pgStatEntryRefHash, &i)) != NULL)
    {
        PgStat_EntryRef *entry_ref = ent->entry_ref;
 
        Assert(!entry_ref->shared_stats ||
               entry_ref->shared_stats->magic == 0xdeadbeef);
 
        /*
         * "generation" checks for the case of entries being reinitialized,
         * and "dropped" for the case where these are..  dropped.
         */
        if (!entry_ref->shared_entry->dropped &&
            pg_atomic_read_u32(&entry_ref->shared_entry->generation) ==
            entry_ref->generation)
            continue;
 
        /* cannot gc shared ref that has pending data */
        if (entry_ref->pending != NULL)
            continue;
 
        pgstat_release_entry_ref(ent->key, entry_ref, false);
    }
 
    pgStatSharedRefAge = curage;
}
 
static void
pgstat_release_matching_entry_refs(bool discard_pending, ReleaseMatchCB match,
                                   Datum match_data)
{
    pgstat_entry_ref_hash_iterator i;
    PgStat_EntryRefHashEntry *ent;
 
    if (pgStatEntryRefHash == NULL)
        return;
 
    pgstat_entry_ref_hash_start_iterate(pgStatEntryRefHash, &i);
 
    while ((ent = pgstat_entry_ref_hash_iterate(pgStatEntryRefHash, &i))
           != NULL)
    {
        Assert(ent->entry_ref != NULL);
 
        if (match && !match(ent, match_data))
            continue;
 
        pgstat_release_entry_ref(ent->key, ent->entry_ref, discard_pending);
    }
}
 
/*
 * Release all local references to shared stats entries.
 *
 * When a process exits it cannot do so while still holding references onto
 * stats entries, otherwise the shared stats entries could never be freed.
 */
static void
pgstat_release_all_entry_refs(bool discard_pending)
{
    if (pgStatEntryRefHash == NULL)
        return;
 
    pgstat_release_matching_entry_refs(discard_pending, NULL, 0);
    Assert(pgStatEntryRefHash->members == 0);
    pgstat_entry_ref_hash_destroy(pgStatEntryRefHash);
    pgStatEntryRefHash = NULL;
}
 
static bool
match_db(PgStat_EntryRefHashEntry *ent, Datum match_data)
{
    Oid         dboid = DatumGetObjectId(match_data);
 
    return ent->key.dboid == dboid;
}
 
static void
pgstat_release_db_entry_refs(Oid dboid)
{
    pgstat_release_matching_entry_refs( /* discard pending = */ true,
                                       match_db,
                                       ObjectIdGetDatum(dboid));
}
 
 
/* ------------------------------------------------------------
 * Dropping and resetting of stats entries
 * ------------------------------------------------------------
 */
 
static void
pgstat_free_entry(PgStatShared_HashEntry *shent, dshash_seq_status *hstat)
{
    dsa_pointer pdsa;
    PgStat_Kind kind = shent->key.kind;
 
    /*
     * Fetch dsa pointer before deleting entry - that way we can free the
     * memory after releasing the lock.
     */
    pdsa = shent->body;
 
    if (!hstat)
        dshash_delete_entry(pgStatLocal.shared_hash, shent);
    else
        dshash_delete_current(hstat);
 
    dsa_free(pgStatLocal.dsa, pdsa);
 
    /* Decrement entry count, if required. */
    if (pgstat_get_kind_info(kind)->track_entry_count)
        pg_atomic_sub_fetch_u64(&pgStatLocal.shmem->entry_counts[kind - 1], 1);
}
 
/*
 * Helper for both pgstat_drop_database_and_contents() and
 * pgstat_drop_entry(). If hstat is non-null delete the shared entry using
 * dshash_delete_current(), otherwise use dshash_delete_entry(). In either
 * case the entry needs to be already locked.
 */
static bool
pgstat_drop_entry_internal(PgStatShared_HashEntry *shent,
                           dshash_seq_status *hstat)
{
    Assert(shent->body != InvalidDsaPointer);
 
    /* should already have released local reference */
    if (pgStatEntryRefHash)
        Assert(!pgstat_entry_ref_hash_lookup(pgStatEntryRefHash, shent->key));
 
    /*
     * Signal that the entry is dropped - this will eventually cause other
     * backends to release their references.
     */
    if (shent->dropped)
        elog(ERROR,
             "trying to drop stats entry already dropped: kind=%s dboid=%u objid=%" PRIu64 " refcount=%u generation=%u",
             pgstat_get_kind_info(shent->key.kind)->name,
             shent->key.dboid,
             shent->key.objid,
             pg_atomic_read_u32(&shent->refcount),
             pg_atomic_read_u32(&shent->generation));
    shent->dropped = true;
 
    /* release refcount marking entry as not dropped */
    if (pg_atomic_sub_fetch_u32(&shent->refcount, 1) == 0)
    {
        pgstat_free_entry(shent, hstat);
        return true;
    }
    else
    {
        if (!hstat)
            dshash_release_lock(pgStatLocal.shared_hash, shent);
        return false;
    }
}
 
/*
 * Drop stats for the database and all the objects inside that database.
 */
static void
pgstat_drop_database_and_contents(Oid dboid)
{
    dshash_seq_status hstat;
    PgStatShared_HashEntry *p;
    uint64      not_freed_count = 0;
 
    Assert(OidIsValid(dboid));
 
    Assert(pgStatLocal.shared_hash != NULL);
 
    /*
     * This backend might very well be the only backend holding a reference to
     * about-to-be-dropped entries. Ensure that we're not preventing it from
     * being cleaned up till later.
     *
     * Doing this separately from the dshash iteration below avoids having to
     * do so while holding a partition lock on the shared hashtable.
     */
    pgstat_release_db_entry_refs(dboid);
 
    /* some of the dshash entries are to be removed, take exclusive lock. */
    dshash_seq_init(&hstat, pgStatLocal.shared_hash, true);
    while ((p = dshash_seq_next(&hstat)) != NULL)
    {
        if (p->dropped)
            continue;
 
        if (p->key.dboid != dboid)
            continue;
 
        if (!pgstat_drop_entry_internal(p, &hstat))
        {
            /*
             * Even statistics for a dropped database might currently be
             * accessed (consider e.g. database stats for pg_stat_database).
             */
            not_freed_count++;
        }
    }
    dshash_seq_term(&hstat);
 
    /*
     * If some of the stats data could not be freed, signal the reference
     * holders to run garbage collection of their cached pgStatLocal.shmem.
     */
    if (not_freed_count > 0)
        pgstat_request_entry_refs_gc();
}
 
/*
 * Drop a single stats entry.
 *
 * This routine returns false if the stats entry of the dropped object could
 * not be freed, true otherwise.
 *
 * The callers of this function should call pgstat_request_entry_refs_gc()
 * if the stats entry could not be freed, to ensure that this entry's memory
 * can be reclaimed later by a different backend calling
 * pgstat_gc_entry_refs().
 */
bool
pgstat_drop_entry(PgStat_Kind kind, Oid dboid, uint64 objid)
{
    PgStat_HashKey key = {0};
    PgStatShared_HashEntry *shent;
    bool        freed = true;
 
    key.kind = kind;
    key.dboid = dboid;
    key.objid = objid;
 
    /* delete local reference */
    if (pgStatEntryRefHash)
    {
        PgStat_EntryRefHashEntry *lohashent =
            pgstat_entry_ref_hash_lookup(pgStatEntryRefHash, key);
 
        if (lohashent)
            pgstat_release_entry_ref(lohashent->key, lohashent->entry_ref,
                                     true);
    }
 
    /* mark entry in shared hashtable as deleted, drop if possible */
    shent = dshash_find(pgStatLocal.shared_hash, &key, true);
    if (shent)
    {
        freed = pgstat_drop_entry_internal(shent, NULL);
 
        /*
         * Database stats contain other stats. Drop those as well when
         * dropping the database. XXX: Perhaps this should be done in a
         * slightly more principled way? But not obvious what that'd look
         * like, and so far this is the only case...
         */
        if (key.kind == PGSTAT_KIND_DATABASE)
            pgstat_drop_database_and_contents(key.dboid);
    }
 
    return freed;
}
 
/*
 * Scan through the shared hashtable of stats, dropping statistics if
 * approved by the optional do_drop() function.
 */
void
pgstat_drop_matching_entries(bool (*do_drop) (PgStatShared_HashEntry *, Datum),
                             Datum match_data)
{
    dshash_seq_status hstat;
    PgStatShared_HashEntry *ps;
    uint64      not_freed_count = 0;
 
    /* entries are removed, take an exclusive lock */
    dshash_seq_init(&hstat, pgStatLocal.shared_hash, true);
    while ((ps = dshash_seq_next(&hstat)) != NULL)
    {
        if (ps->dropped)
            continue;
 
        if (do_drop != NULL && !do_drop(ps, match_data))
            continue;
 
        /* delete local reference */
        if (pgStatEntryRefHash)
        {
            PgStat_EntryRefHashEntry *lohashent =
                pgstat_entry_ref_hash_lookup(pgStatEntryRefHash, ps->key);
 
            if (lohashent)
                pgstat_release_entry_ref(lohashent->key, lohashent->entry_ref,
                                         true);
        }
 
        if (!pgstat_drop_entry_internal(ps, &hstat))
            not_freed_count++;
    }
    dshash_seq_term(&hstat);
 
    if (not_freed_count > 0)
        pgstat_request_entry_refs_gc();
}
 
/*
 * Scan through the shared hashtable of stats and drop all entries.
 */
void
pgstat_drop_all_entries(void)
{
    pgstat_drop_matching_entries(NULL, 0);
}
 
static void
shared_stat_reset_contents(PgStat_Kind kind, PgStatShared_Common *header,
                           TimestampTz ts)
{
    const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
 
    memset(pgstat_get_entry_data(kind, header), 0,
           pgstat_get_entry_len(kind));
 
    if (kind_info->reset_timestamp_cb)
        kind_info->reset_timestamp_cb(header, ts);
}
 
/*
 * Reset one variable-numbered stats entry.
 */
void
pgstat_reset_entry(PgStat_Kind kind, Oid dboid, uint64 objid, TimestampTz ts)
{
    PgStat_EntryRef *entry_ref;
 
    Assert(!pgstat_get_kind_info(kind)->fixed_amount);
 
    entry_ref = pgstat_get_entry_ref(kind, dboid, objid, false, NULL);
    if (!entry_ref || entry_ref->shared_entry->dropped)
        return;
 
    (void) pgstat_lock_entry(entry_ref, false);
    shared_stat_reset_contents(kind, entry_ref->shared_stats, ts);
    pgstat_unlock_entry(entry_ref);
}
 
/*
 * Scan through the shared hashtable of stats, resetting statistics if
 * approved by the provided do_reset() function.
 */
void
pgstat_reset_matching_entries(bool (*do_reset) (PgStatShared_HashEntry *, Datum),
                              Datum match_data, TimestampTz ts)
{
    dshash_seq_status hstat;
    PgStatShared_HashEntry *p;
 
    /* dshash entry is not modified, take shared lock */
    dshash_seq_init(&hstat, pgStatLocal.shared_hash, false);
    while ((p = dshash_seq_next(&hstat)) != NULL)
    {
        PgStatShared_Common *header;
 
        if (p->dropped)
            continue;
 
        if (!do_reset(p, match_data))
            continue;
 
        header = dsa_get_address(pgStatLocal.dsa, p->body);
 
        LWLockAcquire(&header->lock, LW_EXCLUSIVE);
 
        shared_stat_reset_contents(p->key.kind, header, ts);
 
        LWLockRelease(&header->lock);
    }
    dshash_seq_term(&hstat);
}
 
static bool
match_kind(PgStatShared_HashEntry *p, Datum match_data)
{
    return p->key.kind == DatumGetInt32(match_data);
}
 
void
pgstat_reset_entries_of_kind(PgStat_Kind kind, TimestampTz ts)
{
    pgstat_reset_matching_entries(match_kind, Int32GetDatum(kind), ts);
}
 
static void
pgstat_setup_memcxt(void)
{
    if (unlikely(!pgStatSharedRefContext))
        pgStatSharedRefContext =
            AllocSetContextCreate(TopMemoryContext,
                                  "PgStat Shared Ref",
                                  ALLOCSET_SMALL_SIZES);
    if (unlikely(!pgStatEntryRefHashContext))
        pgStatEntryRefHashContext =
            AllocSetContextCreate(TopMemoryContext,
                                  "PgStat Shared Ref Hash",
                                  ALLOCSET_SMALL_SIZES);
}