pgstat_internal.h

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/* ----------
 * pgstat_internal.h
 *
 * Definitions for the PostgreSQL cumulative statistics system that should
 * only be needed by files implementing statistics support (rather than ones
 * reporting / querying stats).
 *
 * Copyright (c) 2001-2024, PostgreSQL Global Development Group
 *
 * src/include/utils/pgstat_internal.h
 * ----------
 */
#ifndef PGSTAT_INTERNAL_H
#define PGSTAT_INTERNAL_H
 
 
#include "common/hashfn_unstable.h"
#include "lib/dshash.h"
#include "lib/ilist.h"
#include "pgstat.h"
#include "storage/lwlock.h"
#include "utils/dsa.h"
 
 
/*
 * Types related to shared memory storage of statistics.
 *
 * Per-object statistics are stored in the "shared stats" hashtable. That
 * table's entries (PgStatShared_HashEntry) contain a pointer to the actual stats
 * data for the object (the size of the stats data varies depending on the
 * kind of stats). The table is keyed by PgStat_HashKey.
 *
 * Once a backend has a reference to a shared stats entry, it increments the
 * entry's refcount. Even after stats data is dropped (e.g., due to a DROP
 * TABLE), the entry itself can only be deleted once all references have been
 * released.
 *
 * These refcounts, in combination with a backend local hashtable
 * (pgStatEntryRefHash, with entries pointing to PgStat_EntryRef) in front of
 * the shared hash table, mean that most stats work can happen without
 * touching the shared hash table, reducing contention.
 *
 * Once there are pending stats updates for a table PgStat_EntryRef->pending
 * is allocated to contain a working space for as-of-yet-unapplied stats
 * updates. Once the stats are flushed, PgStat_EntryRef->pending is freed.
 *
 * Each stat kind in the shared hash table has a fixed member
 * PgStatShared_Common as the first element.
 */
 
/* struct for shared statistics hash entry key. */
typedef struct PgStat_HashKey
{
    PgStat_Kind kind;           /* statistics entry kind */
    Oid         dboid;          /* database ID. InvalidOid for shared objects. */
    Oid         objoid;         /* object ID, either table or function. */
} PgStat_HashKey;
 
/*
 * Shared statistics hash entry. Doesn't itself contain any stats, but points
 * to them (with ->body). That allows the stats entries themselves to be of
 * variable size.
 */
typedef struct PgStatShared_HashEntry
{
    PgStat_HashKey key;         /* hash key */
 
    /*
     * If dropped is set, backends need to release their references so that
     * the memory for the entry can be freed. No new references may be made
     * once marked as dropped.
     */
    bool        dropped;
 
    /*
     * Refcount managing lifetime of the entry itself (as opposed to the
     * dshash entry pointing to it). The stats lifetime has to be separate
     * from the hash table entry lifetime because we allow backends to point
     * to a stats entry without holding a hash table lock (and some other
     * reasons).
     *
     * As long as the entry is not dropped, 1 is added to the refcount
     * representing that the entry should not be dropped. In addition each
     * backend that has a reference to the entry needs to increment the
     * refcount as long as it does.
     *
     * May only be incremented / decremented while holding at least a shared
     * lock on the dshash partition containing the entry. It needs to be an
     * atomic variable because multiple backends can increment the refcount
     * with just a shared lock.
     *
     * When the refcount reaches 0 the entry needs to be freed.
     */
    pg_atomic_uint32 refcount;
 
    /*
     * Pointer to shared stats. The stats entry always starts with
     * PgStatShared_Common, embedded in a larger struct containing the
     * PgStat_Kind specific stats fields.
     */
    dsa_pointer body;
} PgStatShared_HashEntry;
 
/*
 * Common header struct for PgStatShared_*.
 */
typedef struct PgStatShared_Common
{
    uint32      magic;          /* just a validity cross-check */
    /* lock protecting stats contents (i.e. data following the header) */
    LWLock      lock;
} PgStatShared_Common;
 
/*
 * A backend local reference to a shared stats entry. As long as at least one
 * such reference exists, the shared stats entry will not be released.
 *
 * If there are pending stats update to the shared stats, these are stored in
 * ->pending.
 */
typedef struct PgStat_EntryRef
{
    /*
     * Pointer to the PgStatShared_HashEntry entry in the shared stats
     * hashtable.
     */
    PgStatShared_HashEntry *shared_entry;
 
    /*
     * Pointer to the stats data (i.e. PgStatShared_HashEntry->body), resolved
     * as a local pointer, to avoid repeated dsa_get_address() calls.
     */
    PgStatShared_Common *shared_stats;
 
    /*
     * Pending statistics data that will need to be flushed to shared memory
     * stats eventually. Each stats kind utilizing pending data defines what
     * format its pending data has and needs to provide a
     * PgStat_KindInfo->flush_pending_cb callback to merge pending into shared
     * stats.
     */
    void       *pending;
    dlist_node  pending_node;   /* membership in pgStatPending list */
} PgStat_EntryRef;
 
 
/*
 * Some stats changes are transactional. To maintain those, a stack of
 * PgStat_SubXactStatus entries is maintained, which contain data pertaining
 * to the current transaction and its active subtransactions.
 */
typedef struct PgStat_SubXactStatus
{
    int         nest_level;     /* subtransaction nest level */
 
    struct PgStat_SubXactStatus *prev;  /* higher-level subxact if any */
 
    /*
     * Statistics for transactionally dropped objects need to be
     * transactionally dropped as well. Collect the stats dropped in the
     * current (sub-)transaction and only execute the stats drop when we know
     * if the transaction commits/aborts. To handle replicas and crashes,
     * stats drops are included in commit / abort records.
     */
    dclist_head pending_drops;
 
    /*
     * Tuple insertion/deletion counts for an open transaction can't be
     * propagated into PgStat_TableStatus counters until we know if it is
     * going to commit or abort.  Hence, we keep these counts in per-subxact
     * structs that live in TopTransactionContext.  This data structure is
     * designed on the assumption that subxacts won't usually modify very many
     * tables.
     */
    PgStat_TableXactStatus *first;  /* head of list for this subxact */
} PgStat_SubXactStatus;
 
 
/*
 * Metadata for a specific kind of statistics.
 */
typedef struct PgStat_KindInfo
{
    /*
     * Do a fixed number of stats objects exist for this kind of stats (e.g.
     * bgwriter stats) or not (e.g. tables).
     */
    bool        fixed_amount:1;
 
    /*
     * Can stats of this kind be accessed from another database? Determines
     * whether a stats object gets included in stats snapshots.
     */
    bool        accessed_across_databases:1;
 
    /*
     * The size of an entry in the shared stats hash table (pointed to by
     * PgStatShared_HashEntry->body).
     */
    uint32      shared_size;
 
    /*
     * The offset of the statistics struct in the cached statistics snapshot
     * PgStat_Snapshot, for fixed-numbered statistics.
     */
    uint32      snapshot_ctl_off;
 
    /*
     * The offset of the statistics struct in the containing shared memory
     * control structure PgStat_ShmemControl, for fixed-numbered statistics.
     */
    uint32      shared_ctl_off;
 
    /*
     * The offset/size of statistics inside the shared stats entry. Used when
     * [de-]serializing statistics to / from disk respectively. Separate from
     * shared_size because [de-]serialization may not include in-memory state
     * like lwlocks.
     */
    uint32      shared_data_off;
    uint32      shared_data_len;
 
    /*
     * The size of the pending data for this kind. E.g. how large
     * PgStat_EntryRef->pending is. Used for allocations.
     *
     * 0 signals that an entry of this kind should never have a pending entry.
     */
    uint32      pending_size;
 
    /*
     * For variable-numbered stats: flush pending stats. Required if pending
     * data is used.
     */
    bool        (*flush_pending_cb) (PgStat_EntryRef *sr, bool nowait);
 
    /*
     * For variable-numbered stats: delete pending stats. Optional.
     */
    void        (*delete_pending_cb) (PgStat_EntryRef *sr);
 
    /*
     * For variable-numbered stats: reset the reset timestamp. Optional.
     */
    void        (*reset_timestamp_cb) (PgStatShared_Common *header, TimestampTz ts);
 
    /*
     * For variable-numbered stats. Optional.
     */
    void        (*to_serialized_name) (const PgStat_HashKey *key,
                                       const PgStatShared_Common *header, NameData *name);
    bool        (*from_serialized_name) (const NameData *name, PgStat_HashKey *key);
 
    /*
     * For fixed-numbered statistics: Initialize shared memory state.
     *
     * "stats" is the pointer to the allocated shared memory area.
     */
    void        (*init_shmem_cb) (void *stats);
 
    /*
     * For fixed-numbered statistics: Reset All.
     */
    void        (*reset_all_cb) (TimestampTz ts);
 
    /*
     * For fixed-numbered statistics: Build snapshot for entry
     */
    void        (*snapshot_cb) (void);
 
    /* name of the kind of stats */
    const char *const name;
} PgStat_KindInfo;
 
 
/*
 * List of SLRU names that we keep stats for.  There is no central registry of
 * SLRUs, so we use this fixed list instead.  The "other" entry is used for
 * all SLRUs without an explicit entry (e.g. SLRUs in extensions).
 *
 * This is only defined here so that SLRU_NUM_ELEMENTS is known for later type
 * definitions.
 */
static const char *const slru_names[] = {
    "commit_timestamp",
    "multixact_member",
    "multixact_offset",
    "notify",
    "serializable",
    "subtransaction",
    "transaction",
    "other"                     /* has to be last */
};
 
#define SLRU_NUM_ELEMENTS   lengthof(slru_names)
 
 
/* ----------
 * Types and definitions for different kinds of fixed-amount stats.
 *
 * Single-writer stats use the changecount mechanism to achieve low-overhead
 * writes - they're obviously more performance critical than reads. Check the
 * definition of struct PgBackendStatus for some explanation of the
 * changecount mechanism.
 *
 * Because the obvious implementation of resetting single-writer stats isn't
 * compatible with that (another backend needs to write), we don't scribble on
 * shared stats while resetting. Instead, just record the current counter
 * values in a copy of the stats data, which is protected by ->lock. See
 * pgstat_fetch_stat_(archiver|bgwriter|checkpointer) for the reader side.
 *
 * The only exception to that is the stat_reset_timestamp in these structs,
 * which is protected by ->lock, because it has to be written by another
 * backend while resetting.
 * ----------
 */
 
typedef struct PgStatShared_Archiver
{
    /* lock protects ->reset_offset as well as stats->stat_reset_timestamp */
    LWLock      lock;
    uint32      changecount;
    PgStat_ArchiverStats stats;
    PgStat_ArchiverStats reset_offset;
} PgStatShared_Archiver;
 
typedef struct PgStatShared_BgWriter
{
    /* lock protects ->reset_offset as well as stats->stat_reset_timestamp */
    LWLock      lock;
    uint32      changecount;
    PgStat_BgWriterStats stats;
    PgStat_BgWriterStats reset_offset;
} PgStatShared_BgWriter;
 
typedef struct PgStatShared_Checkpointer
{
    /* lock protects ->reset_offset as well as stats->stat_reset_timestamp */
    LWLock      lock;
    uint32      changecount;
    PgStat_CheckpointerStats stats;
    PgStat_CheckpointerStats reset_offset;
} PgStatShared_Checkpointer;
 
/* Shared-memory ready PgStat_IO */
typedef struct PgStatShared_IO
{
    /*
     * locks[i] protects stats.stats[i]. locks[0] also protects
     * stats.stat_reset_timestamp.
     */
    LWLock      locks[BACKEND_NUM_TYPES];
    PgStat_IO   stats;
} PgStatShared_IO;
 
typedef struct PgStatShared_SLRU
{
    /* lock protects ->stats */
    LWLock      lock;
    PgStat_SLRUStats stats[SLRU_NUM_ELEMENTS];
} PgStatShared_SLRU;
 
typedef struct PgStatShared_Wal
{
    /* lock protects ->stats */
    LWLock      lock;
    PgStat_WalStats stats;
} PgStatShared_Wal;
 
 
 
/* ----------
 * Types and definitions for different kinds of variable-amount stats.
 *
 * Each struct has to start with PgStatShared_Common, containing information
 * common across the different types of stats. Kind-specific data follows.
 * ----------
 */
 
typedef struct PgStatShared_Database
{
    PgStatShared_Common header;
    PgStat_StatDBEntry stats;
} PgStatShared_Database;
 
typedef struct PgStatShared_Relation
{
    PgStatShared_Common header;
    PgStat_StatTabEntry stats;
} PgStatShared_Relation;
 
typedef struct PgStatShared_Function
{
    PgStatShared_Common header;
    PgStat_StatFuncEntry stats;
} PgStatShared_Function;
 
typedef struct PgStatShared_Subscription
{
    PgStatShared_Common header;
    PgStat_StatSubEntry stats;
} PgStatShared_Subscription;
 
typedef struct PgStatShared_ReplSlot
{
    PgStatShared_Common header;
    PgStat_StatReplSlotEntry stats;
} PgStatShared_ReplSlot;
 
 
/*
 * Central shared memory entry for the cumulative stats system.
 *
 * Fixed amount stats, the dynamic shared memory hash table for
 * non-fixed-amount stats, as well as remaining bits and pieces are all
 * reached from here.
 */
typedef struct PgStat_ShmemControl
{
    void       *raw_dsa_area;
 
    /*
     * Stats for variable-numbered objects are kept in this shared hash table.
     * See comment above PgStat_Kind for details.
     */
    dshash_table_handle hash_handle;    /* shared dbstat hash */
 
    /* Has the stats system already been shut down? Just a debugging check. */
    bool        is_shutdown;
 
    /*
     * Whenever statistics for dropped objects could not be freed - because
     * backends still have references - the dropping backend calls
     * pgstat_request_entry_refs_gc() incrementing this counter. Eventually
     * that causes backends to run pgstat_gc_entry_refs(), allowing memory to
     * be reclaimed.
     */
    pg_atomic_uint64 gc_request_count;
 
    /*
     * Stats data for fixed-numbered objects.
     */
    PgStatShared_Archiver archiver;
    PgStatShared_BgWriter bgwriter;
    PgStatShared_Checkpointer checkpointer;
    PgStatShared_IO io;
    PgStatShared_SLRU slru;
    PgStatShared_Wal wal;
} PgStat_ShmemControl;
 
 
/*
 * Cached statistics snapshot
 */
typedef struct PgStat_Snapshot
{
    PgStat_FetchConsistency mode;
 
    /* time at which snapshot was taken */
    TimestampTz snapshot_timestamp;
 
    bool        fixed_valid[PGSTAT_NUM_KINDS];
 
    PgStat_ArchiverStats archiver;
 
    PgStat_BgWriterStats bgwriter;
 
    PgStat_CheckpointerStats checkpointer;
 
    PgStat_IO   io;
 
    PgStat_SLRUStats slru[SLRU_NUM_ELEMENTS];
 
    PgStat_WalStats wal;
 
    /* to free snapshot in bulk */
    MemoryContext context;
    struct pgstat_snapshot_hash *stats;
} PgStat_Snapshot;
 
 
/*
 * Collection of backend-local stats state.
 */
typedef struct PgStat_LocalState
{
    PgStat_ShmemControl *shmem;
    dsa_area   *dsa;
    dshash_table *shared_hash;
 
    /* the current statistics snapshot */
    PgStat_Snapshot snapshot;
} PgStat_LocalState;
 
 
/*
 * Inline functions defined further below.
 */
 
static inline void pgstat_begin_changecount_write(uint32 *cc);
static inline void pgstat_end_changecount_write(uint32 *cc);
static inline uint32 pgstat_begin_changecount_read(uint32 *cc);
static inline bool pgstat_end_changecount_read(uint32 *cc, uint32 cc_before);
 
static inline void pgstat_copy_changecounted_stats(void *dst, void *src, size_t len,
                                                   uint32 *cc);
 
static inline int pgstat_cmp_hash_key(const void *a, const void *b, size_t size, void *arg);
static inline uint32 pgstat_hash_hash_key(const void *d, size_t size, void *arg);
static inline size_t pgstat_get_entry_len(PgStat_Kind kind);
static inline void *pgstat_get_entry_data(PgStat_Kind kind, PgStatShared_Common *entry);
 
 
/*
 * Functions in pgstat.c
 */
 
extern const PgStat_KindInfo *pgstat_get_kind_info(PgStat_Kind kind);
 
#ifdef USE_ASSERT_CHECKING
extern void pgstat_assert_is_up(void);
#else
#define pgstat_assert_is_up() ((void)true)
#endif
 
extern void pgstat_delete_pending_entry(PgStat_EntryRef *entry_ref);
extern PgStat_EntryRef *pgstat_prep_pending_entry(PgStat_Kind kind, Oid dboid, Oid objoid, bool *created_entry);
extern PgStat_EntryRef *pgstat_fetch_pending_entry(PgStat_Kind kind, Oid dboid, Oid objoid);
 
extern void *pgstat_fetch_entry(PgStat_Kind kind, Oid dboid, Oid objoid);
extern void pgstat_snapshot_fixed(PgStat_Kind kind);
 
 
/*
 * Functions in pgstat_archiver.c
 */
 
extern void pgstat_archiver_init_shmem_cb(void *stats);
extern void pgstat_archiver_reset_all_cb(TimestampTz ts);
extern void pgstat_archiver_snapshot_cb(void);
 
 
/*
 * Functions in pgstat_bgwriter.c
 */
 
extern void pgstat_bgwriter_init_shmem_cb(void *stats);
extern void pgstat_bgwriter_reset_all_cb(TimestampTz ts);
extern void pgstat_bgwriter_snapshot_cb(void);
 
 
/*
 * Functions in pgstat_checkpointer.c
 */
 
extern void pgstat_checkpointer_init_shmem_cb(void *stats);
extern void pgstat_checkpointer_reset_all_cb(TimestampTz ts);
extern void pgstat_checkpointer_snapshot_cb(void);
 
 
/*
 * Functions in pgstat_database.c
 */
 
extern void pgstat_report_disconnect(Oid dboid);
extern void pgstat_update_dbstats(TimestampTz ts);
extern void AtEOXact_PgStat_Database(bool isCommit, bool parallel);
 
extern PgStat_StatDBEntry *pgstat_prep_database_pending(Oid dboid);
extern void pgstat_reset_database_timestamp(Oid dboid, TimestampTz ts);
extern bool pgstat_database_flush_cb(PgStat_EntryRef *entry_ref, bool nowait);
extern void pgstat_database_reset_timestamp_cb(PgStatShared_Common *header, TimestampTz ts);
 
 
/*
 * Functions in pgstat_function.c
 */
 
extern bool pgstat_function_flush_cb(PgStat_EntryRef *entry_ref, bool nowait);
 
 
/*
 * Functions in pgstat_io.c
 */
 
extern bool pgstat_flush_io(bool nowait);
extern void pgstat_io_init_shmem_cb(void *stats);
extern void pgstat_io_reset_all_cb(TimestampTz ts);
extern void pgstat_io_snapshot_cb(void);
 
 
/*
 * Functions in pgstat_relation.c
 */
 
extern void AtEOXact_PgStat_Relations(PgStat_SubXactStatus *xact_state, bool isCommit);
extern void AtEOSubXact_PgStat_Relations(PgStat_SubXactStatus *xact_state, bool isCommit, int nestDepth);
extern void AtPrepare_PgStat_Relations(PgStat_SubXactStatus *xact_state);
extern void PostPrepare_PgStat_Relations(PgStat_SubXactStatus *xact_state);
 
extern bool pgstat_relation_flush_cb(PgStat_EntryRef *entry_ref, bool nowait);
extern void pgstat_relation_delete_pending_cb(PgStat_EntryRef *entry_ref);
 
 
/*
 * Functions in pgstat_replslot.c
 */
 
extern void pgstat_replslot_reset_timestamp_cb(PgStatShared_Common *header, TimestampTz ts);
extern void pgstat_replslot_to_serialized_name_cb(const PgStat_HashKey *key, const PgStatShared_Common *header, NameData *name);
extern bool pgstat_replslot_from_serialized_name_cb(const NameData *name, PgStat_HashKey *key);
 
 
/*
 * Functions in pgstat_shmem.c
 */
 
extern void pgstat_attach_shmem(void);
extern void pgstat_detach_shmem(void);
 
extern PgStat_EntryRef *pgstat_get_entry_ref(PgStat_Kind kind, Oid dboid, Oid objoid,
                                             bool create, bool *created_entry);
extern bool pgstat_lock_entry(PgStat_EntryRef *entry_ref, bool nowait);
extern bool pgstat_lock_entry_shared(PgStat_EntryRef *entry_ref, bool nowait);
extern void pgstat_unlock_entry(PgStat_EntryRef *entry_ref);
extern bool pgstat_drop_entry(PgStat_Kind kind, Oid dboid, Oid objoid);
extern void pgstat_drop_all_entries(void);
extern PgStat_EntryRef *pgstat_get_entry_ref_locked(PgStat_Kind kind, Oid dboid, Oid objoid,
                                                    bool nowait);
extern void pgstat_reset_entry(PgStat_Kind kind, Oid dboid, Oid objoid, TimestampTz ts);
extern void pgstat_reset_entries_of_kind(PgStat_Kind kind, TimestampTz ts);
extern void pgstat_reset_matching_entries(bool (*do_reset) (PgStatShared_HashEntry *, Datum),
                                          Datum match_data,
                                          TimestampTz ts);
 
extern void pgstat_request_entry_refs_gc(void);
extern PgStatShared_Common *pgstat_init_entry(PgStat_Kind kind,
                                              PgStatShared_HashEntry *shhashent);
 
 
/*
 * Functions in pgstat_slru.c
 */
 
extern bool pgstat_slru_flush(bool nowait);
extern void pgstat_slru_init_shmem_cb(void *stats);
extern void pgstat_slru_reset_all_cb(TimestampTz ts);
extern void pgstat_slru_snapshot_cb(void);
 
 
/*
 * Functions in pgstat_wal.c
 */
 
extern bool pgstat_flush_wal(bool nowait);
extern void pgstat_init_wal(void);
extern bool pgstat_have_pending_wal(void);
 
extern void pgstat_wal_init_shmem_cb(void *stats);
extern void pgstat_wal_reset_all_cb(TimestampTz ts);
extern void pgstat_wal_snapshot_cb(void);
 
 
/*
 * Functions in pgstat_subscription.c
 */
 
extern bool pgstat_subscription_flush_cb(PgStat_EntryRef *entry_ref, bool nowait);
extern void pgstat_subscription_reset_timestamp_cb(PgStatShared_Common *header, TimestampTz ts);
 
 
/*
 * Functions in pgstat_xact.c
 */
 
extern PgStat_SubXactStatus *pgstat_get_xact_stack_level(int nest_level);
extern void pgstat_drop_transactional(PgStat_Kind kind, Oid dboid, Oid objoid);
extern void pgstat_create_transactional(PgStat_Kind kind, Oid dboid, Oid objoid);
 
 
/*
 * Variables in pgstat.c
 */
 
extern PGDLLIMPORT PgStat_LocalState pgStatLocal;
 
 
/*
 * Variables in pgstat_io.c
 */
 
extern PGDLLIMPORT bool have_iostats;
 
 
/*
 * Variables in pgstat_slru.c
 */
 
extern PGDLLIMPORT bool have_slrustats;
 
 
/*
 * Implementation of inline functions declared above.
 */
 
/*
 * Helpers for changecount manipulation. See comments around struct
 * PgBackendStatus for details.
 */
 
static inline void
pgstat_begin_changecount_write(uint32 *cc)
{
    Assert((*cc & 1) == 0);
 
    START_CRIT_SECTION();
    (*cc)++;
    pg_write_barrier();
}
 
static inline void
pgstat_end_changecount_write(uint32 *cc)
{
    Assert((*cc & 1) == 1);
 
    pg_write_barrier();
 
    (*cc)++;
 
    END_CRIT_SECTION();
}
 
static inline uint32
pgstat_begin_changecount_read(uint32 *cc)
{
    uint32      before_cc = *cc;
 
    CHECK_FOR_INTERRUPTS();
 
    pg_read_barrier();
 
    return before_cc;
}
 
/*
 * Returns true if the read succeeded, false if it needs to be repeated.
 */
static inline bool
pgstat_end_changecount_read(uint32 *cc, uint32 before_cc)
{
    uint32      after_cc;
 
    pg_read_barrier();
 
    after_cc = *cc;
 
    /* was a write in progress when we started? */
    if (before_cc & 1)
        return false;
 
    /* did writes start and complete while we read? */
    return before_cc == after_cc;
}
 
 
/*
 * helper function for PgStat_KindInfo->snapshot_cb
 * PgStat_KindInfo->reset_all_cb callbacks.
 *
 * Copies out the specified memory area following change-count protocol.
 */
static inline void
pgstat_copy_changecounted_stats(void *dst, void *src, size_t len,
                                uint32 *cc)
{
    uint32      cc_before;
 
    do
    {
        cc_before = pgstat_begin_changecount_read(cc);
 
        memcpy(dst, src, len);
    }
    while (!pgstat_end_changecount_read(cc, cc_before));
}
 
/* helpers for dshash / simplehash hashtables */
static inline int
pgstat_cmp_hash_key(const void *a, const void *b, size_t size, void *arg)
{
    Assert(size == sizeof(PgStat_HashKey) && arg == NULL);
    return memcmp(a, b, sizeof(PgStat_HashKey));
}
 
static inline uint32
pgstat_hash_hash_key(const void *d, size_t size, void *arg)
{
    const char *key = (const char *) d;
 
    Assert(size == sizeof(PgStat_HashKey) && arg == NULL);
    return fasthash32(key, size, 0);
}
 
/*
 * The length of the data portion of a shared memory stats entry (i.e. without
 * transient data such as refcounts, lwlocks, ...).
 */
static inline size_t
pgstat_get_entry_len(PgStat_Kind kind)
{
    return pgstat_get_kind_info(kind)->shared_data_len;
}
 
/*
 * Returns a pointer to the data portion of a shared memory stats entry.
 */
static inline void *
pgstat_get_entry_data(PgStat_Kind kind, PgStatShared_Common *entry)
{
    size_t      off = pgstat_get_kind_info(kind)->shared_data_off;
 
    Assert(off != 0 && off < PG_UINT32_MAX);
 
    return ((char *) (entry)) + off;
}
 
#endif                          /* PGSTAT_INTERNAL_H */