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

/*-------------------------------------------------------------------------
 *
 * proc.c
 *        routines to manage per-process shared memory data structure
 *
 * Portions Copyright (c) 1996-2002, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *        $Header: /cvsroot/pgsql/src/backend/storage/lmgr/proc.c,v 1.122 2002/07/13 01:02:14 momjian Exp $
 *
 *-------------------------------------------------------------------------
 */
/*
 * Interface (a):
 *              ProcSleep(), ProcWakeup(),
 *              ProcQueueAlloc() -- create a shm queue for sleeping processes
 *              ProcQueueInit() -- create a queue without allocing memory
 *
 * Locking and waiting for buffers can cause the backend to be
 * put to sleep.  Whoever releases the lock, etc. 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.
 *
 * 5/15/91 -- removed the buffer pool based lock chain in favor
 *              of a shared memory lock chain.  The write-protection is
 *              more expensive if the lock chain is in the buffer pool.
 *              The only reason I kept the lock chain in the buffer pool
 *              in the first place was to allow the lock table to grow larger
 *              than available shared memory and that isn't going to work
 *              without a lot of unimplemented support anyway.
 */
#include "postgres.h"

#include <errno.h>
#include <signal.h>
#include <unistd.h>
#include <sys/time.h>

#include "miscadmin.h"
#include "access/xact.h"
#include "storage/ipc.h"
#include "storage/proc.h"
#include "storage/sinval.h"
#include "storage/spin.h"

int                     DeadlockTimeout = 1000;
int                     StatementTimeout = 0;
int                     RemainingStatementTimeout = 0;
bool            alarm_is_statement_timeout = false;

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.
 */
static slock_t *ProcStructLock = NULL;

static PROC_HDR *ProcGlobal = NULL;

static PGPROC *DummyProc = NULL;

static bool waitingForLock = false;
static bool waitingForSignal = false;

static void ProcKill(void);
static void DummyProcKill(void);


/*
 * Report number of semaphores needed by InitProcGlobal.
 */
int
ProcGlobalSemas(int maxBackends)
{
        /* We need a sema per backend, plus one for the dummy process. */
        return maxBackends + 1;
}

/*
 * InitProcGlobal -
 *        initializes the global process table. We put it here so that
 *        the postmaster can do this initialization.
 *
 *        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
 *        MaxBackends 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.
 */
void
InitProcGlobal(int maxBackends)
{
        bool            found = false;

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

        /* --------------------
         * We're the first - initialize.
         * XXX if found should ever be true, it is a sign of impending doom ...
         * ought to complain if so?
         * --------------------
         */
        if (!found)
        {
                int                     i;

                ProcGlobal->freeProcs = INVALID_OFFSET;

                /*
                 * Pre-create the PGPROC structures and create a semaphore for each.
                 */
                for (i = 0; i < maxBackends; i++)
                {
                        PGPROC   *proc;

                        proc = (PGPROC *) ShmemAlloc(sizeof(PGPROC));
                        if (!proc)
                                elog(FATAL, "cannot create new proc: out of memory");
                        MemSet(proc, 0, sizeof(PGPROC));
                        PGSemaphoreCreate(&proc->sem);
                        proc->links.next = ProcGlobal->freeProcs;
                        ProcGlobal->freeProcs = MAKE_OFFSET(proc);
                }

                /*
                 * Pre-allocate a PGPROC structure for dummy (checkpoint) processes,
                 * too.  This does not get linked into the freeProcs list.
                 */
                DummyProc = (PGPROC *) ShmemAlloc(sizeof(PGPROC));
                if (!DummyProc)
                        elog(FATAL, "cannot create new proc: out of memory");
                MemSet(DummyProc, 0, sizeof(PGPROC));
                DummyProc->pid = 0;             /* marks DummyProc as not in use */
                PGSemaphoreCreate(&DummyProc->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)
{
        SHMEM_OFFSET myOffset;
        /* use volatile pointer to prevent code rearrangement */
        volatile PROC_HDR *procglobal = ProcGlobal;

        /*
         * ProcGlobal should be set by a previous call to InitProcGlobal (if
         * we are a backend, we inherit this by fork() from the postmaster).
         */
        if (procglobal == NULL)
                elog(PANIC, "InitProcess: Proc Header uninitialized");

        if (MyProc != NULL)
                elog(ERROR, "InitProcess: 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).
         */
        SpinLockAcquire(ProcStructLock);

        myOffset = procglobal->freeProcs;

        if (myOffset != INVALID_OFFSET)
        {
                MyProc = (PGPROC *) MAKE_PTR(myOffset);
                procglobal->freeProcs = 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.
                 */
                SpinLockRelease(ProcStructLock);
                elog(FATAL, "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->errType = STATUS_OK;
        MyProc->xid = InvalidTransactionId;
        MyProc->xmin = InvalidTransactionId;
        MyProc->pid = MyProcPid;
        MyProc->databaseId = MyDatabaseId;
        MyProc->logRec.xrecoff = 0;
        MyProc->lwWaiting = false;
        MyProc->lwExclusive = false;
        MyProc->lwWaitLink = NULL;
        MyProc->waitLock = NULL;
        MyProc->waitHolder = NULL;
        SHMQueueInit(&(MyProc->procHolders));

        /*
         * Arrange to clean up at backend exit.
         */
        on_shmem_exit(ProcKill, 0);

        /*
         * We might be reusing a semaphore that belonged to a failed process.
         * So be careful and reinitialize its value here.
         */
        PGSemaphoreReset(&MyProc->sem);

        /*
         * Now that we have a PGPROC, we could try to acquire locks, so
         * initialize the deadlock checker.
         */
        InitDeadLockChecking();
}

/*
 * InitDummyProcess -- create a dummy per-process data structure
 *
 * This is called by checkpoint 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 the extra ones created during InitProcGlobal.
 */
void
InitDummyProcess(void)
{
        /*
         * ProcGlobal should be set by a previous call to InitProcGlobal (we
         * inherit this by fork() from the postmaster).
         */
        if (ProcGlobal == NULL || DummyProc == NULL)
                elog(PANIC, "InitDummyProcess: Proc Header uninitialized");

        if (MyProc != NULL)
                elog(ERROR, "InitDummyProcess: you already exist");

        /*
         * DummyProc should not presently be in use by anyone else
         */
        if (DummyProc->pid != 0)
                elog(FATAL, "InitDummyProcess: DummyProc is in use by PID %d",
                         DummyProc->pid);
        MyProc = DummyProc;

        /*
         * Initialize all fields of MyProc, except MyProc->sem which was set
         * up by InitProcGlobal.
         */
        MyProc->pid = MyProcPid;        /* marks DummyProc as in use by me */
        SHMQueueElemInit(&(MyProc->links));
        MyProc->errType = STATUS_OK;
        MyProc->xid = InvalidTransactionId;
        MyProc->xmin = InvalidTransactionId;
        MyProc->databaseId = MyDatabaseId;
        MyProc->logRec.xrecoff = 0;
        MyProc->lwWaiting = false;
        MyProc->lwExclusive = false;
        MyProc->lwWaitLink = NULL;
        MyProc->waitLock = NULL;
        MyProc->waitHolder = NULL;
        SHMQueueInit(&(MyProc->procHolders));

        /*
         * Arrange to clean up at process exit.
         */
        on_shmem_exit(DummyProcKill, 0);

        /*
         * We might be reusing a semaphore that belonged to a failed process.
         * So be careful and reinitialize its value here.
         */
        PGSemaphoreReset(&MyProc->sem);
}

/*
 * Cancel any pending wait for lock, when aborting a transaction.
 *
 * Returns true if we had been waiting for a lock, else false.
 *
 * (Normally, this would only happen if we accept a cancel/die
 * interrupt while waiting; but an elog(ERROR) while waiting is
 * within the realm of possibility, too.)
 */
bool
LockWaitCancel(void)
{
        /* Nothing to do if we weren't waiting for a lock */
        if (!waitingForLock)
                return false;

        waitingForLock = false;

        /* 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!) */
        LWLockAcquire(LockMgrLock, LW_EXCLUSIVE);
        if (MyProc->links.next != INVALID_OFFSET)
                RemoveFromWaitQueue(MyProc);
        LWLockRelease(LockMgrLock);

        /*
         * Reset the proc wait semaphore to zero.  This is necessary in the
         * scenario where someone else granted us the lock we wanted before we
         * were able to remove ourselves from the wait-list.  The semaphore
         * will have been bumped to 1 by the would-be grantor, and since we
         * are no longer going to wait on the sema, we have to force it back
         * to zero. Otherwise, our next attempt to wait for a lock will fall
         * through prematurely.
         */
        PGSemaphoreReset(&MyProc->sem);

        /*
         * Return true even if we were kicked off the lock before we were able
         * to remove ourselves.
         */
        return true;
}


/*
 * ProcReleaseLocks() -- release locks associated with current transaction
 *                      at transaction commit or abort
 *
 * At commit, we release only locks tagged with the current transaction's XID,
 * leaving those marked with XID 0 (ie, session locks) undisturbed.  At abort,
 * we release all locks including XID 0, because we need to clean up after
 * a failure.  This logic will need extension if we ever support nested
 * transactions.
 *
 * Note that user locks are not released in either 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, MyProc,
                                   !isCommit, GetCurrentTransactionId());
}


/*
 * ProcKill() -- Destroy the per-proc data structure for
 *              this process. Release any of its held LW locks.
 */
static void
ProcKill(void)
{
        /* use volatile pointer to prevent code rearrangement */
        volatile PROC_HDR *procglobal = ProcGlobal;

        Assert(MyProc != NULL);

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

        /* Abort any buffer I/O in progress */
        AbortBufferIO();

        /* Get off any wait queue I might be on */
        LockWaitCancel();

        /* Remove from the standard lock table */
        LockReleaseAll(DEFAULT_LOCKMETHOD, MyProc, true, InvalidTransactionId);

#ifdef USER_LOCKS
        /* Remove from the user lock table */
        LockReleaseAll(USER_LOCKMETHOD, MyProc, true, InvalidTransactionId);
#endif

        SpinLockAcquire(ProcStructLock);

        /* Return PGPROC structure (and semaphore) to freelist */
        MyProc->links.next = procglobal->freeProcs;
        procglobal->freeProcs = MAKE_OFFSET(MyProc);

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

        SpinLockRelease(ProcStructLock);
}

/*
 * DummyProcKill() -- Cut-down version of ProcKill for dummy (checkpoint)
 *              processes.      The PGPROC and sema are not released, only marked
 *              as not-in-use.
 */
static void
DummyProcKill(void)
{
        Assert(MyProc != NULL && MyProc == DummyProc);

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

        /* Abort any buffer I/O in progress */
        AbortBufferIO();

        /* I can't be on regular lock queues, so needn't check */

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

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


/*
 * 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
 *
 * Caller must have set MyProc->heldLocks to reflect locks already held
 * on the lockable object by this process (under all XIDs).
 *
 * Locktable's masterLock 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 masterLock.
 *
 * 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(LOCKMETHODTABLE *lockMethodTable,
                  LOCKMODE lockmode,
                  LOCK *lock,
                  HOLDER *holder)
{
        LOCKMETHODCTL *lockctl = lockMethodTable->ctl;
        LWLockId        masterLock = lockctl->masterLock;
        PROC_QUEUE *waitQueue = &(lock->waitProcs);
        int                     myHeldLocks = MyProc->heldLocks;
        bool            early_deadlock = false;
        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)
        {
                int                     aheadRequests = 0;

                proc = (PGPROC *) MAKE_PTR(waitQueue->links.next);
                for (i = 0; i < waitQueue->size; i++)
                {
                        /* Must he wait for me? */
                        if (lockctl->conflictTab[proc->waitLockMode] & myHeldLocks)
                        {
                                /* Must I wait for him ? */
                                if (lockctl->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.
                                         */
                                        early_deadlock = true;
                                        break;
                                }
                                /* I must go before this waiter.  Check special case. */
                                if ((lockctl->conflictTab[lockmode] & aheadRequests) == 0 &&
                                        LockCheckConflicts(lockMethodTable,
                                                                           lockmode,
                                                                           lock,
                                                                           holder,
                                                                           MyProc,
                                                                           NULL) == STATUS_OK)
                                {
                                        /* Skip the wait and just grant myself the lock. */
                                        GrantLock(lock, holder, lockmode);
                                        return STATUS_OK;
                                }
                                /* Break out of loop to put myself before him */
                                break;
                        }
                        /* Nope, so advance to next waiter */
                        aheadRequests |= (1 << proc->waitLockMode);
                        proc = (PGPROC *) MAKE_PTR(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 |= (1 << lockmode);

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

        MyProc->errType = STATUS_OK;    /* initialize result for success */

        /*
         * 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);
                MyProc->errType = STATUS_ERROR;
                return STATUS_ERROR;
        }

        /* mark that we are waiting for a lock */
        waitingForLock = true;

        /*
         * Release the locktable's masterLock.
         *
         * 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(masterLock);

        /*
         * 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->errType = 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, "ProcSleep: Unable to 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.  Note also that if CheckDeadLock is
         * invoked but does not detect a deadlock, PGSemaphoreLock() will
         * continue to wait.  There used to be a loop here, but it was useless
         * code...
         *
         * 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 state-change work to do after
         * being granted the lock (the grantor did it all).
         */
        PGSemaphoreLock(&MyProc->sem, true);

        /*
         * Disable the timer, if it's still running
         */
        if (!disable_sig_alarm(false))
                elog(FATAL, "ProcSleep: Unable to disable timer for process wakeup");

        /*
         * Now there is nothing for LockWaitCancel to do.
         */
        waitingForLock = false;

        /*
         * Re-acquire the locktable's masterLock.
         */
        LWLockAcquire(masterLock, LW_EXCLUSIVE);

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


/*
 * 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.
 *
 * 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.
 */
PGPROC *
ProcWakeup(PGPROC *proc, int errType)
{
        PGPROC     *retProc;

        /* assume that masterLock has been acquired */

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

        /* Save next process before we zap the list link */
        retProc = (PGPROC *) MAKE_PTR(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->waitHolder = NULL;
        proc->errType = errType;

        /* 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.
 */
void
ProcLockWakeup(LOCKMETHODTABLE *lockMethodTable, LOCK *lock)
{
        LOCKMETHODCTL *lockctl = lockMethodTable->ctl;
        PROC_QUEUE *waitQueue = &(lock->waitProcs);
        int                     queue_size = waitQueue->size;
        PGPROC     *proc;
        int                     aheadRequests = 0;

        Assert(queue_size >= 0);

        if (queue_size == 0)
                return;

        proc = (PGPROC *) MAKE_PTR(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 ((lockctl->conflictTab[lockmode] & aheadRequests) == 0 &&
                        LockCheckConflicts(lockMethodTable,
                                                           lockmode,
                                                           lock,
                                                           proc->waitHolder,
                                                           proc,
                                                           NULL) == STATUS_OK)
                {
                        /* OK to waken */
                        GrantLock(lock, proc->waitHolder, 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 |= (1 << lockmode);
                        proc = (PGPROC *) MAKE_PTR(proc->links.next);
                }
        }

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

/* --------------------
 * 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.
 * If we have a real deadlock, remove ourselves from the lock's wait queue
 * and signal an error to ProcSleep.
 * --------------------
 */
void
CheckDeadLock(void)
{
        int                     save_errno = errno;

        /*
         * Acquire locktable lock.      Note that the SIGALRM interrupt had better
         * not be enabled anywhere that this process itself holds the
         * locktable lock, else this will wait forever.  Also note that
         * LWLockAcquire creates a critical section, so that this routine
         * cannot be interrupted by cancel/die interrupts.
         */
        LWLockAcquire(LockMgrLock, 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 locktable
         * lock.
         *
         */
        if (MyProc->links.prev == INVALID_OFFSET ||
                MyProc->links.next == INVALID_OFFSET)
        {
                LWLockRelease(LockMgrLock);
                errno = save_errno;
                return;
        }

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

        if (!DeadLockCheck(MyProc))
        {
                /* No deadlock, so keep waiting */
                LWLockRelease(LockMgrLock);
                errno = save_errno;
                return;
        }

        /*
         * Oops.  We have a deadlock.
         *
         * Get this process out of wait state.
         */
        RemoveFromWaitQueue(MyProc);

        /*
         * Set MyProc->errType to STATUS_ERROR so that ProcSleep will report
         * an error after we return from this signal handler.
         */
        MyProc->errType = STATUS_ERROR;

        /*
         * 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.
         */
        LWLockRelease(LockMgrLock);
        errno = save_errno;
}


/*
 * 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.
 */
void
ProcWaitForSignal(void)
{
        waitingForSignal = true;
        PGSemaphoreLock(&MyProc->sem, true);
        waitingForSignal = false;
}

/*
 * ProcCancelWaitForSignal - clean up an aborted wait for signal
 *
 * We need this in case the signal arrived after we aborted waiting,
 * or if it arrived but we never reached ProcWaitForSignal() at all.
 * Caller should call this after resetting the signal request status.
 */
void
ProcCancelWaitForSignal(void)
{
        PGSemaphoreReset(&MyProc->sem);
        waitingForSignal = false;
}

/*
 * ProcSendSignal - send a signal to a backend identified by BackendId
 */
void
ProcSendSignal(BackendId procId)
{
        PGPROC     *proc = BackendIdGetProc(procId);

        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 multiple alarms when the statement_timeout
 * alarm is specified first.
 *
 * Returns TRUE if okay, FALSE on failure.
 */
bool
enable_sig_alarm(int delayms, bool is_statement_timeout)
{
#ifndef __BEOS__
        struct itimerval timeval, remaining;
#else
        bigtime_t       time_interval, remaining;
#endif

        /* Don't set timer if the statement timeout scheduled before next alarm. */
        if (alarm_is_statement_timeout &&
                !is_statement_timeout &&
                RemainingStatementTimeout <= delayms)
                return true;

#ifndef __BEOS__
        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, &remaining))
                return false;
#else
        /* BeOS doesn't have setitimer, but has set_alarm */
        time_interval = delayms * 1000;         /* usecs */
        if ((remaining = set_alarm(time_interval, B_ONE_SHOT_RELATIVE_ALARM)) < 0)
                return false;
#endif

        if (is_statement_timeout)
                RemainingStatementTimeout = StatementTimeout;
        else
        {
                /* Switching to non-statement-timeout alarm, get remaining time */
                if (alarm_is_statement_timeout)
                {
#ifndef __BEOS__
                        /* We lose precision here because we convert to milliseconds */
                        RemainingStatementTimeout = remaining.it_value.tv_sec * 1000 +
                                                                                remaining.it_value.tv_usec / 1000;
#else
                        RemainingStatementTimeout = remaining / 1000;
#endif
                        /* Rounding could cause a zero */
                        if (RemainingStatementTimeout == 0)
                                RemainingStatementTimeout = 1;
                }

                if (RemainingStatementTimeout)
                {
                        /* Remaining timeout alarm < delayms? */
                        if (RemainingStatementTimeout <= delayms)
                        {
                                /* reinstall statement timeout alarm */
                                alarm_is_statement_timeout = true;
#ifndef __BEOS__
                                remaining.it_value.tv_sec = RemainingStatementTimeout / 1000;
                                remaining.it_value.tv_usec = (RemainingStatementTimeout % 1000) * 1000;
                                if (setitimer(ITIMER_REAL, &remaining, &timeval))
                                        return false;
                                else
                                        return true;
#else
                                remaining = RemainingStatementTimeout * 1000;
                                if ((timeval = set_alarm(remaining, B_ONE_SHOT_RELATIVE_ALARM)) < 0)
                                        return false;
                                else
                                        return true;
#endif
                        }
                        else
                                RemainingStatementTimeout -= delayms;
                }
        }

        if (is_statement_timeout)
                alarm_is_statement_timeout = true;
        else
                alarm_is_statement_timeout = false;

        return true;
}

/*
 * Cancel the SIGALRM timer.
 *
 * This is also called if the timer has fired to reschedule
 * the statement_timeout timer.
 *
 * Returns TRUE if okay, FALSE on failure.
 */
bool
disable_sig_alarm(bool is_statement_timeout)
{
#ifndef __BEOS__
        struct itimerval timeval, remaining;
        MemSet(&timeval, 0, sizeof(struct itimerval));
#else
        bigtime_t time_interval = 0;
#endif

        if (!is_statement_timeout && RemainingStatementTimeout)
        {
#ifndef __BEOS__
                /* turn off timer and get remaining time, if any */
                if (setitimer(ITIMER_REAL, &timeval, &remaining))
                        return false;
                /* Add remaining time back because the timer didn't complete */
                RemainingStatementTimeout += remaining.it_value.tv_sec * 1000 +
                                                                         remaining.it_value.tv_usec / 1000;
                /* Prepare to set timer */
                timeval.it_value.tv_sec = RemainingStatementTimeout / 1000;
                timeval.it_value.tv_usec = (RemainingStatementTimeout % 1000) * 1000;
#else
                /* BeOS doesn't have setitimer, but has set_alarm */
                if ((time_interval = set_alarm(B_INFINITE_TIMEOUT, B_PERIODIC_ALARM)) < 0)
                        return false;
                RemainingStatementTimeout += time_interval / 1000;
                time_interval = RemainingStatementTimeout * 1000;
#endif
                /* Restore remaining statement timeout value */
                alarm_is_statement_timeout = true;
        }
        /*
         *      Optimization: is_statement_timeout && RemainingStatementTimeout == 0
         *  does nothing.  This is for cases where no timeout was set.
         */
        if (!is_statement_timeout || RemainingStatementTimeout)
        {
#ifndef __BEOS__
                if (setitimer(ITIMER_REAL, &timeval, &remaining))
                        return false;
#else
                if (time_interval)
                {
                        if (set_alarm(time_interval, B_ONE_SHOT_RELATIVE_ALARM) < 0)
                                return false;
                }
                else
                {
                        if (set_alarm(B_INFINITE_TIMEOUT, B_PERIODIC_ALARM) < 0)
                                return false;
                }
#endif
        }

        if (is_statement_timeout)
                RemainingStatementTimeout = 0;

        return true;
}


/*
 * Call alarm handler, either StatementCancel or Deadlock checker.
 */
void
handle_sig_alarm(SIGNAL_ARGS)
{
        if (alarm_is_statement_timeout)
        {
                RemainingStatementTimeout = 0;
                alarm_is_statement_timeout = false;
                kill(MyProcPid, SIGINT);
        }
        else
        {
                CheckDeadLock();
                /* Reactivate any statement_timeout alarm. */
                disable_sig_alarm(false);
        }
}