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

/*-------------------------------------------------------------------------
 *
 * proc.c
 *        routines to manage per-process shared memory data structure
 *
 * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *        $PostgreSQL: pgsql/src/backend/storage/lmgr/proc.c,v 1.204 2008/12/09 15:59:39 heikki Exp $
 *
 *-------------------------------------------------------------------------
 */
/*
 * 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/xact.h"
#include "miscadmin.h"
#include "postmaster/autovacuum.h"
#include "storage/ipc.h"
#include "storage/lmgr.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "storage/spin.h"


/* GUC variables */
int                     DeadlockTimeout = 1000;
int                     StatementTimeout = 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 */
NON_EXEC_STATIC PROC_HDR *ProcGlobal = NULL;
NON_EXEC_STATIC PGPROC *AuxiliaryProcs = NULL;

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

/* Mark these volatile because they can be changed by signal handler */
static volatile bool statement_timeout_active = false;
static volatile bool deadlock_timeout_active = false;
static volatile DeadLockState deadlock_state = DS_NOT_YET_CHECKED;
volatile bool cancel_from_timeout = false;

/* timeout_start_time is set when log_lock_waits is true */
static TimestampTz timeout_start_time;

/* statement_fin_time is valid only if statement_timeout_active is true */
static TimestampTz statement_fin_time;


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


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

        /* ProcGlobal */
        size = add_size(size, sizeof(PROC_HDR));
        /* AuxiliaryProcs */
        size = add_size(size, mul_size(NUM_AUXILIARY_PROCS, sizeof(PGPROC)));
        /* MyProcs, including autovacuum */
        size = add_size(size, mul_size(MaxBackends, sizeof(PGPROC)));
        /* ProcStructLock */
        size = add_size(size, sizeof(slock_t));

        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 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;
        bool            found;

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

        /*
         * Create the PGPROC structures for auxiliary (bgwriter) processes, too.
         * These do not get linked into the freeProcs list.
         */
        AuxiliaryProcs = (PGPROC *)
                ShmemInitStruct("AuxiliaryProcs", NUM_AUXILIARY_PROCS * sizeof(PGPROC),
                                                &found);
        Assert(!found);

        /*
         * Initialize the data structures.
         */
        ProcGlobal->freeProcs = NULL;
        ProcGlobal->autovacFreeProcs = NULL;

        ProcGlobal->spins_per_delay = DEFAULT_SPINS_PER_DELAY;

        /*
         * Pre-create the PGPROC structures and create a semaphore for each.
         */
        procs = (PGPROC *) ShmemAlloc((MaxConnections) * sizeof(PGPROC));
        if (!procs)
                ereport(FATAL,
                                (errcode(ERRCODE_OUT_OF_MEMORY),
                                 errmsg("out of shared memory")));
        MemSet(procs, 0, MaxConnections * sizeof(PGPROC));
        for (i = 0; i < MaxConnections; i++)
        {
                PGSemaphoreCreate(&(procs[i].sem));
                procs[i].links.next = (SHM_QUEUE *) ProcGlobal->freeProcs;
                ProcGlobal->freeProcs = &procs[i];
        }

        procs = (PGPROC *) ShmemAlloc((autovacuum_max_workers) * sizeof(PGPROC));
        if (!procs)
                ereport(FATAL,
                                (errcode(ERRCODE_OUT_OF_MEMORY),
                                 errmsg("out of shared memory")));
        MemSet(procs, 0, autovacuum_max_workers * sizeof(PGPROC));
        for (i = 0; i < autovacuum_max_workers; i++)
        {
                PGSemaphoreCreate(&(procs[i].sem));
                procs[i].links.next = (SHM_QUEUE *) ProcGlobal->autovacFreeProcs;
                ProcGlobal->autovacFreeProcs = &procs[i];
        }

        MemSet(AuxiliaryProcs, 0, NUM_AUXILIARY_PROCS * sizeof(PGPROC));
        for (i = 0; i < NUM_AUXILIARY_PROCS; i++)
        {
                AuxiliaryProcs[i].pid = 0;              /* marks auxiliary proc as not in use */
                PGSemaphoreCreate(&(AuxiliaryProcs[i].sem));
        }

        /* 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;
        int                     i;

        /*
         * 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");

        /*
         * 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 (IsAutoVacuumWorkerProcess())
                MyProc = procglobal->autovacFreeProcs;
        else
                MyProc = procglobal->freeProcs;

        if (MyProc != NULL)
        {
                if (IsAutoVacuumWorkerProcess())
                        procglobal->autovacFreeProcs = (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")));
        }

        /*
         * Initialize all fields of MyProc, except for the semaphore which was
         * prepared for us by InitProcGlobal.
         */
        SHMQueueElemInit(&(MyProc->links));
        MyProc->waitStatus = STATUS_OK;
        MyProc->lxid = 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->inCommit = false;
        MyProc->vacuumFlags = 0;
        if (IsAutoVacuumWorkerProcess())
                MyProc->vacuumFlags |= PROC_IS_AUTOVACUUM;
        MyProc->lwWaiting = false;
        MyProc->lwExclusive = false;
        MyProc->lwWaitLink = NULL;
        MyProc->waitLock = NULL;
        MyProc->waitProcLock = NULL;
        for (i = 0; i < NUM_LOCK_PARTITIONS; i++)
                SHMQueueInit(&(MyProc->myProcLocks[i]));

        /*
         * 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 there is a good deal
 * of stuff to be done before this step that will require LWLock access.
 */
void
InitProcessPhase2(void)
{
        Assert(MyProc != NULL);

        /*
         * We should now know what database we're in, so advertise that.  (We need
         * not do any locking here, since no other backend can yet see our
         * PGPROC.)
         */
        Assert(OidIsValid(MyDatabaseId));
        MyProc->databaseId = MyDatabaseId;

        /*
         * 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.
 */
void
InitAuxiliaryProcess(void)
{
        PGPROC     *auxproc;
        int                     proctype;
        int                     i;

        /*
         * 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 the semaphore which was
         * prepared for us by InitProcGlobal.
         */
        SHMQueueElemInit(&(MyProc->links));
        MyProc->waitStatus = STATUS_OK;
        MyProc->lxid = InvalidLocalTransactionId;
        MyProc->xid = InvalidTransactionId;
        MyProc->xmin = InvalidTransactionId;
        MyProc->backendId = InvalidBackendId;
        MyProc->databaseId = InvalidOid;
        MyProc->roleId = InvalidOid;
        MyProc->inCommit = false;
        /* we don't set the "is autovacuum" flag in the launcher */
        MyProc->vacuumFlags = 0;
        MyProc->lwWaiting = false;
        MyProc->lwExclusive = false;
        MyProc->lwWaitLink = NULL;
        MyProc->waitLock = NULL;
        MyProc->waitProcLock = NULL;
        for (i = 0; i < NUM_LOCK_PARTITIONS; i++)
                SHMQueueInit(&(MyProc->myProcLocks[i]));

        /*
         * 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));
}

/*
 * 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);
}

/*
 * Cancel any pending wait for lock, when aborting a transaction.
 *
 * (Normally, this would only happen if we accept a cancel/die
 * interrupt while waiting; but an ereport(ERROR) while waiting is
 * within the realm of possibility, too.)
 */
void
LockWaitCancel(void)
{
        LWLockId        partitionLock;

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

        /* Turn off the deadlock timer, if it's still running (see ProcSleep) */
        disable_sig_alarm(false);

        /* 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 all locks except session locks.
 * At main transaction abort, we release all locks including session locks;
 * this lets us clean up after a VACUUM FULL failure.
 *
 * 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.
 *
 * Note that user locks are not released in any case.
 */
void
ProcReleaseLocks(bool isCommit)
{
        if (!MyProc)
                return;
        /* If waiting, get off wait queue (should only be needed after error) */
        LockWaitCancel();
        /* Release locks */
        LockReleaseAll(DEFAULT_LOCKMETHOD, !isCommit);
}


/*
 * 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);

        /*
         * 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();

        SpinLockAcquire(ProcStructLock);

        /* Return PGPROC structure (and semaphore) to freelist */
        if (IsAutoVacuumWorkerProcess())
        {
                MyProc->links.next = (SHM_QUEUE *) procglobal->autovacFreeProcs;
                procglobal->autovacFreeProcs = 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);

        /* wake autovac launcher if needed -- see comments in FreeWorkerInfo */
        if (AutovacuumLauncherPid != 0)
                kill(AutovacuumLauncherPid, SIGUSR1);
}

/*
 * 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;

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

        auxproc = &AuxiliaryProcs[proctype];

        Assert(MyProc == auxproc);

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

        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 or NULL
 * Side Effects: Initializes the queue if we allocated one
 */
#ifdef NOT_USED
PROC_QUEUE *
ProcQueueAlloc(char *name)
{
        bool            found;
        PROC_QUEUE *queue = (PROC_QUEUE *)
        ShmemInitStruct(name, sizeof(PROC_QUEUE), &found);

        if (!queue)
                return NULL;
        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,
                                                                           MyProc) == 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
         * LockWaitCancel will clean up if cancel/die happens.
         */
        LWLockRelease(partitionLock);

        /* Reset deadlock_state before enabling the signal 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 (!enable_sig_alarm(DeadlockTimeout, false))
                elog(FATAL, "could not set timer for process wakeup");

        /*
         * 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
         * updating the locallock table, but if we lose control to an error,
         * LockWaitCancel 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();

                        LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);

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

                                elog(DEBUG2, "sending cancel to blocking autovacuum pid = %d",
                                         pid);

                                /* don't hold the lock across the kill() syscall */
                                LWLockRelease(ProcArrayLock);

                                /* 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(timeout_start_time, 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 timer, if it's still running
         */
        if (!disable_sig_alarm(false))
                elog(FATAL, "could not disable timer for process wakeup");

        /*
         * 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 LockWaitCancel 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,
                                                           proc) == 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 we got SIGALRM after DeadlockTimeout
 * 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.
 */
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(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 cancelling 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 = BackendPidGetProc(pid);

        if (proc != NULL)
                PGSemaphoreUnlock(&proc->sem);
}


/*****************************************************************************
 * SIGALRM interrupt support
 *
 * Maybe these should be in pqsignal.c?
 *****************************************************************************/

/*
 * Enable the SIGALRM interrupt to fire after the specified delay
 *
 * Delay is given in milliseconds.      Caller should be sure a SIGALRM
 * signal handler is installed before this is called.
 *
 * This code properly handles nesting of deadlock timeout alarms within
 * statement timeout alarms.
 *
 * Returns TRUE if okay, FALSE on failure.
 */
bool
enable_sig_alarm(int delayms, bool is_statement_timeout)
{
        TimestampTz fin_time;
        struct itimerval timeval;

        if (is_statement_timeout)
        {
                /*
                 * Begin statement-level timeout
                 *
                 * Note that we compute statement_fin_time with reference to the
                 * statement_timestamp, but apply the specified delay without any
                 * correction; that is, we ignore whatever time has elapsed since
                 * statement_timestamp was set.  In the normal case only a small
                 * interval will have elapsed and so this doesn't matter, but there
                 * are corner cases (involving multi-statement query strings with
                 * embedded COMMIT or ROLLBACK) where we might re-initialize the
                 * statement timeout long after initial receipt of the message. In
                 * such cases the enforcement of the statement timeout will be a bit
                 * inconsistent.  This annoyance is judged not worth the cost of
                 * performing an additional gettimeofday() here.
                 */
                Assert(!deadlock_timeout_active);
                fin_time = GetCurrentStatementStartTimestamp();
                fin_time = TimestampTzPlusMilliseconds(fin_time, delayms);
                statement_fin_time = fin_time;
                cancel_from_timeout = false;
                statement_timeout_active = true;
        }
        else if (statement_timeout_active)
        {
                /*
                 * Begin deadlock timeout with statement-level timeout active
                 *
                 * Here, we want to interrupt at the closer of the two timeout times.
                 * If fin_time >= statement_fin_time then we need not touch the
                 * existing timer setting; else set up to interrupt at the deadlock
                 * timeout time.
                 *
                 * NOTE: in this case it is possible that this routine will be
                 * interrupted by the previously-set timer alarm.  This is okay
                 * because the signal handler will do only what it should do according
                 * to the state variables.      The deadlock checker may get run earlier
                 * than normal, but that does no harm.
                 */
                timeout_start_time = GetCurrentTimestamp();
                fin_time = TimestampTzPlusMilliseconds(timeout_start_time, delayms);
                deadlock_timeout_active = true;
                if (fin_time >= statement_fin_time)
                        return true;
        }
        else
        {
                /* Begin deadlock timeout with no statement-level timeout */
                deadlock_timeout_active = true;
                /* GetCurrentTimestamp can be expensive, so only do it if we must */
                if (log_lock_waits)
                        timeout_start_time = GetCurrentTimestamp();
        }

        /* If we reach here, okay to set the timer interrupt */
        MemSet(&timeval, 0, sizeof(struct itimerval));
        timeval.it_value.tv_sec = delayms / 1000;
        timeval.it_value.tv_usec = (delayms % 1000) * 1000;
        if (setitimer(ITIMER_REAL, &timeval, NULL))
                return false;
        return true;
}

/*
 * Cancel the SIGALRM timer, either for a deadlock timeout or a statement
 * timeout.  If a deadlock timeout is canceled, any active statement timeout
 * remains in force.
 *
 * Returns TRUE if okay, FALSE on failure.
 */
bool
disable_sig_alarm(bool is_statement_timeout)
{
        /*
         * Always disable the interrupt if it is active; this avoids being
         * interrupted by the signal handler and thereby possibly getting
         * confused.
         *
         * We will re-enable the interrupt if necessary in CheckStatementTimeout.
         */
        if (statement_timeout_active || deadlock_timeout_active)
        {
                struct itimerval timeval;

                MemSet(&timeval, 0, sizeof(struct itimerval));
                if (setitimer(ITIMER_REAL, &timeval, NULL))
                {
                        statement_timeout_active = false;
                        cancel_from_timeout = false;
                        deadlock_timeout_active = false;
                        return false;
                }
        }

        /* Always cancel deadlock timeout, in case this is error cleanup */
        deadlock_timeout_active = false;

        /* Cancel or reschedule statement timeout */
        if (is_statement_timeout)
        {
                statement_timeout_active = false;
                cancel_from_timeout = false;
        }
        else if (statement_timeout_active)
        {
                if (!CheckStatementTimeout())
                        return false;
        }
        return true;
}


/*
 * Check for statement timeout.  If the timeout time has come,
 * trigger a query-cancel interrupt; if not, reschedule the SIGALRM
 * interrupt to occur at the right time.
 *
 * Returns true if okay, false if failed to set the interrupt.
 */
static bool
CheckStatementTimeout(void)
{
        TimestampTz now;

        if (!statement_timeout_active)
                return true;                    /* do nothing if not active */

        now = GetCurrentTimestamp();

        if (now >= statement_fin_time)
        {
                /* Time to die */
                statement_timeout_active = false;
                cancel_from_timeout = true;
#ifdef HAVE_SETSID
                /* try to signal whole process group */
                kill(-MyProcPid, SIGINT);
#endif
                kill(MyProcPid, SIGINT);
        }
        else
        {
                /* Not time yet, so (re)schedule the interrupt */
                long            secs;
                int                     usecs;
                struct itimerval timeval;

                TimestampDifference(now, statement_fin_time,
                                                        &secs, &usecs);

                /*
                 * It's possible that the difference is less than a microsecond;
                 * ensure we don't cancel, rather than set, the interrupt.
                 */
                if (secs == 0 && usecs == 0)
                        usecs = 1;
                MemSet(&timeval, 0, sizeof(struct itimerval));
                timeval.it_value.tv_sec = secs;
                timeval.it_value.tv_usec = usecs;
                if (setitimer(ITIMER_REAL, &timeval, NULL))
                        return false;
        }

        return true;
}


/*
 * Signal handler for SIGALRM
 *
 * Process deadlock check and/or statement timeout check, as needed.
 * To avoid various edge cases, we must be careful to do nothing
 * when there is nothing to be done.  We also need to be able to
 * reschedule the timer interrupt if called before end of statement.
 */
void
handle_sig_alarm(SIGNAL_ARGS)
{
        int                     save_errno = errno;

        if (deadlock_timeout_active)
        {
                deadlock_timeout_active = false;
                CheckDeadLock();
        }

        if (statement_timeout_active)
                (void) CheckStatementTimeout();

        errno = save_errno;
}