backend_status.c

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/* ----------
 * backend_status.c
 *    Backend status reporting infrastructure.
 *
 * Copyright (c) 2001-2023, PostgreSQL Global Development Group
 *
 *
 * IDENTIFICATION
 *    src/backend/utils/activity/backend_status.c
 * ----------
 */
#include "postgres.h"
 
#include "access/xact.h"
#include "libpq/libpq.h"
#include "miscadmin.h"
#include "pg_trace.h"
#include "pgstat.h"
#include "port/atomics.h"       /* for memory barriers */
#include "storage/ipc.h"
#include "storage/proc.h"       /* for MyProc */
#include "storage/sinvaladt.h"
#include "utils/ascii.h"
#include "utils/backend_status.h"
#include "utils/guc.h"          /* for application_name */
#include "utils/memutils.h"
 
 
/* ----------
 * 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;
int         pgstat_track_activity_query_size = 1024;
 
 
/* exposed so that backend_progress.c can access it */
PgBackendStatus *MyBEEntry = NULL;
 
 
static PgBackendStatus *BackendStatusArray = 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
#ifdef ENABLE_GSS
static PgBackendGSSStatus *BackendGssStatusBuffer = NULL;
#endif
 
 
/* Status for backends including auxiliary */
static LocalPgBackendStatus *localBackendStatusTable = NULL;
 
/* Total number of backends including auxiliary */
static int  localNumBackends = 0;
 
static MemoryContext backendStatusSnapContext;
 
 
static void pgstat_beshutdown_hook(int code, Datum arg);
static void pgstat_read_current_status(void);
static void pgstat_setup_backend_status_context(void);
 
 
/*
 * 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
#ifdef ENABLE_GSS
    /* BackendGssStatusBuffer: */
    size = add_size(size,
                    mul_size(sizeof(PgBackendGSSStatus), 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, NumBackendStatSlots);
    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, NumBackendStatSlots);
    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, BackendActivityBufferSize);
 
        /* 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
 
#ifdef ENABLE_GSS
    /* Create or attach to the shared GSSAPI status buffer */
    size = mul_size(sizeof(PgBackendGSSStatus), NumBackendStatSlots);
    BackendGssStatusBuffer = (PgBackendGSSStatus *)
        ShmemInitStruct("Backend GSS Status Buffer", size, &found);
 
    if (!found)
    {
        PgBackendGSSStatus *ptr;
 
        MemSet(BackendGssStatusBuffer, 0, size);
 
        /* Initialize st_gssstatus pointers. */
        ptr = BackendGssStatusBuffer;
        for (i = 0; i < NumBackendStatSlots; i++)
        {
            BackendStatusArray[i].st_gssstatus = ptr;
            ptr++;
        }
    }
#endif
}
 
/*
 * Initialize pgstats backend activity 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_beinit(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_beinit).
 *  Note also that we must be inside a transaction if this isn't an aux
 *  process, as we may need to do encoding conversion on some strings.
 * ----------
 */
void
pgstat_bestart(void)
{
    volatile PgBackendStatus *vbeentry = MyBEEntry;
    PgBackendStatus lbeentry;
#ifdef USE_SSL
    PgBackendSSLStatus lsslstatus;
#endif
#ifdef ENABLE_GSS
    PgBackendGSSStatus lgssstatus;
#endif
 
    /* pgstats state must be initialized from pgstat_beinit() */
    Assert(vbeentry != NULL);
 
    /*
     * To minimize the time spent modifying the PgBackendStatus entry, and
     * avoid risk of errors inside the critical section, we first copy the
     * shared-memory struct to a local variable, then modify the data in the
     * local variable, then copy the local variable back to shared memory.
     * Only the last step has to be inside the critical section.
     *
     * Most of the data we copy from shared memory is just going to be
     * overwritten, but the struct's not so large that it's worth the
     * maintenance hassle to copy only the needful fields.
     */
    memcpy(&lbeentry,
           unvolatize(PgBackendStatus *, vbeentry),
           sizeof(PgBackendStatus));
 
    /* These structs can just start from zeroes each time, though */
#ifdef USE_SSL
    memset(&lsslstatus, 0, sizeof(lsslstatus));
#endif
#ifdef ENABLE_GSS
    memset(&lgssstatus, 0, sizeof(lgssstatus));
#endif
 
    /*
     * Now fill in all the fields of lbeentry, except for strings that are
     * out-of-line data.  Those have to be handled separately, below.
     */
    lbeentry.st_procpid = MyProcPid;
    lbeentry.st_backendType = MyBackendType;
    lbeentry.st_proc_start_timestamp = MyStartTimestamp;
    lbeentry.st_activity_start_timestamp = 0;
    lbeentry.st_state_start_timestamp = 0;
    lbeentry.st_xact_start_timestamp = 0;
    lbeentry.st_databaseid = MyDatabaseId;
 
    /* We have userid for client-backends, wal-sender and bgworker processes */
    if (lbeentry.st_backendType == B_BACKEND
        || lbeentry.st_backendType == B_WAL_SENDER
        || lbeentry.st_backendType == B_BG_WORKER)
        lbeentry.st_userid = GetSessionUserId();
    else
        lbeentry.st_userid = InvalidOid;
 
    /*
     * 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(&lbeentry.st_clientaddr, &MyProcPort->raddr,
               sizeof(lbeentry.st_clientaddr));
    else
        MemSet(&lbeentry.st_clientaddr, 0, sizeof(lbeentry.st_clientaddr));
 
#ifdef USE_SSL
    if (MyProcPort && MyProcPort->ssl_in_use)
    {
        lbeentry.st_ssl = true;
        lsslstatus.ssl_bits = be_tls_get_cipher_bits(MyProcPort);
        strlcpy(lsslstatus.ssl_version, be_tls_get_version(MyProcPort), NAMEDATALEN);
        strlcpy(lsslstatus.ssl_cipher, be_tls_get_cipher(MyProcPort), NAMEDATALEN);
        be_tls_get_peer_subject_name(MyProcPort, lsslstatus.ssl_client_dn, NAMEDATALEN);
        be_tls_get_peer_serial(MyProcPort, lsslstatus.ssl_client_serial, NAMEDATALEN);
        be_tls_get_peer_issuer_name(MyProcPort, lsslstatus.ssl_issuer_dn, NAMEDATALEN);
    }
    else
    {
        lbeentry.st_ssl = false;
    }
#else
    lbeentry.st_ssl = false;
#endif
 
#ifdef ENABLE_GSS
    if (MyProcPort && MyProcPort->gss != NULL)
    {
        const char *princ = be_gssapi_get_princ(MyProcPort);
 
        lbeentry.st_gss = true;
        lgssstatus.gss_auth = be_gssapi_get_auth(MyProcPort);
        lgssstatus.gss_enc = be_gssapi_get_enc(MyProcPort);
        lgssstatus.gss_delegation = be_gssapi_get_delegation(MyProcPort);
        if (princ)
            strlcpy(lgssstatus.gss_princ, princ, NAMEDATALEN);
    }
    else
    {
        lbeentry.st_gss = false;
    }
#else
    lbeentry.st_gss = false;
#endif
 
    lbeentry.st_state = STATE_UNDEFINED;
    lbeentry.st_progress_command = PROGRESS_COMMAND_INVALID;
    lbeentry.st_progress_command_target = InvalidOid;
    lbeentry.st_query_id = UINT64CONST(0);
 
    /*
     * 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
     */
 
    /*
     * We're ready to enter the critical section that fills the shared-memory
     * status entry.  We follow 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.
     */
    PGSTAT_BEGIN_WRITE_ACTIVITY(vbeentry);
 
    /* make sure we'll memcpy the same st_changecount back */
    lbeentry.st_changecount = vbeentry->st_changecount;
 
    memcpy(unvolatize(PgBackendStatus *, vbeentry),
           &lbeentry,
           sizeof(PgBackendStatus));
 
    /*
     * We can write the out-of-line strings and structs using the pointers
     * that are in lbeentry; this saves some de-volatilizing messiness.
     */
    lbeentry.st_appname[0] = '\0';
    if (MyProcPort && MyProcPort->remote_hostname)
        strlcpy(lbeentry.st_clienthostname, MyProcPort->remote_hostname,
                NAMEDATALEN);
    else
        lbeentry.st_clienthostname[0] = '\0';
    lbeentry.st_activity_raw[0] = '\0';
    /* Also make sure the last byte in each string area is always 0 */
    lbeentry.st_appname[NAMEDATALEN - 1] = '\0';
    lbeentry.st_clienthostname[NAMEDATALEN - 1] = '\0';
    lbeentry.st_activity_raw[pgstat_track_activity_query_size - 1] = '\0';
 
#ifdef USE_SSL
    memcpy(lbeentry.st_sslstatus, &lsslstatus, sizeof(PgBackendSSLStatus));
#endif
#ifdef ENABLE_GSS
    memcpy(lbeentry.st_gssstatus, &lgssstatus, sizeof(PgBackendGSSStatus));
#endif
 
    PGSTAT_END_WRITE_ACTIVITY(vbeentry);
 
    /* Update app name to current GUC setting */
    if (application_name)
        pgstat_report_appname(application_name);
}
 
/*
 * Clear out our entry in the PgBackendStatus array.
 */
static void
pgstat_beshutdown_hook(int code, Datum arg)
{
    volatile PgBackendStatus *beentry = MyBEEntry;
 
    /*
     * 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_BEGIN_WRITE_ACTIVITY(beentry);
 
    beentry->st_procpid = 0;    /* mark invalid */
 
    PGSTAT_END_WRITE_ACTIVITY(beentry);
 
    /* so that functions can check if backend_status.c is up via MyBEEntry */
    MyBEEntry = NULL;
}
 
/*
 * 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_backend_activity_snapshot(void)
{
    /* Release memory, if any was allocated */
    if (backendStatusSnapContext)
    {
        MemoryContextDelete(backendStatusSnapContext);
        backendStatusSnapContext = NULL;
    }
 
    /* Reset variables */
    localBackendStatusTable = NULL;
    localNumBackends = 0;
}
 
static void
pgstat_setup_backend_status_context(void)
{
    if (!backendStatusSnapContext)
        backendStatusSnapContext = AllocSetContextCreate(TopMemoryContext,
                                                         "Backend Status Snapshot",
                                                         ALLOCSET_SMALL_SIZES);
}
 
 
/* ----------
 * 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_BEGIN_WRITE_ACTIVITY(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;
            beentry->st_query_id = UINT64CONST(0);
            proc->wait_event_info = 0;
            PGSTAT_END_WRITE_ACTIVITY(beentry);
        }
        return;
    }
 
    /*
     * To minimize the time spent modifying the entry, and avoid risk of
     * errors inside the critical section, 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();
 
    /*
     * If the state has changed from "active" or "idle in transaction",
     * calculate the duration.
     */
    if ((beentry->st_state == STATE_RUNNING ||
         beentry->st_state == STATE_FASTPATH ||
         beentry->st_state == STATE_IDLEINTRANSACTION ||
         beentry->st_state == STATE_IDLEINTRANSACTION_ABORTED) &&
        state != beentry->st_state)
    {
        long        secs;
        int         usecs;
 
        TimestampDifference(beentry->st_state_start_timestamp,
                            current_timestamp,
                            &secs, &usecs);
 
        if (beentry->st_state == STATE_RUNNING ||
            beentry->st_state == STATE_FASTPATH)
            pgstat_count_conn_active_time((PgStat_Counter) secs * 1000000 + usecs);
        else
            pgstat_count_conn_txn_idle_time((PgStat_Counter) secs * 1000000 + usecs);
    }
 
    /*
     * Now update the status entry
     */
    PGSTAT_BEGIN_WRITE_ACTIVITY(beentry);
 
    beentry->st_state = state;
    beentry->st_state_start_timestamp = current_timestamp;
 
    /*
     * If a new query is started, we reset the query identifier as it'll only
     * be known after parse analysis, to avoid reporting last query's
     * identifier.
     */
    if (state == STATE_RUNNING)
        beentry->st_query_id = UINT64CONST(0);
 
    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_END_WRITE_ACTIVITY(beentry);
}
 
/* --------
 * pgstat_report_query_id() -
 *
 * Called to update top-level query identifier.
 * --------
 */
void
pgstat_report_query_id(uint64 query_id, bool force)
{
    volatile PgBackendStatus *beentry = MyBEEntry;
 
    /*
     * if track_activities is disabled, st_query_id should already have been
     * reset
     */
    if (!beentry || !pgstat_track_activities)
        return;
 
    /*
     * We only report the top-level query identifiers.  The stored query_id is
     * reset when a backend calls pgstat_report_activity(STATE_RUNNING), or
     * with an explicit call to this function using the force flag.  If the
     * saved query identifier is not zero it means that it's not a top-level
     * command, so ignore the one provided unless it's an explicit call to
     * reset the identifier.
     */
    if (beentry->st_query_id != 0 && !force)
        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_BEGIN_WRITE_ACTIVITY(beentry);
    beentry->st_query_id = query_id;
    PGSTAT_END_WRITE_ACTIVITY(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_BEGIN_WRITE_ACTIVITY(beentry);
 
    memcpy((char *) beentry->st_appname, appname, len);
    beentry->st_appname[len] = '\0';
 
    PGSTAT_END_WRITE_ACTIVITY(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_BEGIN_WRITE_ACTIVITY(beentry);
 
    beentry->st_xact_start_timestamp = tstamp;
 
    PGSTAT_END_WRITE_ACTIVITY(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,
               *localclienthostname,
               *localactivity;
#ifdef USE_SSL
    PgBackendSSLStatus *localsslstatus;
#endif
#ifdef ENABLE_GSS
    PgBackendGSSStatus *localgssstatus;
#endif
    int         i;
 
    if (localBackendStatusTable)
        return;                 /* already done */
 
    pgstat_setup_backend_status_context();
 
    /*
     * Allocate storage for local copy of state data.  We can presume that
     * none of these requests overflow size_t, because we already calculated
     * the same values using mul_size during shmem setup.  However, with
     * probably-silly values of pgstat_track_activity_query_size and
     * max_connections, the localactivity buffer could exceed 1GB, so use
     * "huge" allocation for that one.
     */
    localtable = (LocalPgBackendStatus *)
        MemoryContextAlloc(backendStatusSnapContext,
                           sizeof(LocalPgBackendStatus) * NumBackendStatSlots);
    localappname = (char *)
        MemoryContextAlloc(backendStatusSnapContext,
                           NAMEDATALEN * NumBackendStatSlots);
    localclienthostname = (char *)
        MemoryContextAlloc(backendStatusSnapContext,
                           NAMEDATALEN * NumBackendStatSlots);
    localactivity = (char *)
        MemoryContextAllocHuge(backendStatusSnapContext,
                               pgstat_track_activity_query_size * NumBackendStatSlots);
#ifdef USE_SSL
    localsslstatus = (PgBackendSSLStatus *)
        MemoryContextAlloc(backendStatusSnapContext,
                           sizeof(PgBackendSSLStatus) * NumBackendStatSlots);
#endif
#ifdef ENABLE_GSS
    localgssstatus = (PgBackendGSSStatus *)
        MemoryContextAlloc(backendStatusSnapContext,
                           sizeof(PgBackendGSSStatus) * 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_begin_read_activity(beentry, before_changecount);
 
            localentry->backendStatus.st_procpid = beentry->st_procpid;
            /* Skip all the data-copying work if entry is not in use */
            if (localentry->backendStatus.st_procpid > 0)
            {
                memcpy(&localentry->backendStatus, unvolatize(PgBackendStatus *, beentry), sizeof(PgBackendStatus));
 
                /*
                 * For each PgBackendStatus field that is a pointer, copy the
                 * pointed-to data, then adjust the local copy of the pointer
                 * field to point at the local copy of the data.
                 *
                 * 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(localclienthostname, (char *) beentry->st_clienthostname);
                localentry->backendStatus.st_clienthostname = localclienthostname;
                strcpy(localactivity, (char *) beentry->st_activity_raw);
                localentry->backendStatus.st_activity_raw = localactivity;
#ifdef USE_SSL
                if (beentry->st_ssl)
                {
                    memcpy(localsslstatus, beentry->st_sslstatus, sizeof(PgBackendSSLStatus));
                    localentry->backendStatus.st_sslstatus = localsslstatus;
                }
#endif
#ifdef ENABLE_GSS
                if (beentry->st_gss)
                {
                    memcpy(localgssstatus, beentry->st_gssstatus, sizeof(PgBackendGSSStatus));
                    localentry->backendStatus.st_gssstatus = localgssstatus;
                }
#endif
            }
 
            pgstat_end_read_activity(beentry, after_changecount);
 
            if (pgstat_read_activity_complete(before_changecount,
                                              after_changecount))
                break;
 
            /* Make sure we can break out of loop if stuck... */
            CHECK_FOR_INTERRUPTS();
        }
 
        /* Only valid entries get included into the local array */
        if (localentry->backendStatus.st_procpid > 0)
        {
            /*
             * The BackendStatusArray index is exactly the BackendId of the
             * source backend.  Note that this means localBackendStatusTable
             * is in order by backend_id.  pgstat_get_beentry_by_backend_id()
             * depends on that.
             */
            localentry->backend_id = i;
            BackendIdGetTransactionIds(i,
                                       &localentry->backend_xid,
                                       &localentry->backend_xmin,
                                       &localentry->backend_subxact_count,
                                       &localentry->backend_subxact_overflowed);
 
            localentry++;
            localappname += NAMEDATALEN;
            localclienthostname += NAMEDATALEN;
            localactivity += pgstat_track_activity_query_size;
#ifdef USE_SSL
            localsslstatus++;
#endif
#ifdef ENABLE_GSS
            localgssstatus++;
#endif
            localNumBackends++;
        }
 
        beentry++;
    }
 
    /* Set the pointer only after completion of a valid table */
    localBackendStatusTable = localtable;
}
 
 
/* ----------
 * 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_begin_read_activity(vbeentry, before_changecount);
 
            found = (vbeentry->st_procpid == pid);
 
            pgstat_end_read_activity(vbeentry, after_changecount);
 
            if (pgstat_read_activity_complete(before_changecount,
                                              after_changecount))
                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;
}
 
/* ----------
 * pgstat_get_my_query_id() -
 *
 * Return current backend's query identifier.
 */
uint64
pgstat_get_my_query_id(void)
{
    if (!MyBEEntry)
        return 0;
 
    /*
     * There's no need for a lock around pgstat_begin_read_activity /
     * pgstat_end_read_activity here as it's only called from
     * pg_stat_get_activity which is already protected, or from the same
     * backend which means that there won't be concurrent writes.
     */
    return MyBEEntry->st_query_id;
}
 
/* ----------
 * cmp_lbestatus
 *
 *  Comparison function for bsearch() on an array of LocalPgBackendStatus.
 *  The backend_id field is used to compare the arguments.
 * ----------
 */
static int
cmp_lbestatus(const void *a, const void *b)
{
    const LocalPgBackendStatus *lbestatus1 = (const LocalPgBackendStatus *) a;
    const LocalPgBackendStatus *lbestatus2 = (const LocalPgBackendStatus *) b;
 
    return lbestatus1->backend_id - lbestatus2->backend_id;
}
 
/* ----------
 * pgstat_get_beentry_by_backend_id() -
 *
 *  Support function for the SQL-callable pgstat* functions. Returns
 *  our local copy of the current-activity entry for one backend,
 *  or NULL if the given beid doesn't identify any known session.
 *
 *  The beid argument is the BackendId of the desired session
 *  (note that this is unlike pgstat_get_local_beentry_by_index()).
 *
 *  NB: caller is responsible for a check if the user is permitted to see
 *  this info (especially the querystring).
 * ----------
 */
PgBackendStatus *
pgstat_get_beentry_by_backend_id(BackendId beid)
{
    LocalPgBackendStatus *ret = pgstat_get_local_beentry_by_backend_id(beid);
 
    if (ret)
        return &ret->backendStatus;
 
    return NULL;
}
 
 
/* ----------
 * pgstat_get_local_beentry_by_backend_id() -
 *
 *  Like pgstat_get_beentry_by_backend_id() but with locally computed additions
 *  (like xid and xmin values of the backend)
 *
 *  The beid argument is the BackendId of the desired session
 *  (note that this is unlike pgstat_get_local_beentry_by_index()).
 *
 *  NB: caller is responsible for checking if the user is permitted to see this
 *  info (especially the querystring).
 * ----------
 */
LocalPgBackendStatus *
pgstat_get_local_beentry_by_backend_id(BackendId beid)
{
    LocalPgBackendStatus key;
 
    pgstat_read_current_status();
 
    /*
     * Since the localBackendStatusTable is in order by backend_id, we can use
     * bsearch() to search it efficiently.
     */
    key.backend_id = beid;
    return bsearch(&key, localBackendStatusTable, localNumBackends,
                   sizeof(LocalPgBackendStatus), cmp_lbestatus);
}
 
 
/* ----------
 * pgstat_get_local_beentry_by_index() -
 *
 *  Like pgstat_get_beentry_by_backend_id() but with locally computed additions
 *  (like xid and xmin values of the backend)
 *
 *  The idx argument is a 1-based index in the localBackendStatusTable
 *  (note that this is unlike pgstat_get_beentry_by_backend_id()).
 *  Returns NULL if the argument is out of range (no current caller does that).
 *
 *  NB: caller is responsible for a check if the user is permitted to see
 *  this info (especially the querystring).
 * ----------
 */
LocalPgBackendStatus *
pgstat_get_local_beentry_by_index(int idx)
{
    pgstat_read_current_status();
 
    if (idx < 1 || idx > localNumBackends)
        return NULL;
 
    return &localBackendStatusTable[idx - 1];
}
 
 
/* ----------
 * pgstat_fetch_stat_numbackends() -
 *
 *  Support function for the SQL-callable pgstat* functions. Returns
 *  the number of sessions known in the localBackendStatusTable, i.e.
 *  the maximum 1-based index to pass to pgstat_get_local_beentry_by_index().
 * ----------
 */
int
pgstat_fetch_stat_numbackends(void)
{
    pgstat_read_current_status();
 
    return localNumBackends;
}
 
/*
 * Convert a potentially unsafely truncated activity string (see
 * PgBackendStatus.st_activity_raw's documentation) into a correctly truncated
 * one.
 *
 * The returned string is allocated in the caller's memory context and may be
 * freed.
 */
char *
pgstat_clip_activity(const char *raw_activity)
{
    char       *activity;
    int         rawlen;
    int         cliplen;
 
    /*
     * Some callers, like pgstat_get_backend_current_activity(), do not
     * guarantee that the buffer isn't concurrently modified. We try to take
     * care that the buffer is always terminated by a NUL byte regardless, but
     * let's still be paranoid about the string's length. In those cases the
     * underlying buffer is guaranteed to be pgstat_track_activity_query_size
     * large.
     */
    activity = pnstrdup(raw_activity, pgstat_track_activity_query_size - 1);
 
    /* now double-guaranteed to be NUL terminated */
    rawlen = strlen(activity);
 
    /*
     * All supported server-encodings make it possible to determine the length
     * of a multi-byte character from its first byte (this is not the case for
     * client encodings, see GB18030). As st_activity is always stored using
     * server encoding, this allows us to perform multi-byte aware truncation,
     * even if the string earlier was truncated in the middle of a multi-byte
     * character.
     */
    cliplen = pg_mbcliplen(activity, rawlen,
                           pgstat_track_activity_query_size - 1);
 
    activity[cliplen] = '\0';
 
    return activity;
}