pgstat.c

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/* ----------
 * pgstat.c
 *
 *  All the statistics collector stuff hacked up in one big, ugly file.
 *
 *  TODO:   - Separate collector, postmaster and backend stuff
 *            into different files.
 *
 *          - Add some automatic call for pgstat vacuuming.
 *
 *          - Add a pgstat config column to pg_database, so this
 *            entire thing can be enabled/disabled on a per db basis.
 *
 *  Copyright (c) 2001-2017, PostgreSQL Global Development Group
 *
 *  src/backend/postmaster/pgstat.c
 * ----------
 */
#include "postgres.h"

#include <unistd.h>
#include <fcntl.h>
#include <sys/param.h>
#include <sys/time.h>
#include <sys/socket.h>
#include <netdb.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <signal.h>
#include <time.h>
#ifdef HAVE_SYS_SELECT_H
#include <sys/select.h>
#endif

#include "pgstat.h"

#include "access/heapam.h"
#include "access/htup_details.h"
#include "access/transam.h"
#include "access/twophase_rmgr.h"
#include "access/xact.h"
#include "catalog/pg_database.h"
#include "catalog/pg_proc.h"
#include "common/ip.h"
#include "libpq/libpq.h"
#include "libpq/pqsignal.h"
#include "mb/pg_wchar.h"
#include "miscadmin.h"
#include "pg_trace.h"
#include "postmaster/autovacuum.h"
#include "postmaster/fork_process.h"
#include "postmaster/postmaster.h"
#include "replication/walsender.h"
#include "storage/backendid.h"
#include "storage/dsm.h"
#include "storage/fd.h"
#include "storage/ipc.h"
#include "storage/latch.h"
#include "storage/lmgr.h"
#include "storage/pg_shmem.h"
#include "storage/procsignal.h"
#include "storage/sinvaladt.h"
#include "utils/ascii.h"
#include "utils/guc.h"
#include "utils/memutils.h"
#include "utils/ps_status.h"
#include "utils/rel.h"
#include "utils/snapmgr.h"
#include "utils/timestamp.h"
#include "utils/tqual.h"


/* ----------
 * Timer definitions.
 * ----------
 */
#define PGSTAT_STAT_INTERVAL    500 /* Minimum time between stats file
                                     * updates; in milliseconds. */

#define PGSTAT_RETRY_DELAY      10  /* How long to wait between checks for a
                                     * new file; in milliseconds. */

#define PGSTAT_MAX_WAIT_TIME    10000   /* Maximum time to wait for a stats
                                         * file update; in milliseconds. */

#define PGSTAT_INQ_INTERVAL     640 /* How often to ping the collector for a
                                     * new file; in milliseconds. */

#define PGSTAT_RESTART_INTERVAL 60  /* How often to attempt to restart a
                                     * failed statistics collector; in
                                     * seconds. */

#define PGSTAT_POLL_LOOP_COUNT  (PGSTAT_MAX_WAIT_TIME / PGSTAT_RETRY_DELAY)
#define PGSTAT_INQ_LOOP_COUNT   (PGSTAT_INQ_INTERVAL / PGSTAT_RETRY_DELAY)

/* Minimum receive buffer size for the collector's socket. */
#define PGSTAT_MIN_RCVBUF       (100 * 1024)


/* ----------
 * The initial size hints for the hash tables used in the collector.
 * ----------
 */
#define PGSTAT_DB_HASH_SIZE     16
#define PGSTAT_TAB_HASH_SIZE    512
#define PGSTAT_FUNCTION_HASH_SIZE   512


/* ----------
 * Total number of backends including auxiliary
 *
 * We reserve a slot for each possible BackendId, plus one for each
 * possible auxiliary process type.  (This scheme assumes there is not
 * more than one of any auxiliary process type at a time.) MaxBackends
 * includes autovacuum workers and background workers as well.
 * ----------
 */
#define NumBackendStatSlots (MaxBackends + NUM_AUXPROCTYPES)


/* ----------
 * GUC parameters
 * ----------
 */
bool        pgstat_track_activities = false;
bool        pgstat_track_counts = false;
int         pgstat_track_functions = TRACK_FUNC_OFF;
int         pgstat_track_activity_query_size = 1024;

/* ----------
 * Built from GUC parameter
 * ----------
 */
char       *pgstat_stat_directory = NULL;
char       *pgstat_stat_filename = NULL;
char       *pgstat_stat_tmpname = NULL;

/*
 * BgWriter global statistics counters (unused in other processes).
 * Stored directly in a stats message structure so it can be sent
 * without needing to copy things around.  We assume this inits to zeroes.
 */
PgStat_MsgBgWriter BgWriterStats;

/* ----------
 * Local data
 * ----------
 */
NON_EXEC_STATIC pgsocket pgStatSock = PGINVALID_SOCKET;

static struct sockaddr_storage pgStatAddr;

static time_t last_pgstat_start_time;

static bool pgStatRunningInCollector = false;

/*
 * Structures in which backends store per-table info that's waiting to be
 * sent to the collector.
 *
 * NOTE: once allocated, TabStatusArray structures are never moved or deleted
 * for the life of the backend.  Also, we zero out the t_id fields of the
 * contained PgStat_TableStatus structs whenever they are not actively in use.
 * This allows relcache pgstat_info pointers to be treated as long-lived data,
 * avoiding repeated searches in pgstat_initstats() when a relation is
 * repeatedly opened during a transaction.
 */
#define TABSTAT_QUANTUM     100 /* we alloc this many at a time */

typedef struct TabStatusArray
{
    struct TabStatusArray *tsa_next;    /* link to next array, if any */
    int         tsa_used;       /* # entries currently used */
    PgStat_TableStatus tsa_entries[TABSTAT_QUANTUM];    /* per-table data */
} TabStatusArray;

static TabStatusArray *pgStatTabList = NULL;

/*
 * pgStatTabHash entry: map from relation OID to PgStat_TableStatus pointer
 */
typedef struct TabStatHashEntry
{
    Oid         t_id;
    PgStat_TableStatus *tsa_entry;
} TabStatHashEntry;

/*
 * Hash table for O(1) t_id -> tsa_entry lookup
 */
static HTAB *pgStatTabHash = NULL;

/*
 * Backends store per-function info that's waiting to be sent to the collector
 * in this hash table (indexed by function OID).
 */
static HTAB *pgStatFunctions = NULL;

/*
 * Indicates if backend has some function stats that it hasn't yet
 * sent to the collector.
 */
static bool have_function_stats = false;

/*
 * 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.
 */
typedef struct PgStat_SubXactStatus
{
    int         nest_level;     /* subtransaction nest level */
    struct PgStat_SubXactStatus *prev;  /* higher-level subxact if any */
    PgStat_TableXactStatus *first;  /* head of list for this subxact */
} PgStat_SubXactStatus;

static PgStat_SubXactStatus *pgStatXactStack = NULL;

static int  pgStatXactCommit = 0;
static int  pgStatXactRollback = 0;
PgStat_Counter pgStatBlockReadTime = 0;
PgStat_Counter pgStatBlockWriteTime = 0;

/* Record that's written to 2PC state file when pgstat state is persisted */
typedef struct TwoPhasePgStatRecord
{
    PgStat_Counter tuples_inserted; /* tuples inserted in xact */
    PgStat_Counter tuples_updated;  /* tuples updated in xact */
    PgStat_Counter tuples_deleted;  /* tuples deleted in xact */
    PgStat_Counter inserted_pre_trunc;  /* tuples inserted prior to truncate */
    PgStat_Counter updated_pre_trunc;   /* tuples updated prior to truncate */
    PgStat_Counter deleted_pre_trunc;   /* tuples deleted prior to truncate */
    Oid         t_id;           /* table's OID */
    bool        t_shared;       /* is it a shared catalog? */
    bool        t_truncated;    /* was the relation truncated? */
} TwoPhasePgStatRecord;

/*
 * Info about current "snapshot" of stats file
 */
static MemoryContext pgStatLocalContext = NULL;
static HTAB *pgStatDBHash = NULL;

/* Status for backends including auxiliary */
static LocalPgBackendStatus *localBackendStatusTable = NULL;

/* Total number of backends including auxiliary */
static int  localNumBackends = 0;

/*
 * Cluster wide statistics, kept in the stats collector.
 * Contains statistics that are not collected per database
 * or per table.
 */
static PgStat_ArchiverStats archiverStats;
static PgStat_GlobalStats globalStats;

/*
 * List of OIDs of databases we need to write out.  If an entry is InvalidOid,
 * it means to write only the shared-catalog stats ("DB 0"); otherwise, we
 * will write both that DB's data and the shared stats.
 */
static List *pending_write_requests = NIL;

/* Signal handler flags */
static volatile bool need_exit = false;
static volatile bool got_SIGHUP = false;

/*
 * Total time charged to functions so far in the current backend.
 * We use this to help separate "self" and "other" time charges.
 * (We assume this initializes to zero.)
 */
static instr_time total_func_time;


/* ----------
 * Local function forward declarations
 * ----------
 */
#ifdef EXEC_BACKEND
static pid_t pgstat_forkexec(void);
#endif

NON_EXEC_STATIC void PgstatCollectorMain(int argc, char *argv[]) pg_attribute_noreturn();
static void pgstat_exit(SIGNAL_ARGS);
static void pgstat_beshutdown_hook(int code, Datum arg);
static void pgstat_sighup_handler(SIGNAL_ARGS);

static PgStat_StatDBEntry *pgstat_get_db_entry(Oid databaseid, bool create);
static PgStat_StatTabEntry *pgstat_get_tab_entry(PgStat_StatDBEntry *dbentry,
                     Oid tableoid, bool create);
static void pgstat_write_statsfiles(bool permanent, bool allDbs);
static void pgstat_write_db_statsfile(PgStat_StatDBEntry *dbentry, bool permanent);
static HTAB *pgstat_read_statsfiles(Oid onlydb, bool permanent, bool deep);
static void pgstat_read_db_statsfile(Oid databaseid, HTAB *tabhash, HTAB *funchash, bool permanent);
static void backend_read_statsfile(void);
static void pgstat_read_current_status(void);

static bool pgstat_write_statsfile_needed(void);
static bool pgstat_db_requested(Oid databaseid);

static void pgstat_send_tabstat(PgStat_MsgTabstat *tsmsg);
static void pgstat_send_funcstats(void);
static HTAB *pgstat_collect_oids(Oid catalogid);

static PgStat_TableStatus *get_tabstat_entry(Oid rel_id, bool isshared);

static void pgstat_setup_memcxt(void);

static const char *pgstat_get_wait_activity(WaitEventActivity w);
static const char *pgstat_get_wait_client(WaitEventClient w);
static const char *pgstat_get_wait_ipc(WaitEventIPC w);
static const char *pgstat_get_wait_timeout(WaitEventTimeout w);
static const char *pgstat_get_wait_io(WaitEventIO w);

static void pgstat_setheader(PgStat_MsgHdr *hdr, StatMsgType mtype);
static void pgstat_send(void *msg, int len);

static void pgstat_recv_inquiry(PgStat_MsgInquiry *msg, int len);
static void pgstat_recv_tabstat(PgStat_MsgTabstat *msg, int len);
static void pgstat_recv_tabpurge(PgStat_MsgTabpurge *msg, int len);
static void pgstat_recv_dropdb(PgStat_MsgDropdb *msg, int len);
static void pgstat_recv_resetcounter(PgStat_MsgResetcounter *msg, int len);
static void pgstat_recv_resetsharedcounter(PgStat_MsgResetsharedcounter *msg, int len);
static void pgstat_recv_resetsinglecounter(PgStat_MsgResetsinglecounter *msg, int len);
static void pgstat_recv_autovac(PgStat_MsgAutovacStart *msg, int len);
static void pgstat_recv_vacuum(PgStat_MsgVacuum *msg, int len);
static void pgstat_recv_analyze(PgStat_MsgAnalyze *msg, int len);
static void pgstat_recv_archiver(PgStat_MsgArchiver *msg, int len);
static void pgstat_recv_bgwriter(PgStat_MsgBgWriter *msg, int len);
static void pgstat_recv_funcstat(PgStat_MsgFuncstat *msg, int len);
static void pgstat_recv_funcpurge(PgStat_MsgFuncpurge *msg, int len);
static void pgstat_recv_recoveryconflict(PgStat_MsgRecoveryConflict *msg, int len);
static void pgstat_recv_deadlock(PgStat_MsgDeadlock *msg, int len);
static void pgstat_recv_tempfile(PgStat_MsgTempFile *msg, int len);

/* ------------------------------------------------------------
 * Public functions called from postmaster follow
 * ------------------------------------------------------------
 */

/* ----------
 * pgstat_init() -
 *
 *  Called from postmaster at startup. Create the resources required
 *  by the statistics collector process.  If unable to do so, do not
 *  fail --- better to let the postmaster start with stats collection
 *  disabled.
 * ----------
 */
void
pgstat_init(void)
{
    ACCEPT_TYPE_ARG3 alen;
    struct addrinfo *addrs = NULL,
               *addr,
                hints;
    int         ret;
    fd_set      rset;
    struct timeval tv;
    char        test_byte;
    int         sel_res;
    int         tries = 0;

#define TESTBYTEVAL ((char) 199)

    /*
     * This static assertion verifies that we didn't mess up the calculations
     * involved in selecting maximum payload sizes for our UDP messages.
     * Because the only consequence of overrunning PGSTAT_MAX_MSG_SIZE would
     * be silent performance loss from fragmentation, it seems worth having a
     * compile-time cross-check that we didn't.
     */
    StaticAssertStmt(sizeof(PgStat_Msg) <= PGSTAT_MAX_MSG_SIZE,
                     "maximum stats message size exceeds PGSTAT_MAX_MSG_SIZE");

    /*
     * Create the UDP socket for sending and receiving statistic messages
     */
    hints.ai_flags = AI_PASSIVE;
    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = SOCK_DGRAM;
    hints.ai_protocol = 0;
    hints.ai_addrlen = 0;
    hints.ai_addr = NULL;
    hints.ai_canonname = NULL;
    hints.ai_next = NULL;
    ret = pg_getaddrinfo_all("localhost", NULL, &hints, &addrs);
    if (ret || !addrs)
    {
        ereport(LOG,
                (errmsg("could not resolve \"localhost\": %s",
                        gai_strerror(ret))));
        goto startup_failed;
    }

    /*
     * On some platforms, pg_getaddrinfo_all() may return multiple addresses
     * only one of which will actually work (eg, both IPv6 and IPv4 addresses
     * when kernel will reject IPv6).  Worse, the failure may occur at the
     * bind() or perhaps even connect() stage.  So we must loop through the
     * results till we find a working combination. We will generate LOG
     * messages, but no error, for bogus combinations.
     */
    for (addr = addrs; addr; addr = addr->ai_next)
    {
#ifdef HAVE_UNIX_SOCKETS
        /* Ignore AF_UNIX sockets, if any are returned. */
        if (addr->ai_family == AF_UNIX)
            continue;
#endif

        if (++tries > 1)
            ereport(LOG,
                    (errmsg("trying another address for the statistics collector")));

        /*
         * Create the socket.
         */
        if ((pgStatSock = socket(addr->ai_family, SOCK_DGRAM, 0)) == PGINVALID_SOCKET)
        {
            ereport(LOG,
                    (errcode_for_socket_access(),
                     errmsg("could not create socket for statistics collector: %m")));
            continue;
        }

        /*
         * Bind it to a kernel assigned port on localhost and get the assigned
         * port via getsockname().
         */
        if (bind(pgStatSock, addr->ai_addr, addr->ai_addrlen) < 0)
        {
            ereport(LOG,
                    (errcode_for_socket_access(),
                     errmsg("could not bind socket for statistics collector: %m")));
            closesocket(pgStatSock);
            pgStatSock = PGINVALID_SOCKET;
            continue;
        }

        alen = sizeof(pgStatAddr);
        if (getsockname(pgStatSock, (struct sockaddr *) &pgStatAddr, &alen) < 0)
        {
            ereport(LOG,
                    (errcode_for_socket_access(),
                     errmsg("could not get address of socket for statistics collector: %m")));
            closesocket(pgStatSock);
            pgStatSock = PGINVALID_SOCKET;
            continue;
        }

        /*
         * Connect the socket to its own address.  This saves a few cycles by
         * not having to respecify the target address on every send. This also
         * provides a kernel-level check that only packets from this same
         * address will be received.
         */
        if (connect(pgStatSock, (struct sockaddr *) &pgStatAddr, alen) < 0)
        {
            ereport(LOG,
                    (errcode_for_socket_access(),
                     errmsg("could not connect socket for statistics collector: %m")));
            closesocket(pgStatSock);
            pgStatSock = PGINVALID_SOCKET;
            continue;
        }

        /*
         * Try to send and receive a one-byte test message on the socket. This
         * is to catch situations where the socket can be created but will not
         * actually pass data (for instance, because kernel packet filtering
         * rules prevent it).
         */
        test_byte = TESTBYTEVAL;

retry1:
        if (send(pgStatSock, &test_byte, 1, 0) != 1)
        {
            if (errno == EINTR)
                goto retry1;    /* if interrupted, just retry */
            ereport(LOG,
                    (errcode_for_socket_access(),
                     errmsg("could not send test message on socket for statistics collector: %m")));
            closesocket(pgStatSock);
            pgStatSock = PGINVALID_SOCKET;
            continue;
        }

        /*
         * There could possibly be a little delay before the message can be
         * received.  We arbitrarily allow up to half a second before deciding
         * it's broken.
         */
        for (;;)                /* need a loop to handle EINTR */
        {
            FD_ZERO(&rset);
            FD_SET(pgStatSock, &rset);

            tv.tv_sec = 0;
            tv.tv_usec = 500000;
            sel_res = select(pgStatSock + 1, &rset, NULL, NULL, &tv);
            if (sel_res >= 0 || errno != EINTR)
                break;
        }
        if (sel_res < 0)
        {
            ereport(LOG,
                    (errcode_for_socket_access(),
                     errmsg("select() failed in statistics collector: %m")));
            closesocket(pgStatSock);
            pgStatSock = PGINVALID_SOCKET;
            continue;
        }
        if (sel_res == 0 || !FD_ISSET(pgStatSock, &rset))
        {
            /*
             * This is the case we actually think is likely, so take pains to
             * give a specific message for it.
             *
             * errno will not be set meaningfully here, so don't use it.
             */
            ereport(LOG,
                    (errcode(ERRCODE_CONNECTION_FAILURE),
                     errmsg("test message did not get through on socket for statistics collector")));
            closesocket(pgStatSock);
            pgStatSock = PGINVALID_SOCKET;
            continue;
        }

        test_byte++;            /* just make sure variable is changed */

retry2:
        if (recv(pgStatSock, &test_byte, 1, 0) != 1)
        {
            if (errno == EINTR)
                goto retry2;    /* if interrupted, just retry */
            ereport(LOG,
                    (errcode_for_socket_access(),
                     errmsg("could not receive test message on socket for statistics collector: %m")));
            closesocket(pgStatSock);
            pgStatSock = PGINVALID_SOCKET;
            continue;
        }

        if (test_byte != TESTBYTEVAL)   /* strictly paranoia ... */
        {
            ereport(LOG,
                    (errcode(ERRCODE_INTERNAL_ERROR),
                     errmsg("incorrect test message transmission on socket for statistics collector")));
            closesocket(pgStatSock);
            pgStatSock = PGINVALID_SOCKET;
            continue;
        }

        /* If we get here, we have a working socket */
        break;
    }

    /* Did we find a working address? */
    if (!addr || pgStatSock == PGINVALID_SOCKET)
        goto startup_failed;

    /*
     * Set the socket to non-blocking IO.  This ensures that if the collector
     * falls behind, statistics messages will be discarded; backends won't
     * block waiting to send messages to the collector.
     */
    if (!pg_set_noblock(pgStatSock))
    {
        ereport(LOG,
                (errcode_for_socket_access(),
                 errmsg("could not set statistics collector socket to nonblocking mode: %m")));
        goto startup_failed;
    }

    /*
     * Try to ensure that the socket's receive buffer is at least
     * PGSTAT_MIN_RCVBUF bytes, so that it won't easily overflow and lose
     * data.  Use of UDP protocol means that we are willing to lose data under
     * heavy load, but we don't want it to happen just because of ridiculously
     * small default buffer sizes (such as 8KB on older Windows versions).
     */
    {
        int         old_rcvbuf;
        int         new_rcvbuf;
        ACCEPT_TYPE_ARG3 rcvbufsize = sizeof(old_rcvbuf);

        if (getsockopt(pgStatSock, SOL_SOCKET, SO_RCVBUF,
                       (char *) &old_rcvbuf, &rcvbufsize) < 0)
        {
            elog(LOG, "getsockopt(SO_RCVBUF) failed: %m");
            /* if we can't get existing size, always try to set it */
            old_rcvbuf = 0;
        }

        new_rcvbuf = PGSTAT_MIN_RCVBUF;
        if (old_rcvbuf < new_rcvbuf)
        {
            if (setsockopt(pgStatSock, SOL_SOCKET, SO_RCVBUF,
                           (char *) &new_rcvbuf, sizeof(new_rcvbuf)) < 0)
                elog(LOG, "setsockopt(SO_RCVBUF) failed: %m");
        }
    }

    pg_freeaddrinfo_all(hints.ai_family, addrs);

    return;

startup_failed:
    ereport(LOG,
            (errmsg("disabling statistics collector for lack of working socket")));

    if (addrs)
        pg_freeaddrinfo_all(hints.ai_family, addrs);

    if (pgStatSock != PGINVALID_SOCKET)
        closesocket(pgStatSock);
    pgStatSock = PGINVALID_SOCKET;

    /*
     * Adjust GUC variables to suppress useless activity, and for debugging
     * purposes (seeing track_counts off is a clue that we failed here). We
     * use PGC_S_OVERRIDE because there is no point in trying to turn it back
     * on from postgresql.conf without a restart.
     */
    SetConfigOption("track_counts", "off", PGC_INTERNAL, PGC_S_OVERRIDE);
}

/*
 * subroutine for pgstat_reset_all
 */
static void
pgstat_reset_remove_files(const char *directory)
{
    DIR        *dir;
    struct dirent *entry;
    char        fname[MAXPGPATH * 2];

    dir = AllocateDir(directory);
    while ((entry = ReadDir(dir, directory)) != NULL)
    {
        int         nchars;
        Oid         tmp_oid;

        /*
         * Skip directory entries that don't match the file names we write.
         * See get_dbstat_filename for the database-specific pattern.
         */
        if (strncmp(entry->d_name, "global.", 7) == 0)
            nchars = 7;
        else
        {
            nchars = 0;
            (void) sscanf(entry->d_name, "db_%u.%n",
                          &tmp_oid, &nchars);
            if (nchars <= 0)
                continue;
            /* %u allows leading whitespace, so reject that */
            if (strchr("0123456789", entry->d_name[3]) == NULL)
                continue;
        }

        if (strcmp(entry->d_name + nchars, "tmp") != 0 &&
            strcmp(entry->d_name + nchars, "stat") != 0)
            continue;

        snprintf(fname, sizeof(fname), "%s/%s", directory,
                 entry->d_name);
        unlink(fname);
    }
    FreeDir(dir);
}

/*
 * pgstat_reset_all() -
 *
 * Remove the stats files.  This is currently used only if WAL
 * recovery is needed after a crash.
 */
void
pgstat_reset_all(void)
{
    pgstat_reset_remove_files(pgstat_stat_directory);
    pgstat_reset_remove_files(PGSTAT_STAT_PERMANENT_DIRECTORY);
}

#ifdef EXEC_BACKEND

/*
 * pgstat_forkexec() -
 *
 * Format up the arglist for, then fork and exec, statistics collector process
 */
static pid_t
pgstat_forkexec(void)
{
    char       *av[10];
    int         ac = 0;

    av[ac++] = "postgres";
    av[ac++] = "--forkcol";
    av[ac++] = NULL;            /* filled in by postmaster_forkexec */

    av[ac] = NULL;
    Assert(ac < lengthof(av));

    return postmaster_forkexec(ac, av);
}
#endif                          /* EXEC_BACKEND */


/*
 * pgstat_start() -
 *
 *  Called from postmaster at startup or after an existing collector
 *  died.  Attempt to fire up a fresh statistics collector.
 *
 *  Returns PID of child process, or 0 if fail.
 *
 *  Note: if fail, we will be called again from the postmaster main loop.
 */
int
pgstat_start(void)
{
    time_t      curtime;
    pid_t       pgStatPid;

    /*
     * Check that the socket is there, else pgstat_init failed and we can do
     * nothing useful.
     */
    if (pgStatSock == PGINVALID_SOCKET)
        return 0;

    /*
     * Do nothing if too soon since last collector start.  This is a safety
     * valve to protect against continuous respawn attempts if the collector
     * is dying immediately at launch.  Note that since we will be re-called
     * from the postmaster main loop, we will get another chance later.
     */
    curtime = time(NULL);
    if ((unsigned int) (curtime - last_pgstat_start_time) <
        (unsigned int) PGSTAT_RESTART_INTERVAL)
        return 0;
    last_pgstat_start_time = curtime;

    /*
     * Okay, fork off the collector.
     */
#ifdef EXEC_BACKEND
    switch ((pgStatPid = pgstat_forkexec()))
#else
    switch ((pgStatPid = fork_process()))
#endif
    {
        case -1:
            ereport(LOG,
                    (errmsg("could not fork statistics collector: %m")));
            return 0;

#ifndef EXEC_BACKEND
        case 0:
            /* in postmaster child ... */
            InitPostmasterChild();

            /* Close the postmaster's sockets */
            ClosePostmasterPorts(false);

            /* Drop our connection to postmaster's shared memory, as well */
            dsm_detach_all();
            PGSharedMemoryDetach();

            PgstatCollectorMain(0, NULL);
            break;
#endif

        default:
            return (int) pgStatPid;
    }

    /* shouldn't get here */
    return 0;
}

void
allow_immediate_pgstat_restart(void)
{
    last_pgstat_start_time = 0;
}

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


/* ----------
 * pgstat_report_stat() -
 *
 *  Must be called by processes that performs DML: tcop/postgres.c, logical
 *  receiver processes, SPI worker, etc. to send the so far collected
 *  per-table and function usage statistics to the collector.  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.
 * ----------
 */
void
pgstat_report_stat(bool force)
{
    /* we assume this inits to all zeroes: */
    static const PgStat_TableCounts all_zeroes;
    static TimestampTz last_report = 0;

    TimestampTz now;
    PgStat_MsgTabstat regular_msg;
    PgStat_MsgTabstat shared_msg;
    TabStatusArray *tsa;
    int         i;

    /* Don't expend a clock check if nothing to do */
    if ((pgStatTabList == NULL || pgStatTabList->tsa_used == 0) &&
        pgStatXactCommit == 0 && pgStatXactRollback == 0 &&
        !have_function_stats)
        return;

    /*
     * Don't send a message unless it's been at least PGSTAT_STAT_INTERVAL
     * msec since we last sent one, or the caller wants to force stats out.
     */
    now = GetCurrentTransactionStopTimestamp();
    if (!force &&
        !TimestampDifferenceExceeds(last_report, now, PGSTAT_STAT_INTERVAL))
        return;
    last_report = now;

    /*
     * Destroy pgStatTabHash before we start invalidating PgStat_TableEntry
     * entries it points to.  (Should we fail partway through the loop below,
     * it's okay to have removed the hashtable already --- the only
     * consequence is we'd get multiple entries for the same table in the
     * pgStatTabList, and that's safe.)
     */
    if (pgStatTabHash)
        hash_destroy(pgStatTabHash);
    pgStatTabHash = NULL;

    /*
     * Scan through the TabStatusArray struct(s) to find tables that actually
     * have counts, and build messages to send.  We have to separate shared
     * relations from regular ones because the databaseid field in the message
     * header has to depend on that.
     */
    regular_msg.m_databaseid = MyDatabaseId;
    shared_msg.m_databaseid = InvalidOid;
    regular_msg.m_nentries = 0;
    shared_msg.m_nentries = 0;

    for (tsa = pgStatTabList; tsa != NULL; tsa = tsa->tsa_next)
    {
        for (i = 0; i < tsa->tsa_used; i++)
        {
            PgStat_TableStatus *entry = &tsa->tsa_entries[i];
            PgStat_MsgTabstat *this_msg;
            PgStat_TableEntry *this_ent;

            /* Shouldn't have any pending transaction-dependent counts */
            Assert(entry->trans == NULL);

            /*
             * Ignore entries that didn't accumulate any actual counts, such
             * as indexes that were opened by the planner but not used.
             */
            if (memcmp(&entry->t_counts, &all_zeroes,
                       sizeof(PgStat_TableCounts)) == 0)
                continue;

            /*
             * OK, insert data into the appropriate message, and send if full.
             */
            this_msg = entry->t_shared ? &shared_msg : &regular_msg;
            this_ent = &this_msg->m_entry[this_msg->m_nentries];
            this_ent->t_id = entry->t_id;
            memcpy(&this_ent->t_counts, &entry->t_counts,
                   sizeof(PgStat_TableCounts));
            if (++this_msg->m_nentries >= PGSTAT_NUM_TABENTRIES)
            {
                pgstat_send_tabstat(this_msg);
                this_msg->m_nentries = 0;
            }
        }
        /* zero out TableStatus structs after use */
        MemSet(tsa->tsa_entries, 0,
               tsa->tsa_used * sizeof(PgStat_TableStatus));
        tsa->tsa_used = 0;
    }

    /*
     * Send partial messages.  Make sure that any pending xact commit/abort
     * gets counted, even if there are no table stats to send.
     */
    if (regular_msg.m_nentries > 0 ||
        pgStatXactCommit > 0 || pgStatXactRollback > 0)
        pgstat_send_tabstat(&regular_msg);
    if (shared_msg.m_nentries > 0)
        pgstat_send_tabstat(&shared_msg);

    /* Now, send function statistics */
    pgstat_send_funcstats();
}

/*
 * Subroutine for pgstat_report_stat: finish and send a tabstat message
 */
static void
pgstat_send_tabstat(PgStat_MsgTabstat *tsmsg)
{
    int         n;
    int         len;

    /* It's unlikely we'd get here with no socket, but maybe not impossible */
    if (pgStatSock == PGINVALID_SOCKET)
        return;

    /*
     * Report and reset accumulated xact commit/rollback and I/O timings
     * whenever we send a normal tabstat message
     */
    if (OidIsValid(tsmsg->m_databaseid))
    {
        tsmsg->m_xact_commit = pgStatXactCommit;
        tsmsg->m_xact_rollback = pgStatXactRollback;
        tsmsg->m_block_read_time = pgStatBlockReadTime;
        tsmsg->m_block_write_time = pgStatBlockWriteTime;
        pgStatXactCommit = 0;
        pgStatXactRollback = 0;
        pgStatBlockReadTime = 0;
        pgStatBlockWriteTime = 0;
    }
    else
    {
        tsmsg->m_xact_commit = 0;
        tsmsg->m_xact_rollback = 0;
        tsmsg->m_block_read_time = 0;
        tsmsg->m_block_write_time = 0;
    }

    n = tsmsg->m_nentries;
    len = offsetof(PgStat_MsgTabstat, m_entry[0]) +
        n * sizeof(PgStat_TableEntry);

    pgstat_setheader(&tsmsg->m_hdr, PGSTAT_MTYPE_TABSTAT);
    pgstat_send(tsmsg, len);
}

/*
 * Subroutine for pgstat_report_stat: populate and send a function stat message
 */
static void
pgstat_send_funcstats(void)
{
    /* we assume this inits to all zeroes: */
    static const PgStat_FunctionCounts all_zeroes;

    PgStat_MsgFuncstat msg;
    PgStat_BackendFunctionEntry *entry;
    HASH_SEQ_STATUS fstat;

    if (pgStatFunctions == NULL)
        return;

    pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_FUNCSTAT);
    msg.m_databaseid = MyDatabaseId;
    msg.m_nentries = 0;

    hash_seq_init(&fstat, pgStatFunctions);
    while ((entry = (PgStat_BackendFunctionEntry *) hash_seq_search(&fstat)) != NULL)
    {
        PgStat_FunctionEntry *m_ent;

        /* Skip it if no counts accumulated since last time */
        if (memcmp(&entry->f_counts, &all_zeroes,
                   sizeof(PgStat_FunctionCounts)) == 0)
            continue;

        /* need to convert format of time accumulators */
        m_ent = &msg.m_entry[msg.m_nentries];
        m_ent->f_id = entry->f_id;
        m_ent->f_numcalls = entry->f_counts.f_numcalls;
        m_ent->f_total_time = INSTR_TIME_GET_MICROSEC(entry->f_counts.f_total_time);
        m_ent->f_self_time = INSTR_TIME_GET_MICROSEC(entry->f_counts.f_self_time);

        if (++msg.m_nentries >= PGSTAT_NUM_FUNCENTRIES)
        {
            pgstat_send(&msg, offsetof(PgStat_MsgFuncstat, m_entry[0]) +
                        msg.m_nentries * sizeof(PgStat_FunctionEntry));
            msg.m_nentries = 0;
        }

        /* reset the entry's counts */
        MemSet(&entry->f_counts, 0, sizeof(PgStat_FunctionCounts));
    }

    if (msg.m_nentries > 0)
        pgstat_send(&msg, offsetof(PgStat_MsgFuncstat, m_entry[0]) +
                    msg.m_nentries * sizeof(PgStat_FunctionEntry));

    have_function_stats = false;
}


/* ----------
 * pgstat_vacuum_stat() -
 *
 *  Will tell the collector about objects he can get rid of.
 * ----------
 */
void
pgstat_vacuum_stat(void)
{
    HTAB       *htab;
    PgStat_MsgTabpurge msg;
    PgStat_MsgFuncpurge f_msg;
    HASH_SEQ_STATUS hstat;
    PgStat_StatDBEntry *dbentry;
    PgStat_StatTabEntry *tabentry;
    PgStat_StatFuncEntry *funcentry;
    int         len;

    if (pgStatSock == PGINVALID_SOCKET)
        return;

    /*
     * If not done for this transaction, read the statistics collector stats
     * file into some hash tables.
     */
    backend_read_statsfile();

    /*
     * Read pg_database and make a list of OIDs of all existing databases
     */
    htab = pgstat_collect_oids(DatabaseRelationId);

    /*
     * Search the database hash table for dead databases and tell the
     * collector to drop them.
     */
    hash_seq_init(&hstat, pgStatDBHash);
    while ((dbentry = (PgStat_StatDBEntry *) hash_seq_search(&hstat)) != NULL)
    {
        Oid         dbid = dbentry->databaseid;

        CHECK_FOR_INTERRUPTS();

        /* the DB entry for shared tables (with InvalidOid) is never dropped */
        if (OidIsValid(dbid) &&
            hash_search(htab, (void *) &dbid, HASH_FIND, NULL) == NULL)
            pgstat_drop_database(dbid);
    }

    /* Clean up */
    hash_destroy(htab);

    /*
     * Lookup our own database entry; if not found, nothing more to do.
     */
    dbentry = (PgStat_StatDBEntry *) hash_search(pgStatDBHash,
                                                 (void *) &MyDatabaseId,
                                                 HASH_FIND, NULL);
    if (dbentry == NULL || dbentry->tables == NULL)
        return;

    /*
     * Similarly to above, make a list of all known relations in this DB.
     */
    htab = pgstat_collect_oids(RelationRelationId);

    /*
     * Initialize our messages table counter to zero
     */
    msg.m_nentries = 0;

    /*
     * Check for all tables listed in stats hashtable if they still exist.
     */
    hash_seq_init(&hstat, dbentry->tables);
    while ((tabentry = (PgStat_StatTabEntry *) hash_seq_search(&hstat)) != NULL)
    {
        Oid         tabid = tabentry->tableid;

        CHECK_FOR_INTERRUPTS();

        if (hash_search(htab, (void *) &tabid, HASH_FIND, NULL) != NULL)
            continue;

        /*
         * Not there, so add this table's Oid to the message
         */
        msg.m_tableid[msg.m_nentries++] = tabid;

        /*
         * If the message is full, send it out and reinitialize to empty
         */
        if (msg.m_nentries >= PGSTAT_NUM_TABPURGE)
        {
            len = offsetof(PgStat_MsgTabpurge, m_tableid[0])
                + msg.m_nentries * sizeof(Oid);

            pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_TABPURGE);
            msg.m_databaseid = MyDatabaseId;
            pgstat_send(&msg, len);

            msg.m_nentries = 0;
        }
    }

    /*
     * Send the rest
     */
    if (msg.m_nentries > 0)
    {
        len = offsetof(PgStat_MsgTabpurge, m_tableid[0])
            + msg.m_nentries * sizeof(Oid);

        pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_TABPURGE);
        msg.m_databaseid = MyDatabaseId;
        pgstat_send(&msg, len);
    }

    /* Clean up */
    hash_destroy(htab);

    /*
     * Now repeat the above steps for functions.  However, we needn't bother
     * in the common case where no function stats are being collected.
     */
    if (dbentry->functions != NULL &&
        hash_get_num_entries(dbentry->functions) > 0)
    {
        htab = pgstat_collect_oids(ProcedureRelationId);

        pgstat_setheader(&f_msg.m_hdr, PGSTAT_MTYPE_FUNCPURGE);
        f_msg.m_databaseid = MyDatabaseId;
        f_msg.m_nentries = 0;

        hash_seq_init(&hstat, dbentry->functions);
        while ((funcentry = (PgStat_StatFuncEntry *) hash_seq_search(&hstat)) != NULL)
        {
            Oid         funcid = funcentry->functionid;

            CHECK_FOR_INTERRUPTS();

            if (hash_search(htab, (void *) &funcid, HASH_FIND, NULL) != NULL)
                continue;

            /*
             * Not there, so add this function's Oid to the message
             */
            f_msg.m_functionid[f_msg.m_nentries++] = funcid;

            /*
             * If the message is full, send it out and reinitialize to empty
             */
            if (f_msg.m_nentries >= PGSTAT_NUM_FUNCPURGE)
            {
                len = offsetof(PgStat_MsgFuncpurge, m_functionid[0])
                    + f_msg.m_nentries * sizeof(Oid);

                pgstat_send(&f_msg, len);

                f_msg.m_nentries = 0;
            }
        }

        /*
         * Send the rest
         */
        if (f_msg.m_nentries > 0)
        {
            len = offsetof(PgStat_MsgFuncpurge, m_functionid[0])
                + f_msg.m_nentries * sizeof(Oid);

            pgstat_send(&f_msg, len);
        }

        hash_destroy(htab);
    }
}


/* ----------
 * pgstat_collect_oids() -
 *
 *  Collect the OIDs of all objects listed in the specified system catalog
 *  into a temporary hash table.  Caller should hash_destroy the result
 *  when done with it.  (However, we make the table in CurrentMemoryContext
 *  so that it will be freed properly in event of an error.)
 * ----------
 */
static HTAB *
pgstat_collect_oids(Oid catalogid)
{
    HTAB       *htab;
    HASHCTL     hash_ctl;
    Relation    rel;
    HeapScanDesc scan;
    HeapTuple   tup;
    Snapshot    snapshot;

    memset(&hash_ctl, 0, sizeof(hash_ctl));
    hash_ctl.keysize = sizeof(Oid);
    hash_ctl.entrysize = sizeof(Oid);
    hash_ctl.hcxt = CurrentMemoryContext;
    htab = hash_create("Temporary table of OIDs",
                       PGSTAT_TAB_HASH_SIZE,
                       &hash_ctl,
                       HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);

    rel = heap_open(catalogid, AccessShareLock);
    snapshot = RegisterSnapshot(GetLatestSnapshot());
    scan = heap_beginscan(rel, snapshot, 0, NULL);
    while ((tup = heap_getnext(scan, ForwardScanDirection)) != NULL)
    {
        Oid         thisoid = HeapTupleGetOid(tup);

        CHECK_FOR_INTERRUPTS();

        (void) hash_search(htab, (void *) &thisoid, HASH_ENTER, NULL);
    }
    heap_endscan(scan);
    UnregisterSnapshot(snapshot);
    heap_close(rel, AccessShareLock);

    return htab;
}


/* ----------
 * pgstat_drop_database() -
 *
 *  Tell the collector that we just dropped a database.
 *  (If the message gets lost, we will still clean the dead DB eventually
 *  via future invocations of pgstat_vacuum_stat().)
 * ----------
 */
void
pgstat_drop_database(Oid databaseid)
{
    PgStat_MsgDropdb msg;

    if (pgStatSock == PGINVALID_SOCKET)
        return;

    pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_DROPDB);
    msg.m_databaseid = databaseid;
    pgstat_send(&msg, sizeof(msg));
}


/* ----------
 * pgstat_drop_relation() -
 *
 *  Tell the collector that we just dropped a relation.
 *  (If the message gets lost, we will still clean the dead entry eventually
 *  via future invocations of pgstat_vacuum_stat().)
 *
 *  Currently not used for lack of any good place to call it; we rely
 *  entirely on pgstat_vacuum_stat() to clean out stats for dead rels.
 * ----------
 */
#ifdef NOT_USED
void
pgstat_drop_relation(Oid relid)
{
    PgStat_MsgTabpurge msg;
    int         len;

    if (pgStatSock == PGINVALID_SOCKET)
        return;

    msg.m_tableid[0] = relid;
    msg.m_nentries = 1;

    len = offsetof(PgStat_MsgTabpurge, m_tableid[0]) + sizeof(Oid);

    pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_TABPURGE);
    msg.m_databaseid = MyDatabaseId;
    pgstat_send(&msg, len);
}
#endif                          /* NOT_USED */


/* ----------
 * pgstat_reset_counters() -
 *
 *  Tell the statistics collector to reset counters for our database.
 *
 *  Permission checking for this function is managed through the normal
 *  GRANT system.
 * ----------
 */
void
pgstat_reset_counters(void)
{
    PgStat_MsgResetcounter msg;

    if (pgStatSock == PGINVALID_SOCKET)
        return;

    pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_RESETCOUNTER);
    msg.m_databaseid = MyDatabaseId;
    pgstat_send(&msg, sizeof(msg));
}

/* ----------
 * pgstat_reset_shared_counters() -
 *
 *  Tell the statistics collector to reset cluster-wide shared counters.
 *
 *  Permission checking for this function is managed through the normal
 *  GRANT system.
 * ----------
 */
void
pgstat_reset_shared_counters(const char *target)
{
    PgStat_MsgResetsharedcounter msg;

    if (pgStatSock == PGINVALID_SOCKET)
        return;

    if (strcmp(target, "archiver") == 0)
        msg.m_resettarget = RESET_ARCHIVER;
    else if (strcmp(target, "bgwriter") == 0)
        msg.m_resettarget = RESET_BGWRITER;
    else
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("unrecognized reset target: \"%s\"", target),
                 errhint("Target must be \"archiver\" or \"bgwriter\".")));

    pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_RESETSHAREDCOUNTER);
    pgstat_send(&msg, sizeof(msg));
}

/* ----------
 * pgstat_reset_single_counter() -
 *
 *  Tell the statistics collector to reset a single counter.
 *
 *  Permission checking for this function is managed through the normal
 *  GRANT system.
 * ----------
 */
void
pgstat_reset_single_counter(Oid objoid, PgStat_Single_Reset_Type type)
{
    PgStat_MsgResetsinglecounter msg;

    if (pgStatSock == PGINVALID_SOCKET)
        return;

    pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_RESETSINGLECOUNTER);
    msg.m_databaseid = MyDatabaseId;
    msg.m_resettype = type;
    msg.m_objectid = objoid;

    pgstat_send(&msg, sizeof(msg));
}

/* ----------
 * pgstat_report_autovac() -
 *
 *  Called from autovacuum.c to report startup of an autovacuum process.
 *  We are called before InitPostgres is done, so can't rely on MyDatabaseId;
 *  the db OID must be passed in, instead.
 * ----------
 */
void
pgstat_report_autovac(Oid dboid)
{
    PgStat_MsgAutovacStart msg;

    if (pgStatSock == PGINVALID_SOCKET)
        return;

    pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_AUTOVAC_START);
    msg.m_databaseid = dboid;
    msg.m_start_time = GetCurrentTimestamp();

    pgstat_send(&msg, sizeof(msg));
}


/* ---------
 * pgstat_report_vacuum() -
 *
 *  Tell the collector about the table we just vacuumed.
 * ---------
 */
void
pgstat_report_vacuum(Oid tableoid, bool shared,
                     PgStat_Counter livetuples, PgStat_Counter deadtuples)
{
    PgStat_MsgVacuum msg;

    if (pgStatSock == PGINVALID_SOCKET || !pgstat_track_counts)
        return;

    pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_VACUUM);
    msg.m_databaseid = shared ? InvalidOid : MyDatabaseId;
    msg.m_tableoid = tableoid;
    msg.m_autovacuum = IsAutoVacuumWorkerProcess();
    msg.m_vacuumtime = GetCurrentTimestamp();
    msg.m_live_tuples = livetuples;
    msg.m_dead_tuples = deadtuples;
    pgstat_send(&msg, sizeof(msg));
}

/* --------
 * pgstat_report_analyze() -
 *
 *  Tell the collector about the table we just analyzed.
 *
 * Caller must provide new live- and dead-tuples estimates, as well as a
 * flag indicating whether to reset the changes_since_analyze counter.
 * --------
 */
void
pgstat_report_analyze(Relation rel,
                      PgStat_Counter livetuples, PgStat_Counter deadtuples,
                      bool resetcounter)
{
    PgStat_MsgAnalyze msg;

    if (pgStatSock == PGINVALID_SOCKET || !pgstat_track_counts)
        return;

    /*
     * Unlike VACUUM, ANALYZE might be running inside a transaction that has
     * already inserted and/or deleted rows in the target table. ANALYZE will
     * have counted such rows as live or dead respectively. Because we will
     * report our counts of such rows at transaction end, we should subtract
     * off these counts from what we send to the collector now, else they'll
     * be double-counted after commit.  (This approach also ensures that the
     * collector ends up with the right numbers if we abort instead of
     * committing.)
     */
    if (rel->pgstat_info != NULL)
    {
        PgStat_TableXactStatus *trans;

        for (trans = rel->pgstat_info->trans; trans; trans = trans->upper)
        {
            livetuples -= trans->tuples_inserted - trans->tuples_deleted;
            deadtuples -= trans->tuples_updated + trans->tuples_deleted;
        }
        /* count stuff inserted by already-aborted subxacts, too */
        deadtuples -= rel->pgstat_info->t_counts.t_delta_dead_tuples;
        /* Since ANALYZE's counts are estimates, we could have underflowed */
        livetuples = Max(livetuples, 0);
        deadtuples = Max(deadtuples, 0);
    }

    pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_ANALYZE);
    msg.m_databaseid = rel->rd_rel->relisshared ? InvalidOid : MyDatabaseId;
    msg.m_tableoid = RelationGetRelid(rel);
    msg.m_autovacuum = IsAutoVacuumWorkerProcess();
    msg.m_resetcounter = resetcounter;
    msg.m_analyzetime = GetCurrentTimestamp();
    msg.m_live_tuples = livetuples;
    msg.m_dead_tuples = deadtuples;
    pgstat_send(&msg, sizeof(msg));
}

/* --------
 * pgstat_report_recovery_conflict() -
 *
 *  Tell the collector about a Hot Standby recovery conflict.
 * --------
 */
void
pgstat_report_recovery_conflict(int reason)
{
    PgStat_MsgRecoveryConflict msg;

    if (pgStatSock == PGINVALID_SOCKET || !pgstat_track_counts)
        return;

    pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_RECOVERYCONFLICT);
    msg.m_databaseid = MyDatabaseId;
    msg.m_reason = reason;
    pgstat_send(&msg, sizeof(msg));
}

/* --------
 * pgstat_report_deadlock() -
 *
 *  Tell the collector about a deadlock detected.
 * --------
 */
void
pgstat_report_deadlock(void)
{
    PgStat_MsgDeadlock msg;

    if (pgStatSock == PGINVALID_SOCKET || !pgstat_track_counts)
        return;

    pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_DEADLOCK);
    msg.m_databaseid = MyDatabaseId;
    pgstat_send(&msg, sizeof(msg));
}

/* --------
 * pgstat_report_tempfile() -
 *
 *  Tell the collector about a temporary file.
 * --------
 */
void
pgstat_report_tempfile(size_t filesize)
{
    PgStat_MsgTempFile msg;

    if (pgStatSock == PGINVALID_SOCKET || !pgstat_track_counts)
        return;

    pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_TEMPFILE);
    msg.m_databaseid = MyDatabaseId;
    msg.m_filesize = filesize;
    pgstat_send(&msg, sizeof(msg));
}


/* ----------
 * pgstat_ping() -
 *
 *  Send some junk data to the collector to increase traffic.
 * ----------
 */
void
pgstat_ping(void)
{
    PgStat_MsgDummy msg;

    if (pgStatSock == PGINVALID_SOCKET)
        return;

    pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_DUMMY);
    pgstat_send(&msg, sizeof(msg));
}

/* ----------
 * pgstat_send_inquiry() -
 *
 *  Notify collector that we need fresh data.
 * ----------
 */
static void
pgstat_send_inquiry(TimestampTz clock_time, TimestampTz cutoff_time, Oid databaseid)
{
    PgStat_MsgInquiry msg;

    pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_INQUIRY);
    msg.clock_time = clock_time;
    msg.cutoff_time = cutoff_time;
    msg.databaseid = databaseid;
    pgstat_send(&msg, sizeof(msg));
}


/*
 * Initialize function call usage data.
 * Called by the executor before invoking a function.
 */
void
pgstat_init_function_usage(FunctionCallInfoData *fcinfo,
                           PgStat_FunctionCallUsage *fcu)
{
    PgStat_BackendFunctionEntry *htabent;
    bool        found;

    if (pgstat_track_functions <= fcinfo->flinfo->fn_stats)
    {
        /* stats not wanted */
        fcu->fs = NULL;
        return;
    }

    if (!pgStatFunctions)
    {
        /* First time through - initialize function stat table */
        HASHCTL     hash_ctl;

        memset(&hash_ctl, 0, sizeof(hash_ctl));
        hash_ctl.keysize = sizeof(Oid);
        hash_ctl.entrysize = sizeof(PgStat_BackendFunctionEntry);
        pgStatFunctions = hash_create("Function stat entries",
                                      PGSTAT_FUNCTION_HASH_SIZE,
                                      &hash_ctl,
                                      HASH_ELEM | HASH_BLOBS);
    }

    /* Get the stats entry for this function, create if necessary */
    htabent = hash_search(pgStatFunctions, &fcinfo->flinfo->fn_oid,
                          HASH_ENTER, &found);
    if (!found)
        MemSet(&htabent->f_counts, 0, sizeof(PgStat_FunctionCounts));

    fcu->fs = &htabent->f_counts;

    /* save stats for this function, later used to compensate for recursion */
    fcu->save_f_total_time = htabent->f_counts.f_total_time;

    /* save current backend-wide total time */
    fcu->save_total = total_func_time;

    /* get clock time as of function start */
    INSTR_TIME_SET_CURRENT(fcu->f_start);
}

/*
 * find_funcstat_entry - find any existing PgStat_BackendFunctionEntry entry
 *      for specified function
 *
 * If no entry, return NULL, don't create a new one
 */
PgStat_BackendFunctionEntry *
find_funcstat_entry(Oid func_id)
{
    if (pgStatFunctions == NULL)
        return NULL;

    return (PgStat_BackendFunctionEntry *) hash_search(pgStatFunctions,
                                                       (void *) &func_id,
                                                       HASH_FIND, NULL);
}

/*
 * Calculate function call usage and update stat counters.
 * Called by the executor after invoking a function.
 *
 * In the case of a set-returning function that runs in value-per-call mode,
 * we will see multiple pgstat_init_function_usage/pgstat_end_function_usage
 * calls for what the user considers a single call of the function.  The
 * finalize flag should be TRUE on the last call.
 */
void
pgstat_end_function_usage(PgStat_FunctionCallUsage *fcu, bool finalize)
{
    PgStat_FunctionCounts *fs = fcu->fs;
    instr_time  f_total;
    instr_time  f_others;
    instr_time  f_self;

    /* stats not wanted? */
    if (fs == NULL)
        return;

    /* total elapsed time in this function call */
    INSTR_TIME_SET_CURRENT(f_total);
    INSTR_TIME_SUBTRACT(f_total, fcu->f_start);

    /* self usage: elapsed minus anything already charged to other calls */
    f_others = total_func_time;
    INSTR_TIME_SUBTRACT(f_others, fcu->save_total);
    f_self = f_total;
    INSTR_TIME_SUBTRACT(f_self, f_others);

    /* update backend-wide total time */
    INSTR_TIME_ADD(total_func_time, f_self);

    /*
     * Compute the new f_total_time as the total elapsed time added to the
     * pre-call value of f_total_time.  This is necessary to avoid
     * double-counting any time taken by recursive calls of myself.  (We do
     * not need any similar kluge for self time, since that already excludes
     * any recursive calls.)
     */
    INSTR_TIME_ADD(f_total, fcu->save_f_total_time);

    /* update counters in function stats table */
    if (finalize)
        fs->f_numcalls++;
    fs->f_total_time = f_total;
    INSTR_TIME_ADD(fs->f_self_time, f_self);

    /* indicate that we have something to send */
    have_function_stats = true;
}


/* ----------
 * pgstat_initstats() -
 *
 *  Initialize a relcache entry to count access statistics.
 *  Called whenever a relation is opened.
 *
 *  We assume that a relcache entry's pgstat_info field is zeroed by
 *  relcache.c when the relcache entry is made; thereafter it is long-lived
 *  data.  We can avoid repeated searches of the TabStatus arrays when the
 *  same relation is touched repeatedly within a transaction.
 * ----------
 */
void
pgstat_initstats(Relation rel)
{
    Oid         rel_id = rel->rd_id;
    char        relkind = rel->rd_rel->relkind;

    /* We only count stats for things that have storage */
    if (!(relkind == RELKIND_RELATION ||
          relkind == RELKIND_MATVIEW ||
          relkind == RELKIND_INDEX ||
          relkind == RELKIND_TOASTVALUE ||
          relkind == RELKIND_SEQUENCE))
    {
        rel->pgstat_info = NULL;
        return;
    }

    if (pgStatSock == PGINVALID_SOCKET || !pgstat_track_counts)
    {
        /* We're not counting at all */
        rel->pgstat_info = NULL;
        return;
    }

    /*
     * If we already set up this relation in the current transaction, nothing
     * to do.
     */
    if (rel->pgstat_info != NULL &&
        rel->pgstat_info->t_id == rel_id)
        return;

    /* Else find or make the PgStat_TableStatus entry, and update link */
    rel->pgstat_info = get_tabstat_entry(rel_id, rel->rd_rel->relisshared);
}

/*
 * get_tabstat_entry - find or create a PgStat_TableStatus entry for rel
 */
static PgStat_TableStatus *
get_tabstat_entry(Oid rel_id, bool isshared)
{
    TabStatHashEntry *hash_entry;
    PgStat_TableStatus *entry;
    TabStatusArray *tsa;
    bool        found;

    /*
     * Create hash table if we don't have it already.
     */
    if (pgStatTabHash == NULL)
    {
        HASHCTL     ctl;

        memset(&ctl, 0, sizeof(ctl));
        ctl.keysize = sizeof(Oid);
        ctl.entrysize = sizeof(TabStatHashEntry);

        pgStatTabHash = hash_create("pgstat TabStatusArray lookup hash table",
                                    TABSTAT_QUANTUM,
                                    &ctl,
                                    HASH_ELEM | HASH_BLOBS);
    }

    /*
     * Find an entry or create a new one.
     */
    hash_entry = hash_search(pgStatTabHash, &rel_id, HASH_ENTER, &found);
    if (!found)
    {
        /* initialize new entry with null pointer */
        hash_entry->tsa_entry = NULL;
    }

    /*
     * If entry is already valid, we're done.
     */
    if (hash_entry->tsa_entry)
        return hash_entry->tsa_entry;

    /*
     * Locate the first pgStatTabList entry with free space, making a new list
     * entry if needed.  Note that we could get an OOM failure here, but if so
     * we have left the hashtable and the list in a consistent state.
     */
    if (pgStatTabList == NULL)
    {
        /* Set up first pgStatTabList entry */
        pgStatTabList = (TabStatusArray *)
            MemoryContextAllocZero(TopMemoryContext,
                                   sizeof(TabStatusArray));
    }

    tsa = pgStatTabList;
    while (tsa->tsa_used >= TABSTAT_QUANTUM)
    {
        if (tsa->tsa_next == NULL)
            tsa->tsa_next = (TabStatusArray *)
                MemoryContextAllocZero(TopMemoryContext,
                                       sizeof(TabStatusArray));
        tsa = tsa->tsa_next;
    }

    /*
     * Allocate a PgStat_TableStatus entry within this list entry.  We assume
     * the entry was already zeroed, either at creation or after last use.
     */
    entry = &tsa->tsa_entries[tsa->tsa_used++];
    entry->t_id = rel_id;
    entry->t_shared = isshared;

    /*
     * Now we can fill the entry in pgStatTabHash.
     */
    hash_entry->tsa_entry = entry;

    return entry;
}

/*
 * find_tabstat_entry - find any existing PgStat_TableStatus entry for rel
 *
 * If no entry, return NULL, don't create a new one
 *
 * Note: if we got an error in the most recent execution of pgstat_report_stat,
 * it's possible that an entry exists but there's no hashtable entry for it.
 * That's okay, we'll treat this case as "doesn't exist".
 */
PgStat_TableStatus *
find_tabstat_entry(Oid rel_id)
{
    TabStatHashEntry *hash_entry;

    /* If hashtable doesn't exist, there are no entries at all */
    if (!pgStatTabHash)
        return NULL;

    hash_entry = hash_search(pgStatTabHash, &rel_id, HASH_FIND, NULL);
    if (!hash_entry)
        return NULL;

    /* Note that this step could also return NULL, but that's correct */
    return hash_entry->tsa_entry;
}

/*
 * get_tabstat_stack_level - add a new (sub)transaction stack entry if needed
 */
static PgStat_SubXactStatus *
get_tabstat_stack_level(int nest_level)
{
    PgStat_SubXactStatus *xact_state;

    xact_state = pgStatXactStack;
    if (xact_state == NULL || xact_state->nest_level != nest_level)
    {
        xact_state = (PgStat_SubXactStatus *)
            MemoryContextAlloc(TopTransactionContext,
                               sizeof(PgStat_SubXactStatus));
        xact_state->nest_level = nest_level;
        xact_state->prev = pgStatXactStack;
        xact_state->first = NULL;
        pgStatXactStack = xact_state;
    }
    return xact_state;
}

/*
 * add_tabstat_xact_level - add a new (sub)transaction state record
 */
static void
add_tabstat_xact_level(PgStat_TableStatus *pgstat_info, int nest_level)
{
    PgStat_SubXactStatus *xact_state;
    PgStat_TableXactStatus *trans;

    /*
     * If this is the first rel to be modified at the current nest level, we
     * first have to push a transaction stack entry.
     */
    xact_state = get_tabstat_stack_level(nest_level);

    /* Now make a per-table stack entry */
    trans = (PgStat_TableXactStatus *)
        MemoryContextAllocZero(TopTransactionContext,
                               sizeof(PgStat_TableXactStatus));
    trans->nest_level = nest_level;
    trans->upper = pgstat_info->trans;
    trans->parent = pgstat_info;
    trans->next = xact_state->first;
    xact_state->first = trans;
    pgstat_info->trans = trans;
}

/*
 * pgstat_count_heap_insert - count a tuple insertion of n tuples
 */
void
pgstat_count_heap_insert(Relation rel, PgStat_Counter n)
{
    PgStat_TableStatus *pgstat_info = rel->pgstat_info;

    if (pgstat_info != NULL)
    {
        /* We have to log the effect at the proper transactional level */
        int         nest_level = GetCurrentTransactionNestLevel();

        if (pgstat_info->trans == NULL ||
            pgstat_info->trans->nest_level != nest_level)
            add_tabstat_xact_level(pgstat_info, nest_level);

        pgstat_info->trans->tuples_inserted += n;
    }
}

/*
 * pgstat_count_heap_update - count a tuple update
 */
void
pgstat_count_heap_update(Relation rel, bool hot)
{
    PgStat_TableStatus *pgstat_info = rel->pgstat_info;

    if (pgstat_info != NULL)
    {
        /* We have to log the effect at the proper transactional level */
        int         nest_level = GetCurrentTransactionNestLevel();

        if (pgstat_info->trans == NULL ||
            pgstat_info->trans->nest_level != nest_level)
            add_tabstat_xact_level(pgstat_info, nest_level);

        pgstat_info->trans->tuples_updated++;

        /* t_tuples_hot_updated is nontransactional, so just advance it */
        if (hot)
            pgstat_info->t_counts.t_tuples_hot_updated++;
    }
}

/*
 * pgstat_count_heap_delete - count a tuple deletion
 */
void
pgstat_count_heap_delete(Relation rel)
{
    PgStat_TableStatus *pgstat_info = rel->pgstat_info;

    if (pgstat_info != NULL)
    {
        /* We have to log the effect at the proper transactional level */
        int         nest_level = GetCurrentTransactionNestLevel();

        if (pgstat_info->trans == NULL ||
            pgstat_info->trans->nest_level != nest_level)
            add_tabstat_xact_level(pgstat_info, nest_level);

        pgstat_info->trans->tuples_deleted++;
    }
}

/*
 * pgstat_truncate_save_counters
 *
 * Whenever a table is truncated, we save its i/u/d counters so that they can
 * be cleared, and if the (sub)xact that executed the truncate later aborts,
 * the counters can be restored to the saved (pre-truncate) values.  Note we do
 * this on the first truncate in any particular subxact level only.
 */
static void
pgstat_truncate_save_counters(PgStat_TableXactStatus *trans)
{
    if (!trans->truncated)
    {
        trans->inserted_pre_trunc = trans->tuples_inserted;
        trans->updated_pre_trunc = trans->tuples_updated;
        trans->deleted_pre_trunc = trans->tuples_deleted;
        trans->truncated = true;
    }
}

/*
 * pgstat_truncate_restore_counters - restore counters when a truncate aborts
 */
static void
pgstat_truncate_restore_counters(PgStat_TableXactStatus *trans)
{
    if (trans->truncated)
    {
        trans->tuples_inserted = trans->inserted_pre_trunc;
        trans->tuples_updated = trans->updated_pre_trunc;
        trans->tuples_deleted = trans->deleted_pre_trunc;
    }
}

/*
 * pgstat_count_truncate - update tuple counters due to truncate
 */
void
pgstat_count_truncate(Relation rel)
{
    PgStat_TableStatus *pgstat_info = rel->pgstat_info;

    if (pgstat_info != NULL)
    {
        /* We have to log the effect at the proper transactional level */
        int         nest_level = GetCurrentTransactionNestLevel();

        if (pgstat_info->trans == NULL ||
            pgstat_info->trans->nest_level != nest_level)
            add_tabstat_xact_level(pgstat_info, nest_level);

        pgstat_truncate_save_counters(pgstat_info->trans);
        pgstat_info->trans->tuples_inserted = 0;
        pgstat_info->trans->tuples_updated = 0;
        pgstat_info->trans->tuples_deleted = 0;
    }
}

/*
 * pgstat_update_heap_dead_tuples - update dead-tuples count
 *
 * The semantics of this are that we are reporting the nontransactional
 * recovery of "delta" dead tuples; so t_delta_dead_tuples decreases
 * rather than increasing, and the change goes straight into the per-table
 * counter, not into transactional state.
 */
void
pgstat_update_heap_dead_tuples(Relation rel, int delta)
{
    PgStat_TableStatus *pgstat_info = rel->pgstat_info;

    if (pgstat_info != NULL)
        pgstat_info->t_counts.t_delta_dead_tuples -= delta;
}


/* ----------
 * AtEOXact_PgStat
 *
 *  Called from access/transam/xact.c at top-level transaction commit/abort.
 * ----------
 */
void
AtEOXact_PgStat(bool isCommit)
{
    PgStat_SubXactStatus *xact_state;

    /*
     * Count transaction commit or abort.  (We use counters, not just bools,
     * in case the reporting message isn't sent right away.)
     */
    if (isCommit)
        pgStatXactCommit++;
    else
        pgStatXactRollback++;

    /*
     * Transfer transactional insert/update counts into the base tabstat
     * entries.  We don't bother to free any of the transactional state, since
     * it's all in TopTransactionContext and will go away anyway.
     */
    xact_state = pgStatXactStack;
    if (xact_state != NULL)
    {
        PgStat_TableXactStatus *trans;

        Assert(xact_state->nest_level == 1);
        Assert(xact_state->prev == NULL);
        for (trans = xact_state->first; trans != NULL; trans = trans->next)
        {
            PgStat_TableStatus *tabstat;

            Assert(trans->nest_level == 1);
            Assert(trans->upper == NULL);
            tabstat = trans->parent;
            Assert(tabstat->trans == trans);
            /* restore pre-truncate stats (if any) in case of aborted xact */
            if (!isCommit)
                pgstat_truncate_restore_counters(trans);
            /* count attempted actions regardless of commit/abort */
            tabstat->t_counts.t_tuples_inserted += trans->tuples_inserted;
            tabstat->t_counts.t_tuples_updated += trans->tuples_updated;
            tabstat->t_counts.t_tuples_deleted += trans->tuples_deleted;
            if (isCommit)
            {
                tabstat->t_counts.t_truncated = trans->truncated;
                if (trans->truncated)
                {
                    /* forget live/dead stats seen by backend thus far */
                    tabstat->t_counts.t_delta_live_tuples = 0;
                    tabstat->t_counts.t_delta_dead_tuples = 0;
                }
                /* insert adds a live tuple, delete removes one */
                tabstat->t_counts.t_delta_live_tuples +=
                    trans->tuples_inserted - trans->tuples_deleted;
                /* update and delete each create a dead tuple */
                tabstat->t_counts.t_delta_dead_tuples +=
                    trans->tuples_updated + trans->tuples_deleted;
                /* insert, update, delete each count as one change event */
                tabstat->t_counts.t_changed_tuples +=
                    trans->tuples_inserted + trans->tuples_updated +
                    trans->tuples_deleted;
            }
            else
            {
                /* inserted tuples are dead, deleted tuples are unaffected */
                tabstat->t_counts.t_delta_dead_tuples +=
                    trans->tuples_inserted + trans->tuples_updated;
                /* an aborted xact generates no changed_tuple events */
            }
            tabstat->trans = NULL;
        }
    }
    pgStatXactStack = NULL;

    /* Make sure any stats snapshot is thrown away */
    pgstat_clear_snapshot();
}

/* ----------
 * AtEOSubXact_PgStat
 *
 *  Called from access/transam/xact.c at subtransaction commit/abort.
 * ----------
 */
void
AtEOSubXact_PgStat(bool isCommit, int nestDepth)
{
    PgStat_SubXactStatus *xact_state;

    /*
     * Transfer transactional insert/update counts into the next higher
     * subtransaction state.
     */
    xact_state = pgStatXactStack;
    if (xact_state != NULL &&
        xact_state->nest_level >= nestDepth)
    {
        PgStat_TableXactStatus *trans;
        PgStat_TableXactStatus *next_trans;

        /* delink xact_state from stack immediately to simplify reuse case */
        pgStatXactStack = xact_state->prev;

        for (trans = xact_state->first; trans != NULL; trans = next_trans)
        {
            PgStat_TableStatus *tabstat;

            next_trans = trans->next;
            Assert(trans->nest_level == nestDepth);
            tabstat = trans->parent;
            Assert(tabstat->trans == trans);
            if (isCommit)
            {
                if (trans->upper && trans->upper->nest_level == nestDepth - 1)
                {
                    if (trans->truncated)
                    {
                        /* propagate the truncate status one level up */
                        pgstat_truncate_save_counters(trans->upper);
                        /* replace upper xact stats with ours */
                        trans->upper->tuples_inserted = trans->tuples_inserted;
                        trans->upper->tuples_updated = trans->tuples_updated;
                        trans->upper->tuples_deleted = trans->tuples_deleted;
                    }
                    else
                    {
                        trans->upper->tuples_inserted += trans->tuples_inserted;
                        trans->upper->tuples_updated += trans->tuples_updated;
                        trans->upper->tuples_deleted += trans->tuples_deleted;
                    }
                    tabstat->trans = trans->upper;
                    pfree(trans);
                }
                else
                {
                    /*
                     * When there isn't an immediate parent state, we can just
                     * reuse the record instead of going through a
                     * palloc/pfree pushup (this works since it's all in
                     * TopTransactionContext anyway).  We have to re-link it
                     * into the parent level, though, and that might mean
                     * pushing a new entry into the pgStatXactStack.
                     */
                    PgStat_SubXactStatus *upper_xact_state;

                    upper_xact_state = get_tabstat_stack_level(nestDepth - 1);
                    trans->next = upper_xact_state->first;
                    upper_xact_state->first = trans;
                    trans->nest_level = nestDepth - 1;
                }
            }
            else
            {
                /*
                 * On abort, update top-level tabstat counts, then forget the
                 * subtransaction
                 */

                /* first restore values obliterated by truncate */
                pgstat_truncate_restore_counters(trans);
                /* count attempted actions regardless of commit/abort */
                tabstat->t_counts.t_tuples_inserted += trans->tuples_inserted;
                tabstat->t_counts.t_tuples_updated += trans->tuples_updated;
                tabstat->t_counts.t_tuples_deleted += trans->tuples_deleted;
                /* inserted tuples are dead, deleted tuples are unaffected */
                tabstat->t_counts.t_delta_dead_tuples +=
                    trans->tuples_inserted + trans->tuples_updated;
                tabstat->trans = trans->upper;
                pfree(trans);
            }
        }
        pfree(xact_state);
    }
}


/*
 * AtPrepare_PgStat
 *      Save the transactional stats state at 2PC transaction prepare.
 *
 * In this phase we just generate 2PC records for all the pending
 * transaction-dependent stats work.
 */
void
AtPrepare_PgStat(void)
{
    PgStat_SubXactStatus *xact_state;

    xact_state = pgStatXactStack;
    if (xact_state != NULL)
    {
        PgStat_TableXactStatus *trans;

        Assert(xact_state->nest_level == 1);
        Assert(xact_state->prev == NULL);
        for (trans = xact_state->first; trans != NULL; trans = trans->next)
        {
            PgStat_TableStatus *tabstat;
            TwoPhasePgStatRecord record;

            Assert(trans->nest_level == 1);
            Assert(trans->upper == NULL);
            tabstat = trans->parent;
            Assert(tabstat->trans == trans);

            record.tuples_inserted = trans->tuples_inserted;
            record.tuples_updated = trans->tuples_updated;
            record.tuples_deleted = trans->tuples_deleted;
            record.inserted_pre_trunc = trans->inserted_pre_trunc;
            record.updated_pre_trunc = trans->updated_pre_trunc;
            record.deleted_pre_trunc = trans->deleted_pre_trunc;
            record.t_id = tabstat->t_id;
            record.t_shared = tabstat->t_shared;
            record.t_truncated = trans->truncated;

            RegisterTwoPhaseRecord(TWOPHASE_RM_PGSTAT_ID, 0,
                                   &record, sizeof(TwoPhasePgStatRecord));
        }
    }
}

/*
 * PostPrepare_PgStat
 *      Clean up after successful PREPARE.
 *
 * All we need do here is unlink the transaction stats state from the
 * nontransactional state.  The nontransactional action counts will be
 * reported to the stats collector immediately, while the effects on live
 * and dead tuple counts are preserved in the 2PC state file.
 *
 * Note: AtEOXact_PgStat is not called during PREPARE.
 */
void
PostPrepare_PgStat(void)
{
    PgStat_SubXactStatus *xact_state;

    /*
     * We don't bother to free any of the transactional state, since it's all
     * in TopTransactionContext and will go away anyway.
     */
    xact_state = pgStatXactStack;
    if (xact_state != NULL)
    {
        PgStat_TableXactStatus *trans;

        for (trans = xact_state->first; trans != NULL; trans = trans->next)
        {
            PgStat_TableStatus *tabstat;

            tabstat = trans->parent;
            tabstat->trans = NULL;
        }
    }
    pgStatXactStack = NULL;

    /* Make sure any stats snapshot is thrown away */
    pgstat_clear_snapshot();
}

/*
 * 2PC processing routine for COMMIT PREPARED case.
 *
 * Load the saved counts into our local pgstats state.
 */
void
pgstat_twophase_postcommit(TransactionId xid, uint16 info,
                           void *recdata, uint32 len)
{
    TwoPhasePgStatRecord *rec = (TwoPhasePgStatRecord *) recdata;
    PgStat_TableStatus *pgstat_info;

    /* Find or create a tabstat entry for the rel */
    pgstat_info = get_tabstat_entry(rec->t_id, rec->t_shared);

    /* Same math as in AtEOXact_PgStat, commit case */
    pgstat_info->t_counts.t_tuples_inserted += rec->tuples_inserted;
    pgstat_info->t_counts.t_tuples_updated += rec->tuples_updated;
    pgstat_info->t_counts.t_tuples_deleted += rec->tuples_deleted;
    pgstat_info->t_counts.t_truncated = rec->t_truncated;
    if (rec->t_truncated)
    {
        /* forget live/dead stats seen by backend thus far */
        pgstat_info->t_counts.t_delta_live_tuples = 0;
        pgstat_info->t_counts.t_delta_dead_tuples = 0;
    }
    pgstat_info->t_counts.t_delta_live_tuples +=
        rec->tuples_inserted - rec->tuples_deleted;
    pgstat_info->t_counts.t_delta_dead_tuples +=
        rec->tuples_updated + rec->tuples_deleted;
    pgstat_info->t_counts.t_changed_tuples +=
        rec->tuples_inserted + rec->tuples_updated +
        rec->tuples_deleted;
}

/*
 * 2PC processing routine for ROLLBACK PREPARED case.
 *
 * Load the saved counts into our local pgstats state, but treat them
 * as aborted.
 */
void
pgstat_twophase_postabort(TransactionId xid, uint16 info,
                          void *recdata, uint32 len)
{
    TwoPhasePgStatRecord *rec = (TwoPhasePgStatRecord *) recdata;
    PgStat_TableStatus *pgstat_info;

    /* Find or create a tabstat entry for the rel */
    pgstat_info = get_tabstat_entry(rec->t_id, rec->t_shared);

    /* Same math as in AtEOXact_PgStat, abort case */
    if (rec->t_truncated)
    {
        rec->tuples_inserted = rec->inserted_pre_trunc;
        rec->tuples_updated = rec->updated_pre_trunc;
        rec->tuples_deleted = rec->deleted_pre_trunc;
    }
    pgstat_info->t_counts.t_tuples_inserted += rec->tuples_inserted;
    pgstat_info->t_counts.t_tuples_updated += rec->tuples_updated;
    pgstat_info->t_counts.t_tuples_deleted += rec->tuples_deleted;
    pgstat_info->t_counts.t_delta_dead_tuples +=
        rec->tuples_inserted + rec->tuples_updated;
}


/* ----------
 * pgstat_fetch_stat_dbentry() -
 *
 *  Support function for the SQL-callable pgstat* functions. Returns
 *  the collected statistics for one database or NULL. NULL doesn't mean
 *  that the database doesn't exist, it is just not yet known by the
 *  collector, so the caller is better off to report ZERO instead.
 * ----------
 */
PgStat_StatDBEntry *
pgstat_fetch_stat_dbentry(Oid dbid)
{
    /*
     * If not done for this transaction, read the statistics collector stats
     * file into some hash tables.
     */
    backend_read_statsfile();

    /*
     * Lookup the requested database; return NULL if not found
     */
    return (PgStat_StatDBEntry *) hash_search(pgStatDBHash,
                                              (void *) &dbid,
                                              HASH_FIND, NULL);
}


/* ----------
 * pgstat_fetch_stat_tabentry() -
 *
 *  Support function for the SQL-callable pgstat* functions. Returns
 *  the collected statistics for one table or NULL. NULL doesn't mean
 *  that the table doesn't exist, it is just not yet known by the
 *  collector, so the caller is better off to report ZERO instead.
 * ----------
 */
PgStat_StatTabEntry *
pgstat_fetch_stat_tabentry(Oid relid)
{
    Oid         dbid;
    PgStat_StatDBEntry *dbentry;
    PgStat_StatTabEntry *tabentry;

    /*
     * If not done for this transaction, read the statistics collector stats
     * file into some hash tables.
     */
    backend_read_statsfile();

    /*
     * Lookup our database, then look in its table hash table.
     */
    dbid = MyDatabaseId;
    dbentry = (PgStat_StatDBEntry *) hash_search(pgStatDBHash,
                                                 (void *) &dbid,
                                                 HASH_FIND, NULL);
    if (dbentry != NULL && dbentry->tables != NULL)
    {
        tabentry = (PgStat_StatTabEntry *) hash_search(dbentry->tables,
                                                       (void *) &relid,
                                                       HASH_FIND, NULL);
        if (tabentry)
            return tabentry;
    }

    /*
     * If we didn't find it, maybe it's a shared table.
     */
    dbid = InvalidOid;
    dbentry = (PgStat_StatDBEntry *) hash_search(pgStatDBHash,
                                                 (void *) &dbid,
                                                 HASH_FIND, NULL);
    if (dbentry != NULL && dbentry->tables != NULL)
    {
        tabentry = (PgStat_StatTabEntry *) hash_search(dbentry->tables,
                                                       (void *) &relid,
                                                       HASH_FIND, NULL);
        if (tabentry)
            return tabentry;
    }

    return NULL;
}


/* ----------
 * pgstat_fetch_stat_funcentry() -
 *
 *  Support function for the SQL-callable pgstat* functions. Returns
 *  the collected statistics for one function or NULL.
 * ----------
 */
PgStat_StatFuncEntry *
pgstat_fetch_stat_funcentry(Oid func_id)
{
    PgStat_StatDBEntry *dbentry;
    PgStat_StatFuncEntry *funcentry = NULL;

    /* load the stats file if needed */
    backend_read_statsfile();

    /* Lookup our database, then find the requested function.  */
    dbentry = pgstat_fetch_stat_dbentry(MyDatabaseId);
    if (dbentry != NULL && dbentry->functions != NULL)
    {
        funcentry = (PgStat_StatFuncEntry *) hash_search(dbentry->functions,
                                                         (void *) &func_id,
                                                         HASH_FIND, NULL);
    }

    return funcentry;
}


/* ----------
 * pgstat_fetch_stat_beentry() -
 *
 *  Support function for the SQL-callable pgstat* functions. Returns
 *  our local copy of the current-activity entry for one backend.
 *
 *  NB: caller is responsible for a check if the user is permitted to see
 *  this info (especially the querystring).
 * ----------
 */
PgBackendStatus *
pgstat_fetch_stat_beentry(int beid)
{
    pgstat_read_current_status();

    if (beid < 1 || beid > localNumBackends)
        return NULL;

    return &localBackendStatusTable[beid - 1].backendStatus;
}


/* ----------
 * pgstat_fetch_stat_local_beentry() -
 *
 *  Like pgstat_fetch_stat_beentry() but with locally computed additions (like
 *  xid and xmin values of the backend)
 *
 *  NB: caller is responsible for a check if the user is permitted to see
 *  this info (especially the querystring).
 * ----------
 */
LocalPgBackendStatus *
pgstat_fetch_stat_local_beentry(int beid)
{
    pgstat_read_current_status();

    if (beid < 1 || beid > localNumBackends)
        return NULL;

    return &localBackendStatusTable[beid - 1];
}


/* ----------
 * pgstat_fetch_stat_numbackends() -
 *
 *  Support function for the SQL-callable pgstat* functions. Returns
 *  the maximum current backend id.
 * ----------
 */
int
pgstat_fetch_stat_numbackends(void)
{
    pgstat_read_current_status();

    return localNumBackends;
}

/*
 * ---------
 * pgstat_fetch_stat_archiver() -
 *
 *  Support function for the SQL-callable pgstat* functions. Returns
 *  a pointer to the archiver statistics struct.
 * ---------
 */
PgStat_ArchiverStats *
pgstat_fetch_stat_archiver(void)
{
    backend_read_statsfile();

    return &archiverStats;
}


/*
 * ---------
 * pgstat_fetch_global() -
 *
 *  Support function for the SQL-callable pgstat* functions. Returns
 *  a pointer to the global statistics struct.
 * ---------
 */
PgStat_GlobalStats *
pgstat_fetch_global(void)
{
    backend_read_statsfile();

    return &globalStats;
}


/* ------------------------------------------------------------
 * Functions for management of the shared-memory PgBackendStatus array
 * ------------------------------------------------------------
 */

static PgBackendStatus *BackendStatusArray = NULL;
static PgBackendStatus *MyBEEntry = NULL;
static char *BackendAppnameBuffer = NULL;
static char *BackendClientHostnameBuffer = NULL;
static char *BackendActivityBuffer = NULL;
static Size BackendActivityBufferSize = 0;
#ifdef USE_SSL
static PgBackendSSLStatus *BackendSslStatusBuffer = NULL;
#endif


/*
 * Report shared-memory space needed by CreateSharedBackendStatus.
 */
Size
BackendStatusShmemSize(void)
{
    Size        size;

    /* BackendStatusArray: */
    size = mul_size(sizeof(PgBackendStatus), NumBackendStatSlots);
    /* BackendAppnameBuffer: */
    size = add_size(size,
                    mul_size(NAMEDATALEN, NumBackendStatSlots));
    /* BackendClientHostnameBuffer: */
    size = add_size(size,
                    mul_size(NAMEDATALEN, NumBackendStatSlots));
    /* BackendActivityBuffer: */
    size = add_size(size,
                    mul_size(pgstat_track_activity_query_size, NumBackendStatSlots));
#ifdef USE_SSL
    /* BackendSslStatusBuffer: */
    size = add_size(size,
                    mul_size(sizeof(PgBackendSSLStatus), NumBackendStatSlots));
#endif
    return size;
}

/*
 * Initialize the shared status array and several string buffers
 * during postmaster startup.
 */
void
CreateSharedBackendStatus(void)
{
    Size        size;
    bool        found;
    int         i;
    char       *buffer;

    /* Create or attach to the shared array */
    size = mul_size(sizeof(PgBackendStatus), NumBackendStatSlots);
    BackendStatusArray = (PgBackendStatus *)
        ShmemInitStruct("Backend Status Array", size, &found);

    if (!found)
    {
        /*
         * We're the first - initialize.
         */
        MemSet(BackendStatusArray, 0, size);
    }

    /* Create or attach to the shared appname buffer */
    size = mul_size(NAMEDATALEN, MaxBackends);
    BackendAppnameBuffer = (char *)
        ShmemInitStruct("Backend Application Name Buffer", size, &found);

    if (!found)
    {
        MemSet(BackendAppnameBuffer, 0, size);

        /* Initialize st_appname pointers. */
        buffer = BackendAppnameBuffer;
        for (i = 0; i < NumBackendStatSlots; i++)
        {
            BackendStatusArray[i].st_appname = buffer;
            buffer += NAMEDATALEN;
        }
    }

    /* Create or attach to the shared client hostname buffer */
    size = mul_size(NAMEDATALEN, MaxBackends);
    BackendClientHostnameBuffer = (char *)
        ShmemInitStruct("Backend Client Host Name Buffer", size, &found);

    if (!found)
    {
        MemSet(BackendClientHostnameBuffer, 0, size);

        /* Initialize st_clienthostname pointers. */
        buffer = BackendClientHostnameBuffer;
        for (i = 0; i < NumBackendStatSlots; i++)
        {
            BackendStatusArray[i].st_clienthostname = buffer;
            buffer += NAMEDATALEN;
        }
    }

    /* Create or attach to the shared activity buffer */
    BackendActivityBufferSize = mul_size(pgstat_track_activity_query_size,
                                         NumBackendStatSlots);
    BackendActivityBuffer = (char *)
        ShmemInitStruct("Backend Activity Buffer",
                        BackendActivityBufferSize,
                        &found);

    if (!found)
    {
        MemSet(BackendActivityBuffer, 0, size);

        /* Initialize st_activity pointers. */
        buffer = BackendActivityBuffer;
        for (i = 0; i < NumBackendStatSlots; i++)
        {
            BackendStatusArray[i].st_activity_raw = buffer;
            buffer += pgstat_track_activity_query_size;
        }
    }

#ifdef USE_SSL
    /* Create or attach to the shared SSL status buffer */
    size = mul_size(sizeof(PgBackendSSLStatus), NumBackendStatSlots);
    BackendSslStatusBuffer = (PgBackendSSLStatus *)
        ShmemInitStruct("Backend SSL Status Buffer", size, &found);

    if (!found)
    {
        PgBackendSSLStatus *ptr;

        MemSet(BackendSslStatusBuffer, 0, size);

        /* Initialize st_sslstatus pointers. */
        ptr = BackendSslStatusBuffer;
        for (i = 0; i < NumBackendStatSlots; i++)
        {
            BackendStatusArray[i].st_sslstatus = ptr;
            ptr++;
        }
    }
#endif
}


/* ----------
 * pgstat_initialize() -
 *
 *  Initialize pgstats state, and set up our on-proc-exit hook.
 *  Called from InitPostgres and AuxiliaryProcessMain. For auxiliary process,
 *  MyBackendId is invalid. Otherwise, MyBackendId must be set,
 *  but we must not have started any transaction yet (since the
 *  exit hook must run after the last transaction exit).
 *  NOTE: MyDatabaseId isn't set yet; so the shutdown hook has to be careful.
 * ----------
 */
void
pgstat_initialize(void)
{
    /* Initialize MyBEEntry */
    if (MyBackendId != InvalidBackendId)
    {
        Assert(MyBackendId >= 1 && MyBackendId <= MaxBackends);
        MyBEEntry = &BackendStatusArray[MyBackendId - 1];
    }
    else
    {
        /* Must be an auxiliary process */
        Assert(MyAuxProcType != NotAnAuxProcess);

        /*
         * Assign the MyBEEntry for an auxiliary process.  Since it doesn't
         * have a BackendId, the slot is statically allocated based on the
         * auxiliary process type (MyAuxProcType).  Backends use slots indexed
         * in the range from 1 to MaxBackends (inclusive), so we use
         * MaxBackends + AuxBackendType + 1 as the index of the slot for an
         * auxiliary process.
         */
        MyBEEntry = &BackendStatusArray[MaxBackends + MyAuxProcType];
    }

    /* Set up a process-exit hook to clean up */
    on_shmem_exit(pgstat_beshutdown_hook, 0);
}

/* ----------
 * pgstat_bestart() -
 *
 *  Initialize this backend's entry in the PgBackendStatus array.
 *  Called from InitPostgres.
 *
 *  Apart from auxiliary processes, MyBackendId, MyDatabaseId,
 *  session userid, and application_name must be set for a
 *  backend (hence, this cannot be combined with pgstat_initialize).
 * ----------
 */
void
pgstat_bestart(void)
{
    TimestampTz proc_start_timestamp;
    SockAddr    clientaddr;
    volatile PgBackendStatus *beentry;

    /*
     * To minimize the time spent modifying the PgBackendStatus entry, fetch
     * all the needed data first.
     *
     * If we have a MyProcPort, use its session start time (for consistency,
     * and to save a kernel call).
     */
    if (MyProcPort)
        proc_start_timestamp = MyProcPort->SessionStartTime;
    else
        proc_start_timestamp = GetCurrentTimestamp();

    /*
     * We may not have a MyProcPort (eg, if this is the autovacuum process).
     * If so, use all-zeroes client address, which is dealt with specially in
     * pg_stat_get_backend_client_addr and pg_stat_get_backend_client_port.
     */
    if (MyProcPort)
        memcpy(&clientaddr, &MyProcPort->raddr, sizeof(clientaddr));
    else
        MemSet(&clientaddr, 0, sizeof(clientaddr));

    /*
     * Initialize my status entry, following the protocol of bumping
     * st_changecount before and after; and make sure it's even afterwards. We
     * use a volatile pointer here to ensure the compiler doesn't try to get
     * cute.
     */
    beentry = MyBEEntry;

    /* pgstats state must be initialized from pgstat_initialize() */
    Assert(beentry != NULL);

    if (MyBackendId != InvalidBackendId)
    {
        if (IsAutoVacuumLauncherProcess())
        {
            /* Autovacuum Launcher */
            beentry->st_backendType = B_AUTOVAC_LAUNCHER;
        }
        else if (IsAutoVacuumWorkerProcess())
        {
            /* Autovacuum Worker */
            beentry->st_backendType = B_AUTOVAC_WORKER;
        }
        else if (am_walsender)
        {
            /* Wal sender */
            beentry->st_backendType = B_WAL_SENDER;
        }
        else if (IsBackgroundWorker)
        {
            /* bgworker */
            beentry->st_backendType = B_BG_WORKER;
        }
        else
        {
            /* client-backend */
            beentry->st_backendType = B_BACKEND;
        }
    }
    else
    {
        /* Must be an auxiliary process */
        Assert(MyAuxProcType != NotAnAuxProcess);
        switch (MyAuxProcType)
        {
            case StartupProcess:
                beentry->st_backendType = B_STARTUP;
                break;
            case BgWriterProcess:
                beentry->st_backendType = B_BG_WRITER;
                break;
            case CheckpointerProcess:
                beentry->st_backendType = B_CHECKPOINTER;
                break;
            case WalWriterProcess:
                beentry->st_backendType = B_WAL_WRITER;
                break;
            case WalReceiverProcess:
                beentry->st_backendType = B_WAL_RECEIVER;
                break;
            default:
                elog(FATAL, "unrecognized process type: %d",
                     (int) MyAuxProcType);
                proc_exit(1);
        }
    }

    do
    {
        pgstat_increment_changecount_before(beentry);
    } while ((beentry->st_changecount & 1) == 0);

    beentry->st_procpid = MyProcPid;
    beentry->st_proc_start_timestamp = proc_start_timestamp;
    beentry->st_activity_start_timestamp = 0;
    beentry->st_state_start_timestamp = 0;
    beentry->st_xact_start_timestamp = 0;
    beentry->st_databaseid = MyDatabaseId;

    /* We have userid for client-backends, wal-sender and bgworker processes */
    if (beentry->st_backendType == B_BACKEND
        || beentry->st_backendType == B_WAL_SENDER
        || beentry->st_backendType == B_BG_WORKER)
        beentry->st_userid = GetSessionUserId();
    else
        beentry->st_userid = InvalidOid;

    beentry->st_clientaddr = clientaddr;
    if (MyProcPort && MyProcPort->remote_hostname)
        strlcpy(beentry->st_clienthostname, MyProcPort->remote_hostname,
                NAMEDATALEN);
    else
        beentry->st_clienthostname[0] = '\0';
#ifdef USE_SSL
    if (MyProcPort && MyProcPort->ssl != NULL)
    {
        beentry->st_ssl = true;
        beentry->st_sslstatus->ssl_bits = be_tls_get_cipher_bits(MyProcPort);
        beentry->st_sslstatus->ssl_compression = be_tls_get_compression(MyProcPort);
        be_tls_get_version(MyProcPort, beentry->st_sslstatus->ssl_version, NAMEDATALEN);
        be_tls_get_cipher(MyProcPort, beentry->st_sslstatus->ssl_cipher, NAMEDATALEN);
        be_tls_get_peerdn_name(MyProcPort, beentry->st_sslstatus->ssl_clientdn, NAMEDATALEN);
    }
    else
    {
        beentry->st_ssl = false;
    }
#else
    beentry->st_ssl = false;
#endif
    beentry->st_state = STATE_UNDEFINED;
    beentry->st_appname[0] = '\0';
    beentry->st_activity_raw[0] = '\0';
    /* Also make sure the last byte in each string area is always 0 */
    beentry->st_clienthostname[NAMEDATALEN - 1] = '\0';
    beentry->st_appname[NAMEDATALEN - 1] = '\0';
    beentry->st_activity_raw[pgstat_track_activity_query_size - 1] = '\0';
    beentry->st_progress_command = PROGRESS_COMMAND_INVALID;
    beentry->st_progress_command_target = InvalidOid;

    /*
     * we don't zero st_progress_param here to save cycles; nobody should
     * examine it until st_progress_command has been set to something other
     * than PROGRESS_COMMAND_INVALID
     */

    pgstat_increment_changecount_after(beentry);

    /* Update app name to current GUC setting */
    if (application_name)
        pgstat_report_appname(application_name);
}

/*
 * Shut down a single backend's statistics reporting at process exit.
 *
 * Flush any remaining statistics counts out to the collector.
 * Without this, operations triggered during backend exit (such as
 * temp table deletions) won't be counted.
 *
 * Lastly, clear out our entry in the PgBackendStatus array.
 */
static void
pgstat_beshutdown_hook(int code, Datum arg)
{
    volatile PgBackendStatus *beentry = MyBEEntry;

    /*
     * If we got as far as discovering our own database ID, we can report what
     * we did to the collector.  Otherwise, we'd be sending 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_stat(true);

    /*
     * Clear my status entry, following the protocol of bumping st_changecount
     * before and after.  We use a volatile pointer here to ensure the
     * compiler doesn't try to get cute.
     */
    pgstat_increment_changecount_before(beentry);

    beentry->st_procpid = 0;    /* mark invalid */

    pgstat_increment_changecount_after(beentry);
}


/* ----------
 * pgstat_report_activity() -
 *
 *  Called from tcop/postgres.c to report what the backend is actually doing
 *  (but note cmd_str can be NULL for certain cases).
 *
 * All updates of the status entry follow the protocol of bumping
 * st_changecount before and after.  We use a volatile pointer here to
 * ensure the compiler doesn't try to get cute.
 * ----------
 */
void
pgstat_report_activity(BackendState state, const char *cmd_str)
{
    volatile PgBackendStatus *beentry = MyBEEntry;
    TimestampTz start_timestamp;
    TimestampTz current_timestamp;
    int         len = 0;

    TRACE_POSTGRESQL_STATEMENT_STATUS(cmd_str);

    if (!beentry)
        return;

    if (!pgstat_track_activities)
    {
        if (beentry->st_state != STATE_DISABLED)
        {
            volatile PGPROC *proc = MyProc;

            /*
             * track_activities is disabled, but we last reported a
             * non-disabled state.  As our final update, change the state and
             * clear fields we will not be updating anymore.
             */
            pgstat_increment_changecount_before(beentry);
            beentry->st_state = STATE_DISABLED;
            beentry->st_state_start_timestamp = 0;
            beentry->st_activity_raw[0] = '\0';
            beentry->st_activity_start_timestamp = 0;
            /* st_xact_start_timestamp and wait_event_info are also disabled */
            beentry->st_xact_start_timestamp = 0;
            proc->wait_event_info = 0;
            pgstat_increment_changecount_after(beentry);
        }
        return;
    }

    /*
     * To minimize the time spent modifying the entry, fetch all the needed
     * data first.
     */
    start_timestamp = GetCurrentStatementStartTimestamp();
    if (cmd_str != NULL)
    {
        /*
         * Compute length of to-be-stored string unaware of multi-byte
         * characters. For speed reasons that'll get corrected on read, rather
         * than computed every write.
         */
        len = Min(strlen(cmd_str), pgstat_track_activity_query_size - 1);
    }
    current_timestamp = GetCurrentTimestamp();

    /*
     * Now update the status entry
     */
    pgstat_increment_changecount_before(beentry);

    beentry->st_state = state;
    beentry->st_state_start_timestamp = current_timestamp;

    if (cmd_str != NULL)
    {
        memcpy((char *) beentry->st_activity_raw, cmd_str, len);
        beentry->st_activity_raw[len] = '\0';
        beentry->st_activity_start_timestamp = start_timestamp;
    }

    pgstat_increment_changecount_after(beentry);
}

/*-----------
 * pgstat_progress_start_command() -
 *
 * Set st_progress_command (and st_progress_command_target) in own backend
 * entry.  Also, zero-initialize st_progress_param array.
 *-----------
 */
void
pgstat_progress_start_command(ProgressCommandType cmdtype, Oid relid)
{
    volatile PgBackendStatus *beentry = MyBEEntry;

    if (!beentry || !pgstat_track_activities)
        return;

    pgstat_increment_changecount_before(beentry);
    beentry->st_progress_command = cmdtype;
    beentry->st_progress_command_target = relid;
    MemSet(&beentry->st_progress_param, 0, sizeof(beentry->st_progress_param));
    pgstat_increment_changecount_after(beentry);
}

/*-----------
 * pgstat_progress_update_param() -
 *
 * Update index'th member in st_progress_param[] of own backend entry.
 *-----------
 */
void
pgstat_progress_update_param(int index, int64 val)
{
    volatile PgBackendStatus *beentry = MyBEEntry;

    Assert(index >= 0 && index < PGSTAT_NUM_PROGRESS_PARAM);

    if (!beentry || !pgstat_track_activities)
        return;

    pgstat_increment_changecount_before(beentry);
    beentry->st_progress_param[index] = val;
    pgstat_increment_changecount_after(beentry);
}

/*-----------
 * pgstat_progress_update_multi_param() -
 *
 * Update multiple members in st_progress_param[] of own backend entry.
 * This is atomic; readers won't see intermediate states.
 *-----------
 */
void
pgstat_progress_update_multi_param(int nparam, const int *index,
                                   const int64 *val)
{
    volatile PgBackendStatus *beentry = MyBEEntry;
    int         i;

    if (!beentry || !pgstat_track_activities || nparam == 0)
        return;

    pgstat_increment_changecount_before(beentry);

    for (i = 0; i < nparam; ++i)
    {
        Assert(index[i] >= 0 && index[i] < PGSTAT_NUM_PROGRESS_PARAM);

        beentry->st_progress_param[index[i]] = val[i];
    }

    pgstat_increment_changecount_after(beentry);
}

/*-----------
 * pgstat_progress_end_command() -
 *
 * Reset st_progress_command (and st_progress_command_target) in own backend
 * entry.  This signals the end of the command.
 *-----------
 */
void
pgstat_progress_end_command(void)
{
    volatile PgBackendStatus *beentry = MyBEEntry;

    if (!beentry)
        return;
    if (!pgstat_track_activities
        && beentry->st_progress_command == PROGRESS_COMMAND_INVALID)
        return;

    pgstat_increment_changecount_before(beentry);
    beentry->st_progress_command = PROGRESS_COMMAND_INVALID;
    beentry->st_progress_command_target = InvalidOid;
    pgstat_increment_changecount_after(beentry);
}

/* ----------
 * pgstat_report_appname() -
 *
 *  Called to update our application name.
 * ----------
 */
void
pgstat_report_appname(const char *appname)
{
    volatile PgBackendStatus *beentry = MyBEEntry;
    int         len;

    if (!beentry)
        return;

    /* This should be unnecessary if GUC did its job, but be safe */
    len = pg_mbcliplen(appname, strlen(appname), NAMEDATALEN - 1);

    /*
     * Update my status entry, following the protocol of bumping
     * st_changecount before and after.  We use a volatile pointer here to
     * ensure the compiler doesn't try to get cute.
     */
    pgstat_increment_changecount_before(beentry);

    memcpy((char *) beentry->st_appname, appname, len);
    beentry->st_appname[len] = '\0';

    pgstat_increment_changecount_after(beentry);
}

/*
 * Report current transaction start timestamp as the specified value.
 * Zero means there is no active transaction.
 */
void
pgstat_report_xact_timestamp(TimestampTz tstamp)
{
    volatile PgBackendStatus *beentry = MyBEEntry;

    if (!pgstat_track_activities || !beentry)
        return;

    /*
     * Update my status entry, following the protocol of bumping
     * st_changecount before and after.  We use a volatile pointer here to
     * ensure the compiler doesn't try to get cute.
     */
    pgstat_increment_changecount_before(beentry);
    beentry->st_xact_start_timestamp = tstamp;
    pgstat_increment_changecount_after(beentry);
}

/* ----------
 * pgstat_read_current_status() -
 *
 *  Copy the current contents of the PgBackendStatus array to local memory,
 *  if not already done in this transaction.
 * ----------
 */
static void
pgstat_read_current_status(void)
{
    volatile PgBackendStatus *beentry;
    LocalPgBackendStatus *localtable;
    LocalPgBackendStatus *localentry;
    char       *localappname,
               *localactivity;
#ifdef USE_SSL
    PgBackendSSLStatus *localsslstatus;
#endif
    int         i;

    Assert(!pgStatRunningInCollector);
    if (localBackendStatusTable)
        return;                 /* already done */

    pgstat_setup_memcxt();

    localtable = (LocalPgBackendStatus *)
        MemoryContextAlloc(pgStatLocalContext,
                           sizeof(LocalPgBackendStatus) * NumBackendStatSlots);
    localappname = (char *)
        MemoryContextAlloc(pgStatLocalContext,
                           NAMEDATALEN * NumBackendStatSlots);
    localactivity = (char *)
        MemoryContextAlloc(pgStatLocalContext,
                           pgstat_track_activity_query_size * NumBackendStatSlots);
#ifdef USE_SSL
    localsslstatus = (PgBackendSSLStatus *)
        MemoryContextAlloc(pgStatLocalContext,
                           sizeof(PgBackendSSLStatus) * NumBackendStatSlots);
#endif

    localNumBackends = 0;

    beentry = BackendStatusArray;
    localentry = localtable;
    for (i = 1; i <= NumBackendStatSlots; i++)
    {
        /*
         * Follow the protocol of retrying if st_changecount changes while we
         * copy the entry, or if it's odd.  (The check for odd is needed to
         * cover the case where we are able to completely copy the entry while
         * the source backend is between increment steps.)  We use a volatile
         * pointer here to ensure the compiler doesn't try to get cute.
         */
        for (;;)
        {
            int         before_changecount;
            int         after_changecount;

            pgstat_save_changecount_before(beentry, before_changecount);

            localentry->backendStatus.st_procpid = beentry->st_procpid;
            if (localentry->backendStatus.st_procpid > 0)
            {
                memcpy(&localentry->backendStatus, (char *) beentry, sizeof(PgBackendStatus));

                /*
                 * strcpy is safe even if the string is modified concurrently,
                 * because there's always a \0 at the end of the buffer.
                 */
                strcpy(localappname, (char *) beentry->st_appname);
                localentry->backendStatus.st_appname = localappname;
                strcpy(localactivity, (char *) beentry->st_activity_raw);
                localentry->backendStatus.st_activity_raw = localactivity;
                localentry->backendStatus.st_ssl = beentry->st_ssl;
#ifdef USE_SSL
                if (beentry->st_ssl)
                {
                    memcpy(localsslstatus, beentry->st_sslstatus, sizeof(PgBackendSSLStatus));
                    localentry->backendStatus.st_sslstatus = localsslstatus;
                }
#endif
            }

            pgstat_save_changecount_after(beentry, after_changecount);
            if (before_changecount == after_changecount &&
                (before_changecount & 1) == 0)
                break;

            /* Make sure we can break out of loop if stuck... */
            CHECK_FOR_INTERRUPTS();
        }

        beentry++;
        /* Only valid entries get included into the local array */
        if (localentry->backendStatus.st_procpid > 0)
        {
            BackendIdGetTransactionIds(i,
                                       &localentry->backend_xid,
                                       &localentry->backend_xmin);

            localentry++;
            localappname += NAMEDATALEN;
            localactivity += pgstat_track_activity_query_size;
#ifdef USE_SSL
            localsslstatus++;
#endif
            localNumBackends++;
        }
    }

    /* Set the pointer only after completion of a valid table */
    localBackendStatusTable = localtable;
}

/* ----------
 * pgstat_get_wait_event_type() -
 *
 *  Return a string representing the current wait event type, backend is
 *  waiting on.
 */
const char *
pgstat_get_wait_event_type(uint32 wait_event_info)
{
    uint32      classId;
    const char *event_type;

    /* report process as not waiting. */
    if (wait_event_info == 0)
        return NULL;

    classId = wait_event_info & 0xFF000000;

    switch (classId)
    {
        case PG_WAIT_LWLOCK:
            event_type = "LWLock";
            break;
        case PG_WAIT_LOCK:
            event_type = "Lock";
            break;
        case PG_WAIT_BUFFER_PIN:
            event_type = "BufferPin";
            break;
        case PG_WAIT_ACTIVITY:
            event_type = "Activity";
            break;
        case PG_WAIT_CLIENT:
            event_type = "Client";
            break;
        case PG_WAIT_EXTENSION:
            event_type = "Extension";
            break;
        case PG_WAIT_IPC:
            event_type = "IPC";
            break;
        case PG_WAIT_TIMEOUT:
            event_type = "Timeout";
            break;
        case PG_WAIT_IO:
            event_type = "IO";
            break;
        default:
            event_type = "???";
            break;
    }

    return event_type;
}

/* ----------
 * pgstat_get_wait_event() -
 *
 *  Return a string representing the current wait event, backend is
 *  waiting on.
 */
const char *
pgstat_get_wait_event(uint32 wait_event_info)
{
    uint32      classId;
    uint16      eventId;
    const char *event_name;

    /* report process as not waiting. */
    if (wait_event_info == 0)
        return NULL;

    classId = wait_event_info & 0xFF000000;
    eventId = wait_event_info & 0x0000FFFF;

    switch (classId)
    {
        case PG_WAIT_LWLOCK:
            event_name = GetLWLockIdentifier(classId, eventId);
            break;
        case PG_WAIT_LOCK:
            event_name = GetLockNameFromTagType(eventId);
            break;
        case PG_WAIT_BUFFER_PIN:
            event_name = "BufferPin";
            break;
        case PG_WAIT_ACTIVITY:
            {
                WaitEventActivity w = (WaitEventActivity) wait_event_info;

                event_name = pgstat_get_wait_activity(w);
                break;
            }
        case PG_WAIT_CLIENT:
            {
                WaitEventClient w = (WaitEventClient) wait_event_info;

                event_name = pgstat_get_wait_client(w);
                break;
            }
        case PG_WAIT_EXTENSION:
            event_name = "Extension";
            break;
        case PG_WAIT_IPC:
            {
                WaitEventIPC w = (WaitEventIPC) wait_event_info;

                event_name = pgstat_get_wait_ipc(w);
                break;
            }
        case PG_WAIT_TIMEOUT:
            {
                WaitEventTimeout w = (WaitEventTimeout) wait_event_info;

                event_name = pgstat_get_wait_timeout(w);
                break;
            }
        case PG_WAIT_IO:
            {
                WaitEventIO w = (WaitEventIO) wait_event_info;

                event_name = pgstat_get_wait_io(w);
                break;
            }
        default:
            event_name = "unknown wait event";
            break;
    }

    return event_name;
}

/* ----------
 * pgstat_get_wait_activity() -
 *
 * Convert WaitEventActivity to string.
 * ----------
 */
static const char *
pgstat_get_wait_activity(WaitEventActivity w)
{
    const char *event_name = "unknown wait event";

    switch (w)
    {
        case WAIT_EVENT_ARCHIVER_MAIN:
            event_name = "ArchiverMain";
            break;
        case WAIT_EVENT_AUTOVACUUM_MAIN:
            event_name = "AutoVacuumMain";
            break;
        case WAIT_EVENT_BGWRITER_HIBERNATE:
            event_name = "BgWriterHibernate";
            break;
        case WAIT_EVENT_BGWRITER_MAIN:
            event_name = "BgWriterMain";
            break;
        case WAIT_EVENT_CHECKPOINTER_MAIN:
            event_name = "CheckpointerMain";
            break;
        case WAIT_EVENT_LOGICAL_LAUNCHER_MAIN:
            event_name = "LogicalLauncherMain";
            break;
        case WAIT_EVENT_LOGICAL_APPLY_MAIN:
            event_name = "LogicalApplyMain";
            break;
        case WAIT_EVENT_PGSTAT_MAIN:
            event_name = "PgStatMain";
            break;
        case WAIT_EVENT_RECOVERY_WAL_ALL:
            event_name = "RecoveryWalAll";
            break;
        case WAIT_EVENT_RECOVERY_WAL_STREAM:
            event_name = "RecoveryWalStream";
            break;
        case WAIT_EVENT_SYSLOGGER_MAIN:
            event_name = "SysLoggerMain";
            break;
        case WAIT_EVENT_WAL_RECEIVER_MAIN:
            event_name = "WalReceiverMain";
            break;
        case WAIT_EVENT_WAL_SENDER_MAIN:
            event_name = "WalSenderMain";
            break;
        case WAIT_EVENT_WAL_WRITER_MAIN:
            event_name = "WalWriterMain";
            break;
            /* no default case, so that compiler will warn */
    }

    return event_name;
}

/* ----------
 * pgstat_get_wait_client() -
 *
 * Convert WaitEventClient to string.
 * ----------
 */
static const char *
pgstat_get_wait_client(WaitEventClient w)
{
    const char *event_name = "unknown wait event";

    switch (w)
    {
        case WAIT_EVENT_CLIENT_READ:
            event_name = "ClientRead";
            break;
        case WAIT_EVENT_CLIENT_WRITE:
            event_name = "ClientWrite";
            break;
        case WAIT_EVENT_LIBPQWALRECEIVER_CONNECT:
            event_name = "LibPQWalReceiverConnect";
            break;
        case WAIT_EVENT_LIBPQWALRECEIVER_RECEIVE:
            event_name = "LibPQWalReceiverReceive";
            break;
        case WAIT_EVENT_SSL_OPEN_SERVER:
            event_name = "SSLOpenServer";
            break;
        case WAIT_EVENT_WAL_RECEIVER_WAIT_START:
            event_name = "WalReceiverWaitStart";
            break;
        case WAIT_EVENT_WAL_SENDER_WAIT_WAL:
            event_name = "WalSenderWaitForWAL";
            break;
        case WAIT_EVENT_WAL_SENDER_WRITE_DATA:
            event_name = "WalSenderWriteData";
            break;
            /* no default case, so that compiler will warn */
    }

    return event_name;
}

/* ----------
 * pgstat_get_wait_ipc() -
 *
 * Convert WaitEventIPC to string.
 * ----------
 */
static const char *
pgstat_get_wait_ipc(WaitEventIPC w)
{
    const char *event_name = "unknown wait event";

    switch (w)
    {
        case WAIT_EVENT_BGWORKER_SHUTDOWN:
            event_name = "BgWorkerShutdown";
            break;
        case WAIT_EVENT_BGWORKER_STARTUP:
            event_name = "BgWorkerStartup";
            break;
        case WAIT_EVENT_BTREE_PAGE:
            event_name = "BtreePage";
            break;
        case WAIT_EVENT_EXECUTE_GATHER:
            event_name = "ExecuteGather";
            break;
        case WAIT_EVENT_LOGICAL_SYNC_DATA:
            event_name = "LogicalSyncData";
            break;
        case WAIT_EVENT_LOGICAL_SYNC_STATE_CHANGE:
            event_name = "LogicalSyncStateChange";
            break;
        case WAIT_EVENT_MQ_INTERNAL:
            event_name = "MessageQueueInternal";
            break;
        case WAIT_EVENT_MQ_PUT_MESSAGE:
            event_name = "MessageQueuePutMessage";
            break;
        case WAIT_EVENT_MQ_RECEIVE:
            event_name = "MessageQueueReceive";
            break;
        case WAIT_EVENT_MQ_SEND:
            event_name = "MessageQueueSend";
            break;
        case WAIT_EVENT_PARALLEL_FINISH:
            event_name = "ParallelFinish";
            break;
        case WAIT_EVENT_PARALLEL_BITMAP_SCAN:
            event_name = "ParallelBitmapScan";
            break;
        case WAIT_EVENT_PROCARRAY_GROUP_UPDATE:
            event_name = "ProcArrayGroupUpdate";
            break;
        case WAIT_EVENT_CLOG_GROUP_UPDATE:
            event_name = "ClogGroupUpdate";
            break;
        case WAIT_EVENT_REPLICATION_ORIGIN_DROP:
            event_name = "ReplicationOriginDrop";
            break;
        case WAIT_EVENT_REPLICATION_SLOT_DROP:
            event_name = "ReplicationSlotDrop";
            break;
        case WAIT_EVENT_SAFE_SNAPSHOT:
            event_name = "SafeSnapshot";
            break;
        case WAIT_EVENT_SYNC_REP:
            event_name = "SyncRep";
            break;
            /* no default case, so that compiler will warn */
    }

    return event_name;
}

/* ----------
 * pgstat_get_wait_timeout() -
 *
 * Convert WaitEventTimeout to string.
 * ----------
 */
static const char *
pgstat_get_wait_timeout(WaitEventTimeout w)
{
    const char *event_name = "unknown wait event";

    switch (w)
    {
        case WAIT_EVENT_BASE_BACKUP_THROTTLE:
            event_name = "BaseBackupThrottle";
            break;
        case WAIT_EVENT_PG_SLEEP:
            event_name = "PgSleep";
            break;
        case WAIT_EVENT_RECOVERY_APPLY_DELAY:
            event_name = "RecoveryApplyDelay";
            break;
            /* no default case, so that compiler will warn */
    }

    return event_name;
}

/* ----------
 * pgstat_get_wait_io() -
 *
 * Convert WaitEventIO to string.
 * ----------
 */
static const char *
pgstat_get_wait_io(WaitEventIO w)
{
    const char *event_name = "unknown wait event";

    switch (w)
    {
        case WAIT_EVENT_BUFFILE_READ:
            event_name = "BufFileRead";
            break;
        case WAIT_EVENT_BUFFILE_WRITE:
            event_name = "BufFileWrite";
            break;
        case WAIT_EVENT_CONTROL_FILE_READ:
            event_name = "ControlFileRead";
            break;
        case WAIT_EVENT_CONTROL_FILE_SYNC:
            event_name = "ControlFileSync";
            break;
        case WAIT_EVENT_CONTROL_FILE_SYNC_UPDATE:
            event_name = "ControlFileSyncUpdate";
            break;
        case WAIT_EVENT_CONTROL_FILE_WRITE:
            event_name = "ControlFileWrite";
            break;
        case WAIT_EVENT_CONTROL_FILE_WRITE_UPDATE:
            event_name = "ControlFileWriteUpdate";
            break;
        case WAIT_EVENT_COPY_FILE_READ:
            event_name = "CopyFileRead";
            break;
        case WAIT_EVENT_COPY_FILE_WRITE:
            event_name = "CopyFileWrite";
            break;
        case WAIT_EVENT_DATA_FILE_EXTEND:
            event_name = "DataFileExtend";
            break;
        case WAIT_EVENT_DATA_FILE_FLUSH:
            event_name = "DataFileFlush";
            break;
        case WAIT_EVENT_DATA_FILE_IMMEDIATE_SYNC:
            event_name = "DataFileImmediateSync";
            break;
        case WAIT_EVENT_DATA_FILE_PREFETCH:
            event_name = "DataFilePrefetch";
            break;
        case WAIT_EVENT_DATA_FILE_READ:
            event_name = "DataFileRead";
            break;
        case WAIT_EVENT_DATA_FILE_SYNC:
            event_name = "DataFileSync";
            break;
        case WAIT_EVENT_DATA_FILE_TRUNCATE:
            event_name = "DataFileTruncate";
            break;
        case WAIT_EVENT_DATA_FILE_WRITE:
            event_name = "DataFileWrite";
            break;
        case WAIT_EVENT_DSM_FILL_ZERO_WRITE:
            event_name = "DSMFillZeroWrite";
            break;
        case WAIT_EVENT_LOCK_FILE_ADDTODATADIR_READ:
            event_name = "LockFileAddToDataDirRead";
            break;
        case WAIT_EVENT_LOCK_FILE_ADDTODATADIR_SYNC:
            event_name = "LockFileAddToDataDirSync";
            break;
        case WAIT_EVENT_LOCK_FILE_ADDTODATADIR_WRITE:
            event_name = "LockFileAddToDataDirWrite";
            break;
        case WAIT_EVENT_LOCK_FILE_CREATE_READ:
            event_name = "LockFileCreateRead";
            break;
        case WAIT_EVENT_LOCK_FILE_CREATE_SYNC:
            event_name = "LockFileCreateSync";
            break;
        case WAIT_EVENT_LOCK_FILE_CREATE_WRITE:
            event_name = "LockFileCreateWRITE";
            break;
        case WAIT_EVENT_LOCK_FILE_RECHECKDATADIR_READ:
            event_name = "LockFileReCheckDataDirRead";
            break;
        case WAIT_EVENT_LOGICAL_REWRITE_CHECKPOINT_SYNC:
            event_name = "LogicalRewriteCheckpointSync";
            break;
        case WAIT_EVENT_LOGICAL_REWRITE_MAPPING_SYNC:
            event_name = "LogicalRewriteMappingSync";
            break;
        case WAIT_EVENT_LOGICAL_REWRITE_MAPPING_WRITE:
            event_name = "LogicalRewriteMappingWrite";
            break;
        case WAIT_EVENT_LOGICAL_REWRITE_SYNC:
            event_name = "LogicalRewriteSync";
            break;
        case WAIT_EVENT_LOGICAL_REWRITE_TRUNCATE:
            event_name = "LogicalRewriteTruncate";
            break;
        case WAIT_EVENT_LOGICAL_REWRITE_WRITE:
            event_name = "LogicalRewriteWrite";
            break;
        case WAIT_EVENT_RELATION_MAP_READ:
            event_name = "RelationMapRead";
            break;
        case WAIT_EVENT_RELATION_MAP_SYNC:
            event_name = "RelationMapSync";
            break;
        case WAIT_EVENT_RELATION_MAP_WRITE:
            event_name = "RelationMapWrite";
            break;
        case WAIT_EVENT_REORDER_BUFFER_READ:
            event_name = "ReorderBufferRead";
            break;
        case WAIT_EVENT_REORDER_BUFFER_WRITE:
            event_name = "ReorderBufferWrite";
            break;
        case WAIT_EVENT_REORDER_LOGICAL_MAPPING_READ:
            event_name = "ReorderLogicalMappingRead";
            break;
        case WAIT_EVENT_REPLICATION_SLOT_READ:
            event_name = "ReplicationSlotRead";
            break;
        case WAIT_EVENT_REPLICATION_SLOT_RESTORE_SYNC:
            event_name = "ReplicationSlotRestoreSync";
            break;
        case WAIT_EVENT_REPLICATION_SLOT_SYNC:
            event_name = "ReplicationSlotSync";
            break;
        case WAIT_EVENT_REPLICATION_SLOT_WRITE:
            event_name = "ReplicationSlotWrite";
            break;
        case WAIT_EVENT_SLRU_FLUSH_SYNC:
            event_name = "SLRUFlushSync";
            break;
        case WAIT_EVENT_SLRU_READ:
            event_name = "SLRURead";
            break;
        case WAIT_EVENT_SLRU_SYNC:
            event_name = "SLRUSync";
            break;
        case WAIT_EVENT_SLRU_WRITE:
            event_name = "SLRUWrite";
            break;
        case WAIT_EVENT_SNAPBUILD_READ:
            event_name = "SnapbuildRead";
            break;
        case WAIT_EVENT_SNAPBUILD_SYNC:
            event_name = "SnapbuildSync";
            break;
        case WAIT_EVENT_SNAPBUILD_WRITE:
            event_name = "SnapbuildWrite";
            break;
        case WAIT_EVENT_TIMELINE_HISTORY_FILE_SYNC:
            event_name = "TimelineHistoryFileSync";
            break;
        case WAIT_EVENT_TIMELINE_HISTORY_FILE_WRITE:
            event_name = "TimelineHistoryFileWrite";
            break;
        case WAIT_EVENT_TIMELINE_HISTORY_READ:
            event_name = "TimelineHistoryRead";
            break;
        case WAIT_EVENT_TIMELINE_HISTORY_SYNC:
            event_name = "TimelineHistorySync";
            break;
        case WAIT_EVENT_TIMELINE_HISTORY_WRITE:
            event_name = "TimelineHistoryWrite";
            break;
        case WAIT_EVENT_TWOPHASE_FILE_READ:
            event_name = "TwophaseFileRead";
            break;
        case WAIT_EVENT_TWOPHASE_FILE_SYNC:
            event_name = "TwophaseFileSync";
            break;
        case WAIT_EVENT_TWOPHASE_FILE_WRITE:
            event_name = "TwophaseFileWrite";
            break;
        case WAIT_EVENT_WALSENDER_TIMELINE_HISTORY_READ:
            event_name = "WALSenderTimelineHistoryRead";
            break;
        case WAIT_EVENT_WAL_BOOTSTRAP_SYNC:
            event_name = "WALBootstrapSync";
            break;
        case WAIT_EVENT_WAL_BOOTSTRAP_WRITE:
            event_name = "WALBootstrapWrite";
            break;
        case WAIT_EVENT_WAL_COPY_READ:
            event_name = "WALCopyRead";
            break;
        case WAIT_EVENT_WAL_COPY_SYNC:
            event_name = "WALCopySync";
            break;
        case WAIT_EVENT_WAL_COPY_WRITE:
            event_name = "WALCopyWrite";
            break;
        case WAIT_EVENT_WAL_INIT_SYNC:
            event_name = "WALInitSync";
            break;
        case WAIT_EVENT_WAL_INIT_WRITE:
            event_name = "WALInitWrite";
            break;
        case WAIT_EVENT_WAL_READ:
            event_name = "WALRead";
            break;
        case WAIT_EVENT_WAL_SYNC_METHOD_ASSIGN:
            event_name = "WALSyncMethodAssign";
            break;
        case WAIT_EVENT_WAL_WRITE:
            event_name = "WALWrite";
            break;

            /* no default case, so that compiler will warn */
    }

    return event_name;
}


/* ----------
 * pgstat_get_backend_current_activity() -
 *
 *  Return a string representing the current activity of the backend with
 *  the specified PID.  This looks directly at the BackendStatusArray,
 *  and so will provide current information regardless of the age of our
 *  transaction's snapshot of the status array.
 *
 *  It is the caller's responsibility to invoke this only for backends whose
 *  state is expected to remain stable while the result is in use.  The
 *  only current use is in deadlock reporting, where we can expect that
 *  the target backend is blocked on a lock.  (There are corner cases
 *  where the target's wait could get aborted while we are looking at it,
 *  but the very worst consequence is to return a pointer to a string
 *  that's been changed, so we won't worry too much.)
 *
 *  Note: return strings for special cases match pg_stat_get_backend_activity.
 * ----------
 */
const char *
pgstat_get_backend_current_activity(int pid, bool checkUser)
{
    PgBackendStatus *beentry;
    int         i;

    beentry = BackendStatusArray;
    for (i = 1; i <= MaxBackends; i++)
    {
        /*
         * Although we expect the target backend's entry to be stable, that
         * doesn't imply that anyone else's is.  To avoid identifying the
         * wrong backend, while we check for a match to the desired PID we
         * must follow the protocol of retrying if st_changecount changes
         * while we examine the entry, or if it's odd.  (This might be
         * unnecessary, since fetching or storing an int is almost certainly
         * atomic, but let's play it safe.)  We use a volatile pointer here to
         * ensure the compiler doesn't try to get cute.
         */
        volatile PgBackendStatus *vbeentry = beentry;
        bool        found;

        for (;;)
        {
            int         before_changecount;
            int         after_changecount;

            pgstat_save_changecount_before(vbeentry, before_changecount);

            found = (vbeentry->st_procpid == pid);

            pgstat_save_changecount_after(vbeentry, after_changecount);

            if (before_changecount == after_changecount &&
                (before_changecount & 1) == 0)
                break;

            /* Make sure we can break out of loop if stuck... */
            CHECK_FOR_INTERRUPTS();
        }

        if (found)
        {
            /* Now it is safe to use the non-volatile pointer */
            if (checkUser && !superuser() && beentry->st_userid != GetUserId())
                return "<insufficient privilege>";
            else if (*(beentry->st_activity_raw) == '\0')
                return "<command string not enabled>";
            else
            {
                /* this'll leak a bit of memory, but that seems acceptable */
                return pgstat_clip_activity(beentry->st_activity_raw);
            }
        }

        beentry++;
    }

    /* If we get here, caller is in error ... */
    return "<backend information not available>";
}

/* ----------
 * pgstat_get_crashed_backend_activity() -
 *
 *  Return a string representing the current activity of the backend with
 *  the specified PID.  Like the function above, but reads shared memory with
 *  the expectation that it may be corrupt.  On success, copy the string
 *  into the "buffer" argument and return that pointer.  On failure,
 *  return NULL.
 *
 *  This function is only intended to be used by the postmaster to report the
 *  query that crashed a backend.  In particular, no attempt is made to
 *  follow the correct concurrency protocol when accessing the
 *  BackendStatusArray.  But that's OK, in the worst case we'll return a
 *  corrupted message.  We also must take care not to trip on ereport(ERROR).
 * ----------
 */
const char *
pgstat_get_crashed_backend_activity(int pid, char *buffer, int buflen)
{
    volatile PgBackendStatus *beentry;
    int         i;

    beentry = BackendStatusArray;

    /*
     * We probably shouldn't get here before shared memory has been set up,
     * but be safe.
     */
    if (beentry == NULL || BackendActivityBuffer == NULL)
        return NULL;

    for (i = 1; i <= MaxBackends; i++)
    {
        if (beentry->st_procpid == pid)
        {
            /* Read pointer just once, so it can't change after validation */
            const char *activity = beentry->st_activity_raw;
            const char *activity_last;

            /*
             * We mustn't access activity string before we verify that it
             * falls within the BackendActivityBuffer. To make sure that the
             * entire string including its ending is contained within the
             * buffer, subtract one activity length from the buffer size.
             */
            activity_last = BackendActivityBuffer + BackendActivityBufferSize
                - pgstat_track_activity_query_size;

            if (activity < BackendActivityBuffer ||
                activity > activity_last)
                return NULL;

            /* If no string available, no point in a report */
            if (activity[0] == '\0')
                return NULL;

            /*
             * Copy only ASCII-safe characters so we don't run into encoding
             * problems when reporting the message; and be sure not to run off
             * the end of memory.  As only ASCII characters are reported, it
             * doesn't seem necessary to perform multibyte aware clipping.
             */
            ascii_safe_strlcpy(buffer, activity,
                               Min(buflen, pgstat_track_activity_query_size));

            return buffer;
        }

        beentry++;
    }

    /* PID not found */
    return NULL;
}

const char *
pgstat_get_backend_desc(BackendType backendType)
{
    const char *backendDesc = "unknown process type";

    switch (backendType)
    {
        case B_AUTOVAC_LAUNCHER:
            backendDesc = "autovacuum launcher";
            break;
        case B_AUTOVAC_WORKER:
            backendDesc = "autovacuum worker";
            break;
        case B_BACKEND:
            backendDesc = "client backend";
            break;
        case B_BG_WORKER:
            backendDesc = "background worker";
            break;
        case B_BG_WRITER:
            backendDesc = "background writer";
            break;
        case B_CHECKPOINTER:
            backendDesc = "checkpointer";
            break;
        case B_STARTUP:
            backendDesc = "startup";
            break;
        case B_WAL_RECEIVER:
            backendDesc = "walreceiver";
            break;
        case B_WAL_SENDER:
            backendDesc = "walsender";
            break;
        case B_WAL_WRITER:
            backendDesc = "walwriter";
            break;
    }

    return backendDesc;
}

/* ------------------------------------------------------------
 * Local support functions follow
 * ------------------------------------------------------------
 */


/* ----------
 * pgstat_setheader() -
 *
 *      Set common header fields in a statistics message
 * ----------
 */
static void
pgstat_setheader(PgStat_MsgHdr *hdr, StatMsgType mtype)
{
    hdr->m_type = mtype;
}


/* ----------
 * pgstat_send() -
 *
 *      Send out one statistics message to the collector
 * ----------
 */
static void
pgstat_send(void *msg, int len)
{
    int         rc;

    if (pgStatSock == PGINVALID_SOCKET)
        return;

    ((PgStat_MsgHdr *) msg)->m_size = len;

    /* We'll retry after EINTR, but ignore all other failures */
    do
    {
        rc = send(pgStatSock, msg, len, 0);
    } while (rc < 0 && errno == EINTR);

#ifdef USE_ASSERT_CHECKING
    /* In debug builds, log send failures ... */
    if (rc < 0)
        elog(LOG, "could not send to statistics collector: %m");
#endif
}

/* ----------
 * pgstat_send_archiver() -
 *
 *  Tell the collector about the WAL file that we successfully
 *  archived or failed to archive.
 * ----------
 */
void
pgstat_send_archiver(const char *xlog, bool failed)
{
    PgStat_MsgArchiver msg;

    /*
     * Prepare and send the message
     */
    pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_ARCHIVER);
    msg.m_failed = failed;
    StrNCpy(msg.m_xlog, xlog, sizeof(msg.m_xlog));
    msg.m_timestamp = GetCurrentTimestamp();
    pgstat_send(&msg, sizeof(msg));
}

/* ----------
 * pgstat_send_bgwriter() -
 *
 *      Send bgwriter statistics to the collector
 * ----------
 */
void
pgstat_send_bgwriter(void)
{
    /* We assume this initializes to zeroes */
    static const PgStat_MsgBgWriter all_zeroes;

    /*
     * This function can be called even if nothing at all has happened. In
     * this case, avoid sending a completely empty message to the stats
     * collector.
     */
    if (memcmp(&BgWriterStats, &all_zeroes, sizeof(PgStat_MsgBgWriter)) == 0)
        return;

    /*
     * Prepare and send the message
     */
    pgstat_setheader(&BgWriterStats.m_hdr, PGSTAT_MTYPE_BGWRITER);
    pgstat_send(&BgWriterStats, sizeof(BgWriterStats));

    /*
     * Clear out the statistics buffer, so it can be re-used.
     */
    MemSet(&BgWriterStats, 0, sizeof(BgWriterStats));
}


/* ----------
 * PgstatCollectorMain() -
 *
 *  Start up the statistics collector process.  This is the body of the
 *  postmaster child process.
 *
 *  The argc/argv parameters are valid only in EXEC_BACKEND case.
 * ----------
 */
NON_EXEC_STATIC void
PgstatCollectorMain(int argc, char *argv[])
{
    int         len;
    PgStat_Msg  msg;
    int         wr;

    /*
     * Ignore all signals usually bound to some action in the postmaster,
     * except SIGHUP and SIGQUIT.  Note we don't need a SIGUSR1 handler to
     * support latch operations, because we only use a local latch.
     */
    pqsignal(SIGHUP, pgstat_sighup_handler);
    pqsignal(SIGINT, SIG_IGN);
    pqsignal(SIGTERM, SIG_IGN);
    pqsignal(SIGQUIT, pgstat_exit);
    pqsignal(SIGALRM, SIG_IGN);
    pqsignal(SIGPIPE, SIG_IGN);
    pqsignal(SIGUSR1, SIG_IGN);
    pqsignal(SIGUSR2, SIG_IGN);
    pqsignal(SIGCHLD, SIG_DFL);
    pqsignal(SIGTTIN, SIG_DFL);
    pqsignal(SIGTTOU, SIG_DFL);
    pqsignal(SIGCONT, SIG_DFL);
    pqsignal(SIGWINCH, SIG_DFL);
    PG_SETMASK(&UnBlockSig);

    /*
     * Identify myself via ps
     */
    init_ps_display("stats collector", "", "", "");

    /*
     * Read in existing stats files or initialize the stats to zero.
     */
    pgStatRunningInCollector = true;
    pgStatDBHash = pgstat_read_statsfiles(InvalidOid, true, true);

    /*
     * Loop to process messages until we get SIGQUIT or detect ungraceful
     * death of our parent postmaster.
     *
     * For performance reasons, we don't want to do ResetLatch/WaitLatch after
     * every message; instead, do that only after a recv() fails to obtain a
     * message.  (This effectively means that if backends are sending us stuff
     * like mad, we won't notice postmaster death until things slack off a
     * bit; which seems fine.)  To do that, we have an inner loop that
     * iterates as long as recv() succeeds.  We do recognize got_SIGHUP inside
     * the inner loop, which means that such interrupts will get serviced but
     * the latch won't get cleared until next time there is a break in the
     * action.
     */
    for (;;)
    {
        /* Clear any already-pending wakeups */
        ResetLatch(MyLatch);

        /*
         * Quit if we get SIGQUIT from the postmaster.
         */
        if (need_exit)
            break;

        /*
         * Inner loop iterates as long as we keep getting messages, or until
         * need_exit becomes set.
         */
        while (!need_exit)
        {
            /*
             * Reload configuration if we got SIGHUP from the postmaster.
             */
            if (got_SIGHUP)
            {
                got_SIGHUP = false;
                ProcessConfigFile(PGC_SIGHUP);
            }

            /*
             * Write the stats file(s) if a new request has arrived that is
             * not satisfied by existing file(s).
             */
            if (pgstat_write_statsfile_needed())
                pgstat_write_statsfiles(false, false);

            /*
             * Try to receive and process a message.  This will not block,
             * since the socket is set to non-blocking mode.
             *
             * XXX On Windows, we have to force pgwin32_recv to cooperate,
             * despite the previous use of pg_set_noblock() on the socket.
             * This is extremely broken and should be fixed someday.
             */
#ifdef WIN32
            pgwin32_noblock = 1;
#endif

            len = recv(pgStatSock, (char *) &msg,
                       sizeof(PgStat_Msg), 0);

#ifdef WIN32
            pgwin32_noblock = 0;
#endif

            if (len < 0)
            {
                if (errno == EAGAIN || errno == EWOULDBLOCK || errno == EINTR)
                    break;      /* out of inner loop */
                ereport(ERROR,
                        (errcode_for_socket_access(),
                         errmsg("could not read statistics message: %m")));
            }

            /*
             * We ignore messages that are smaller than our common header
             */
            if (len < sizeof(PgStat_MsgHdr))
                continue;

            /*
             * The received length must match the length in the header
             */
            if (msg.msg_hdr.m_size != len)
                continue;

            /*
             * O.K. - we accept this message.  Process it.
             */
            switch (msg.msg_hdr.m_type)
            {
                case PGSTAT_MTYPE_DUMMY:
                    break;

                case PGSTAT_MTYPE_INQUIRY:
                    pgstat_recv_inquiry((PgStat_MsgInquiry *) &msg, len);
                    break;

                case PGSTAT_MTYPE_TABSTAT:
                    pgstat_recv_tabstat((PgStat_MsgTabstat *) &msg, len);
                    break;

                case PGSTAT_MTYPE_TABPURGE:
                    pgstat_recv_tabpurge((PgStat_MsgTabpurge *) &msg, len);
                    break;

                case PGSTAT_MTYPE_DROPDB:
                    pgstat_recv_dropdb((PgStat_MsgDropdb *) &msg, len);
                    break;

                case PGSTAT_MTYPE_RESETCOUNTER:
                    pgstat_recv_resetcounter((PgStat_MsgResetcounter *) &msg,
                                             len);
                    break;

                case PGSTAT_MTYPE_RESETSHAREDCOUNTER:
                    pgstat_recv_resetsharedcounter(
                                                   (PgStat_MsgResetsharedcounter *) &msg,
                                                   len);
                    break;

                case PGSTAT_MTYPE_RESETSINGLECOUNTER:
                    pgstat_recv_resetsinglecounter(
                                                   (PgStat_MsgResetsinglecounter *) &msg,
                                                   len);
                    break;

                case PGSTAT_MTYPE_AUTOVAC_START:
                    pgstat_recv_autovac((PgStat_MsgAutovacStart *) &msg, len);
                    break;

                case PGSTAT_MTYPE_VACUUM:
                    pgstat_recv_vacuum((PgStat_MsgVacuum *) &msg, len);
                    break;

                case PGSTAT_MTYPE_ANALYZE:
                    pgstat_recv_analyze((PgStat_MsgAnalyze *) &msg, len);
                    break;

                case PGSTAT_MTYPE_ARCHIVER:
                    pgstat_recv_archiver((PgStat_MsgArchiver *) &msg, len);
                    break;

                case PGSTAT_MTYPE_BGWRITER:
                    pgstat_recv_bgwriter((PgStat_MsgBgWriter *) &msg, len);
                    break;

                case PGSTAT_MTYPE_FUNCSTAT:
                    pgstat_recv_funcstat((PgStat_MsgFuncstat *) &msg, len);
                    break;

                case PGSTAT_MTYPE_FUNCPURGE:
                    pgstat_recv_funcpurge((PgStat_MsgFuncpurge *) &msg, len);
                    break;

                case PGSTAT_MTYPE_RECOVERYCONFLICT:
                    pgstat_recv_recoveryconflict((PgStat_MsgRecoveryConflict *) &msg, len);
                    break;

                case PGSTAT_MTYPE_DEADLOCK:
                    pgstat_recv_deadlock((PgStat_MsgDeadlock *) &msg, len);
                    break;

                case PGSTAT_MTYPE_TEMPFILE:
                    pgstat_recv_tempfile((PgStat_MsgTempFile *) &msg, len);
                    break;

                default:
                    break;
            }
        }                       /* end of inner message-processing loop */

        /* Sleep until there's something to do */
#ifndef WIN32
        wr = WaitLatchOrSocket(MyLatch,
                               WL_LATCH_SET | WL_POSTMASTER_DEATH | WL_SOCKET_READABLE,
                               pgStatSock, -1L,
                               WAIT_EVENT_PGSTAT_MAIN);
#else

        /*
         * Windows, at least in its Windows Server 2003 R2 incarnation,
         * sometimes loses FD_READ events.  Waking up and retrying the recv()
         * fixes that, so don't sleep indefinitely.  This is a crock of the
         * first water, but until somebody wants to debug exactly what's
         * happening there, this is the best we can do.  The two-second
         * timeout matches our pre-9.2 behavior, and needs to be short enough
         * to not provoke "using stale statistics" complaints from
         * backend_read_statsfile.
         */
        wr = WaitLatchOrSocket(MyLatch,
                               WL_LATCH_SET | WL_POSTMASTER_DEATH | WL_SOCKET_READABLE | WL_TIMEOUT,
                               pgStatSock,
                               2 * 1000L /* msec */ ,
                               WAIT_EVENT_PGSTAT_MAIN);
#endif

        /*
         * Emergency bailout if postmaster has died.  This is to avoid the
         * necessity for manual cleanup of all postmaster children.
         */
        if (wr & WL_POSTMASTER_DEATH)
            break;
    }                           /* end of outer loop */

    /*
     * Save the final stats to reuse at next startup.
     */
    pgstat_write_statsfiles(true, true);

    exit(0);
}


/* SIGQUIT signal handler for collector process */
static void
pgstat_exit(SIGNAL_ARGS)
{
    int         save_errno = errno;

    need_exit = true;
    SetLatch(MyLatch);

    errno = save_errno;
}

/* SIGHUP handler for collector process */
static void
pgstat_sighup_handler(SIGNAL_ARGS)
{
    int         save_errno = errno;

    got_SIGHUP = true;
    SetLatch(MyLatch);

    errno = save_errno;
}

/*
 * Subroutine to clear stats in a database entry
 *
 * Tables and functions hashes are initialized to empty.
 */
static void
reset_dbentry_counters(PgStat_StatDBEntry *dbentry)
{
    HASHCTL     hash_ctl;

    dbentry->n_xact_commit = 0;
    dbentry->n_xact_rollback = 0;
    dbentry->n_blocks_fetched = 0;
    dbentry->n_blocks_hit = 0;
    dbentry->n_tuples_returned = 0;
    dbentry->n_tuples_fetched = 0;
    dbentry->n_tuples_inserted = 0;
    dbentry->n_tuples_updated = 0;
    dbentry->n_tuples_deleted = 0;
    dbentry->last_autovac_time = 0;
    dbentry->n_conflict_tablespace = 0;
    dbentry->n_conflict_lock = 0;
    dbentry->n_conflict_snapshot = 0;
    dbentry->n_conflict_bufferpin = 0;
    dbentry->n_conflict_startup_deadlock = 0;
    dbentry->n_temp_files = 0;
    dbentry->n_temp_bytes = 0;
    dbentry->n_deadlocks = 0;
    dbentry->n_block_read_time = 0;
    dbentry->n_block_write_time = 0;

    dbentry->stat_reset_timestamp = GetCurrentTimestamp();
    dbentry->stats_timestamp = 0;

    memset(&hash_ctl, 0, sizeof(hash_ctl));
    hash_ctl.keysize = sizeof(Oid);
    hash_ctl.entrysize = sizeof(PgStat_StatTabEntry);
    dbentry->tables = hash_create("Per-database table",
                                  PGSTAT_TAB_HASH_SIZE,
                                  &hash_ctl,
                                  HASH_ELEM | HASH_BLOBS);

    hash_ctl.keysize = sizeof(Oid);
    hash_ctl.entrysize = sizeof(PgStat_StatFuncEntry);
    dbentry->functions = hash_create("Per-database function",
                                     PGSTAT_FUNCTION_HASH_SIZE,
                                     &hash_ctl,
                                     HASH_ELEM | HASH_BLOBS);
}

/*
 * Lookup the hash table entry for the specified database. If no hash
 * table entry exists, initialize it, if the create parameter is true.
 * Else, return NULL.
 */
static PgStat_StatDBEntry *
pgstat_get_db_entry(Oid databaseid, bool create)
{
    PgStat_StatDBEntry *result;
    bool        found;
    HASHACTION  action = (create ? HASH_ENTER : HASH_FIND);

    /* Lookup or create the hash table entry for this database */
    result = (PgStat_StatDBEntry *) hash_search(pgStatDBHash,
                                                &databaseid,
                                                action, &found);

    if (!create && !found)
        return NULL;

    /*
     * If not found, initialize the new one.  This creates empty hash tables
     * for tables and functions, too.
     */
    if (!found)
        reset_dbentry_counters(result);

    return result;
}


/*
 * Lookup the hash table entry for the specified table. If no hash
 * table entry exists, initialize it, if the create parameter is true.
 * Else, return NULL.
 */
static PgStat_StatTabEntry *
pgstat_get_tab_entry(PgStat_StatDBEntry *dbentry, Oid tableoid, bool create)
{
    PgStat_StatTabEntry *result;
    bool        found;
    HASHACTION  action = (create ? HASH_ENTER : HASH_FIND);

    /* Lookup or create the hash table entry for this table */
    result = (PgStat_StatTabEntry *) hash_search(dbentry->tables,
                                                 &tableoid,
                                                 action, &found);

    if (!create && !found)
        return NULL;

    /* If not found, initialize the new one. */
    if (!found)
    {
        result->numscans = 0;
        result->tuples_returned = 0;
        result->tuples_fetched = 0;
        result->tuples_inserted = 0;
        result->tuples_updated = 0;
        result->tuples_deleted = 0;
        result->tuples_hot_updated = 0;
        result->n_live_tuples = 0;
        result->n_dead_tuples = 0;
        result->changes_since_analyze = 0;
        result->blocks_fetched = 0;
        result->blocks_hit = 0;
        result->vacuum_timestamp = 0;
        result->vacuum_count = 0;
        result->autovac_vacuum_timestamp = 0;
        result->autovac_vacuum_count = 0;
        result->analyze_timestamp = 0;
        result->analyze_count = 0;
        result->autovac_analyze_timestamp = 0;
        result->autovac_analyze_count = 0;
    }

    return result;
}


/* ----------
 * pgstat_write_statsfiles() -
 *      Write the global statistics file, as well as requested DB files.
 *
 *  'permanent' specifies writing to the permanent files not temporary ones.
 *  When true (happens only when the collector is shutting down), also remove
 *  the temporary files so that backends starting up under a new postmaster
 *  can't read old data before the new collector is ready.
 *
 *  When 'allDbs' is false, only the requested databases (listed in
 *  pending_write_requests) will be written; otherwise, all databases
 *  will be written.
 * ----------
 */
static void
pgstat_write_statsfiles(bool permanent, bool allDbs)
{
    HASH_SEQ_STATUS hstat;
    PgStat_StatDBEntry *dbentry;
    FILE       *fpout;
    int32       format_id;
    const char *tmpfile = permanent ? PGSTAT_STAT_PERMANENT_TMPFILE : pgstat_stat_tmpname;
    const char *statfile = permanent ? PGSTAT_STAT_PERMANENT_FILENAME : pgstat_stat_filename;
    int         rc;

    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;
    }

    /*
     * Set the timestamp of the stats file.
     */
    globalStats.stats_timestamp = GetCurrentTimestamp();

    /*
     * Write the file header --- currently just a format ID.
     */
    format_id = PGSTAT_FILE_FORMAT_ID;
    rc = fwrite(&format_id, sizeof(format_id), 1, fpout);
    (void) rc;                  /* we'll check for error with ferror */

    /*
     * Write global stats struct
     */
    rc = fwrite(&globalStats, sizeof(globalStats), 1, fpout);
    (void) rc;                  /* we'll check for error with ferror */

    /*
     * Write archiver stats struct
     */
    rc = fwrite(&archiverStats, sizeof(archiverStats), 1, fpout);
    (void) rc;                  /* we'll check for error with ferror */

    /*
     * Walk through the database table.
     */
    hash_seq_init(&hstat, pgStatDBHash);
    while ((dbentry = (PgStat_StatDBEntry *) hash_seq_search(&hstat)) != NULL)
    {
        /*
         * Write out the table and function stats for this DB into the
         * appropriate per-DB stat file, if required.
         */
        if (allDbs || pgstat_db_requested(dbentry->databaseid))
        {
            /* Make DB's timestamp consistent with the global stats */
            dbentry->stats_timestamp = globalStats.stats_timestamp;

            pgstat_write_db_statsfile(dbentry, permanent);
        }

        /*
         * Write out the DB entry. We don't write the tables or functions
         * pointers, since they're of no use to any other process.
         */
        fputc('D', fpout);
        rc = fwrite(dbentry, offsetof(PgStat_StatDBEntry, tables), 1, fpout);
        (void) rc;              /* we'll check for error with ferror */
    }

    /*
     * 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 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);
    }

    if (permanent)
        unlink(pgstat_stat_filename);

    /*
     * Now throw away the list of requests.  Note that requests sent after we
     * started the write are still waiting on the network socket.
     */
    list_free(pending_write_requests);
    pending_write_requests = NIL;
}

/*
 * return the filename for a DB stat file; filename is the output buffer,
 * of length len.
 */
static void
get_dbstat_filename(bool permanent, bool tempname, Oid databaseid,
                    char *filename, int len)
{
    int         printed;

    /* NB -- pgstat_reset_remove_files knows about the pattern this uses */
    printed = snprintf(filename, len, "%s/db_%u.%s",
                       permanent ? PGSTAT_STAT_PERMANENT_DIRECTORY :
                       pgstat_stat_directory,
                       databaseid,
                       tempname ? "tmp" : "stat");
    if (printed > len)
        elog(ERROR, "overlength pgstat path");
}

/* ----------
 * pgstat_write_db_statsfile() -
 *      Write the stat file for a single database.
 *
 *  If writing to the permanent file (happens when the collector is
 *  shutting down only), remove the temporary file so that backends
 *  starting up under a new postmaster can't read the old data before
 *  the new collector is ready.
 * ----------
 */
static void
pgstat_write_db_statsfile(PgStat_StatDBEntry *dbentry, bool permanent)
{
    HASH_SEQ_STATUS tstat;
    HASH_SEQ_STATUS fstat;
    PgStat_StatTabEntry *tabentry;
    PgStat_StatFuncEntry *funcentry;
    FILE       *fpout;
    int32       format_id;
    Oid         dbid = dbentry->databaseid;
    int         rc;
    char        tmpfile[MAXPGPATH];
    char        statfile[MAXPGPATH];

    get_dbstat_filename(permanent, true, dbid, tmpfile, MAXPGPATH);
    get_dbstat_filename(permanent, false, dbid, statfile, MAXPGPATH);

    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;
    rc = fwrite(&format_id, sizeof(format_id), 1, fpout);
    (void) rc;                  /* we'll check for error with ferror */

    /*
     * Walk through the database's access stats per table.
     */
    hash_seq_init(&tstat, dbentry->tables);
    while ((tabentry = (PgStat_StatTabEntry *) hash_seq_search(&tstat)) != NULL)
    {
        fputc('T', fpout);
        rc = fwrite(tabentry, sizeof(PgStat_StatTabEntry), 1, fpout);
        (void) rc;              /* we'll check for error with ferror */
    }

    /*
     * Walk through the database's function stats table.
     */
    hash_seq_init(&fstat, dbentry->functions);
    while ((funcentry = (PgStat_StatFuncEntry *) hash_seq_search(&fstat)) != NULL)
    {
        fputc('F', fpout);
        rc = fwrite(funcentry, sizeof(PgStat_StatFuncEntry), 1, fpout);
        (void) rc;              /* we'll check for error with ferror */
    }

    /*
     * 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 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);
    }

    if (permanent)
    {
        get_dbstat_filename(false, false, dbid, statfile, MAXPGPATH);

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

/* ----------
 * pgstat_read_statsfiles() -
 *
 *  Reads in some existing statistics collector files and returns the
 *  databases hash table that is the top level of the data.
 *
 *  If 'onlydb' is not InvalidOid, it means we only want data for that DB
 *  plus the shared catalogs ("DB 0").  We'll still populate the DB hash
 *  table for all databases, but we don't bother even creating table/function
 *  hash tables for other databases.
 *
 *  'permanent' specifies reading from the permanent files not temporary ones.
 *  When true (happens only when the collector is starting up), remove the
 *  files after reading; the in-memory status is now authoritative, and the
 *  files would be out of date in case somebody else reads them.
 *
 *  If a 'deep' read is requested, table/function stats are read, otherwise
 *  the table/function hash tables remain empty.
 * ----------
 */
static HTAB *
pgstat_read_statsfiles(Oid onlydb, bool permanent, bool deep)
{
    PgStat_StatDBEntry *dbentry;
    PgStat_StatDBEntry dbbuf;
    HASHCTL     hash_ctl;
    HTAB       *dbhash;
    FILE       *fpin;
    int32       format_id;
    bool        found;
    const char *statfile = permanent ? PGSTAT_STAT_PERMANENT_FILENAME : pgstat_stat_filename;

    /*
     * The tables will live in pgStatLocalContext.
     */
    pgstat_setup_memcxt();

    /*
     * Create the DB hashtable
     */
    memset(&hash_ctl, 0, sizeof(hash_ctl));
    hash_ctl.keysize = sizeof(Oid);
    hash_ctl.entrysize = sizeof(PgStat_StatDBEntry);
    hash_ctl.hcxt = pgStatLocalContext;
    dbhash = hash_create("Databases hash", PGSTAT_DB_HASH_SIZE, &hash_ctl,
                         HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);

    /*
     * Clear out global and archiver statistics so they start from zero in
     * case we can't load an existing statsfile.
     */
    memset(&globalStats, 0, sizeof(globalStats));
    memset(&archiverStats, 0, sizeof(archiverStats));

    /*
     * Set the current timestamp (will be kept only in case we can't load an
     * existing statsfile).
     */
    globalStats.stat_reset_timestamp = GetCurrentTimestamp();
    archiverStats.stat_reset_timestamp = globalStats.stat_reset_timestamp;

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

    /*
     * Verify it's of the expected format.
     */
    if (fread(&format_id, 1, sizeof(format_id), fpin) != sizeof(format_id) ||
        format_id != PGSTAT_FILE_FORMAT_ID)
    {
        ereport(pgStatRunningInCollector ? LOG : WARNING,
                (errmsg("corrupted statistics file \"%s\"", statfile)));
        goto done;
    }

    /*
     * Read global stats struct
     */
    if (fread(&globalStats, 1, sizeof(globalStats), fpin) != sizeof(globalStats))
    {
        ereport(pgStatRunningInCollector ? LOG : WARNING,
                (errmsg("corrupted statistics file \"%s\"", statfile)));
        memset(&globalStats, 0, sizeof(globalStats));
        goto done;
    }

    /*
     * In the collector, disregard the timestamp we read from the permanent
     * stats file; we should be willing to write a temp stats file immediately
     * upon the first request from any backend.  This only matters if the old
     * file's timestamp is less than PGSTAT_STAT_INTERVAL ago, but that's not
     * an unusual scenario.
     */
    if (pgStatRunningInCollector)
        globalStats.stats_timestamp = 0;

    /*
     * Read archiver stats struct
     */
    if (fread(&archiverStats, 1, sizeof(archiverStats), fpin) != sizeof(archiverStats))
    {
        ereport(pgStatRunningInCollector ? LOG : WARNING,
                (errmsg("corrupted statistics file \"%s\"", statfile)));
        memset(&archiverStats, 0, sizeof(archiverStats));
        goto done;
    }

    /*
     * We found an existing collector stats file. Read it and put all the
     * hashtable entries into place.
     */
    for (;;)
    {
        switch (fgetc(fpin))
        {
                /*
                 * 'D'  A PgStat_StatDBEntry struct describing a database
                 * follows.
                 */
            case 'D':
                if (fread(&dbbuf, 1, offsetof(PgStat_StatDBEntry, tables),
                          fpin) != offsetof(PgStat_StatDBEntry, tables))
                {
                    ereport(pgStatRunningInCollector ? LOG : WARNING,
                            (errmsg("corrupted statistics file \"%s\"",
                                    statfile)));
                    goto done;
                }

                /*
                 * Add to the DB hash
                 */
                dbentry = (PgStat_StatDBEntry *) hash_search(dbhash,
                                                             (void *) &dbbuf.databaseid,
                                                             HASH_ENTER,
                                                             &found);
                if (found)
                {
                    ereport(pgStatRunningInCollector ? LOG : WARNING,
                            (errmsg("corrupted statistics file \"%s\"",
                                    statfile)));
                    goto done;
                }

                memcpy(dbentry, &dbbuf, sizeof(PgStat_StatDBEntry));
                dbentry->tables = NULL;
                dbentry->functions = NULL;

                /*
                 * In the collector, disregard the timestamp we read from the
                 * permanent stats file; we should be willing to write a temp
                 * stats file immediately upon the first request from any
                 * backend.
                 */
                if (pgStatRunningInCollector)
                    dbentry->stats_timestamp = 0;

                /*
                 * Don't create tables/functions hashtables for uninteresting
                 * databases.
                 */
                if (onlydb != InvalidOid)
                {
                    if (dbbuf.databaseid != onlydb &&
                        dbbuf.databaseid != InvalidOid)
                        break;
                }

                memset(&hash_ctl, 0, sizeof(hash_ctl));
                hash_ctl.keysize = sizeof(Oid);
                hash_ctl.entrysize = sizeof(PgStat_StatTabEntry);
                hash_ctl.hcxt = pgStatLocalContext;
                dbentry->tables = hash_create("Per-database table",
                                              PGSTAT_TAB_HASH_SIZE,
                                              &hash_ctl,
                                              HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);

                hash_ctl.keysize = sizeof(Oid);
                hash_ctl.entrysize = sizeof(PgStat_StatFuncEntry);
                hash_ctl.hcxt = pgStatLocalContext;
                dbentry->functions = hash_create("Per-database function",
                                                 PGSTAT_FUNCTION_HASH_SIZE,
                                                 &hash_ctl,
                                                 HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);

                /*
                 * If requested, read the data from the database-specific
                 * file.  Otherwise we just leave the hashtables empty.
                 */
                if (deep)
                    pgstat_read_db_statsfile(dbentry->databaseid,
                                             dbentry->tables,
                                             dbentry->functions,
                                             permanent);

                break;

            case 'E':
                goto done;

            default:
                ereport(pgStatRunningInCollector ? LOG : WARNING,
                        (errmsg("corrupted statistics file \"%s\"",
                                statfile)));
                goto done;
        }
    }

done:
    FreeFile(fpin);

    /* If requested to read the permanent file, also get rid of it. */
    if (permanent)
    {
        elog(DEBUG2, "removing permanent stats file \"%s\"", statfile);
        unlink(statfile);
    }

    return dbhash;
}


/* ----------
 * pgstat_read_db_statsfile() -
 *
 *  Reads in the existing statistics collector file for the given database,
 *  filling the passed-in tables and functions hash tables.
 *
 *  As in pgstat_read_statsfiles, if the permanent file is requested, it is
 *  removed after reading.
 *
 *  Note: this code has the ability to skip storing per-table or per-function
 *  data, if NULL is passed for the corresponding hashtable.  That's not used
 *  at the moment though.
 * ----------
 */
static void
pgstat_read_db_statsfile(Oid databaseid, HTAB *tabhash, HTAB *funchash,
                         bool permanent)
{
    PgStat_StatTabEntry *tabentry;
    PgStat_StatTabEntry tabbuf;
    PgStat_StatFuncEntry funcbuf;
    PgStat_StatFuncEntry *funcentry;
    FILE       *fpin;
    int32       format_id;
    bool        found;
    char        statfile[MAXPGPATH];

    get_dbstat_filename(permanent, false, databaseid, statfile, MAXPGPATH);

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

    /*
     * Verify it's of the expected format.
     */
    if (fread(&format_id, 1, sizeof(format_id), fpin) != sizeof(format_id) ||
        format_id != PGSTAT_FILE_FORMAT_ID)
    {
        ereport(pgStatRunningInCollector ? LOG : WARNING,
                (errmsg("corrupted statistics file \"%s\"", statfile)));
        goto done;
    }

    /*
     * We found an existing collector stats file. Read it and put all the
     * hashtable entries into place.
     */
    for (;;)
    {
        switch (fgetc(fpin))
        {
                /*
                 * 'T'  A PgStat_StatTabEntry follows.
                 */
            case 'T':
                if (fread(&tabbuf, 1, sizeof(PgStat_StatTabEntry),
                          fpin) != sizeof(PgStat_StatTabEntry))
                {
                    ereport(pgStatRunningInCollector ? LOG : WARNING,
                            (errmsg("corrupted statistics file \"%s\"",
                                    statfile)));
                    goto done;
                }

                /*
                 * Skip if table data not wanted.
                 */
                if (tabhash == NULL)
                    break;

                tabentry = (PgStat_StatTabEntry *) hash_search(tabhash,
                                                               (void *) &tabbuf.tableid,
                                                               HASH_ENTER, &found);

                if (found)
                {
                    ereport(pgStatRunningInCollector ? LOG : WARNING,
                            (errmsg("corrupted statistics file \"%s\"",
                                    statfile)));
                    goto done;
                }

                memcpy(tabentry, &tabbuf, sizeof(tabbuf));
                break;

                /*
                 * 'F'  A PgStat_StatFuncEntry follows.
                 */
            case 'F':
                if (fread(&funcbuf, 1, sizeof(PgStat_StatFuncEntry),
                          fpin) != sizeof(PgStat_StatFuncEntry))
                {
                    ereport(pgStatRunningInCollector ? LOG : WARNING,
                            (errmsg("corrupted statistics file \"%s\"",
                                    statfile)));
                    goto done;
                }

                /*
                 * Skip if function data not wanted.
                 */
                if (funchash == NULL)
                    break;

                funcentry = (PgStat_StatFuncEntry *) hash_search(funchash,
                                                                 (void *) &funcbuf.functionid,
                                                                 HASH_ENTER, &found);

                if (found)
                {
                    ereport(pgStatRunningInCollector ? LOG : WARNING,
                            (errmsg("corrupted statistics file \"%s\"",
                                    statfile)));
                    goto done;
                }

                memcpy(funcentry, &funcbuf, sizeof(funcbuf));
                break;

                /*
                 * 'E'  The EOF marker of a complete stats file.
                 */
            case 'E':
                goto done;

            default:
                ereport(pgStatRunningInCollector ? LOG : WARNING,
                        (errmsg("corrupted statistics file \"%s\"",
                                statfile)));
                goto done;
        }
    }

done:
    FreeFile(fpin);

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

/* ----------
 * pgstat_read_db_statsfile_timestamp() -
 *
 *  Attempt to determine the timestamp of the last db statfile write.
 *  Returns TRUE if successful; the timestamp is stored in *ts.
 *
 *  This needs to be careful about handling databases for which no stats file
 *  exists, such as databases without a stat entry or those not yet written:
 *
 *  - if there's a database entry in the global file, return the corresponding
 *  stats_timestamp value.
 *
 *  - if there's no db stat entry (e.g. for a new or inactive database),
 *  there's no stats_timestamp value, but also nothing to write so we return
 *  the timestamp of the global statfile.
 * ----------
 */
static bool
pgstat_read_db_statsfile_timestamp(Oid databaseid, bool permanent,
                                   TimestampTz *ts)
{
    PgStat_StatDBEntry dbentry;
    PgStat_GlobalStats myGlobalStats;
    PgStat_ArchiverStats myArchiverStats;
    FILE       *fpin;
    int32       format_id;
    const char *statfile = permanent ? PGSTAT_STAT_PERMANENT_FILENAME : pgstat_stat_filename;

    /*
     * Try to open the stats file.  As above, anything but ENOENT is worthy of
     * complaining about.
     */
    if ((fpin = AllocateFile(statfile, PG_BINARY_R)) == NULL)
    {
        if (errno != ENOENT)
            ereport(pgStatRunningInCollector ? LOG : WARNING,
                    (errcode_for_file_access(),
                     errmsg("could not open statistics file \"%s\": %m",
                            statfile)));
        return false;
    }

    /*
     * Verify it's of the expected format.
     */
    if (fread(&format_id, 1, sizeof(format_id), fpin) != sizeof(format_id) ||
        format_id != PGSTAT_FILE_FORMAT_ID)
    {
        ereport(pgStatRunningInCollector ? LOG : WARNING,
                (errmsg("corrupted statistics file \"%s\"", statfile)));
        FreeFile(fpin);
        return false;
    }

    /*
     * Read global stats struct
     */
    if (fread(&myGlobalStats, 1, sizeof(myGlobalStats),
              fpin) != sizeof(myGlobalStats))
    {
        ereport(pgStatRunningInCollector ? LOG : WARNING,
                (errmsg("corrupted statistics file \"%s\"", statfile)));
        FreeFile(fpin);
        return false;
    }

    /*
     * Read archiver stats struct
     */
    if (fread(&myArchiverStats, 1, sizeof(myArchiverStats),
              fpin) != sizeof(myArchiverStats))
    {
        ereport(pgStatRunningInCollector ? LOG : WARNING,
                (errmsg("corrupted statistics file \"%s\"", statfile)));
        FreeFile(fpin);
        return false;
    }

    /* By default, we're going to return the timestamp of the global file. */
    *ts = myGlobalStats.stats_timestamp;

    /*
     * We found an existing collector stats file.  Read it and look for a
     * record for the requested database.  If found, use its timestamp.
     */
    for (;;)
    {
        switch (fgetc(fpin))
        {
                /*
                 * 'D'  A PgStat_StatDBEntry struct describing a database
                 * follows.
                 */
            case 'D':
                if (fread(&dbentry, 1, offsetof(PgStat_StatDBEntry, tables),
                          fpin) != offsetof(PgStat_StatDBEntry, tables))
                {
                    ereport(pgStatRunningInCollector ? LOG : WARNING,
                            (errmsg("corrupted statistics file \"%s\"",
                                    statfile)));
                    goto done;
                }

                /*
                 * If this is the DB we're looking for, save its timestamp and
                 * we're done.
                 */
                if (dbentry.databaseid == databaseid)
                {
                    *ts = dbentry.stats_timestamp;
                    goto done;
                }

                break;

            case 'E':
                goto done;

            default:
                ereport(pgStatRunningInCollector ? LOG : WARNING,
                        (errmsg("corrupted statistics file \"%s\"",
                                statfile)));
                goto done;
        }
    }

done:
    FreeFile(fpin);
    return true;
}

/*
 * If not already done, read the statistics collector stats file into
 * some hash tables.  The results will be kept until pgstat_clear_snapshot()
 * is called (typically, at end of transaction).
 */
static void
backend_read_statsfile(void)
{
    TimestampTz min_ts = 0;
    TimestampTz ref_ts = 0;
    Oid         inquiry_db;
    int         count;

    /* already read it? */
    if (pgStatDBHash)
        return;
    Assert(!pgStatRunningInCollector);

    /*
     * In a normal backend, we check staleness of the data for our own DB, and
     * so we send MyDatabaseId in inquiry messages.  In the autovac launcher,
     * check staleness of the shared-catalog data, and send InvalidOid in
     * inquiry messages so as not to force writing unnecessary data.
     */
    if (IsAutoVacuumLauncherProcess())
        inquiry_db = InvalidOid;
    else
        inquiry_db = MyDatabaseId;

    /*
     * Loop until fresh enough stats file is available or we ran out of time.
     * The stats inquiry message is sent repeatedly in case collector drops
     * it; but not every single time, as that just swamps the collector.
     */
    for (count = 0; count < PGSTAT_POLL_LOOP_COUNT; count++)
    {
        bool        ok;
        TimestampTz file_ts = 0;
        TimestampTz cur_ts;

        CHECK_FOR_INTERRUPTS();

        ok = pgstat_read_db_statsfile_timestamp(inquiry_db, false, &file_ts);

        cur_ts = GetCurrentTimestamp();
        /* Calculate min acceptable timestamp, if we didn't already */
        if (count == 0 || cur_ts < ref_ts)
        {
            /*
             * We set the minimum acceptable timestamp to PGSTAT_STAT_INTERVAL
             * msec before now.  This indirectly ensures that the collector
             * needn't write the file more often than PGSTAT_STAT_INTERVAL. In
             * an autovacuum worker, however, we want a lower delay to avoid
             * using stale data, so we use PGSTAT_RETRY_DELAY (since the
             * number of workers is low, this shouldn't be a problem).
             *
             * We don't recompute min_ts after sleeping, except in the
             * unlikely case that cur_ts went backwards.  So we might end up
             * accepting a file a bit older than PGSTAT_STAT_INTERVAL.  In
             * practice that shouldn't happen, though, as long as the sleep
             * time is less than PGSTAT_STAT_INTERVAL; and we don't want to
             * tell the collector that our cutoff time is less than what we'd
             * actually accept.
             */
            ref_ts = cur_ts;
            if (IsAutoVacuumWorkerProcess())
                min_ts = TimestampTzPlusMilliseconds(ref_ts,
                                                     -PGSTAT_RETRY_DELAY);
            else
                min_ts = TimestampTzPlusMilliseconds(ref_ts,
                                                     -PGSTAT_STAT_INTERVAL);
        }

        /*
         * If the file timestamp is actually newer than cur_ts, we must have
         * had a clock glitch (system time went backwards) or there is clock
         * skew between our processor and the stats collector's processor.
         * Accept the file, but send an inquiry message anyway to make
         * pgstat_recv_inquiry do a sanity check on the collector's time.
         */
        if (ok && file_ts > cur_ts)
        {
            /*
             * A small amount of clock skew between processors isn't terribly
             * surprising, but a large difference is worth logging.  We
             * arbitrarily define "large" as 1000 msec.
             */
            if (file_ts >= TimestampTzPlusMilliseconds(cur_ts, 1000))
            {
                char       *filetime;
                char       *mytime;

                /* Copy because timestamptz_to_str returns a static buffer */
                filetime = pstrdup(timestamptz_to_str(file_ts));
                mytime = pstrdup(timestamptz_to_str(cur_ts));
                elog(LOG, "stats collector's time %s is later than backend local time %s",
                     filetime, mytime);
                pfree(filetime);
                pfree(mytime);
            }

            pgstat_send_inquiry(cur_ts, min_ts, inquiry_db);
            break;
        }

        /* Normal acceptance case: file is not older than cutoff time */
        if (ok && file_ts >= min_ts)
            break;

        /* Not there or too old, so kick the collector and wait a bit */
        if ((count % PGSTAT_INQ_LOOP_COUNT) == 0)
            pgstat_send_inquiry(cur_ts, min_ts, inquiry_db);

        pg_usleep(PGSTAT_RETRY_DELAY * 1000L);
    }

    if (count >= PGSTAT_POLL_LOOP_COUNT)
        ereport(LOG,
                (errmsg("using stale statistics instead of current ones "
                        "because stats collector is not responding")));

    /*
     * Autovacuum launcher wants stats about all databases, but a shallow read
     * is sufficient.  Regular backends want a deep read for just the tables
     * they can see (MyDatabaseId + shared catalogs).
     */
    if (IsAutoVacuumLauncherProcess())
        pgStatDBHash = pgstat_read_statsfiles(InvalidOid, false, false);
    else
        pgStatDBHash = pgstat_read_statsfiles(MyDatabaseId, false, true);
}


/* ----------
 * pgstat_setup_memcxt() -
 *
 *  Create pgStatLocalContext, if not already done.
 * ----------
 */
static void
pgstat_setup_memcxt(void)
{
    if (!pgStatLocalContext)
        pgStatLocalContext = AllocSetContextCreate(TopMemoryContext,
                                                   "Statistics snapshot",
                                                   ALLOCSET_SMALL_SIZES);
}


/* ----------
 * pgstat_clear_snapshot() -
 *
 *  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.
 * ----------
 */
void
pgstat_clear_snapshot(void)
{
    /* Release memory, if any was allocated */
    if (pgStatLocalContext)
        MemoryContextDelete(pgStatLocalContext);

    /* Reset variables */
    pgStatLocalContext = NULL;
    pgStatDBHash = NULL;
    localBackendStatusTable = NULL;
    localNumBackends = 0;
}


/* ----------
 * pgstat_recv_inquiry() -
 *
 *  Process stat inquiry requests.
 * ----------
 */
static void
pgstat_recv_inquiry(PgStat_MsgInquiry *msg, int len)
{
    PgStat_StatDBEntry *dbentry;

    elog(DEBUG2, "received inquiry for database %u", msg->databaseid);

    /*
     * If there's already a write request for this DB, there's nothing to do.
     *
     * Note that if a request is found, we return early and skip the below
     * check for clock skew.  This is okay, since the only way for a DB
     * request to be present in the list is that we have been here since the
     * last write round.  It seems sufficient to check for clock skew once per
     * write round.
     */
    if (list_member_oid(pending_write_requests, msg->databaseid))
        return;

    /*
     * Check to see if we last wrote this database at a time >= the requested
     * cutoff time.  If so, this is a stale request that was generated before
     * we updated the DB file, and we don't need to do so again.
     *
     * If the requestor's local clock time is older than stats_timestamp, we
     * should suspect a clock glitch, ie system time going backwards; though
     * the more likely explanation is just delayed message receipt.  It is
     * worth expending a GetCurrentTimestamp call to be sure, since a large
     * retreat in the system clock reading could otherwise cause us to neglect
     * to update the stats file for a long time.
     */
    dbentry = pgstat_get_db_entry(msg->databaseid, false);
    if (dbentry == NULL)
    {
        /*
         * We have no data for this DB.  Enter a write request anyway so that
         * the global stats will get updated.  This is needed to prevent
         * backend_read_statsfile from waiting for data that we cannot supply,
         * in the case of a new DB that nobody has yet reported any stats for.
         * See the behavior of pgstat_read_db_statsfile_timestamp.
         */
    }
    else if (msg->clock_time < dbentry->stats_timestamp)
    {
        TimestampTz cur_ts = GetCurrentTimestamp();

        if (cur_ts < dbentry->stats_timestamp)
        {
            /*
             * Sure enough, time went backwards.  Force a new stats file write
             * to get back in sync; but first, log a complaint.
             */
            char       *writetime;
            char       *mytime;

            /* Copy because timestamptz_to_str returns a static buffer */
            writetime = pstrdup(timestamptz_to_str(dbentry->stats_timestamp));
            mytime = pstrdup(timestamptz_to_str(cur_ts));
            elog(LOG,
                 "stats_timestamp %s is later than collector's time %s for database %u",
                 writetime, mytime, dbentry->databaseid);
            pfree(writetime);
            pfree(mytime);
        }
        else
        {
            /*
             * Nope, it's just an old request.  Assuming msg's clock_time is
             * >= its cutoff_time, it must be stale, so we can ignore it.
             */
            return;
        }
    }
    else if (msg->cutoff_time <= dbentry->stats_timestamp)
    {
        /* Stale request, ignore it */
        return;
    }

    /*
     * We need to write this DB, so create a request.
     */
    pending_write_requests = lappend_oid(pending_write_requests,
                                         msg->databaseid);
}


/* ----------
 * pgstat_recv_tabstat() -
 *
 *  Count what the backend has done.
 * ----------
 */
static void
pgstat_recv_tabstat(PgStat_MsgTabstat *msg, int len)
{
    PgStat_StatDBEntry *dbentry;
    PgStat_StatTabEntry *tabentry;
    int         i;
    bool        found;

    dbentry = pgstat_get_db_entry(msg->m_databaseid, true);

    /*
     * Update database-wide stats.
     */
    dbentry->n_xact_commit += (PgStat_Counter) (msg->m_xact_commit);
    dbentry->n_xact_rollback += (PgStat_Counter) (msg->m_xact_rollback);
    dbentry->n_block_read_time += msg->m_block_read_time;
    dbentry->n_block_write_time += msg->m_block_write_time;

    /*
     * Process all table entries in the message.
     */
    for (i = 0; i < msg->m_nentries; i++)
    {
        PgStat_TableEntry *tabmsg = &(msg->m_entry[i]);

        tabentry = (PgStat_StatTabEntry *) hash_search(dbentry->tables,
                                                       (void *) &(tabmsg->t_id),
                                                       HASH_ENTER, &found);

        if (!found)
        {
            /*
             * If it's a new table entry, initialize counters to the values we
             * just got.
             */
            tabentry->numscans = tabmsg->t_counts.t_numscans;
            tabentry->tuples_returned = tabmsg->t_counts.t_tuples_returned;
            tabentry->tuples_fetched = tabmsg->t_counts.t_tuples_fetched;
            tabentry->tuples_inserted = tabmsg->t_counts.t_tuples_inserted;
            tabentry->tuples_updated = tabmsg->t_counts.t_tuples_updated;
            tabentry->tuples_deleted = tabmsg->t_counts.t_tuples_deleted;
            tabentry->tuples_hot_updated = tabmsg->t_counts.t_tuples_hot_updated;
            tabentry->n_live_tuples = tabmsg->t_counts.t_delta_live_tuples;
            tabentry->n_dead_tuples = tabmsg->t_counts.t_delta_dead_tuples;
            tabentry->changes_since_analyze = tabmsg->t_counts.t_changed_tuples;
            tabentry->blocks_fetched = tabmsg->t_counts.t_blocks_fetched;
            tabentry->blocks_hit = tabmsg->t_counts.t_blocks_hit;

            tabentry->vacuum_timestamp = 0;
            tabentry->vacuum_count = 0;
            tabentry->autovac_vacuum_timestamp = 0;
            tabentry->autovac_vacuum_count = 0;
            tabentry->analyze_timestamp = 0;
            tabentry->analyze_count = 0;
            tabentry->autovac_analyze_timestamp = 0;
            tabentry->autovac_analyze_count = 0;
        }
        else
        {
            /*
             * Otherwise add the values to the existing entry.
             */
            tabentry->numscans += tabmsg->t_counts.t_numscans;
            tabentry->tuples_returned += tabmsg->t_counts.t_tuples_returned;
            tabentry->tuples_fetched += tabmsg->t_counts.t_tuples_fetched;
            tabentry->tuples_inserted += tabmsg->t_counts.t_tuples_inserted;
            tabentry->tuples_updated += tabmsg->t_counts.t_tuples_updated;
            tabentry->tuples_deleted += tabmsg->t_counts.t_tuples_deleted;
            tabentry->tuples_hot_updated += tabmsg->t_counts.t_tuples_hot_updated;
            /* If table was truncated, first reset the live/dead counters */
            if (tabmsg->t_counts.t_truncated)
            {
                tabentry->n_live_tuples = 0;
                tabentry->n_dead_tuples = 0;
            }
            tabentry->n_live_tuples += tabmsg->t_counts.t_delta_live_tuples;
            tabentry->n_dead_tuples += tabmsg->t_counts.t_delta_dead_tuples;
            tabentry->changes_since_analyze += tabmsg->t_counts.t_changed_tuples;
            tabentry->blocks_fetched += tabmsg->t_counts.t_blocks_fetched;
            tabentry->blocks_hit += tabmsg->t_counts.t_blocks_hit;
        }

        /* Clamp n_live_tuples in case of negative delta_live_tuples */
        tabentry->n_live_tuples = Max(tabentry->n_live_tuples, 0);
        /* Likewise for n_dead_tuples */
        tabentry->n_dead_tuples = Max(tabentry->n_dead_tuples, 0);

        /*
         * Add per-table stats to the per-database entry, too.
         */
        dbentry->n_tuples_returned += tabmsg->t_counts.t_tuples_returned;
        dbentry->n_tuples_fetched += tabmsg->t_counts.t_tuples_fetched;
        dbentry->n_tuples_inserted += tabmsg->t_counts.t_tuples_inserted;
        dbentry->n_tuples_updated += tabmsg->t_counts.t_tuples_updated;
        dbentry->n_tuples_deleted += tabmsg->t_counts.t_tuples_deleted;
        dbentry->n_blocks_fetched += tabmsg->t_counts.t_blocks_fetched;
        dbentry->n_blocks_hit += tabmsg->t_counts.t_blocks_hit;
    }
}


/* ----------
 * pgstat_recv_tabpurge() -
 *
 *  Arrange for dead table removal.
 * ----------
 */
static void
pgstat_recv_tabpurge(PgStat_MsgTabpurge *msg, int len)
{
    PgStat_StatDBEntry *dbentry;
    int         i;

    dbentry = pgstat_get_db_entry(msg->m_databaseid, false);

    /*
     * No need to purge if we don't even know the database.
     */
    if (!dbentry || !dbentry->tables)
        return;

    /*
     * Process all table entries in the message.
     */
    for (i = 0; i < msg->m_nentries; i++)
    {
        /* Remove from hashtable if present; we don't care if it's not. */
        (void) hash_search(dbentry->tables,
                           (void *) &(msg->m_tableid[i]),
                           HASH_REMOVE, NULL);
    }
}


/* ----------
 * pgstat_recv_dropdb() -
 *
 *  Arrange for dead database removal
 * ----------
 */
static void
pgstat_recv_dropdb(PgStat_MsgDropdb *msg, int len)
{
    Oid         dbid = msg->m_databaseid;
    PgStat_StatDBEntry *dbentry;

    /*
     * Lookup the database in the hashtable.
     */
    dbentry = pgstat_get_db_entry(dbid, false);

    /*
     * If found, remove it (along with the db statfile).
     */
    if (dbentry)
    {
        char        statfile[MAXPGPATH];

        get_dbstat_filename(false, false, dbid, statfile, MAXPGPATH);

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

        if (dbentry->tables != NULL)
            hash_destroy(dbentry->tables);
        if (dbentry->functions != NULL)
            hash_destroy(dbentry->functions);

        if (hash_search(pgStatDBHash,
                        (void *) &dbid,
                        HASH_REMOVE, NULL) == NULL)