proc.c

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/*-------------------------------------------------------------------------
 *
 * proc.c
 *    routines to manage per-process shared memory data structure
 *
 * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/storage/lmgr/proc.c
 *
 *-------------------------------------------------------------------------
 */
/*
 * Interface (a):
 *      ProcSleep(), ProcWakeup(),
 *      ProcQueueAlloc() -- create a shm queue for sleeping processes
 *      ProcQueueInit() -- create a queue without allocing memory
 *
 * 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/transam.h"
#include "access/twophase.h"
#include "access/xact.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "postmaster/autovacuum.h"
#include "replication/slot.h"
#include "replication/syncrep.h"
#include "replication/walsender.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 "utils/timeout.h"
#include "utils/timestamp.h"

/* GUC variables */
int         DeadlockTimeout = 1000;
int         StatementTimeout = 0;
int         LockTimeout = 0;
int         IdleInTransactionSessionTimeout = 0;
bool        log_lock_waits = false;

/* Pointer to this process's PGPROC struct, if any */
PGPROC     *MyProc = NULL;

/*
 * This spinlock protects the freelist of recycled PGPROC structures.
 * We cannot use an LWLock because the LWLock manager depends on already
 * having a PGPROC and a wait semaphore!  But these structures are touched
 * relatively infrequently (only at backend startup or shutdown) and not for
 * very long, so a spinlock is okay.
 */
NON_EXEC_STATIC slock_t *ProcStructLock = NULL;

/* Pointers to shared-memory structures */
PROC_HDR   *ProcGlobal = NULL;
NON_EXEC_STATIC PGPROC *AuxiliaryProcs = NULL;
PGPROC     *PreparedXactProcs = NULL;

/* If we are waiting for a lock, this points to the associated LOCALLOCK */
static LOCALLOCK *lockAwaited = NULL;

static DeadLockState deadlock_state = DS_NOT_YET_CHECKED;

/* 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 void CheckDeadLock(void);


/*
 * Report shared-memory space needed by InitProcGlobal.
 */
Size
ProcGlobalShmemSize(void)
{
    Size        size = 0;
    Size        TotalProcs =
        add_size(MaxBackends, add_size(NUM_AUXILIARY_PROCS, max_prepared_xacts));

    /* ProcGlobal */
    size = add_size(size, sizeof(PROC_HDR));
    size = add_size(size, mul_size(TotalProcs, sizeof(PGPROC)));
    size = add_size(size, sizeof(slock_t));

    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)));

    return size;
}

/*
 * Report number of semaphores needed by InitProcGlobal.
 */
int
ProcGlobalSemas(void)
{
    /*
     * We need a sema per backend (including autovacuum), plus one for each
     * auxiliary process.
     */
    return MaxBackends + NUM_AUXILIARY_PROCS;
}

/*
 * InitProcGlobal -
 *    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_max_workers 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.
 *
 * Note: this is NOT called by individual backends under a postmaster,
 * not even in the EXEC_BACKEND case.  The ProcGlobal and AuxiliaryProcs
 * pointers must be propagated specially for EXEC_BACKEND operation.
 */
void
InitProcGlobal(void)
{
    PGPROC     *procs;
    int         i,
                j;
    bool        found;
    uint32      TotalProcs = MaxBackends + NUM_AUXILIARY_PROCS + max_prepared_xacts;

    /* Create the ProcGlobal shared structure */
    ProcGlobal = (PROC_HDR *)
        ShmemInitStruct("Proc Header", sizeof(PROC_HDR), &found);
    Assert(!found);

    /*
     * Initialize the data structures.
     */
    ProcGlobal->spins_per_delay = DEFAULT_SPINS_PER_DELAY;
    ProcGlobal->freeProcs = NULL;
    ProcGlobal->autovacFreeProcs = NULL;
    ProcGlobal->bgworkerFreeProcs = NULL;
    ProcGlobal->walsenderFreeProcs = NULL;
    ProcGlobal->startupProc = NULL;
    ProcGlobal->startupProcPid = 0;
    ProcGlobal->startupBufferPinWaitBufId = -1;
    ProcGlobal->walwriterLatch = NULL;
    ProcGlobal->checkpointerLatch = NULL;
    pg_atomic_init_u32(&ProcGlobal->procArrayGroupFirst, INVALID_PGPROCNO);
    pg_atomic_init_u32(&ProcGlobal->clogGroupFirst, INVALID_PGPROCNO);

    /*
     * Create and initialize all the PGPROC structures we'll need.  There are
     * five separate consumers: (1) normal backends, (2) autovacuum workers
     * and the autovacuum launcher, (3) background workers, (4) auxiliary
     * processes, and (5) prepared transactions.  Each PGPROC structure is
     * dedicated to exactly one of these purposes, and they do not move
     * between groups.
     */
    procs = (PGPROC *) ShmemAlloc(TotalProcs * sizeof(PGPROC));
    MemSet(procs, 0, TotalProcs * sizeof(PGPROC));
    ProcGlobal->allProcs = procs;
    /* XXX allProcCount isn't really all of them; it excludes prepared xacts */
    ProcGlobal->allProcCount = MaxBackends + NUM_AUXILIARY_PROCS;

    /*
     * Allocate 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 *) ShmemAlloc(TotalProcs * sizeof(*ProcGlobal->xids));
    MemSet(ProcGlobal->xids, 0, TotalProcs * sizeof(*ProcGlobal->xids));
    ProcGlobal->subxidStates = (XidCacheStatus *) ShmemAlloc(TotalProcs * sizeof(*ProcGlobal->subxidStates));
    MemSet(ProcGlobal->subxidStates, 0, TotalProcs * sizeof(*ProcGlobal->subxidStates));
    ProcGlobal->statusFlags = (uint8 *) ShmemAlloc(TotalProcs * sizeof(*ProcGlobal->statusFlags));
    MemSet(ProcGlobal->statusFlags, 0, TotalProcs * sizeof(*ProcGlobal->statusFlags));

    for (i = 0; i < TotalProcs; i++)
    {
        /* Common initialization for all PGPROCs, regardless of type. */

        /*
         * 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 < MaxBackends + NUM_AUXILIARY_PROCS)
        {
            procs[i].sem = PGSemaphoreCreate();
            InitSharedLatch(&(procs[i].procLatch));
            LWLockInitialize(&(procs[i].fpInfoLock), LWTRANCHE_LOCK_FASTPATH);
        }
        procs[i].pgprocno = i;

        /*
         * Newly created PGPROCs for normal backends, autovacuum and bgworkers
         * 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 */
            procs[i].links.next = (SHM_QUEUE *) ProcGlobal->freeProcs;
            ProcGlobal->freeProcs = &procs[i];
            procs[i].procgloballist = &ProcGlobal->freeProcs;
        }
        else if (i < MaxConnections + autovacuum_max_workers + 1)
        {
            /* PGPROC for AV launcher/worker, add to autovacFreeProcs list */
            procs[i].links.next = (SHM_QUEUE *) ProcGlobal->autovacFreeProcs;
            ProcGlobal->autovacFreeProcs = &procs[i];
            procs[i].procgloballist = &ProcGlobal->autovacFreeProcs;
        }
        else if (i < MaxConnections + autovacuum_max_workers + 1 + max_worker_processes)
        {
            /* PGPROC for bgworker, add to bgworkerFreeProcs list */
            procs[i].links.next = (SHM_QUEUE *) ProcGlobal->bgworkerFreeProcs;
            ProcGlobal->bgworkerFreeProcs = &procs[i];
            procs[i].procgloballist = &ProcGlobal->bgworkerFreeProcs;
        }
        else if (i < MaxBackends)
        {
            /* PGPROC for walsender, add to walsenderFreeProcs list */
            procs[i].links.next = (SHM_QUEUE *) ProcGlobal->walsenderFreeProcs;
            ProcGlobal->walsenderFreeProcs = &procs[i];
            procs[i].procgloballist = &ProcGlobal->walsenderFreeProcs;
        }

        /* Initialize myProcLocks[] shared memory queues. */
        for (j = 0; j < NUM_LOCK_PARTITIONS; j++)
            SHMQueueInit(&(procs[i].myProcLocks[j]));

        /* Initialize lockGroupMembers list. */
        dlist_init(&procs[i].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(&(procs[i].procArrayGroupNext), INVALID_PGPROCNO);
        pg_atomic_init_u32(&(procs[i].clogGroupNext), INVALID_PGPROCNO);
    }

    /*
     * Save pointers to the blocks of PGPROC structures reserved for auxiliary
     * processes and prepared transactions.
     */
    AuxiliaryProcs = &procs[MaxBackends];
    PreparedXactProcs = &procs[MaxBackends + NUM_AUXILIARY_PROCS];

    /* Create ProcStructLock spinlock, too */
    ProcStructLock = (slock_t *) ShmemAlloc(sizeof(slock_t));
    SpinLockInit(ProcStructLock);
}

/*
 * InitProcess -- initialize a per-process data structure for this backend
 */
void
InitProcess(void)
{
    PGPROC     *volatile *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");

    /* Decide which list should supply our PGPROC. */
    if (IsAnyAutoVacuumProcess())
        procgloballist = &ProcGlobal->autovacFreeProcs;
    else if (IsBackgroundWorker)
        procgloballist = &ProcGlobal->bgworkerFreeProcs;
    else if (am_walsender)
        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 ProcStructLock, also copy the current shared
     * estimate of spins_per_delay to local storage.
     */
    SpinLockAcquire(ProcStructLock);

    set_spins_per_delay(ProcGlobal->spins_per_delay);

    MyProc = *procgloballist;

    if (MyProc != NULL)
    {
        *procgloballist = (PGPROC *) MyProc->links.next;
        SpinLockRelease(ProcStructLock);
    }
    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(ProcStructLock);
        if (am_walsender)
            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")));
    }

    /*
     * 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);

    /*
     * Now that we have a PGPROC, mark ourselves as an active postmaster
     * child; this is so that the postmaster can detect it if we exit without
     * cleaning up.  (XXX autovac launcher currently doesn't participate in
     * this; it probably should.)
     */
    if (IsUnderPostmaster && !IsAutoVacuumLauncherProcess())
        MarkPostmasterChildActive();

    /*
     * Initialize all fields of MyProc, except for those previously
     * initialized by InitProcGlobal.
     */
    SHMQueueElemInit(&(MyProc->links));
    MyProc->waitStatus = PROC_WAIT_STATUS_OK;
    MyProc->lxid = InvalidLocalTransactionId;
    MyProc->fpVXIDLock = false;
    MyProc->fpLocalTransactionId = InvalidLocalTransactionId;
    MyProc->xid = InvalidTransactionId;
    MyProc->xmin = InvalidTransactionId;
    MyProc->pid = MyProcPid;
    /* backendId, databaseId and roleId will be filled in later */
    MyProc->backendId = InvalidBackendId;
    MyProc->databaseId = InvalidOid;
    MyProc->roleId = InvalidOid;
    MyProc->tempNamespaceId = InvalidOid;
    MyProc->isBackgroundWorker = IsBackgroundWorker;
    MyProc->delayChkpt = false;
    MyProc->statusFlags = 0;
    /* NB -- autovac launcher intentionally does not set IS_AUTOVACUUM */
    if (IsAutoVacuumWorkerProcess())
        MyProc->statusFlags |= PROC_IS_AUTOVACUUM;
    MyProc->lwWaiting = false;
    MyProc->lwWaitMode = 0;
    MyProc->waitLock = NULL;
    MyProc->waitProcLock = NULL;
#ifdef USE_ASSERT_CHECKING
    {
        int         i;

        /* Last process should have released all locks. */
        for (i = 0; i < NUM_LOCK_PARTITIONS; i++)
            Assert(SHMQueueEmpty(&(MyProc->myProcLocks[i])));
    }
#endif
    MyProc->recoveryConflictPending = false;

    /* Initialize fields for sync rep */
    MyProc->waitLSN = 0;
    MyProc->syncRepState = SYNC_REP_NOT_WAITING;
    SHMQueueElemInit(&(MyProc->syncRepLinks));

    /* Initialize fields for group XID clearing. */
    MyProc->procArrayGroupMember = false;
    MyProc->procArrayGroupMemberXid = InvalidTransactionId;
    Assert(pg_atomic_read_u32(&MyProc->procArrayGroupNext) == INVALID_PGPROCNO);

    /* 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_PGPROCNO);

    /*
     * 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();

    /*
     * 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);

    /*
     * 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();
}

/*
 * 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 CreateSharedMemoryAndSemaphores.
 */
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 per-auxiliary-process data structure
 *
 * 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
 * InitProcGlobal.
 *
 * 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");

    /*
     * We use the ProcStructLock to protect assignment and releasing of
     * AuxiliaryProcs entries.
     *
     * While we are holding the ProcStructLock, also copy the current shared
     * estimate of spins_per_delay to local storage.
     */
    SpinLockAcquire(ProcStructLock);

    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(ProcStructLock);
        elog(FATAL, "all AuxiliaryProcs are in use");
    }

    /* Mark auxiliary proc as in use by me */
    /* use volatile pointer to prevent code rearrangement */
    ((volatile PGPROC *) auxproc)->pid = MyProcPid;

    MyProc = auxproc;

    SpinLockRelease(ProcStructLock);

    /*
     * Initialize all fields of MyProc, except for those previously
     * initialized by InitProcGlobal.
     */
    SHMQueueElemInit(&(MyProc->links));
    MyProc->waitStatus = PROC_WAIT_STATUS_OK;
    MyProc->lxid = InvalidLocalTransactionId;
    MyProc->fpVXIDLock = false;
    MyProc->fpLocalTransactionId = InvalidLocalTransactionId;
    MyProc->xid = InvalidTransactionId;
    MyProc->xmin = InvalidTransactionId;
    MyProc->backendId = InvalidBackendId;
    MyProc->databaseId = InvalidOid;
    MyProc->roleId = InvalidOid;
    MyProc->tempNamespaceId = InvalidOid;
    MyProc->isBackgroundWorker = IsBackgroundWorker;
    MyProc->delayChkpt = false;
    MyProc->statusFlags = 0;
    MyProc->lwWaiting = false;
    MyProc->lwWaitMode = 0;
    MyProc->waitLock = NULL;
    MyProc->waitProcLock = NULL;
#ifdef USE_ASSERT_CHECKING
    {
        int         i;

        /* Last process should have released all locks. */
        for (i = 0; i < NUM_LOCK_PARTITIONS; i++)
            Assert(SHMQueueEmpty(&(MyProc->myProcLocks[i])));
    }
#endif

    /*
     * 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();

    /* 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);

    /*
     * Arrange to clean up at process exit.
     */
    on_shmem_exit(AuxiliaryProcKill, Int32GetDatum(proctype));
}

/*
 * Record the PID and PGPROC structures for the Startup process, for use in
 * ProcSendSignal().  See comments there for further explanation.
 */
void
PublishStartupProcessInformation(void)
{
    SpinLockAcquire(ProcStructLock);

    ProcGlobal->startupProc = MyProc;
    ProcGlobal->startupProcPid = MyProcPid;

    SpinLockRelease(ProcStructLock);
}

/*
 * 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.
 *
 * Note: this is designed on the assumption that N will generally be small.
 */
bool
HaveNFreeProcs(int n)
{
    PGPROC     *proc;

    SpinLockAcquire(ProcStructLock);

    proc = ProcGlobal->freeProcs;

    while (n > 0 && proc != NULL)
    {
        proc = (PGPROC *) proc->links.next;
        n--;
    }

    SpinLockRelease(ProcStructLock);

    return (n <= 0);
}

/*
 * Check if the current process is awaiting a lock.
 */
bool
IsWaitingForLock(void)
{
    if (lockAwaited == NULL)
        return false;

    return true;
}

/*
 * 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)
{
    LWLock     *partitionLock;
    DisableTimeoutParams timeouts[2];

    HOLD_INTERRUPTS();

    AbortStrongLockAcquire();

    /* Nothing to do if we weren't waiting for a lock */
    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 (MyProc->links.next != NULL)
    {
        /* 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();
    }

    lockAwaited = NULL;

    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     *volatile *procgloballist;

    Assert(MyProc != NULL);

    /* 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(SHMQueueEmpty(&(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();

    /* Cancel any pending condition variable sleep, too */
    ConditionVariableCancelSleep();

    /* Make sure active replication slots are released */
    if (MyReplicationSlot != NULL)
        ReplicationSlotRelease();

    /* Also cleanup all the temporary slots. */
    ReplicationSlotCleanup();

    /*
     * Detach from any lock group of which we are a member.  If the leader
     * exist before all other group members, its PGPROC will remain allocated
     * until the last group process exits; that process must return the
     * leader's PGPROC to the appropriate list.
     */
    if (MyProc->lockGroupLeader != NULL)
    {
        PGPROC     *leader = MyProc->lockGroupLeader;
        LWLock     *leader_lwlock = LockHashPartitionLockByProc(leader);

        LWLockAcquire(leader_lwlock, LW_EXCLUSIVE);
        Assert(!dlist_is_empty(&leader->lockGroupMembers));
        dlist_delete(&MyProc->lockGroupLink);
        if (dlist_is_empty(&leader->lockGroupMembers))
        {
            leader->lockGroupLeader = NULL;
            if (leader != MyProc)
            {
                procgloballist = leader->procgloballist;

                /* Leader exited first; return its PGPROC. */
                SpinLockAcquire(ProcStructLock);
                leader->links.next = (SHM_QUEUE *) *procgloballist;
                *procgloballist = leader;
                SpinLockRelease(ProcStructLock);
            }
        }
        else if (leader != MyProc)
            MyProc->lockGroupLeader = NULL;
        LWLockRelease(leader_lwlock);
    }

    /*
     * Reset MyLatch to the process local one.  This is so that signal
     * handlers et al can continue using the latch after the shared latch
     * isn't ours anymore. After that clear MyProc and disown the shared
     * latch.
     */
    SwitchBackToLocalLatch();
    proc = MyProc;
    MyProc = NULL;
    DisownLatch(&proc->procLatch);

    procgloballist = proc->procgloballist;
    SpinLockAcquire(ProcStructLock);

    /*
     * If we're still a member of a locking group, that means we're a leader
     * which has somehow exited before its children.  The last remaining child
     * will release our PGPROC.  Otherwise, release it now.
     */
    if (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 */
        proc->links.next = (SHM_QUEUE *) *procgloballist;
        *procgloballist = proc;
    }

    /* Update shared estimate of spins_per_delay */
    ProcGlobal->spins_per_delay = update_spins_per_delay(ProcGlobal->spins_per_delay);

    SpinLockRelease(ProcStructLock);

    /*
     * This process is no longer present in shared memory in any meaningful
     * way, so tell the postmaster we've cleaned up acceptably well. (XXX
     * autovac launcher should be included here someday)
     */
    if (IsUnderPostmaster && !IsAutoVacuumLauncherProcess())
        MarkPostmasterChildInactive();

    /* wake autovac launcher if needed -- see comments in FreeWorkerInfo */
    if (AutovacuumLauncherPid != 0)
        kill(AutovacuumLauncherPid, SIGUSR2);
}

/*
 * 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);

    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();

    /*
     * Reset MyLatch to the process local one.  This is so that signal
     * handlers et al can continue using the latch after the shared latch
     * isn't ours anymore. After that clear MyProc and disown the shared
     * latch.
     */
    SwitchBackToLocalLatch();
    proc = MyProc;
    MyProc = NULL;
    DisownLatch(&proc->procLatch);

    SpinLockAcquire(ProcStructLock);

    /* Mark auxiliary proc no longer in use */
    proc->pid = 0;

    /* Update shared estimate of spins_per_delay */
    ProcGlobal->spins_per_delay = update_spins_per_delay(ProcGlobal->spins_per_delay);

    SpinLockRelease(ProcStructLock);
}

/*
 * 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;
}

/*
 * ProcQueue package: routines for putting processes to sleep
 *      and  waking them up
 */

/*
 * ProcQueueAlloc -- alloc/attach to a shared memory process queue
 *
 * Returns: a pointer to the queue
 * Side Effects: Initializes the queue if it wasn't there before
 */
#ifdef NOT_USED
PROC_QUEUE *
ProcQueueAlloc(const char *name)
{
    PROC_QUEUE *queue;
    bool        found;

    queue = (PROC_QUEUE *)
        ShmemInitStruct(name, sizeof(PROC_QUEUE), &found);

    if (!found)
        ProcQueueInit(queue);

    return queue;
}
#endif

/*
 * ProcQueueInit -- initialize a shared memory process queue
 */
void
ProcQueueInit(PROC_QUEUE *queue)
{
    SHMQueueInit(&(queue->links));
    queue->size = 0;
}


/*
 * ProcSleep -- put a process to sleep on the specified lock
 *
 * Caller must have set MyProc->heldLocks to reflect locks already held
 * on the lockable object by this process (under all XIDs).
 *
 * The lock table's partition lock must be held at entry, and will be held
 * at exit.
 *
 * Result: PROC_WAIT_STATUS_OK if we acquired the lock, PROC_WAIT_STATUS_ERROR if not (deadlock).
 *
 * ASSUME: that no one will fiddle with the queue until after
 *      we release the partition lock.
 *
 * NOTES: The process queue is now a priority queue for locking.
 */
ProcWaitStatus
ProcSleep(LOCALLOCK *locallock, LockMethod lockMethodTable)
{
    LOCKMODE    lockmode = locallock->tag.mode;
    LOCK       *lock = locallock->lock;
    PROCLOCK   *proclock = locallock->proclock;
    uint32      hashcode = locallock->hashcode;
    LWLock     *partitionLock = LockHashPartitionLock(hashcode);
    PROC_QUEUE *waitQueue = &(lock->waitProcs);
    LOCKMASK    myHeldLocks = MyProc->heldLocks;
    bool        early_deadlock = false;
    bool        allow_autovacuum_cancel = true;
    ProcWaitStatus myWaitStatus;
    PGPROC     *proc;
    PGPROC     *leader = MyProc->lockGroupLeader;
    int         i;

    /*
     * 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 or page locks 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.
     */
    if (leader != NULL)
    {
        SHM_QUEUE  *procLocks = &(lock->procLocks);
        PROCLOCK   *otherproclock;

        otherproclock = (PROCLOCK *)
            SHMQueueNext(procLocks, procLocks, offsetof(PROCLOCK, lockLink));
        while (otherproclock != NULL)
        {
            if (otherproclock->groupLeader == leader)
                myHeldLocks |= otherproclock->holdMask;
            otherproclock = (PROCLOCK *)
                SHMQueueNext(procLocks, &otherproclock->lockLink,
                             offsetof(PROCLOCK, lockLink));
        }
    }

    /*
     * 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)
    {
        LOCKMASK    aheadRequests = 0;

        proc = (PGPROC *) waitQueue->links.next;
        for (i = 0; i < waitQueue->size; i++)
        {
            /*
             * 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)
            {
                proc = (PGPROC *) proc->links.next;
                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);
                    GrantAwaitedLock();
                    return PROC_WAIT_STATUS_OK;
                }
                /* Break out of loop to put myself before him */
                break;
            }
            /* Nope, so advance to next waiter */
            aheadRequests |= LOCKBIT_ON(proc->waitLockMode);
            proc = (PGPROC *) proc->links.next;
        }

        /*
         * If we fall out of loop normally, proc points to waitQueue head, so
         * we will insert at tail of queue as desired.
         */
    }
    else
    {
        /* I hold no locks, so I can't push in front of anyone. */
        proc = (PGPROC *) &(waitQueue->links);
    }

    /*
     * Insert self into queue, ahead of the given proc (or at tail of queue).
     */
    SHMQueueInsertBefore(&(proc->links), &(MyProc->links));
    waitQueue->size++;

    lock->waitMask |= LOCKBIT_ON(lockmode);

    /* Set up wait information in PGPROC object, too */
    MyProc->waitLock = lock;
    MyProc->waitProcLock = proclock;
    MyProc->waitLockMode = lockmode;

    MyProc->waitStatus = PROC_WAIT_STATUS_WAITING;

    /*
     * If we detected deadlock, give up without waiting.  This must agree with
     * CheckDeadLock's recovery code.
     */
    if (early_deadlock)
    {
        RemoveFromWaitQueue(MyProc, hashcode);
        return PROC_WAIT_STATUS_ERROR;
    }

    /* mark that we are waiting for a lock */
    lockAwaited = locallock;

    /*
     * Release the lock table's partition lock.
     *
     * NOTE: this may also cause us to exit critical-section state, possibly
     * allowing a cancel/die interrupt to be accepted. This is OK because we
     * have recorded the fact that we are waiting for a lock, and so
     * LockErrorCleanup will clean up if cancel/die happens.
     */
    LWLockRelease(partitionLock);

    /*
     * Also, 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);
    }

    /*
     * 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)
        {
            /* Set a timer and wait for that or for the Lock to be granted */
            ResolveRecoveryConflictWithLock(locallock->tag.lock);
        }
        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)
            {
                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 cancelled.  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("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, and
         * log_lock_waits is on, then report about the wait.
         */
        if (log_lock_waits && deadlock_state != DS_NOT_YET_CHECKED)
        {
            StringInfoData buf,
                        lock_waiters_sbuf,
                        lock_holders_sbuf;
            const char *modename;
            long        secs;
            int         usecs;
            long        msecs;
            SHM_QUEUE  *procLocks;
            PROCLOCK   *proclock;
            bool        first_holder = true,
                        first_waiter = true;
            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);
            TimestampDifference(get_timeout_start_time(DEADLOCK_TIMEOUT),
                                GetCurrentTimestamp(),
                                &secs, &usecs);
            msecs = secs * 1000 + usecs / 1000;
            usecs = usecs % 1000;

            /*
             * we 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.
             */

            LWLockAcquire(partitionLock, LW_SHARED);

            procLocks = &(lock->procLocks);
            proclock = (PROCLOCK *) SHMQueueNext(procLocks, procLocks,
                                                 offsetof(PROCLOCK, lockLink));

            while (proclock)
            {
                /*
                 * we are a waiter if myProc->waitProcLock == proclock; we are
                 * a holder if it is NULL or something different
                 */
                if (proclock->tag.myProc->waitProcLock == proclock)
                {
                    if (first_waiter)
                    {
                        appendStringInfo(&lock_waiters_sbuf, "%d",
                                         proclock->tag.myProc->pid);
                        first_waiter = false;
                    }
                    else
                        appendStringInfo(&lock_waiters_sbuf, ", %d",
                                         proclock->tag.myProc->pid);
                }
                else
                {
                    if (first_holder)
                    {
                        appendStringInfo(&lock_holders_sbuf, "%d",
                                         proclock->tag.myProc->pid);
                        first_holder = false;
                    }
                    else
                        appendStringInfo(&lock_holders_sbuf, ", %d",
                                         proclock->tag.myProc->pid);

                    lockHoldersNum++;
                }

                proclock = (PROCLOCK *) SHMQueueNext(procLocks, &proclock->lockLink,
                                                     offsetof(PROCLOCK, lockLink));
            }

            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)
                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))));
            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))));
            }

            /*
             * At this point we might still need to wait for the lock. Reset
             * state so we don't print the above messages again.
             */
            deadlock_state = DS_NO_DEADLOCK;

            pfree(buf.data);
            pfree(lock_holders_sbuf.data);
            pfree(lock_waiters_sbuf.data);
        }
    } 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);
    }

    /*
     * Re-acquire the lock table's partition lock.  We have to do this to hold
     * off cancel/die interrupts before we can mess with lockAwaited (else we
     * might have a missed or duplicated locallock update).
     */
    LWLockAcquire(partitionLock, LW_EXCLUSIVE);

    /*
     * We no longer want LockErrorCleanup to do anything.
     */
    lockAwaited = NULL;

    /*
     * If we got the lock, be sure to remember it in the locallock table.
     */
    if (MyProc->waitStatus == PROC_WAIT_STATUS_OK)
        GrantAwaitedLock();

    /*
     * We don't have to do anything else, because the awaker did all the
     * necessary update of the lock table and MyProc.
     */
    return MyProc->waitStatus;
}


/*
 * ProcWakeup -- wake up a process by setting its latch.
 *
 *   Also remove the process from the wait queue and set its links invalid.
 *   RETURN: the next process in the wait queue.
 *
 * 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.
 */
PGPROC *
ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus)
{
    PGPROC     *retProc;

    /* Proc should be sleeping ... */
    if (proc->links.prev == NULL ||
        proc->links.next == NULL)
        return NULL;
    Assert(proc->waitStatus == PROC_WAIT_STATUS_WAITING);

    /* Save next process before we zap the list link */
    retProc = (PGPROC *) proc->links.next;

    /* Remove process from wait queue */
    SHMQueueDelete(&(proc->links));
    (proc->waitLock->waitProcs.size)--;

    /* Clean up process' state and pass it the ok/fail signal */
    proc->waitLock = NULL;
    proc->waitProcLock = NULL;
    proc->waitStatus = waitStatus;

    /* And awaken it */
    SetLatch(&proc->procLatch);

    return retProc;
}

/*
 * 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)
{
    PROC_QUEUE *waitQueue = &(lock->waitProcs);
    int         queue_size = waitQueue->size;
    PGPROC     *proc;
    LOCKMASK    aheadRequests = 0;

    Assert(queue_size >= 0);

    if (queue_size == 0)
        return;

    proc = (PGPROC *) waitQueue->links.next;

    while (queue_size-- > 0)
    {
        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);
            proc = ProcWakeup(proc, PROC_WAIT_STATUS_OK);

            /*
             * ProcWakeup removes proc from the lock's waiting process queue
             * and returns the next proc in chain; don't use proc's next-link,
             * because it's been cleared.
             */
        }
        else
        {
            /*
             * Cannot wake this guy. Remember his request for later checks.
             */
            aheadRequests |= LOCKBIT_ON(lockmode);
            proc = (PGPROC *) proc->links.next;
        }
    }

    Assert(waitQueue->size >= 0);
}

/*
 * 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.  (But signal ProcSleep to log a message, if
 * log_lock_waits is true.)  If we have a real deadlock, remove ourselves from
 * the lock's wait queue and signal an error to ProcSleep.
 */
static void
CheckDeadLock(void)
{
    int         i;

    /*
     * 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 (MyProc->links.prev == NULL ||
        MyProc->links.next == NULL)
        goto check_done;

#ifdef LOCK_DEBUG
    if (Debug_deadlocks)
        DumpAllLocks();
#endif

    /* Run the deadlock check, and set deadlock_state for use by ProcSleep */
    deadlock_state = DeadLockCheck(MyProc);

    if (deadlock_state == 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 from the signal
         * handler.
         */
        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));
}

/*
 * 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.
     */
    SetLatch(MyLatch);
    errno = save_errno;
}

/*
 * 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 - send a signal to a backend identified by PID
 */
void
ProcSendSignal(int pid)
{
    PGPROC     *proc = NULL;

    if (RecoveryInProgress())
    {
        SpinLockAcquire(ProcStructLock);

        /*
         * Check to see whether it is the Startup process we wish to signal.
         * This call is made by the buffer manager when it wishes to wake up a
         * process that has been waiting for a pin in so it can obtain a
         * cleanup lock using LockBufferForCleanup(). Startup is not a normal
         * backend, so BackendPidGetProc() will not return any pid at all. So
         * we remember the information for this special case.
         */
        if (pid == ProcGlobal->startupProcPid)
            proc = ProcGlobal->startupProc;

        SpinLockRelease(ProcStructLock);
    }

    if (proc == NULL)
        proc = BackendPidGetProc(pid);

    if (proc != NULL)
    {
        SetLatch(&proc->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
     * accesses leader->pgprocno in a PGPROC that might be free.  This is safe
     * because all PGPROCs' pgprocno fields are set during shared memory
     * initialization and never change thereafter; 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;
}