proc.c

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/*-------------------------------------------------------------------------
 *
 * proc.c
 *    routines to manage per-process shared memory data structure
 *
 * Portions Copyright (c) 1996-2026, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/storage/lmgr/proc.c
 *
 *-------------------------------------------------------------------------
 */
/*
 * Interface (a):
 *      JoinWaitQueue(), ProcSleep(), ProcWakeup()
 *
 * Waiting for a lock causes the backend to be put to sleep.  Whoever releases
 * the lock wakes the process up again (and gives it an error code so it knows
 * whether it was awoken on an error condition).
 *
 * Interface (b):
 *
 * ProcReleaseLocks -- frees the locks associated with current transaction
 *
 * ProcKill -- destroys the shared memory state (and locks)
 * associated with the process.
 */
#include "postgres.h"
 
#include <signal.h>
#include <unistd.h>
#include <sys/time.h>
 
#include "access/clog.h"
#include "access/transam.h"
#include "access/twophase.h"
#include "access/xlogutils.h"
#include "access/xlogwait.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "postmaster/autovacuum.h"
#include "replication/slotsync.h"
#include "replication/syncrep.h"
#include "storage/condition_variable.h"
#include "storage/ipc.h"
#include "storage/lmgr.h"
#include "storage/pmsignal.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "storage/procsignal.h"
#include "storage/spin.h"
#include "storage/standby.h"
#include "storage/subsystems.h"
#include "utils/injection_point.h"
#include "utils/timeout.h"
#include "utils/timestamp.h"
#include "utils/wait_event.h"
 
/* GUC variables */
int         DeadlockTimeout = 1000;
int         StatementTimeout = 0;
int         LockTimeout = 0;
int         IdleInTransactionSessionTimeout = 0;
int         TransactionTimeout = 0;
int         IdleSessionTimeout = 0;
bool        log_lock_waits = true;
 
/* Pointer to this process's PGPROC struct, if any */
PGPROC     *MyProc = NULL;
 
/* Pointers to shared-memory structures */
PROC_HDR   *ProcGlobal = NULL;
static void *AllProcsShmemPtr;
static void *FastPathLockArrayShmemPtr;
NON_EXEC_STATIC PGPROC *AuxiliaryProcs = NULL;
PGPROC     *PreparedXactProcs = NULL;
 
static void ProcGlobalShmemRequest(void *arg);
static void ProcGlobalShmemInit(void *arg);
 
const ShmemCallbacks ProcGlobalShmemCallbacks = {
    .request_fn = ProcGlobalShmemRequest,
    .init_fn = ProcGlobalShmemInit,
};
 
static uint32 TotalProcs;
static size_t ProcGlobalAllProcsShmemSize;
static size_t FastPathLockArrayShmemSize;
 
/* Is a deadlock check pending? */
static volatile sig_atomic_t got_deadlock_timeout;
 
static void RemoveProcFromArray(int code, Datum arg);
static void ProcKill(int code, Datum arg);
static void AuxiliaryProcKill(int code, Datum arg);
static DeadLockState CheckDeadLock(void);
 
 
/*
 * Calculate shared-memory space needed by Fast-Path locks.
 */
static Size
CalculateFastPathLockShmemSize(void)
{
    Size        size = 0;
    Size        fpLockBitsSize,
                fpRelIdSize;
 
    /*
     * Memory needed for PGPROC fast-path lock arrays. Make sure the sizes are
     * nicely aligned in each backend.
     */
    fpLockBitsSize = MAXALIGN(FastPathLockGroupsPerBackend * sizeof(uint64));
    fpRelIdSize = MAXALIGN(FastPathLockSlotsPerBackend() * sizeof(Oid));
 
    size = add_size(size, mul_size(TotalProcs, (fpLockBitsSize + fpRelIdSize)));
 
    Assert(TotalProcs > 0);
    Assert(size > 0);
 
    return size;
}
 
/*
 * Report number of semaphores needed by ProcGlobalShmemInit.
 */
int
ProcGlobalSemas(void)
{
    /*
     * We need a sema per backend (including autovacuum), plus one for each
     * auxiliary process.
     */
    return MaxBackends + NUM_AUXILIARY_PROCS;
}
 
/*
 * ProcGlobalShmemRequest
 *    Register shared memory needs.
 *
 * This is called during postmaster or standalone backend startup, and also
 * during backend startup in EXEC_BACKEND mode.
 */
static void
ProcGlobalShmemRequest(void *arg)
{
    Size        size;
 
    /*
     * Reserve all the PGPROC structures we'll need.  There are six separate
     * consumers: (1) normal backends, (2) autovacuum workers and special
     * workers, (3) background workers, (4) walsenders, (5) auxiliary
     * processes, and (6) prepared transactions.  (For largely-historical
     * reasons, we combine autovacuum and special workers into one category
     * with a single freelist.)  Each PGPROC structure is dedicated to exactly
     * one of these purposes, and they do not move between groups.
     */
    TotalProcs =
        add_size(MaxBackends, add_size(NUM_AUXILIARY_PROCS, max_prepared_xacts));
 
    size = 0;
    size = add_size(size, mul_size(TotalProcs, sizeof(PGPROC)));
    size = add_size(size, mul_size(TotalProcs, sizeof(*ProcGlobal->xids)));
    size = add_size(size, mul_size(TotalProcs, sizeof(*ProcGlobal->subxidStates)));
    size = add_size(size, mul_size(TotalProcs, sizeof(*ProcGlobal->statusFlags)));
    ProcGlobalAllProcsShmemSize = size;
    ShmemRequestStruct(.name = "PGPROC structures",
                       .size = ProcGlobalAllProcsShmemSize,
                       .ptr = &AllProcsShmemPtr,
        );
 
    if (!IsUnderPostmaster)
        size = FastPathLockArrayShmemSize = CalculateFastPathLockShmemSize();
    else
        size = SHMEM_ATTACH_UNKNOWN_SIZE;
    ShmemRequestStruct(.name = "Fast-Path Lock Array",
                       .size = size,
                       .ptr = &FastPathLockArrayShmemPtr,
        );
 
    /*
     * ProcGlobal is registered here in .ptr as usual, but it needs to be
     * propagated specially in EXEC_BACKEND mode, because ProcGlobal needs to
     * be accessed early at backend startup, before ShmemAttachRequested() has
     * been called.
     */
    ShmemRequestStruct(.name = "Proc Header",
                       .size = sizeof(PROC_HDR),
                       .ptr = (void **) &ProcGlobal,
        );
 
    /* Let the semaphore implementation register its shared memory needs */
    PGSemaphoreShmemRequest(ProcGlobalSemas());
}
 
 
/*
 * ProcGlobalShmemInit -
 *    Initialize the global process table during postmaster or standalone
 *    backend startup.
 *
 *    We also create all the per-process semaphores we will need to support
 *    the requested number of backends.  We used to allocate semaphores
 *    only when backends were actually started up, but that is bad because
 *    it lets Postgres fail under load --- a lot of Unix systems are
 *    (mis)configured with small limits on the number of semaphores, and
 *    running out when trying to start another backend is a common failure.
 *    So, now we grab enough semaphores to support the desired max number
 *    of backends immediately at initialization --- if the sysadmin has set
 *    MaxConnections, max_worker_processes, max_wal_senders, or
 *    autovacuum_worker_slots higher than his kernel will support, he'll
 *    find out sooner rather than later.
 *
 *    Another reason for creating semaphores here is that the semaphore
 *    implementation typically requires us to create semaphores in the
 *    postmaster, not in backends.
 */
static void
ProcGlobalShmemInit(void *arg)
{
    char       *ptr;
    size_t      requestSize;
    PGPROC     *procs;
    int         i,
                j;
 
    /* Used for setup of per-backend fast-path slots. */
    char       *fpPtr,
               *fpEndPtr PG_USED_FOR_ASSERTS_ONLY;
    Size        fpLockBitsSize,
                fpRelIdSize;
 
    Assert(ProcGlobal);
    ProcGlobal->spins_per_delay = DEFAULT_SPINS_PER_DELAY;
    SpinLockInit(&ProcGlobal->freeProcsLock);
    dlist_init(&ProcGlobal->freeProcs);
    dlist_init(&ProcGlobal->autovacFreeProcs);
    dlist_init(&ProcGlobal->bgworkerFreeProcs);
    dlist_init(&ProcGlobal->walsenderFreeProcs);
    ProcGlobal->startupBufferPinWaitBufId = -1;
    pg_atomic_init_u32(&ProcGlobal->avLauncherProc, INVALID_PROC_NUMBER);
    pg_atomic_init_u32(&ProcGlobal->walwriterProc, INVALID_PROC_NUMBER);
    pg_atomic_init_u32(&ProcGlobal->checkpointerProc, INVALID_PROC_NUMBER);
    pg_atomic_init_u32(&ProcGlobal->procArrayGroupFirst, INVALID_PROC_NUMBER);
    pg_atomic_init_u32(&ProcGlobal->clogGroupFirst, INVALID_PROC_NUMBER);
 
    ptr = AllProcsShmemPtr;
    requestSize = ProcGlobalAllProcsShmemSize;
    MemSet(ptr, 0, requestSize);
 
    /* Carve out the allProcs array from the shared memory area */
    procs = (PGPROC *) ptr;
    ptr = ptr + TotalProcs * sizeof(PGPROC);
 
    ProcGlobal->allProcs = procs;
    /* XXX allProcCount isn't really all of them; it excludes prepared xacts */
    ProcGlobal->allProcCount = MaxBackends + NUM_AUXILIARY_PROCS;
 
    /*
     * Carve out arrays mirroring PGPROC fields in a dense manner. See
     * PROC_HDR.
     *
     * XXX: It might make sense to increase padding for these arrays, given
     * how hotly they are accessed.
     */
    ProcGlobal->xids = (TransactionId *) ptr;
    ptr = ptr + (TotalProcs * sizeof(*ProcGlobal->xids));
 
    ProcGlobal->subxidStates = (XidCacheStatus *) ptr;
    ptr = ptr + (TotalProcs * sizeof(*ProcGlobal->subxidStates));
 
    ProcGlobal->statusFlags = (uint8 *) ptr;
    ptr = ptr + (TotalProcs * sizeof(*ProcGlobal->statusFlags));
 
    /* make sure we didn't overflow */
    Assert((ptr > (char *) procs) && (ptr <= (char *) procs + requestSize));
 
    /*
     * Initialize arrays for fast-path locks. Those are variable-length, so
     * can't be included in PGPROC directly. We allocate a separate piece of
     * shared memory and then divide that between backends.
     */
    fpLockBitsSize = MAXALIGN(FastPathLockGroupsPerBackend * sizeof(uint64));
    fpRelIdSize = MAXALIGN(FastPathLockSlotsPerBackend() * sizeof(Oid));
 
    fpPtr = FastPathLockArrayShmemPtr;
    requestSize = FastPathLockArrayShmemSize;
    memset(fpPtr, 0, requestSize);
 
    /* For asserts checking we did not overflow. */
    fpEndPtr = fpPtr + requestSize;
 
    /* Initialize semaphores */
    PGSemaphoreInit(ProcGlobalSemas());
 
    for (i = 0; i < TotalProcs; i++)
    {
        PGPROC     *proc = &procs[i];
 
        /* Common initialization for all PGPROCs, regardless of type. */
 
        /*
         * Set the fast-path lock arrays, and move the pointer. We interleave
         * the two arrays, to (hopefully) get some locality for each backend.
         */
        proc->fpLockBits = (uint64 *) fpPtr;
        fpPtr += fpLockBitsSize;
 
        proc->fpRelId = (Oid *) fpPtr;
        fpPtr += fpRelIdSize;
 
        Assert(fpPtr <= fpEndPtr);
 
        /*
         * Set up per-PGPROC semaphore, latch, and fpInfoLock.  Prepared xact
         * dummy PGPROCs don't need these though - they're never associated
         * with a real process
         */
        if (i < FIRST_PREPARED_XACT_PROC_NUMBER)
        {
            proc->sem = PGSemaphoreCreate();
            InitSharedLatch(&(proc->procLatch));
            LWLockInitialize(&(proc->fpInfoLock), LWTRANCHE_LOCK_FASTPATH);
        }
 
        /*
         * Newly created PGPROCs for normal backends, autovacuum workers,
         * special workers, bgworkers, and walsenders must be queued up on the
         * appropriate free list.  Because there can only ever be a small,
         * fixed number of auxiliary processes, no free list is used in that
         * case; InitAuxiliaryProcess() instead uses a linear search.  PGPROCs
         * for prepared transactions are added to a free list by
         * TwoPhaseShmemInit().
         */
        if (i < MaxConnections)
        {
            /* PGPROC for normal backend, add to freeProcs list */
            dlist_push_tail(&ProcGlobal->freeProcs, &proc->freeProcsLink);
            proc->procgloballist = &ProcGlobal->freeProcs;
        }
        else if (i < MaxConnections + autovacuum_worker_slots + NUM_SPECIAL_WORKER_PROCS)
        {
            /* PGPROC for AV or special worker, add to autovacFreeProcs list */
            dlist_push_tail(&ProcGlobal->autovacFreeProcs, &proc->freeProcsLink);
            proc->procgloballist = &ProcGlobal->autovacFreeProcs;
        }
        else if (i < MaxConnections + autovacuum_worker_slots + NUM_SPECIAL_WORKER_PROCS + max_worker_processes)
        {
            /* PGPROC for bgworker, add to bgworkerFreeProcs list */
            dlist_push_tail(&ProcGlobal->bgworkerFreeProcs, &proc->freeProcsLink);
            proc->procgloballist = &ProcGlobal->bgworkerFreeProcs;
        }
        else if (i < MaxBackends)
        {
            /* PGPROC for walsender, add to walsenderFreeProcs list */
            dlist_push_tail(&ProcGlobal->walsenderFreeProcs, &proc->freeProcsLink);
            proc->procgloballist = &ProcGlobal->walsenderFreeProcs;
        }
 
        /* Initialize myProcLocks[] shared memory queues. */
        for (j = 0; j < NUM_LOCK_PARTITIONS; j++)
            dlist_init(&(proc->myProcLocks[j]));
 
        /* Initialize lockGroupMembers list. */
        dlist_init(&proc->lockGroupMembers);
 
        /*
         * Initialize the atomic variables, otherwise, it won't be safe to
         * access them for backends that aren't currently in use.
         */
        pg_atomic_init_u32(&(proc->procArrayGroupNext), INVALID_PROC_NUMBER);
        pg_atomic_init_u32(&(proc->clogGroupNext), INVALID_PROC_NUMBER);
        pg_atomic_init_u64(&(proc->waitStart), 0);
    }
 
    /* Should have consumed exactly the expected amount of fast-path memory. */
    Assert(fpPtr == fpEndPtr);
 
    /*
     * Save pointers to the blocks of PGPROC structures reserved for auxiliary
     * processes and prepared transactions.
     */
    AuxiliaryProcs = &procs[MaxBackends];
    PreparedXactProcs = &procs[FIRST_PREPARED_XACT_PROC_NUMBER];
}
 
/*
 * InitProcess -- initialize a per-process PGPROC entry for this backend
 */
void
InitProcess(void)
{
    dlist_head *procgloballist;
 
    /*
     * ProcGlobal should be set up already (if we are a backend, we inherit
     * this by fork() or EXEC_BACKEND mechanism from the postmaster).
     */
    if (ProcGlobal == NULL)
        elog(PANIC, "proc header uninitialized");
 
    if (MyProc != NULL)
        elog(ERROR, "you already exist");
 
    /*
     * Before we start accessing the shared memory in a serious way, mark
     * ourselves as an active postmaster child; this is so that the postmaster
     * can detect it if we exit without cleaning up.
     */
    if (IsUnderPostmaster)
        RegisterPostmasterChildActive();
 
    /*
     * Decide which list should supply our PGPROC.  This logic must match the
     * way the freelists were constructed in ProcGlobalShmemInit().
     */
    if (AmAutoVacuumWorkerProcess() || AmSpecialWorkerProcess())
        procgloballist = &ProcGlobal->autovacFreeProcs;
    else if (AmBackgroundWorkerProcess())
        procgloballist = &ProcGlobal->bgworkerFreeProcs;
    else if (AmWalSenderProcess())
        procgloballist = &ProcGlobal->walsenderFreeProcs;
    else
        procgloballist = &ProcGlobal->freeProcs;
 
    /*
     * Try to get a proc struct from the appropriate free list.  If this
     * fails, we must be out of PGPROC structures (not to mention semaphores).
     *
     * While we are holding the spinlock, also copy the current shared
     * estimate of spins_per_delay to local storage.
     */
    SpinLockAcquire(&ProcGlobal->freeProcsLock);
 
    set_spins_per_delay(ProcGlobal->spins_per_delay);
 
    if (!dlist_is_empty(procgloballist))
    {
        MyProc = dlist_container(PGPROC, freeProcsLink, dlist_pop_head_node(procgloballist));
        SpinLockRelease(&ProcGlobal->freeProcsLock);
    }
    else
    {
        /*
         * If we reach here, all the PGPROCs are in use.  This is one of the
         * possible places to detect "too many backends", so give the standard
         * error message.  XXX do we need to give a different failure message
         * in the autovacuum case?
         */
        SpinLockRelease(&ProcGlobal->freeProcsLock);
        if (AmWalSenderProcess())
            ereport(FATAL,
                    (errcode(ERRCODE_TOO_MANY_CONNECTIONS),
                     errmsg("number of requested standby connections exceeds \"max_wal_senders\" (currently %d)",
                            max_wal_senders)));
        ereport(FATAL,
                (errcode(ERRCODE_TOO_MANY_CONNECTIONS),
                 errmsg("sorry, too many clients already")));
    }
    MyProcNumber = GetNumberFromPGProc(MyProc);
 
    /*
     * Cross-check that the PGPROC is of the type we expect; if this were not
     * the case, it would get returned to the wrong list.
     */
    Assert(MyProc->procgloballist == procgloballist);
 
    /*
     * Initialize all fields of MyProc, except for those previously
     * initialized by ProcGlobalShmemInit.
     */
    dlist_node_init(&MyProc->freeProcsLink);
    MyProc->waitStatus = PROC_WAIT_STATUS_OK;
    MyProc->fpVXIDLock = false;
    MyProc->fpLocalTransactionId = InvalidLocalTransactionId;
    MyProc->xid = InvalidTransactionId;
    MyProc->xmin = InvalidTransactionId;
    MyProc->pid = MyProcPid;
    MyProc->vxid.procNumber = MyProcNumber;
    MyProc->vxid.lxid = InvalidLocalTransactionId;
    /* databaseId and roleId will be filled in later */
    MyProc->databaseId = InvalidOid;
    MyProc->roleId = InvalidOid;
    MyProc->tempNamespaceId = InvalidOid;
    MyProc->backendType = MyBackendType;
    MyProc->delayChkptFlags = 0;
    MyProc->statusFlags = 0;
    /* NB -- autovac launcher intentionally does not set IS_AUTOVACUUM */
    if (AmAutoVacuumWorkerProcess())
        MyProc->statusFlags |= PROC_IS_AUTOVACUUM;
    MyProc->lwWaiting = LW_WS_NOT_WAITING;
    MyProc->lwWaitMode = 0;
    MyProc->waitLock = NULL;
    dlist_node_init(&MyProc->waitLink);
    MyProc->waitProcLock = NULL;
    pg_atomic_write_u64(&MyProc->waitStart, 0);
#ifdef USE_ASSERT_CHECKING
    {
        int         i;
 
        /* Last process should have released all locks. */
        for (i = 0; i < NUM_LOCK_PARTITIONS; i++)
            Assert(dlist_is_empty(&(MyProc->myProcLocks[i])));
    }
#endif
    pg_atomic_write_u32(&MyProc->pendingRecoveryConflicts, 0);
 
    /* Initialize fields for sync rep */
    MyProc->waitLSN = InvalidXLogRecPtr;
    MyProc->syncRepState = SYNC_REP_NOT_WAITING;
    dlist_node_init(&MyProc->syncRepLinks);
 
    /* Initialize fields for group XID clearing. */
    MyProc->procArrayGroupMember = false;
    MyProc->procArrayGroupMemberXid = InvalidTransactionId;
    Assert(pg_atomic_read_u32(&MyProc->procArrayGroupNext) == INVALID_PROC_NUMBER);
 
    /* Check that group locking fields are in a proper initial state. */
    Assert(MyProc->lockGroupLeader == NULL);
    Assert(dlist_is_empty(&MyProc->lockGroupMembers));
 
    /* Initialize wait event information. */
    MyProc->wait_event_info = 0;
 
    /* Initialize fields for group transaction status update. */
    MyProc->clogGroupMember = false;
    MyProc->clogGroupMemberXid = InvalidTransactionId;
    MyProc->clogGroupMemberXidStatus = TRANSACTION_STATUS_IN_PROGRESS;
    MyProc->clogGroupMemberPage = -1;
    MyProc->clogGroupMemberLsn = InvalidXLogRecPtr;
    Assert(pg_atomic_read_u32(&MyProc->clogGroupNext) == INVALID_PROC_NUMBER);
 
    /*
     * Acquire ownership of the PGPROC's latch, so that we can use WaitLatch
     * on it.  That allows us to repoint the process latch, which so far
     * points to process local one, to the shared one.
     */
    OwnLatch(&MyProc->procLatch);
    SwitchToSharedLatch();
 
    /* now that we have a proc, report wait events to shared memory */
    pgstat_set_wait_event_storage(&MyProc->wait_event_info);
 
    /*
     * We might be reusing a semaphore that belonged to a failed process. So
     * be careful and reinitialize its value here.  (This is not strictly
     * necessary anymore, but seems like a good idea for cleanliness.)
     */
    PGSemaphoreReset(MyProc->sem);
 
    /* autovacuum launcher is specially advertised in ProcGlobal */
    if (MyBackendType == B_AUTOVAC_LAUNCHER)
        pg_atomic_write_u32(&ProcGlobal->avLauncherProc, MyProcNumber);
 
    /*
     * Arrange to clean up at backend exit.
     */
    on_shmem_exit(ProcKill, 0);
 
    /*
     * Now that we have a PGPROC, we could try to acquire locks, so initialize
     * local state needed for LWLocks, and the deadlock checker.
     */
    InitLWLockAccess();
    InitDeadLockChecking();
 
#ifdef EXEC_BACKEND
 
    /*
     * Initialize backend-local pointers to all the shared data structures.
     * (We couldn't do this until now because it needs LWLocks.)
     */
    if (IsUnderPostmaster)
        AttachSharedMemoryStructs();
#endif
}
 
/*
 * InitProcessPhase2 -- make MyProc visible in the shared ProcArray.
 *
 * This is separate from InitProcess because we can't acquire LWLocks until
 * we've created a PGPROC, but in the EXEC_BACKEND case ProcArrayAdd won't
 * work until after we've done AttachSharedMemoryStructs.
 */
void
InitProcessPhase2(void)
{
    Assert(MyProc != NULL);
 
    /*
     * Add our PGPROC to the PGPROC array in shared memory.
     */
    ProcArrayAdd(MyProc);
 
    /*
     * Arrange to clean that up at backend exit.
     */
    on_shmem_exit(RemoveProcFromArray, 0);
}
 
/*
 * InitAuxiliaryProcess -- create a PGPROC entry for an auxiliary process
 *
 * This is called by bgwriter and similar processes so that they will have a
 * MyProc value that's real enough to let them wait for LWLocks.  The PGPROC
 * and sema that are assigned are one of the extra ones created during
 * ProcGlobalShmemInit.
 *
 * Auxiliary processes are presently not expected to wait for real (lockmgr)
 * locks, so we need not set up the deadlock checker.  They are never added
 * to the ProcArray or the sinval messaging mechanism, either.  They also
 * don't get a VXID assigned, since this is only useful when we actually
 * hold lockmgr locks.
 *
 * Startup process however uses locks but never waits for them in the
 * normal backend sense. Startup process also takes part in sinval messaging
 * as a sendOnly process, so never reads messages from sinval queue. So
 * Startup process does have a VXID and does show up in pg_locks.
 */
void
InitAuxiliaryProcess(void)
{
    PGPROC     *auxproc;
    int         proctype;
 
    /*
     * ProcGlobal should be set up already (if we are a backend, we inherit
     * this by fork() or EXEC_BACKEND mechanism from the postmaster).
     */
    if (ProcGlobal == NULL || AuxiliaryProcs == NULL)
        elog(PANIC, "proc header uninitialized");
 
    if (MyProc != NULL)
        elog(ERROR, "you already exist");
 
    if (IsUnderPostmaster)
        RegisterPostmasterChildActive();
 
    /*
     * We use the freeProcsLock to protect assignment and releasing of
     * AuxiliaryProcs entries.
     *
     * While we are holding the spinlock, also copy the current shared
     * estimate of spins_per_delay to local storage.
     */
    SpinLockAcquire(&ProcGlobal->freeProcsLock);
 
    set_spins_per_delay(ProcGlobal->spins_per_delay);
 
    /*
     * Find a free auxproc ... *big* trouble if there isn't one ...
     */
    for (proctype = 0; proctype < NUM_AUXILIARY_PROCS; proctype++)
    {
        auxproc = &AuxiliaryProcs[proctype];
        if (auxproc->pid == 0)
            break;
    }
    if (proctype >= NUM_AUXILIARY_PROCS)
    {
        SpinLockRelease(&ProcGlobal->freeProcsLock);
        elog(FATAL, "all AuxiliaryProcs are in use");
    }
 
    /* Mark auxiliary proc as in use by me */
    auxproc->pid = MyProcPid;
 
    SpinLockRelease(&ProcGlobal->freeProcsLock);
 
    MyProc = auxproc;
    MyProcNumber = GetNumberFromPGProc(MyProc);
 
    /*
     * Initialize all fields of MyProc, except for those previously
     * initialized by ProcGlobalShmemInit.
     */
    dlist_node_init(&MyProc->freeProcsLink);
    MyProc->waitStatus = PROC_WAIT_STATUS_OK;
    MyProc->fpVXIDLock = false;
    MyProc->fpLocalTransactionId = InvalidLocalTransactionId;
    MyProc->xid = InvalidTransactionId;
    MyProc->xmin = InvalidTransactionId;
    MyProc->vxid.procNumber = INVALID_PROC_NUMBER;
    MyProc->vxid.lxid = InvalidLocalTransactionId;
    MyProc->databaseId = InvalidOid;
    MyProc->roleId = InvalidOid;
    MyProc->tempNamespaceId = InvalidOid;
    MyProc->backendType = MyBackendType;
    MyProc->delayChkptFlags = 0;
    MyProc->statusFlags = 0;
    MyProc->lwWaiting = LW_WS_NOT_WAITING;
    MyProc->lwWaitMode = 0;
    MyProc->waitLock = NULL;
    dlist_node_init(&MyProc->waitLink);
    MyProc->waitProcLock = NULL;
    pg_atomic_write_u64(&MyProc->waitStart, 0);
#ifdef USE_ASSERT_CHECKING
    {
        int         i;
 
        /* Last process should have released all locks. */
        for (i = 0; i < NUM_LOCK_PARTITIONS; i++)
            Assert(dlist_is_empty(&(MyProc->myProcLocks[i])));
    }
#endif
    pg_atomic_write_u32(&MyProc->pendingRecoveryConflicts, 0);
 
    /*
     * Acquire ownership of the PGPROC's latch, so that we can use WaitLatch
     * on it.  That allows us to repoint the process latch, which so far
     * points to process local one, to the shared one.
     */
    OwnLatch(&MyProc->procLatch);
    SwitchToSharedLatch();
 
    /* now that we have a proc, report wait events to shared memory */
    pgstat_set_wait_event_storage(&MyProc->wait_event_info);
 
    /* Check that group locking fields are in a proper initial state. */
    Assert(MyProc->lockGroupLeader == NULL);
    Assert(dlist_is_empty(&MyProc->lockGroupMembers));
 
    /*
     * We might be reusing a semaphore that belonged to a failed process. So
     * be careful and reinitialize its value here.  (This is not strictly
     * necessary anymore, but seems like a good idea for cleanliness.)
     */
    PGSemaphoreReset(MyProc->sem);
 
    /* Some aux processes are also advertised in ProcGlobal */
    if (MyBackendType == B_WAL_WRITER)
        pg_atomic_write_u32(&ProcGlobal->walwriterProc, MyProcNumber);
    if (MyBackendType == B_CHECKPOINTER)
        pg_atomic_write_u32(&ProcGlobal->checkpointerProc, MyProcNumber);
 
    /*
     * Arrange to clean up at process exit.
     */
    on_shmem_exit(AuxiliaryProcKill, Int32GetDatum(proctype));
 
    /*
     * Now that we have a PGPROC, we could try to acquire lightweight locks.
     * Initialize local state needed for them.  (Heavyweight locks cannot be
     * acquired in aux processes.)
     */
    InitLWLockAccess();
 
#ifdef EXEC_BACKEND
 
    /*
     * Initialize backend-local pointers to all the shared data structures.
     * (We couldn't do this until now because it needs LWLocks.)
     */
    if (IsUnderPostmaster)
        AttachSharedMemoryStructs();
#endif
}
 
/*
 * Used from bufmgr to share the value of the buffer that Startup waits on,
 * or to reset the value to "not waiting" (-1). This allows processing
 * of recovery conflicts for buffer pins. Set is made before backends look
 * at this value, so locking not required, especially since the set is
 * an atomic integer set operation.
 */
void
SetStartupBufferPinWaitBufId(int bufid)
{
    /* use volatile pointer to prevent code rearrangement */
    volatile PROC_HDR *procglobal = ProcGlobal;
 
    procglobal->startupBufferPinWaitBufId = bufid;
}
 
/*
 * Used by backends when they receive a request to check for buffer pin waits.
 */
int
GetStartupBufferPinWaitBufId(void)
{
    /* use volatile pointer to prevent code rearrangement */
    volatile PROC_HDR *procglobal = ProcGlobal;
 
    return procglobal->startupBufferPinWaitBufId;
}
 
/*
 * Check whether there are at least N free PGPROC objects.  If false is
 * returned, *nfree will be set to the number of free PGPROC objects.
 * Otherwise, *nfree will be set to n.
 *
 * Note: this is designed on the assumption that N will generally be small.
 */
bool
HaveNFreeProcs(int n, int *nfree)
{
    dlist_iter  iter;
 
    Assert(n > 0);
    Assert(nfree);
 
    SpinLockAcquire(&ProcGlobal->freeProcsLock);
 
    *nfree = 0;
    dlist_foreach(iter, &ProcGlobal->freeProcs)
    {
        (*nfree)++;
        if (*nfree == n)
            break;
    }
 
    SpinLockRelease(&ProcGlobal->freeProcsLock);
 
    return (*nfree == n);
}
 
/*
 * Cancel any pending wait for lock, when aborting a transaction, and revert
 * any strong lock count acquisition for a lock being acquired.
 *
 * (Normally, this would only happen if we accept a cancel/die
 * interrupt while waiting; but an ereport(ERROR) before or during the lock
 * wait is within the realm of possibility, too.)
 */
void
LockErrorCleanup(void)
{
    LOCALLOCK  *lockAwaited;
    LWLock     *partitionLock;
    DisableTimeoutParams timeouts[2];
 
    HOLD_INTERRUPTS();
 
    AbortStrongLockAcquire();
 
    /* Nothing to do if we weren't waiting for a lock */
    lockAwaited = GetAwaitedLock();
    if (lockAwaited == NULL)
    {
        RESUME_INTERRUPTS();
        return;
    }
 
    /*
     * Turn off the deadlock and lock timeout timers, if they are still
     * running (see ProcSleep).  Note we must preserve the LOCK_TIMEOUT
     * indicator flag, since this function is executed before
     * ProcessInterrupts when responding to SIGINT; else we'd lose the
     * knowledge that the SIGINT came from a lock timeout and not an external
     * source.
     */
    timeouts[0].id = DEADLOCK_TIMEOUT;
    timeouts[0].keep_indicator = false;
    timeouts[1].id = LOCK_TIMEOUT;
    timeouts[1].keep_indicator = true;
    disable_timeouts(timeouts, 2);
 
    /* Unlink myself from the wait queue, if on it (might not be anymore!) */
    partitionLock = LockHashPartitionLock(lockAwaited->hashcode);
    LWLockAcquire(partitionLock, LW_EXCLUSIVE);
 
    if (!dlist_node_is_detached(&MyProc->waitLink))
    {
        /* We could not have been granted the lock yet */
        RemoveFromWaitQueue(MyProc, lockAwaited->hashcode);
    }
    else
    {
        /*
         * Somebody kicked us off the lock queue already.  Perhaps they
         * granted us the lock, or perhaps they detected a deadlock. If they
         * did grant us the lock, we'd better remember it in our local lock
         * table.
         */
        if (MyProc->waitStatus == PROC_WAIT_STATUS_OK)
            GrantAwaitedLock();
    }
 
    ResetAwaitedLock();
 
    LWLockRelease(partitionLock);
 
    RESUME_INTERRUPTS();
}
 
 
/*
 * ProcReleaseLocks() -- release locks associated with current transaction
 *          at main transaction commit or abort
 *
 * At main transaction commit, we release standard locks except session locks.
 * At main transaction abort, we release all locks including session locks.
 *
 * Advisory locks are released only if they are transaction-level;
 * session-level holds remain, whether this is a commit or not.
 *
 * At subtransaction commit, we don't release any locks (so this func is not
 * needed at all); we will defer the releasing to the parent transaction.
 * At subtransaction abort, we release all locks held by the subtransaction;
 * this is implemented by retail releasing of the locks under control of
 * the ResourceOwner mechanism.
 */
void
ProcReleaseLocks(bool isCommit)
{
    if (!MyProc)
        return;
    /* If waiting, get off wait queue (should only be needed after error) */
    LockErrorCleanup();
    /* Release standard locks, including session-level if aborting */
    LockReleaseAll(DEFAULT_LOCKMETHOD, !isCommit);
    /* Release transaction-level advisory locks */
    LockReleaseAll(USER_LOCKMETHOD, false);
}
 
 
/*
 * RemoveProcFromArray() -- Remove this process from the shared ProcArray.
 */
static void
RemoveProcFromArray(int code, Datum arg)
{
    Assert(MyProc != NULL);
    ProcArrayRemove(MyProc, InvalidTransactionId);
}
 
/*
 * ProcKill() -- Destroy the per-proc data structure for
 *      this process. Release any of its held LW locks.
 */
static void
ProcKill(int code, Datum arg)
{
    PGPROC     *proc;
    PGPROC     *leader;
    dlist_head *procgloballist;
    bool        push_leader;
    bool        push_self;
 
    Assert(MyProc != NULL);
 
    /* not safe if forked by system(), etc. */
    if (MyProc->pid != (int) getpid())
        elog(PANIC, "ProcKill() called in child process");
 
    /* Make sure we're out of the sync rep lists */
    SyncRepCleanupAtProcExit();
 
#ifdef USE_ASSERT_CHECKING
    {
        int         i;
 
        /* Last process should have released all locks. */
        for (i = 0; i < NUM_LOCK_PARTITIONS; i++)
            Assert(dlist_is_empty(&(MyProc->myProcLocks[i])));
    }
#endif
 
    /*
     * Release any LW locks I am holding.  There really shouldn't be any, but
     * it's cheap to check again before we cut the knees off the LWLock
     * facility by releasing our PGPROC ...
     */
    LWLockReleaseAll();
 
    /*
     * Cleanup waiting for LSN if any.
     */
    WaitLSNCleanup();
 
    /* Cancel any pending condition variable sleep, too */
    ConditionVariableCancelSleep();
 
    /*
     * Reset MyLatch to the process local one and disown the shared latch, so
     * that signal handlers et al can continue using the latch after the
     * shared latch isn't ours anymore.
     *
     * DisownLatch() must happen before our PGPROC can appear on a freelist: a
     * newly-forked backend that pops our slot and calls OwnLatch() would
     * PANIC on a still-owned latch.
     *
     * pgstat_reset_wait_event_storage() is intentionally deferred until after
     * the lock-group block so that wait_event_info remains visible in our
     * PGPROC slot while we may be observed there.  It is safe to defer
     * because our slot is not yet on any freelist at this point, and useful
     * for testing purposes.
     */
    SwitchBackToLocalLatch();
    DisownLatch(&MyProc->procLatch);
 
    if (MyBackendType == B_AUTOVAC_LAUNCHER)
    {
        Assert(pg_atomic_read_u32(&ProcGlobal->avLauncherProc) == MyProcNumber);
        pg_atomic_write_u32(&ProcGlobal->avLauncherProc, INVALID_PROC_NUMBER);
    }
 
    proc = MyProc;
    procgloballist = proc->procgloballist;
 
    /*
     * Detach from any lock group of which we are a member, deciding under
     * leader_lwlock whether we (via push_self) and/or the leader (via
     * push_leader) need to be pushed onto a freelist.  The actual pushes
     * happen after evaluating if any of these are required, under a single
     * ProcGlobal->freeProcsLock.
     *
     * The decision whether any of the freelists needs to be updated is taken
     * under a single leader_lwlock.
     */
    push_leader = false;
    push_self = true;
    leader = NULL;
 
    if (proc->lockGroupLeader != NULL)
    {
        LWLock     *leader_lwlock;
 
        leader = proc->lockGroupLeader;
        leader_lwlock = LockHashPartitionLockByProc(leader);
 
        LWLockAcquire(leader_lwlock, LW_EXCLUSIVE);
        Assert(!dlist_is_empty(&leader->lockGroupMembers));
        dlist_delete(&proc->lockGroupLink);
        if (dlist_is_empty(&leader->lockGroupMembers))
        {
            leader->lockGroupLeader = NULL;
            if (leader != proc)
            {
                /*
                 * We are the last follower and the leader exited earlier; its
                 * PGPROC is still allocated and must be pushed here.
                 */
                push_leader = true;
                proc->lockGroupLeader = NULL;
            }
        }
        else if (leader != proc)
        {
            /* Non-last follower; leader still present in the group. */
            proc->lockGroupLeader = NULL;
        }
        else
        {
            /*
             * We are the leader and followers remain.  Skip our own push; the
             * last follower to exit will push us back to the freelist.
             */
            push_self = false;
        }
        LWLockRelease(leader_lwlock);
    }
 
    /* See comment above, close to DisownLatch() */
    pgstat_reset_wait_event_storage();
 
    MyProc = NULL;
    MyProcNumber = INVALID_PROC_NUMBER;
 
    /* Mark the proc no longer in use */
    proc->pid = 0;
    proc->vxid.procNumber = INVALID_PROC_NUMBER;
    proc->vxid.lxid = InvalidTransactionId;
 
    SpinLockAcquire(&ProcGlobal->freeProcsLock);
    if (push_leader)
    {
        /* Return leader PGPROC (and semaphore) to appropriate freelist */
        dlist_push_head(leader->procgloballist, &leader->freeProcsLink);
    }
    if (push_self)
    {
        Assert(proc->lockGroupLeader == NULL);
        /* Since lockGroupLeader is NULL, lockGroupMembers should be empty. */
        Assert(dlist_is_empty(&proc->lockGroupMembers));
 
        /* Return PGPROC structure (and semaphore) to appropriate freelist */
        dlist_push_tail(procgloballist, &proc->freeProcsLink);
    }
 
    /* Update shared estimate of spins_per_delay */
    ProcGlobal->spins_per_delay = update_spins_per_delay(ProcGlobal->spins_per_delay);
 
    SpinLockRelease(&ProcGlobal->freeProcsLock);
}
 
/*
 * AuxiliaryProcKill() -- Cut-down version of ProcKill for auxiliary
 *      processes (bgwriter, etc).  The PGPROC and sema are not released, only
 *      marked as not-in-use.
 */
static void
AuxiliaryProcKill(int code, Datum arg)
{
    int         proctype = DatumGetInt32(arg);
    PGPROC     *auxproc PG_USED_FOR_ASSERTS_ONLY;
    PGPROC     *proc;
 
    Assert(proctype >= 0 && proctype < NUM_AUXILIARY_PROCS);
 
    /* not safe if forked by system(), etc. */
    if (MyProc->pid != (int) getpid())
        elog(PANIC, "AuxiliaryProcKill() called in child process");
 
    auxproc = &AuxiliaryProcs[proctype];
 
    Assert(MyProc == auxproc);
 
    /* Release any LW locks I am holding (see notes above) */
    LWLockReleaseAll();
 
    /* Cancel any pending condition variable sleep, too */
    ConditionVariableCancelSleep();
 
    /* look at the equivalent ProcKill() code for comments */
    SwitchBackToLocalLatch();
    pgstat_reset_wait_event_storage();
 
    /*
     * If this was one of the aux processes advertised in ProcGlobal, clear it
     */
    if (MyBackendType == B_WAL_WRITER)
    {
        Assert(pg_atomic_read_u32(&ProcGlobal->walwriterProc) == MyProcNumber);
        pg_atomic_write_u32(&ProcGlobal->walwriterProc, INVALID_PROC_NUMBER);
    }
    if (MyBackendType == B_CHECKPOINTER)
    {
        Assert(pg_atomic_read_u32(&ProcGlobal->checkpointerProc) == MyProcNumber);
        pg_atomic_write_u32(&ProcGlobal->checkpointerProc, INVALID_PROC_NUMBER);
    }
 
    proc = MyProc;
    MyProc = NULL;
    MyProcNumber = INVALID_PROC_NUMBER;
    DisownLatch(&proc->procLatch);
 
    SpinLockAcquire(&ProcGlobal->freeProcsLock);
 
    /* Mark auxiliary proc no longer in use */
    proc->pid = 0;
    proc->vxid.procNumber = INVALID_PROC_NUMBER;
    proc->vxid.lxid = InvalidTransactionId;
 
    /* Update shared estimate of spins_per_delay */
    ProcGlobal->spins_per_delay = update_spins_per_delay(ProcGlobal->spins_per_delay);
 
    SpinLockRelease(&ProcGlobal->freeProcsLock);
}
 
/*
 * AuxiliaryPidGetProc -- get PGPROC for an auxiliary process
 * given its PID
 *
 * Returns NULL if not found.
 */
PGPROC *
AuxiliaryPidGetProc(int pid)
{
    PGPROC     *result = NULL;
    int         index;
 
    if (pid == 0)               /* never match dummy PGPROCs */
        return NULL;
 
    for (index = 0; index < NUM_AUXILIARY_PROCS; index++)
    {
        PGPROC     *proc = &AuxiliaryProcs[index];
 
        if (proc->pid == pid)
        {
            result = proc;
            break;
        }
    }
    return result;
}
 
 
/*
 * JoinWaitQueue -- join the wait queue on the specified lock
 *
 * It's not actually guaranteed that we need to wait when this function is
 * called, because it could be that when we try to find a position at which
 * to insert ourself into the wait queue, we discover that we must be inserted
 * ahead of everyone who wants a lock that conflict with ours. In that case,
 * we get the lock immediately. Because of this, it's sensible for this function
 * to have a dontWait argument, despite the name.
 *
 * On entry, the caller has already set up LOCK and PROCLOCK entries to
 * reflect that we have "requested" the lock.  The caller is responsible for
 * cleaning that up, if we end up not joining the queue after all.
 *
 * The lock table's partition lock must be held at entry, and is still held
 * at exit.  The caller must release it before calling ProcSleep().
 *
 * Result is one of the following:
 *
 *  PROC_WAIT_STATUS_OK       - lock was immediately granted
 *  PROC_WAIT_STATUS_WAITING  - joined the wait queue; call ProcSleep()
 *  PROC_WAIT_STATUS_ERROR    - immediate deadlock was detected, or would
 *                              need to wait and dontWait == true
 *
 * NOTES: The process queue is now a priority queue for locking.
 */
ProcWaitStatus
JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
{
    LOCKMODE    lockmode = locallock->tag.mode;
    LOCK       *lock = locallock->lock;
    PROCLOCK   *proclock = locallock->proclock;
    uint32      hashcode = locallock->hashcode;
    LWLock     *partitionLock PG_USED_FOR_ASSERTS_ONLY = LockHashPartitionLock(hashcode);
    dclist_head *waitQueue = &lock->waitProcs;
    PGPROC     *insert_before = NULL;
    LOCKMASK    myProcHeldLocks;
    LOCKMASK    myHeldLocks;
    bool        early_deadlock = false;
    PGPROC     *leader = MyProc->lockGroupLeader;
 
    Assert(LWLockHeldByMeInMode(partitionLock, LW_EXCLUSIVE));
 
    /*
     * Set bitmask of locks this process already holds on this object.
     */
    myHeldLocks = MyProc->heldLocks = proclock->holdMask;
 
    /*
     * Determine which locks we're already holding.
     *
     * If group locking is in use, locks held by members of my locking group
     * need to be included in myHeldLocks.  This is not required for relation
     * extension lock which conflict among group members. However, including
     * them in myHeldLocks will give group members the priority to get those
     * locks as compared to other backends which are also trying to acquire
     * those locks.  OTOH, we can avoid giving priority to group members for
     * that kind of locks, but there doesn't appear to be a clear advantage of
     * the same.
     */
    myProcHeldLocks = proclock->holdMask;
    myHeldLocks = myProcHeldLocks;
    if (leader != NULL)
    {
        dlist_iter  iter;
 
        dlist_foreach(iter, &lock->procLocks)
        {
            PROCLOCK   *otherproclock;
 
            otherproclock = dlist_container(PROCLOCK, lockLink, iter.cur);
 
            if (otherproclock->groupLeader == leader)
                myHeldLocks |= otherproclock->holdMask;
        }
    }
 
    /*
     * Determine where to add myself in the wait queue.
     *
     * Normally I should go at the end of the queue.  However, if I already
     * hold locks that conflict with the request of any previous waiter, put
     * myself in the queue just in front of the first such waiter. This is not
     * a necessary step, since deadlock detection would move me to before that
     * waiter anyway; but it's relatively cheap to detect such a conflict
     * immediately, and avoid delaying till deadlock timeout.
     *
     * Special case: if I find I should go in front of some waiter, check to
     * see if I conflict with already-held locks or the requests before that
     * waiter.  If not, then just grant myself the requested lock immediately.
     * This is the same as the test for immediate grant in LockAcquire, except
     * we are only considering the part of the wait queue before my insertion
     * point.
     */
    if (myHeldLocks != 0 && !dclist_is_empty(waitQueue))
    {
        LOCKMASK    aheadRequests = 0;
        dlist_iter  iter;
 
        dclist_foreach(iter, waitQueue)
        {
            PGPROC     *proc = dlist_container(PGPROC, waitLink, iter.cur);
 
            /*
             * If we're part of the same locking group as this waiter, its
             * locks neither conflict with ours nor contribute to
             * aheadRequests.
             */
            if (leader != NULL && leader == proc->lockGroupLeader)
                continue;
 
            /* Must he wait for me? */
            if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
            {
                /* Must I wait for him ? */
                if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
                {
                    /*
                     * Yes, so we have a deadlock.  Easiest way to clean up
                     * correctly is to call RemoveFromWaitQueue(), but we
                     * can't do that until we are *on* the wait queue. So, set
                     * a flag to check below, and break out of loop.  Also,
                     * record deadlock info for later message.
                     */
                    RememberSimpleDeadLock(MyProc, lockmode, lock, proc);
                    early_deadlock = true;
                    break;
                }
                /* I must go before this waiter.  Check special case. */
                if ((lockMethodTable->conflictTab[lockmode] & aheadRequests) == 0 &&
                    !LockCheckConflicts(lockMethodTable, lockmode, lock,
                                        proclock))
                {
                    /* Skip the wait and just grant myself the lock. */
                    GrantLock(lock, proclock, lockmode);
                    return PROC_WAIT_STATUS_OK;
                }
 
                /* Put myself into wait queue before conflicting process */
                insert_before = proc;
                break;
            }
            /* Nope, so advance to next waiter */
            aheadRequests |= LOCKBIT_ON(proc->waitLockMode);
        }
    }
 
    /*
     * If we detected deadlock, give up without waiting.  This must agree with
     * CheckDeadLock's recovery code.
     */
    if (early_deadlock)
        return PROC_WAIT_STATUS_ERROR;
 
    /*
     * At this point we know that we'd really need to sleep. If we've been
     * commanded not to do that, bail out.
     */
    if (dontWait)
        return PROC_WAIT_STATUS_ERROR;
 
    /*
     * Insert self into queue, at the position determined above.
     */
    if (insert_before)
        dclist_insert_before(waitQueue, &insert_before->waitLink, &MyProc->waitLink);
    else
        dclist_push_tail(waitQueue, &MyProc->waitLink);
 
    lock->waitMask |= LOCKBIT_ON(lockmode);
 
    /* Set up wait information in PGPROC object, too */
    MyProc->heldLocks = myProcHeldLocks;
    MyProc->waitLock = lock;
    MyProc->waitProcLock = proclock;
    MyProc->waitLockMode = lockmode;
 
    MyProc->waitStatus = PROC_WAIT_STATUS_WAITING;
 
    return PROC_WAIT_STATUS_WAITING;
}
 
/*
 * ProcSleep -- put process to sleep waiting on lock
 *
 * This must be called when JoinWaitQueue() returns PROC_WAIT_STATUS_WAITING.
 * Returns after the lock has been granted, or if a deadlock is detected.  Can
 * also bail out with ereport(ERROR), if some other error condition, or a
 * timeout or cancellation is triggered.
 *
 * Result is one of the following:
 *
 *  PROC_WAIT_STATUS_OK      - lock was granted
 *  PROC_WAIT_STATUS_ERROR   - a deadlock was detected
 */
ProcWaitStatus
ProcSleep(LOCALLOCK *locallock)
{
    LOCKMODE    lockmode = locallock->tag.mode;
    LOCK       *lock = locallock->lock;
    uint32      hashcode = locallock->hashcode;
    LWLock     *partitionLock = LockHashPartitionLock(hashcode);
    TimestampTz standbyWaitStart = 0;
    bool        allow_autovacuum_cancel = true;
    bool        logged_recovery_conflict = false;
    bool        logged_lock_wait = false;
    ProcWaitStatus myWaitStatus;
    DeadLockState deadlock_state;
 
    /* The caller must've armed the on-error cleanup mechanism */
    Assert(GetAwaitedLock() == locallock);
    Assert(!LWLockHeldByMe(partitionLock));
 
    /*
     * Now that we will successfully clean up after an ereport, it's safe to
     * check to see if there's a buffer pin deadlock against the Startup
     * process.  Of course, that's only necessary if we're doing Hot Standby
     * and are not the Startup process ourselves.
     */
    if (RecoveryInProgress() && !InRecovery)
        CheckRecoveryConflictDeadlock();
 
    /* Reset deadlock_state before enabling the timeout handler */
    deadlock_state = DS_NOT_YET_CHECKED;
    got_deadlock_timeout = false;
 
    /*
     * Set timer so we can wake up after awhile and check for a deadlock. If a
     * deadlock is detected, the handler sets MyProc->waitStatus =
     * PROC_WAIT_STATUS_ERROR, allowing us to know that we must report failure
     * rather than success.
     *
     * By delaying the check until we've waited for a bit, we can avoid
     * running the rather expensive deadlock-check code in most cases.
     *
     * If LockTimeout is set, also enable the timeout for that.  We can save a
     * few cycles by enabling both timeout sources in one call.
     *
     * If InHotStandby we set lock waits slightly later for clarity with other
     * code.
     */
    if (!InHotStandby)
    {
        if (LockTimeout > 0)
        {
            EnableTimeoutParams timeouts[2];
 
            timeouts[0].id = DEADLOCK_TIMEOUT;
            timeouts[0].type = TMPARAM_AFTER;
            timeouts[0].delay_ms = DeadlockTimeout;
            timeouts[1].id = LOCK_TIMEOUT;
            timeouts[1].type = TMPARAM_AFTER;
            timeouts[1].delay_ms = LockTimeout;
            enable_timeouts(timeouts, 2);
        }
        else
            enable_timeout_after(DEADLOCK_TIMEOUT, DeadlockTimeout);
 
        /*
         * Use the current time obtained for the deadlock timeout timer as
         * waitStart (i.e., the time when this process started waiting for the
         * lock). Since getting the current time newly can cause overhead, we
         * reuse the already-obtained time to avoid that overhead.
         *
         * Note that waitStart is updated without holding the lock table's
         * partition lock, to avoid the overhead by additional lock
         * acquisition. This can cause "waitstart" in pg_locks to become NULL
         * for a very short period of time after the wait started even though
         * "granted" is false. This is OK in practice because we can assume
         * that users are likely to look at "waitstart" when waiting for the
         * lock for a long time.
         */
        pg_atomic_write_u64(&MyProc->waitStart,
                            get_timeout_start_time(DEADLOCK_TIMEOUT));
    }
    else if (log_recovery_conflict_waits)
    {
        /*
         * Set the wait start timestamp if logging is enabled and in hot
         * standby.
         */
        standbyWaitStart = GetCurrentTimestamp();
    }
 
    /*
     * If somebody wakes us between LWLockRelease and WaitLatch, the latch
     * will not wait. But a set latch does not necessarily mean that the lock
     * is free now, as there are many other sources for latch sets than
     * somebody releasing the lock.
     *
     * We process interrupts whenever the latch has been set, so cancel/die
     * interrupts are processed quickly. This means we must not mind losing
     * control to a cancel/die interrupt here.  We don't, because we have no
     * shared-state-change work to do after being granted the lock (the
     * grantor did it all).  We do have to worry about canceling the deadlock
     * timeout and updating the locallock table, but if we lose control to an
     * error, LockErrorCleanup will fix that up.
     */
    do
    {
        if (InHotStandby)
        {
            bool        maybe_log_conflict =
                (standbyWaitStart != 0 && !logged_recovery_conflict);
 
            /* Set a timer and wait for that or for the lock to be granted */
            ResolveRecoveryConflictWithLock(locallock->tag.lock,
                                            maybe_log_conflict);
 
            /*
             * Emit the log message if the startup process is waiting longer
             * than deadlock_timeout for recovery conflict on lock.
             */
            if (maybe_log_conflict)
            {
                TimestampTz now = GetCurrentTimestamp();
 
                if (TimestampDifferenceExceeds(standbyWaitStart, now,
                                               DeadlockTimeout))
                {
                    VirtualTransactionId *vxids;
                    int         cnt;
 
                    vxids = GetLockConflicts(&locallock->tag.lock,
                                             AccessExclusiveLock, &cnt);
 
                    /*
                     * Log the recovery conflict and the list of PIDs of
                     * backends holding the conflicting lock. Note that we do
                     * logging even if there are no such backends right now
                     * because the startup process here has already waited
                     * longer than deadlock_timeout.
                     */
                    LogRecoveryConflict(RECOVERY_CONFLICT_LOCK,
                                        standbyWaitStart, now,
                                        cnt > 0 ? vxids : NULL, true);
                    logged_recovery_conflict = true;
                }
            }
        }
        else
        {
            (void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
                             PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
            ResetLatch(MyLatch);
            /* check for deadlocks first, as that's probably log-worthy */
            if (got_deadlock_timeout)
            {
                deadlock_state = CheckDeadLock();
                got_deadlock_timeout = false;
            }
            CHECK_FOR_INTERRUPTS();
        }
 
        /*
         * waitStatus could change from PROC_WAIT_STATUS_WAITING to something
         * else asynchronously.  Read it just once per loop to prevent
         * surprising behavior (such as missing log messages).
         */
        myWaitStatus = *((volatile ProcWaitStatus *) &MyProc->waitStatus);
 
        /*
         * If we are not deadlocked, but are waiting on an autovacuum-induced
         * task, send a signal to interrupt it.
         */
        if (deadlock_state == DS_BLOCKED_BY_AUTOVACUUM && allow_autovacuum_cancel)
        {
            PGPROC     *autovac = GetBlockingAutoVacuumPgproc();
            uint8       statusFlags;
            uint8       lockmethod_copy;
            LOCKTAG     locktag_copy;
 
            /*
             * Grab info we need, then release lock immediately.  Note this
             * coding means that there is a tiny chance that the process
             * terminates its current transaction and starts a different one
             * before we have a change to send the signal; the worst possible
             * consequence is that a for-wraparound vacuum is canceled.  But
             * that could happen in any case unless we were to do kill() with
             * the lock held, which is much more undesirable.
             */
            LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
            statusFlags = ProcGlobal->statusFlags[autovac->pgxactoff];
            lockmethod_copy = lock->tag.locktag_lockmethodid;
            locktag_copy = lock->tag;
            LWLockRelease(ProcArrayLock);
 
            /*
             * Only do it if the worker is not working to protect against Xid
             * wraparound.
             */
            if ((statusFlags & PROC_IS_AUTOVACUUM) &&
                !(statusFlags & PROC_VACUUM_FOR_WRAPAROUND))
            {
                int         pid = autovac->pid;
 
                /* report the case, if configured to do so */
                if (message_level_is_interesting(DEBUG1))
                {
                    StringInfoData locktagbuf;
                    StringInfoData logbuf;  /* errdetail for server log */
 
                    initStringInfo(&locktagbuf);
                    initStringInfo(&logbuf);
                    DescribeLockTag(&locktagbuf, &locktag_copy);
                    appendStringInfo(&logbuf,
                                     "Process %d waits for %s on %s.",
                                     MyProcPid,
                                     GetLockmodeName(lockmethod_copy, lockmode),
                                     locktagbuf.data);
 
                    ereport(DEBUG1,
                            (errmsg_internal("sending cancel to blocking autovacuum PID %d",
                                             pid),
                             errdetail_log("%s", logbuf.data)));
 
                    pfree(locktagbuf.data);
                    pfree(logbuf.data);
                }
 
                /* send the autovacuum worker Back to Old Kent Road */
                if (kill(pid, SIGINT) < 0)
                {
                    /*
                     * There's a race condition here: once we release the
                     * ProcArrayLock, it's possible for the autovac worker to
                     * close up shop and exit before we can do the kill().
                     * Therefore, we do not whinge about no-such-process.
                     * Other errors such as EPERM could conceivably happen if
                     * the kernel recycles the PID fast enough, but such cases
                     * seem improbable enough that it's probably best to issue
                     * a warning if we see some other errno.
                     */
                    if (errno != ESRCH)
                        ereport(WARNING,
                                (errmsg("could not send signal to process %d: %m",
                                        pid)));
                }
            }
 
            /* prevent signal from being sent again more than once */
            allow_autovacuum_cancel = false;
        }
 
        /*
         * If awoken after the deadlock check interrupt has run, increment the
         * lock statistics counters and if log_lock_waits is on, then report
         * about the wait.
         */
        if (deadlock_state != DS_NOT_YET_CHECKED)
        {
            long        secs;
            int         usecs;
            long        msecs;
 
            INJECTION_POINT("deadlock-timeout-fired", NULL);
            TimestampDifference(get_timeout_start_time(DEADLOCK_TIMEOUT),
                                GetCurrentTimestamp(),
                                &secs, &usecs);
            /* Increment the lock statistics counters if done waiting. */
            if (myWaitStatus == PROC_WAIT_STATUS_OK)
                pgstat_count_lock_waits(locallock->tag.lock.locktag_type,
                                        (PgStat_Counter) secs * 1000000 + usecs);
 
            msecs = secs * 1000 + usecs / 1000;
            usecs = usecs % 1000;
 
            if (log_lock_waits)
            {
                StringInfoData buf,
                            lock_waiters_sbuf,
                            lock_holders_sbuf;
                const char *modename;
                int         lockHoldersNum = 0;
 
                initStringInfo(&buf);
                initStringInfo(&lock_waiters_sbuf);
                initStringInfo(&lock_holders_sbuf);
 
                DescribeLockTag(&buf, &locallock->tag.lock);
                modename = GetLockmodeName(locallock->tag.lock.locktag_lockmethodid,
                                           lockmode);
 
                /* Gather a list of all lock holders and waiters */
                LWLockAcquire(partitionLock, LW_SHARED);
                GetLockHoldersAndWaiters(locallock, &lock_holders_sbuf,
                                         &lock_waiters_sbuf, &lockHoldersNum);
                LWLockRelease(partitionLock);
 
                if (deadlock_state == DS_SOFT_DEADLOCK)
                    ereport(LOG,
                            (errmsg("process %d avoided deadlock for %s on %s by rearranging queue order after %ld.%03d ms",
                                    MyProcPid, modename, buf.data, msecs, usecs),
                             (errdetail_log_plural("Process holding the lock: %s. Wait queue: %s.",
                                                   "Processes holding the lock: %s. Wait queue: %s.",
                                                   lockHoldersNum, lock_holders_sbuf.data, lock_waiters_sbuf.data))));
                else if (deadlock_state == DS_HARD_DEADLOCK)
                {
                    /*
                     * This message is a bit redundant with the error that
                     * will be reported subsequently, but in some cases the
                     * error report might not make it to the log (eg, if it's
                     * caught by an exception handler), and we want to ensure
                     * all long-wait events get logged.
                     */
                    ereport(LOG,
                            (errmsg("process %d detected deadlock while waiting for %s on %s after %ld.%03d ms",
                                    MyProcPid, modename, buf.data, msecs, usecs),
                             (errdetail_log_plural("Process holding the lock: %s. Wait queue: %s.",
                                                   "Processes holding the lock: %s. Wait queue: %s.",
                                                   lockHoldersNum, lock_holders_sbuf.data, lock_waiters_sbuf.data))));
                }
 
                if (myWaitStatus == PROC_WAIT_STATUS_WAITING)
                {
                    /*
                     * Guard the "still waiting on lock" log message so it is
                     * reported at most once while waiting for the lock.
                     *
                     * Without this guard, the message can be emitted whenever
                     * the lock-wait sleep is interrupted (for example by
                     * SIGHUP for config reload or by
                     * client_connection_check_interval). For example, if
                     * client_connection_check_interval is set very low (e.g.,
                     * 100 ms), the message could be logged repeatedly,
                     * flooding the log and making it difficult to use.
                     */
                    if (!logged_lock_wait)
                    {
                        ereport(LOG,
                                (errmsg("process %d still waiting for %s on %s after %ld.%03d ms",
                                        MyProcPid, modename, buf.data, msecs, usecs),
                                 (errdetail_log_plural("Process holding the lock: %s. Wait queue: %s.",
                                                       "Processes holding the lock: %s. Wait queue: %s.",
                                                       lockHoldersNum, lock_holders_sbuf.data, lock_waiters_sbuf.data))));
                        logged_lock_wait = true;
                    }
                }
                else if (myWaitStatus == PROC_WAIT_STATUS_OK)
                    ereport(LOG,
                            (errmsg("process %d acquired %s on %s after %ld.%03d ms",
                                    MyProcPid, modename, buf.data, msecs, usecs)));
                else
                {
                    Assert(myWaitStatus == PROC_WAIT_STATUS_ERROR);
 
                    /*
                     * Currently, the deadlock checker always kicks its own
                     * process, which means that we'll only see
                     * PROC_WAIT_STATUS_ERROR when deadlock_state ==
                     * DS_HARD_DEADLOCK, and there's no need to print
                     * redundant messages.  But for completeness and
                     * future-proofing, print a message if it looks like
                     * someone else kicked us off the lock.
                     */
                    if (deadlock_state != DS_HARD_DEADLOCK)
                        ereport(LOG,
                                (errmsg("process %d failed to acquire %s on %s after %ld.%03d ms",
                                        MyProcPid, modename, buf.data, msecs, usecs),
                                 (errdetail_log_plural("Process holding the lock: %s. Wait queue: %s.",
                                                       "Processes holding the lock: %s. Wait queue: %s.",
                                                       lockHoldersNum, lock_holders_sbuf.data, lock_waiters_sbuf.data))));
                }
                pfree(buf.data);
                pfree(lock_holders_sbuf.data);
                pfree(lock_waiters_sbuf.data);
            }
 
            /*
             * At this point we might still need to wait for the lock. Reset
             * state so we don't print the above messages again if
             * log_lock_waits is on.
             */
            deadlock_state = DS_NO_DEADLOCK;
        }
    } while (myWaitStatus == PROC_WAIT_STATUS_WAITING);
 
    /*
     * Disable the timers, if they are still running.  As in LockErrorCleanup,
     * we must preserve the LOCK_TIMEOUT indicator flag: if a lock timeout has
     * already caused QueryCancelPending to become set, we want the cancel to
     * be reported as a lock timeout, not a user cancel.
     */
    if (!InHotStandby)
    {
        if (LockTimeout > 0)
        {
            DisableTimeoutParams timeouts[2];
 
            timeouts[0].id = DEADLOCK_TIMEOUT;
            timeouts[0].keep_indicator = false;
            timeouts[1].id = LOCK_TIMEOUT;
            timeouts[1].keep_indicator = true;
            disable_timeouts(timeouts, 2);
        }
        else
            disable_timeout(DEADLOCK_TIMEOUT, false);
    }
 
    /*
     * Emit the log message if recovery conflict on lock was resolved but the
     * startup process waited longer than deadlock_timeout for it.
     */
    if (InHotStandby && logged_recovery_conflict)
        LogRecoveryConflict(RECOVERY_CONFLICT_LOCK,
                            standbyWaitStart, GetCurrentTimestamp(),
                            NULL, false);
 
    /*
     * We don't have to do anything else, because the awaker did all the
     * necessary updates of the lock table and MyProc. (The caller is
     * responsible for updating the local lock table.)
     */
    return myWaitStatus;
}
 
 
/*
 * ProcWakeup -- wake up a process by setting its latch.
 *
 *   Also remove the process from the wait queue and set its waitLink invalid.
 *
 * The appropriate lock partition lock must be held by caller.
 *
 * XXX: presently, this code is only used for the "success" case, and only
 * works correctly for that case.  To clean up in failure case, would need
 * to twiddle the lock's request counts too --- see RemoveFromWaitQueue.
 * Hence, in practice the waitStatus parameter must be PROC_WAIT_STATUS_OK.
 */
void
ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus)
{
    if (dlist_node_is_detached(&proc->waitLink))
        return;
 
    Assert(proc->waitStatus == PROC_WAIT_STATUS_WAITING);
 
    /* Remove process from wait queue */
    dclist_delete_from_thoroughly(&proc->waitLock->waitProcs, &proc->waitLink);
 
    /* Clean up process' state and pass it the ok/fail signal */
    proc->waitLock = NULL;
    proc->waitProcLock = NULL;
    proc->waitStatus = waitStatus;
    pg_atomic_write_u64(&proc->waitStart, 0);
 
    /* And awaken it */
    SetLatch(&proc->procLatch);
}
 
/*
 * ProcLockWakeup -- routine for waking up processes when a lock is
 *      released (or a prior waiter is aborted).  Scan all waiters
 *      for lock, waken any that are no longer blocked.
 *
 * The appropriate lock partition lock must be held by caller.
 */
void
ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
{
    dclist_head *waitQueue = &lock->waitProcs;
    LOCKMASK    aheadRequests = 0;
    dlist_mutable_iter miter;
 
    if (dclist_is_empty(waitQueue))
        return;
 
    dclist_foreach_modify(miter, waitQueue)
    {
        PGPROC     *proc = dlist_container(PGPROC, waitLink, miter.cur);
        LOCKMODE    lockmode = proc->waitLockMode;
 
        /*
         * Waken if (a) doesn't conflict with requests of earlier waiters, and
         * (b) doesn't conflict with already-held locks.
         */
        if ((lockMethodTable->conflictTab[lockmode] & aheadRequests) == 0 &&
            !LockCheckConflicts(lockMethodTable, lockmode, lock,
                                proc->waitProcLock))
        {
            /* OK to waken */
            GrantLock(lock, proc->waitProcLock, lockmode);
            /* removes proc from the lock's waiting process queue */
            ProcWakeup(proc, PROC_WAIT_STATUS_OK);
        }
        else
        {
            /*
             * Lock conflicts: Don't wake, but remember requested mode for
             * later checks.
             */
            aheadRequests |= LOCKBIT_ON(lockmode);
        }
    }
}
 
/*
 * CheckDeadLock
 *
 * We only get to this routine, if DEADLOCK_TIMEOUT fired while waiting for a
 * lock to be released by some other process.  Check if there's a deadlock; if
 * not, just return.  If we have a real deadlock, remove ourselves from the
 * lock's wait queue.
 */
static DeadLockState
CheckDeadLock(void)
{
    int         i;
    DeadLockState result;
 
    /*
     * Acquire exclusive lock on the entire shared lock data structures. Must
     * grab LWLocks in partition-number order to avoid LWLock deadlock.
     *
     * Note that the deadlock check interrupt had better not be enabled
     * anywhere that this process itself holds lock partition locks, else this
     * will wait forever.  Also note that LWLockAcquire creates a critical
     * section, so that this routine cannot be interrupted by cancel/die
     * interrupts.
     */
    for (i = 0; i < NUM_LOCK_PARTITIONS; i++)
        LWLockAcquire(LockHashPartitionLockByIndex(i), LW_EXCLUSIVE);
 
    /*
     * Check to see if we've been awoken by anyone in the interim.
     *
     * If we have, we can return and resume our transaction -- happy day.
     * Before we are awoken the process releasing the lock grants it to us so
     * we know that we don't have to wait anymore.
     *
     * We check by looking to see if we've been unlinked from the wait queue.
     * This is safe because we hold the lock partition lock.
     */
    if (dlist_node_is_detached(&MyProc->waitLink))
    {
        result = DS_NO_DEADLOCK;
        goto check_done;
    }
 
#ifdef LOCK_DEBUG
    if (Debug_deadlocks)
        DumpAllLocks();
#endif
 
    /* Run the deadlock check */
    result = DeadLockCheck(MyProc);
 
    if (result == DS_HARD_DEADLOCK)
    {
        /*
         * Oops.  We have a deadlock.
         *
         * Get this process out of wait state. (Note: we could do this more
         * efficiently by relying on lockAwaited, but use this coding to
         * preserve the flexibility to kill some other transaction than the
         * one detecting the deadlock.)
         *
         * RemoveFromWaitQueue sets MyProc->waitStatus to
         * PROC_WAIT_STATUS_ERROR, so ProcSleep will report an error after we
         * return.
         */
        Assert(MyProc->waitLock != NULL);
        RemoveFromWaitQueue(MyProc, LockTagHashCode(&(MyProc->waitLock->tag)));
 
        /*
         * We're done here.  Transaction abort caused by the error that
         * ProcSleep will raise will cause any other locks we hold to be
         * released, thus allowing other processes to wake up; we don't need
         * to do that here.  NOTE: an exception is that releasing locks we
         * hold doesn't consider the possibility of waiters that were blocked
         * behind us on the lock we just failed to get, and might now be
         * wakable because we're not in front of them anymore.  However,
         * RemoveFromWaitQueue took care of waking up any such processes.
         */
    }
 
    /*
     * And release locks.  We do this in reverse order for two reasons: (1)
     * Anyone else who needs more than one of the locks will be trying to lock
     * them in increasing order; we don't want to release the other process
     * until it can get all the locks it needs. (2) This avoids O(N^2)
     * behavior inside LWLockRelease.
     */
check_done:
    for (i = NUM_LOCK_PARTITIONS; --i >= 0;)
        LWLockRelease(LockHashPartitionLockByIndex(i));
 
    return result;
}
 
/*
 * CheckDeadLockAlert - Handle the expiry of deadlock_timeout.
 *
 * NB: Runs inside a signal handler, be careful.
 */
void
CheckDeadLockAlert(void)
{
    int         save_errno = errno;
 
    got_deadlock_timeout = true;
 
    /*
     * Have to set the latch again, even if handle_sig_alarm already did. Back
     * then got_deadlock_timeout wasn't yet set... It's unlikely that this
     * ever would be a problem, but setting a set latch again is cheap.
     *
     * Note that, when this function runs inside procsignal_sigusr1_handler(),
     * the handler function sets the latch again after the latch is set here.
     */
    SetLatch(MyLatch);
    errno = save_errno;
}
 
/*
 * GetLockHoldersAndWaiters - get lock holders and waiters for a lock
 *
 * Fill lock_holders_sbuf and lock_waiters_sbuf with the PIDs of processes holding
 * and waiting for the lock, and set lockHoldersNum to the number of lock holders.
 *
 * The lock table's partition lock must be held on entry and remains held on exit.
 */
void
GetLockHoldersAndWaiters(LOCALLOCK *locallock, StringInfo lock_holders_sbuf,
                         StringInfo lock_waiters_sbuf, int *lockHoldersNum)
{
    dlist_iter  proc_iter;
    PROCLOCK   *curproclock;
    LOCK       *lock = locallock->lock;
    bool        first_holder = true,
                first_waiter = true;
 
#ifdef USE_ASSERT_CHECKING
    {
        uint32      hashcode = locallock->hashcode;
        LWLock     *partitionLock = LockHashPartitionLock(hashcode);
 
        Assert(LWLockHeldByMe(partitionLock));
    }
#endif
 
    *lockHoldersNum = 0;
 
    /*
     * Loop over the lock's procLocks to gather a list of all holders and
     * waiters. Thus we will be able to provide more detailed information for
     * lock debugging purposes.
     *
     * lock->procLocks contains all processes which hold or wait for this
     * lock.
     */
    dlist_foreach(proc_iter, &lock->procLocks)
    {
        curproclock =
            dlist_container(PROCLOCK, lockLink, proc_iter.cur);
 
        /*
         * We are a waiter if myProc->waitProcLock == curproclock; we are a
         * holder if it is NULL or something different.
         */
        if (curproclock->tag.myProc->waitProcLock == curproclock)
        {
            if (first_waiter)
            {
                appendStringInfo(lock_waiters_sbuf, "%d",
                                 curproclock->tag.myProc->pid);
                first_waiter = false;
            }
            else
                appendStringInfo(lock_waiters_sbuf, ", %d",
                                 curproclock->tag.myProc->pid);
        }
        else
        {
            if (first_holder)
            {
                appendStringInfo(lock_holders_sbuf, "%d",
                                 curproclock->tag.myProc->pid);
                first_holder = false;
            }
            else
                appendStringInfo(lock_holders_sbuf, ", %d",
                                 curproclock->tag.myProc->pid);
 
            (*lockHoldersNum)++;
        }
    }
}
 
/*
 * ProcWaitForSignal - wait for a signal from another backend.
 *
 * As this uses the generic process latch the caller has to be robust against
 * unrelated wakeups: Always check that the desired state has occurred, and
 * wait again if not.
 */
void
ProcWaitForSignal(uint32 wait_event_info)
{
    (void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
                     wait_event_info);
    ResetLatch(MyLatch);
    CHECK_FOR_INTERRUPTS();
}
 
/*
 * ProcSendSignal - set the latch of a backend identified by ProcNumber
 */
void
ProcSendSignal(ProcNumber procNumber)
{
    if (procNumber < 0 || procNumber >= ProcGlobal->allProcCount)
        elog(ERROR, "procNumber out of range");
 
    SetLatch(&GetPGProcByNumber(procNumber)->procLatch);
}
 
/*
 * BecomeLockGroupLeader - designate process as lock group leader
 *
 * Once this function has returned, other processes can join the lock group
 * by calling BecomeLockGroupMember.
 */
void
BecomeLockGroupLeader(void)
{
    LWLock     *leader_lwlock;
 
    /* If we already did it, we don't need to do it again. */
    if (MyProc->lockGroupLeader == MyProc)
        return;
 
    /* We had better not be a follower. */
    Assert(MyProc->lockGroupLeader == NULL);
 
    /* Create single-member group, containing only ourselves. */
    leader_lwlock = LockHashPartitionLockByProc(MyProc);
    LWLockAcquire(leader_lwlock, LW_EXCLUSIVE);
    MyProc->lockGroupLeader = MyProc;
    dlist_push_head(&MyProc->lockGroupMembers, &MyProc->lockGroupLink);
    LWLockRelease(leader_lwlock);
}
 
/*
 * BecomeLockGroupMember - designate process as lock group member
 *
 * This is pretty straightforward except for the possibility that the leader
 * whose group we're trying to join might exit before we manage to do so;
 * and the PGPROC might get recycled for an unrelated process.  To avoid
 * that, we require the caller to pass the PID of the intended PGPROC as
 * an interlock.  Returns true if we successfully join the intended lock
 * group, and false if not.
 */
bool
BecomeLockGroupMember(PGPROC *leader, int pid)
{
    LWLock     *leader_lwlock;
    bool        ok = false;
 
    /* Group leader can't become member of group */
    Assert(MyProc != leader);
 
    /* Can't already be a member of a group */
    Assert(MyProc->lockGroupLeader == NULL);
 
    /* PID must be valid. */
    Assert(pid != 0);
 
    /*
     * Get lock protecting the group fields.  Note LockHashPartitionLockByProc
     * calculates the proc number based on the PGPROC slot without looking at
     * its contents, so we will acquire the correct lock even if the leader
     * PGPROC is in process of being recycled.
     */
    leader_lwlock = LockHashPartitionLockByProc(leader);
    LWLockAcquire(leader_lwlock, LW_EXCLUSIVE);
 
    /* Is this the leader we're looking for? */
    if (leader->pid == pid && leader->lockGroupLeader == leader)
    {
        /* OK, join the group */
        ok = true;
        MyProc->lockGroupLeader = leader;
        dlist_push_tail(&leader->lockGroupMembers, &MyProc->lockGroupLink);
    }
    LWLockRelease(leader_lwlock);
 
    return ok;
}