[postgresql] / src / backend / storage / lmgr / proc.c

/*-------------------------------------------------------------------------
 *
 * proc.c
 *        routines to manage per-process shared memory data structure
 *
 * Portions Copyright (c) 1996-2013, 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 "postmaster/autovacuum.h"
#include "replication/syncrep.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 "utils/timeout.h"
#include "utils/timestamp.h"


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

/* Pointer to this process's PGPROC and PGXACT structs, if any */
PGPROC     *MyProc = NULL;
PGXACT     *MyPgXact = 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;

/* Mark this volatile because it can be changed by signal handler */
static volatile DeadLockState deadlock_state = DS_NOT_YET_CHECKED;


static void RemoveProcFromArray(int code, Datum arg);
static void ProcKill(int code, Datum arg);
static void AuxiliaryProcKill(int code, Datum arg);


/*
 * Report shared-memory space needed by InitProcGlobal.
 */
Size
ProcGlobalShmemSize(void)
{
        Size            size = 0;

        /* ProcGlobal */
        size = add_size(size, sizeof(PROC_HDR));
        /* MyProcs, including autovacuum workers and launcher */
        size = add_size(size, mul_size(MaxBackends, sizeof(PGPROC)));
        /* AuxiliaryProcs */
        size = add_size(size, mul_size(NUM_AUXILIARY_PROCS, sizeof(PGPROC)));
        /* Prepared xacts */
        size = add_size(size, mul_size(max_prepared_xacts, sizeof(PGPROC)));
        /* ProcStructLock */
        size = add_size(size, sizeof(slock_t));

        size = add_size(size, mul_size(MaxBackends, sizeof(PGXACT)));
        size = add_size(size, mul_size(NUM_AUXILIARY_PROCS, sizeof(PGXACT)));
        size = add_size(size, mul_size(max_prepared_xacts, sizeof(PGXACT)));

        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, 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;
        PGXACT     *pgxacts;
        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->startupProc = NULL;
        ProcGlobal->startupProcPid = 0;
        ProcGlobal->startupBufferPinWaitBufId = -1;
        ProcGlobal->walwriterLatch = NULL;
        ProcGlobal->checkpointerLatch = NULL;

        /*
         * 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));
        ProcGlobal->allProcs = procs;
        ProcGlobal->allProcCount = TotalProcs;
        if (!procs)
                ereport(FATAL,
                                (errcode(ERRCODE_OUT_OF_MEMORY),
                                 errmsg("out of shared memory")));
        MemSet(procs, 0, TotalProcs * sizeof(PGPROC));

        /*
         * Also allocate a separate array of PGXACT structures.  This is separate
         * from the main PGPROC array so that the most heavily accessed data is
         * stored contiguously in memory in as few cache lines as possible. This
         * provides significant performance benefits, especially on a
         * multiprocessor system.  There is one PGXACT structure for every PGPROC
         * structure.
         */
        pgxacts = (PGXACT *) ShmemAlloc(TotalProcs * sizeof(PGXACT));
        MemSet(pgxacts, 0, TotalProcs * sizeof(PGXACT));
        ProcGlobal->allPgXact = pgxacts;

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

                /*
                 * Set up per-PGPROC semaphore, latch, and backendLock. Prepared xact
                 * dummy PGPROCs don't need these though - they're never associated
                 * with a real process
                 */
                if (i < MaxBackends + NUM_AUXILIARY_PROCS)
                {
                        PGSemaphoreCreate(&(procs[i].sem));
                        InitSharedLatch(&(procs[i].procLatch));
                        procs[i].backendLock = LWLockAssign();
                }
                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];
                }
                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];
                }
                else if (i < MaxBackends)
                {
                        /* PGPROC for bgworker, add to bgworkerFreeProcs list */
                        procs[i].links.next = (SHM_QUEUE *) ProcGlobal->bgworkerFreeProcs;
                        ProcGlobal->bgworkerFreeProcs = &procs[i];
                }

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

        /*
         * 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)
{
        /* use volatile pointer to prevent code rearrangement */
        volatile PROC_HDR *procglobal = ProcGlobal;

        /*
         * 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");

        /*
         * Initialize process-local latch support.      This could fail if the kernel
         * is low on resources, and if so we want to exit cleanly before acquiring
         * any shared-memory resources.
         */
        InitializeLatchSupport();

        /*
         * Try to get a proc struct from the 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);

        if (IsAnyAutoVacuumProcess())
                MyProc = procglobal->autovacFreeProcs;
        else if (IsBackgroundWorker)
                MyProc = procglobal->bgworkerFreeProcs;
        else
                MyProc = procglobal->freeProcs;

        if (MyProc != NULL)
        {
                if (IsAnyAutoVacuumProcess())
                        procglobal->autovacFreeProcs = (PGPROC *) MyProc->links.next;
                else if (IsBackgroundWorker)
                        procglobal->bgworkerFreeProcs = (PGPROC *) MyProc->links.next;
                else
                        procglobal->freeProcs = (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);
                ereport(FATAL,
                                (errcode(ERRCODE_TOO_MANY_CONNECTIONS),
                                 errmsg("sorry, too many clients already")));
        }
        MyPgXact = &ProcGlobal->allPgXact[MyProc->pgprocno];

        /*
         * 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 = STATUS_OK;
        MyProc->lxid = InvalidLocalTransactionId;
        MyProc->fpVXIDLock = false;
        MyProc->fpLocalTransactionId = InvalidLocalTransactionId;
        MyPgXact->xid = InvalidTransactionId;
        MyPgXact->xmin = InvalidTransactionId;
        MyProc->pid = MyProcPid;
        /* backendId, databaseId and roleId will be filled in later */
        MyProc->backendId = InvalidBackendId;
        MyProc->databaseId = InvalidOid;
        MyProc->roleId = InvalidOid;
        MyPgXact->delayChkpt = false;
        MyPgXact->vacuumFlags = 0;
        /* NB -- autovac launcher intentionally does not set IS_AUTOVACUUM */
        if (IsAutoVacuumWorkerProcess())
                MyPgXact->vacuumFlags |= PROC_IS_AUTOVACUUM;
        MyProc->lwWaiting = false;
        MyProc->lwWaitMode = 0;
        MyProc->lwWaitLink = NULL;
        MyProc->waitLock = NULL;
        MyProc->waitProcLock = NULL;
#ifdef USE_ASSERT_CHECKING
        if (assert_enabled)
        {
                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));

        /*
         * Acquire ownership of the PGPROC's latch, so that we can use WaitLatch.
         * Note that there's no particular need to do ResetLatch here.
         */
        OwnLatch(&MyProc->procLatch);

        /*
         * 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
         * the deadlock checker.
         */
        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");

        /*
         * Initialize process-local latch support.      This could fail if the kernel
         * is low on resources, and if so we want to exit cleanly before acquiring
         * any shared-memory resources.
         */
        InitializeLatchSupport();

        /*
         * 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;
        MyPgXact = &ProcGlobal->allPgXact[auxproc->pgprocno];

        SpinLockRelease(ProcStructLock);

        /*
         * Initialize all fields of MyProc, except for those previously
         * initialized by InitProcGlobal.
         */
        SHMQueueElemInit(&(MyProc->links));
        MyProc->waitStatus = STATUS_OK;
        MyProc->lxid = InvalidLocalTransactionId;
        MyProc->fpVXIDLock = false;
        MyProc->fpLocalTransactionId = InvalidLocalTransactionId;
        MyPgXact->xid = InvalidTransactionId;
        MyPgXact->xmin = InvalidTransactionId;
        MyProc->backendId = InvalidBackendId;
        MyProc->databaseId = InvalidOid;
        MyProc->roleId = InvalidOid;
        MyPgXact->delayChkpt = false;
        MyPgXact->vacuumFlags = 0;
        MyProc->lwWaiting = false;
        MyProc->lwWaitMode = 0;
        MyProc->lwWaitLink = NULL;
        MyProc->waitLock = NULL;
        MyProc->waitProcLock = NULL;
#ifdef USE_ASSERT_CHECKING
        if (assert_enabled)
        {
                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.
         * Note that there's no particular need to do ResetLatch here.
         */
        OwnLatch(&MyProc->procLatch);

        /*
         * 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)
{
        /* use volatile pointer to prevent code rearrangement */
        volatile PROC_HDR *procglobal = ProcGlobal;

        SpinLockAcquire(ProcStructLock);

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

        SpinLockRelease(ProcStructLock);
}

/*
 * Used from bufgr 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;

        /* use volatile pointer to prevent code rearrangement */
        volatile PROC_HDR *procglobal = ProcGlobal;

        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)
{
        LWLockId        partitionLock;
        DisableTimeoutParams timeouts[2];

        AbortStrongLockAcquire();

        /* Nothing to do if we weren't waiting for a lock */
        if (lockAwaited == NULL)
                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 == STATUS_OK)
                        GrantAwaitedLock();
        }

        lockAwaited = NULL;

        LWLockRelease(partitionLock);

        /*
         * We used to do PGSemaphoreReset() here to ensure that our proc's wait
         * semaphore is reset to zero.  This prevented a leftover wakeup signal
         * from remaining in the semaphore if someone else had granted us the lock
         * we wanted before we were able to remove ourselves from the wait-list.
         * However, now that ProcSleep loops until waitStatus changes, a leftover
         * wakeup signal isn't harmful, and it seems not worth expending cycles to
         * get rid of a signal that most likely isn't there.
         */
}


/*
 * 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)
{
        /* use volatile pointer to prevent code rearrangement */
        volatile PROC_HDR *procglobal = ProcGlobal;

        Assert(MyProc != NULL);

        /* Make sure we're out of the sync rep lists */
        SyncRepCleanupAtProcExit();

#ifdef USE_ASSERT_CHECKING
        if (assert_enabled)
        {
                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();

        /* Release ownership of the process's latch, too */
        DisownLatch(&MyProc->procLatch);

        SpinLockAcquire(ProcStructLock);

        /* Return PGPROC structure (and semaphore) to appropriate freelist */
        if (IsAnyAutoVacuumProcess())
        {
                MyProc->links.next = (SHM_QUEUE *) procglobal->autovacFreeProcs;
                procglobal->autovacFreeProcs = MyProc;
        }
        else if (IsBackgroundWorker)
        {
                MyProc->links.next = (SHM_QUEUE *) procglobal->bgworkerFreeProcs;
                procglobal->bgworkerFreeProcs = MyProc;
        }
        else
        {
                MyProc->links.next = (SHM_QUEUE *) procglobal->freeProcs;
                procglobal->freeProcs = MyProc;
        }

        /* PGPROC struct isn't mine anymore */
        MyProc = NULL;

        /* 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;

        Assert(proctype >= 0 && proctype < NUM_AUXILIARY_PROCS);

        auxproc = &AuxiliaryProcs[proctype];

        Assert(MyProc == auxproc);

        /* Release any LW locks I am holding (see notes above) */
        LWLockReleaseAll();

        /* Release ownership of the process's latch, too */
        DisownLatch(&MyProc->procLatch);

        SpinLockAcquire(ProcStructLock);

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

        /* PGPROC struct isn't mine anymore */
        MyProc = NULL;

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

        SpinLockRelease(ProcStructLock);
}


/*
 * 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: STATUS_OK if we acquired the lock, 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.
 *
 * P() on the semaphore should put us to sleep.  The process
 * semaphore is normally zero, so when we try to acquire it, we sleep.
 */
int
ProcSleep(LOCALLOCK *locallock, LockMethod lockMethodTable)
{
        LOCKMODE        lockmode = locallock->tag.mode;
        LOCK       *lock = locallock->lock;
        PROCLOCK   *proclock = locallock->proclock;
        uint32          hashcode = locallock->hashcode;
        LWLockId        partitionLock = LockHashPartitionLock(hashcode);
        PROC_QUEUE *waitQueue = &(lock->waitProcs);
        LOCKMASK        myHeldLocks = MyProc->heldLocks;
        bool            early_deadlock = false;
        bool            allow_autovacuum_cancel = true;
        int                     myWaitStatus;
        PGPROC     *proc;
        int                     i;

        /*
         * 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++)
                {
                        /* 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) == STATUS_OK)
                                {
                                        /* Skip the wait and just grant myself the lock. */
                                        GrantLock(lock, proclock, lockmode);
                                        GrantAwaitedLock();
                                        return 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 = STATUS_WAITING;

        /*
         * If we detected deadlock, give up without waiting.  This must agree with
         * CheckDeadLock's recovery code, except that we shouldn't release the
         * semaphore since we haven't tried to lock it yet.
         */
        if (early_deadlock)
        {
                RemoveFromWaitQueue(MyProc, hashcode);
                return 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;

        /*
         * Set timer so we can wake up after awhile and check for a deadlock. If a
         * deadlock is detected, the handler releases the process's semaphore and
         * sets MyProc->waitStatus = 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 (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 someone wakes us between LWLockRelease and PGSemaphoreLock,
         * PGSemaphoreLock will not block.      The wakeup is "saved" by the semaphore
         * implementation.      While this is normally good, there are cases where a
         * saved wakeup might be leftover from a previous operation (for example,
         * we aborted ProcWaitForSignal just before someone did ProcSendSignal).
         * So, loop to wait again if the waitStatus shows we haven't been granted
         * nor denied the lock yet.
         *
         * We pass interruptOK = true, which eliminates a window in which
         * cancel/die interrupts would be held off undesirably.  This is a promise
         * that we don't 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
        {
                PGSemaphoreLock(&MyProc->sem, true);

                /*
                 * waitStatus could change from STATUS_WAITING to something else
                 * asynchronously.      Read it just once per loop to prevent surprising
                 * behavior (such as missing log messages).
                 */
                myWaitStatus = 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();
                        PGXACT     *autovac_pgxact = &ProcGlobal->allPgXact[autovac->pgprocno];

                        LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);

                        /*
                         * Only do it if the worker is not working to protect against Xid
                         * wraparound.
                         */
                        if ((autovac != NULL) &&
                                (autovac_pgxact->vacuumFlags & PROC_IS_AUTOVACUUM) &&
                                !(autovac_pgxact->vacuumFlags & PROC_VACUUM_FOR_WRAPAROUND))
                        {
                                int                     pid = autovac->pid;
                                StringInfoData locktagbuf;
                                StringInfoData logbuf;  /* errdetail for server log */

                                initStringInfo(&locktagbuf);
                                initStringInfo(&logbuf);
                                DescribeLockTag(&locktagbuf, &lock->tag);
                                appendStringInfo(&logbuf,
                                                                 _("Process %d waits for %s on %s."),
                                                                 MyProcPid,
                                                          GetLockmodeName(lock->tag.locktag_lockmethodid,
                                                                                          lockmode),
                                                                 locktagbuf.data);

                                /* release lock as quickly as possible */
                                LWLockRelease(ProcArrayLock);

                                ereport(LOG,
                                          (errmsg("sending cancel to blocking autovacuum PID %d",
                                                          pid),
                                           errdetail_log("%s", logbuf.data)));

                                pfree(logbuf.data);
                                pfree(locktagbuf.data);

                                /* send the autovacuum worker Back to Old Kent Road */
                                if (kill(pid, SIGINT) < 0)
                                {
                                        /* Just a warning to allow multiple callers */
                                        ereport(WARNING,
                                                        (errmsg("could not send signal to process %d: %m",
                                                                        pid)));
                                }
                        }
                        else
                                LWLockRelease(ProcArrayLock);

                        /* prevent signal from being resent 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;
                        const char *modename;
                        long            secs;
                        int                     usecs;
                        long            msecs;

                        initStringInfo(&buf);
                        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;

                        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)));
                        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)));
                        }

                        if (myWaitStatus == STATUS_WAITING)
                                ereport(LOG,
                                                (errmsg("process %d still waiting for %s on %s after %ld.%03d ms",
                                                          MyProcPid, modename, buf.data, msecs, usecs)));
                        else if (myWaitStatus == STATUS_OK)
                                ereport(LOG,
                                        (errmsg("process %d acquired %s on %s after %ld.%03d ms",
                                                        MyProcPid, modename, buf.data, msecs, usecs)));
                        else
                        {
                                Assert(myWaitStatus == STATUS_ERROR);

                                /*
                                 * Currently, the deadlock checker always kicks its own
                                 * process, which means that we'll only see 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)));
                        }

                        /*
                         * 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);
                }
        } while (myWaitStatus == STATUS_WAITING);

        /*
         * Disable the timers, if they are still running
         */
        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 = false;
                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 == 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 releasing its private semaphore.
 *
 *       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 STATUS_OK.
 */
PGPROC *
ProcWakeup(PGPROC *proc, int waitStatus)
{
        PGPROC     *retProc;

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

        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) == STATUS_OK)
                {
                        /* OK to waken */
                        GrantLock(lock, proc->waitProcLock, lockmode);
                        proc = ProcWakeup(proc, 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 the DEADLOCK_TIMEOUT fired
 * while waiting for a lock to be released by some other process.  Look
 * to see if there's a deadlock; if not, just return and continue waiting.
 * (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.
 *
 * NB: this is run inside a signal handler, so be very wary about what is done
 * here or in called routines.
 */
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(FirstLockMgrLock + 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 quicker than checking our semaphore's state, since no kernel
         * call is needed, and it 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 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)));

                /*
                 * Unlock my semaphore so that the interrupted ProcSleep() call can
                 * finish.
                 */
                PGSemaphoreUnlock(&MyProc->sem);

                /*
                 * 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.
                 */
        }
        else if (log_lock_waits || deadlock_state == DS_BLOCKED_BY_AUTOVACUUM)
        {
                /*
                 * Unlock my semaphore so that the interrupted ProcSleep() call can
                 * print the log message (we daren't do it here because we are inside
                 * a signal handler).  It will then sleep again until someone releases
                 * the lock.
                 *
                 * If blocked by autovacuum, this wakeup will enable ProcSleep to send
                 * the canceling signal to the autovacuum worker.
                 */
                PGSemaphoreUnlock(&MyProc->sem);
        }

        /*
         * 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(FirstLockMgrLock + i);
}


/*
 * ProcWaitForSignal - wait for a signal from another backend.
 *
 * This can share the semaphore normally used for waiting for locks,
 * since a backend could never be waiting for a lock and a signal at
 * the same time.  As with locks, it's OK if the signal arrives just
 * before we actually reach the waiting state.  Also as with locks,
 * it's necessary that the caller be robust against bogus wakeups:
 * always check that the desired state has occurred, and wait again
 * if not.      This copes with possible "leftover" wakeups.
 */
void
ProcWaitForSignal(void)
{
        PGSemaphoreLock(&MyProc->sem, true);
}

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

        if (RecoveryInProgress())
        {
                /* use volatile pointer to prevent code rearrangement */
                volatile PROC_HDR *procglobal = ProcGlobal;

                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)
                PGSemaphoreUnlock(&proc->sem);
}