deadlock.c

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/*-------------------------------------------------------------------------
 *
 * deadlock.c
 *    POSTGRES deadlock detection code
 *
 * See src/backend/storage/lmgr/README for a description of the deadlock
 * detection and resolution algorithms.
 *
 *
 * Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/storage/lmgr/deadlock.c
 *
 *  Interface:
 *
 *  DeadLockCheck()
 *  DeadLockReport()
 *  RememberSimpleDeadLock()
 *  InitDeadLockChecking()
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "miscadmin.h"
#include "pg_trace.h"
#include "pgstat.h"
#include "storage/lmgr.h"
#include "storage/proc.h"
#include "utils/memutils.h"


/* One edge in the waits-for graph */
typedef struct
{
    PGPROC     *waiter;         /* the waiting process */
    PGPROC     *blocker;        /* the process it is waiting for */
    int         pred;           /* workspace for TopoSort */
    int         link;           /* workspace for TopoSort */
} EDGE;

/* One potential reordering of a lock's wait queue */
typedef struct
{
    LOCK       *lock;           /* the lock whose wait queue is described */
    PGPROC    **procs;          /* array of PGPROC *'s in new wait order */
    int         nProcs;
} WAIT_ORDER;

/*
 * Information saved about each edge in a detected deadlock cycle.  This
 * is used to print a diagnostic message upon failure.
 *
 * Note: because we want to examine this info after releasing the lock
 * manager's partition locks, we can't just store LOCK and PGPROC pointers;
 * we must extract out all the info we want to be able to print.
 */
typedef struct
{
    LOCKTAG     locktag;        /* ID of awaited lock object */
    LOCKMODE    lockmode;       /* type of lock we're waiting for */
    int         pid;            /* PID of blocked backend */
} DEADLOCK_INFO;


static bool DeadLockCheckRecurse(PGPROC *proc);
static int  TestConfiguration(PGPROC *startProc);
static bool FindLockCycle(PGPROC *checkProc,
              EDGE *softEdges, int *nSoftEdges);
static bool FindLockCycleRecurse(PGPROC *checkProc, int depth,
                     EDGE *softEdges, int *nSoftEdges);
static bool ExpandConstraints(EDGE *constraints, int nConstraints);
static bool TopoSort(LOCK *lock, EDGE *constraints, int nConstraints,
         PGPROC **ordering);

#ifdef DEBUG_DEADLOCK
static void PrintLockQueue(LOCK *lock, const char *info);
#endif


/*
 * Working space for the deadlock detector
 */

/* Workspace for FindLockCycle */
static PGPROC **visitedProcs;   /* Array of visited procs */
static int  nVisitedProcs;

/* Workspace for TopoSort */
static PGPROC **topoProcs;      /* Array of not-yet-output procs */
static int *beforeConstraints;  /* Counts of remaining before-constraints */
static int *afterConstraints;   /* List head for after-constraints */

/* Output area for ExpandConstraints */
static WAIT_ORDER *waitOrders;  /* Array of proposed queue rearrangements */
static int  nWaitOrders;
static PGPROC **waitOrderProcs; /* Space for waitOrders queue contents */

/* Current list of constraints being considered */
static EDGE *curConstraints;
static int  nCurConstraints;
static int  maxCurConstraints;

/* Storage space for results from FindLockCycle */
static EDGE *possibleConstraints;
static int  nPossibleConstraints;
static int  maxPossibleConstraints;
static DEADLOCK_INFO *deadlockDetails;
static int  nDeadlockDetails;

/* PGPROC pointer of any blocking autovacuum worker found */
static PGPROC *blocking_autovacuum_proc = NULL;


/*
 * InitDeadLockChecking -- initialize deadlock checker during backend startup
 *
 * This does per-backend initialization of the deadlock checker; primarily,
 * allocation of working memory for DeadLockCheck.  We do this per-backend
 * since there's no percentage in making the kernel do copy-on-write
 * inheritance of workspace from the postmaster.  We want to allocate the
 * space at startup because (a) the deadlock checker might be invoked when
 * there's no free memory left, and (b) the checker is normally run inside a
 * signal handler, which is a very dangerous place to invoke palloc from.
 */
void
InitDeadLockChecking(void)
{
    MemoryContext oldcxt;

    /* Make sure allocations are permanent */
    oldcxt = MemoryContextSwitchTo(TopMemoryContext);

    /*
     * FindLockCycle needs at most MaxBackends entries in visitedProcs[] and
     * deadlockDetails[].
     */
    visitedProcs = (PGPROC **) palloc(MaxBackends * sizeof(PGPROC *));
    deadlockDetails = (DEADLOCK_INFO *) palloc(MaxBackends * sizeof(DEADLOCK_INFO));

    /*
     * TopoSort needs to consider at most MaxBackends wait-queue entries, and
     * it needn't run concurrently with FindLockCycle.
     */
    topoProcs = visitedProcs;   /* re-use this space */
    beforeConstraints = (int *) palloc(MaxBackends * sizeof(int));
    afterConstraints = (int *) palloc(MaxBackends * sizeof(int));

    /*
     * We need to consider rearranging at most MaxBackends/2 wait queues
     * (since it takes at least two waiters in a queue to create a soft edge),
     * and the expanded form of the wait queues can't involve more than
     * MaxBackends total waiters.
     */
    waitOrders = (WAIT_ORDER *)
        palloc((MaxBackends / 2) * sizeof(WAIT_ORDER));
    waitOrderProcs = (PGPROC **) palloc(MaxBackends * sizeof(PGPROC *));

    /*
     * Allow at most MaxBackends distinct constraints in a configuration. (Is
     * this enough?  In practice it seems it should be, but I don't quite see
     * how to prove it.  If we run out, we might fail to find a workable wait
     * queue rearrangement even though one exists.)  NOTE that this number
     * limits the maximum recursion depth of DeadLockCheckRecurse. Making it
     * really big might potentially allow a stack-overflow problem.
     */
    maxCurConstraints = MaxBackends;
    curConstraints = (EDGE *) palloc(maxCurConstraints * sizeof(EDGE));

    /*
     * Allow up to 3*MaxBackends constraints to be saved without having to
     * re-run TestConfiguration.  (This is probably more than enough, but we
     * can survive if we run low on space by doing excess runs of
     * TestConfiguration to re-compute constraint lists each time needed.) The
     * last MaxBackends entries in possibleConstraints[] are reserved as
     * output workspace for FindLockCycle.
     */
    maxPossibleConstraints = MaxBackends * 4;
    possibleConstraints =
        (EDGE *) palloc(maxPossibleConstraints * sizeof(EDGE));

    MemoryContextSwitchTo(oldcxt);
}

/*
 * DeadLockCheck -- Checks for deadlocks for a given process
 *
 * This code looks for deadlocks involving the given process.  If any
 * are found, it tries to rearrange lock wait queues to resolve the
 * deadlock.  If resolution is impossible, return DS_HARD_DEADLOCK ---
 * the caller is then expected to abort the given proc's transaction.
 *
 * Caller must already have locked all partitions of the lock tables.
 *
 * On failure, deadlock details are recorded in deadlockDetails[] for
 * subsequent printing by DeadLockReport().  That activity is separate
 * because (a) we don't want to do it while holding all those LWLocks,
 * and (b) we are typically invoked inside a signal handler.
 */
DeadLockState
DeadLockCheck(PGPROC *proc)
{
    int         i,
                j;

    /* Initialize to "no constraints" */
    nCurConstraints = 0;
    nPossibleConstraints = 0;
    nWaitOrders = 0;

    /* Initialize to not blocked by an autovacuum worker */
    blocking_autovacuum_proc = NULL;

    /* Search for deadlocks and possible fixes */
    if (DeadLockCheckRecurse(proc))
    {
        /*
         * Call FindLockCycle one more time, to record the correct
         * deadlockDetails[] for the basic state with no rearrangements.
         */
        int         nSoftEdges;

        TRACE_POSTGRESQL_DEADLOCK_FOUND();

        nWaitOrders = 0;
        if (!FindLockCycle(proc, possibleConstraints, &nSoftEdges))
            elog(FATAL, "deadlock seems to have disappeared");

        return DS_HARD_DEADLOCK;    /* cannot find a non-deadlocked state */
    }

    /* Apply any needed rearrangements of wait queues */
    for (i = 0; i < nWaitOrders; i++)
    {
        LOCK       *lock = waitOrders[i].lock;
        PGPROC    **procs = waitOrders[i].procs;
        int         nProcs = waitOrders[i].nProcs;
        PROC_QUEUE *waitQueue = &(lock->waitProcs);

        Assert(nProcs == waitQueue->size);

#ifdef DEBUG_DEADLOCK
        PrintLockQueue(lock, "DeadLockCheck:");
#endif

        /* Reset the queue and re-add procs in the desired order */
        ProcQueueInit(waitQueue);
        for (j = 0; j < nProcs; j++)
        {
            SHMQueueInsertBefore(&(waitQueue->links), &(procs[j]->links));
            waitQueue->size++;
        }

#ifdef DEBUG_DEADLOCK
        PrintLockQueue(lock, "rearranged to:");
#endif

        /* See if any waiters for the lock can be woken up now */
        ProcLockWakeup(GetLocksMethodTable(lock), lock);
    }

    /* Return code tells caller if we had to escape a deadlock or not */
    if (nWaitOrders > 0)
        return DS_SOFT_DEADLOCK;
    else if (blocking_autovacuum_proc != NULL)
        return DS_BLOCKED_BY_AUTOVACUUM;
    else
        return DS_NO_DEADLOCK;
}

/*
 * Return the PGPROC of the autovacuum that's blocking a process.
 *
 * We reset the saved pointer as soon as we pass it back.
 */
PGPROC *
GetBlockingAutoVacuumPgproc(void)
{
    PGPROC     *ptr;

    ptr = blocking_autovacuum_proc;
    blocking_autovacuum_proc = NULL;

    return ptr;
}

/*
 * DeadLockCheckRecurse -- recursively search for valid orderings
 *
 * curConstraints[] holds the current set of constraints being considered
 * by an outer level of recursion.  Add to this each possible solution
 * constraint for any cycle detected at this level.
 *
 * Returns TRUE if no solution exists.  Returns FALSE if a deadlock-free
 * state is attainable, in which case waitOrders[] shows the required
 * rearrangements of lock wait queues (if any).
 */
static bool
DeadLockCheckRecurse(PGPROC *proc)
{
    int         nEdges;
    int         oldPossibleConstraints;
    bool        savedList;
    int         i;

    nEdges = TestConfiguration(proc);
    if (nEdges < 0)
        return true;            /* hard deadlock --- no solution */
    if (nEdges == 0)
        return false;           /* good configuration found */
    if (nCurConstraints >= maxCurConstraints)
        return true;            /* out of room for active constraints? */
    oldPossibleConstraints = nPossibleConstraints;
    if (nPossibleConstraints + nEdges + MaxBackends <= maxPossibleConstraints)
    {
        /* We can save the edge list in possibleConstraints[] */
        nPossibleConstraints += nEdges;
        savedList = true;
    }
    else
    {
        /* Not room; will need to regenerate the edges on-the-fly */
        savedList = false;
    }

    /*
     * Try each available soft edge as an addition to the configuration.
     */
    for (i = 0; i < nEdges; i++)
    {
        if (!savedList && i > 0)
        {
            /* Regenerate the list of possible added constraints */
            if (nEdges != TestConfiguration(proc))
                elog(FATAL, "inconsistent results during deadlock check");
        }
        curConstraints[nCurConstraints] =
            possibleConstraints[oldPossibleConstraints + i];
        nCurConstraints++;
        if (!DeadLockCheckRecurse(proc))
            return false;       /* found a valid solution! */
        /* give up on that added constraint, try again */
        nCurConstraints--;
    }
    nPossibleConstraints = oldPossibleConstraints;
    return true;                /* no solution found */
}


/*--------------------
 * Test a configuration (current set of constraints) for validity.
 *
 * Returns:
 *      0: the configuration is good (no deadlocks)
 *     -1: the configuration has a hard deadlock or is not self-consistent
 *      >0: the configuration has one or more soft deadlocks
 *
 * In the soft-deadlock case, one of the soft cycles is chosen arbitrarily
 * and a list of its soft edges is returned beginning at
 * possibleConstraints+nPossibleConstraints.  The return value is the
 * number of soft edges.
 *--------------------
 */
static int
TestConfiguration(PGPROC *startProc)
{
    int         softFound = 0;
    EDGE       *softEdges = possibleConstraints + nPossibleConstraints;
    int         nSoftEdges;
    int         i;

    /*
     * Make sure we have room for FindLockCycle's output.
     */
    if (nPossibleConstraints + MaxBackends > maxPossibleConstraints)
        return -1;

    /*
     * Expand current constraint set into wait orderings.  Fail if the
     * constraint set is not self-consistent.
     */
    if (!ExpandConstraints(curConstraints, nCurConstraints))
        return -1;

    /*
     * Check for cycles involving startProc or any of the procs mentioned in
     * constraints.  We check startProc last because if it has a soft cycle
     * still to be dealt with, we want to deal with that first.
     */
    for (i = 0; i < nCurConstraints; i++)
    {
        if (FindLockCycle(curConstraints[i].waiter, softEdges, &nSoftEdges))
        {
            if (nSoftEdges == 0)
                return -1;      /* hard deadlock detected */
            softFound = nSoftEdges;
        }
        if (FindLockCycle(curConstraints[i].blocker, softEdges, &nSoftEdges))
        {
            if (nSoftEdges == 0)
                return -1;      /* hard deadlock detected */
            softFound = nSoftEdges;
        }
    }
    if (FindLockCycle(startProc, softEdges, &nSoftEdges))
    {
        if (nSoftEdges == 0)
            return -1;          /* hard deadlock detected */
        softFound = nSoftEdges;
    }
    return softFound;
}


/*
 * FindLockCycle -- basic check for deadlock cycles
 *
 * Scan outward from the given proc to see if there is a cycle in the
 * waits-for graph that includes this proc.  Return TRUE if a cycle
 * is found, else FALSE.  If a cycle is found, we return a list of
 * the "soft edges", if any, included in the cycle.  These edges could
 * potentially be eliminated by rearranging wait queues.  We also fill
 * deadlockDetails[] with information about the detected cycle; this info
 * is not used by the deadlock algorithm itself, only to print a useful
 * message after failing.
 *
 * Since we need to be able to check hypothetical configurations that would
 * exist after wait queue rearrangement, the routine pays attention to the
 * table of hypothetical queue orders in waitOrders[].  These orders will
 * be believed in preference to the actual ordering seen in the locktable.
 */
static bool
FindLockCycle(PGPROC *checkProc,
              EDGE *softEdges,  /* output argument */
              int *nSoftEdges)  /* output argument */
{
    nVisitedProcs = 0;
    nDeadlockDetails = 0;
    *nSoftEdges = 0;
    return FindLockCycleRecurse(checkProc, 0, softEdges, nSoftEdges);
}

static bool
FindLockCycleRecurse(PGPROC *checkProc,
                     int depth,
                     EDGE *softEdges,   /* output argument */
                     int *nSoftEdges)   /* output argument */
{
    PGPROC     *proc;
    PGXACT     *pgxact;
    LOCK       *lock;
    PROCLOCK   *proclock;
    SHM_QUEUE  *procLocks;
    LockMethod  lockMethodTable;
    PROC_QUEUE *waitQueue;
    int         queue_size;
    int         conflictMask;
    int         i;
    int         numLockModes,
                lm;

    /*
     * Have we already seen this proc?
     */
    for (i = 0; i < nVisitedProcs; i++)
    {
        if (visitedProcs[i] == checkProc)
        {
            /* If we return to starting point, we have a deadlock cycle */
            if (i == 0)
            {
                /*
                 * record total length of cycle --- outer levels will now fill
                 * deadlockDetails[]
                 */
                Assert(depth <= MaxBackends);
                nDeadlockDetails = depth;

                return true;
            }

            /*
             * Otherwise, we have a cycle but it does not include the start
             * point, so say "no deadlock".
             */
            return false;
        }
    }
    /* Mark proc as seen */
    Assert(nVisitedProcs < MaxBackends);
    visitedProcs[nVisitedProcs++] = checkProc;

    /*
     * If the proc is not waiting, we have no outgoing waits-for edges.
     */
    if (checkProc->links.next == NULL)
        return false;
    lock = checkProc->waitLock;
    if (lock == NULL)
        return false;
    lockMethodTable = GetLocksMethodTable(lock);
    numLockModes = lockMethodTable->numLockModes;
    conflictMask = lockMethodTable->conflictTab[checkProc->waitLockMode];

    /*
     * Scan for procs that already hold conflicting locks.  These are "hard"
     * edges in the waits-for graph.
     */
    procLocks = &(lock->procLocks);

    proclock = (PROCLOCK *) SHMQueueNext(procLocks, procLocks,
                                         offsetof(PROCLOCK, lockLink));

    while (proclock)
    {
        proc = proclock->tag.myProc;
        pgxact = &ProcGlobal->allPgXact[proc->pgprocno];

        /* A proc never blocks itself */
        if (proc != checkProc)
        {
            for (lm = 1; lm <= numLockModes; lm++)
            {
                if ((proclock->holdMask & LOCKBIT_ON(lm)) &&
                    (conflictMask & LOCKBIT_ON(lm)))
                {
                    /* This proc hard-blocks checkProc */
                    if (FindLockCycleRecurse(proc, depth + 1,
                                             softEdges, nSoftEdges))
                    {
                        /* fill deadlockDetails[] */
                        DEADLOCK_INFO *info = &deadlockDetails[depth];

                        info->locktag = lock->tag;
                        info->lockmode = checkProc->waitLockMode;
                        info->pid = checkProc->pid;

                        return true;
                    }

                    /*
                     * No deadlock here, but see if this proc is an autovacuum
                     * that is directly hard-blocking our own proc.  If so,
                     * report it so that the caller can send a cancel signal
                     * to it, if appropriate.  If there's more than one such
                     * proc, it's indeterminate which one will be reported.
                     *
                     * We don't touch autovacuums that are indirectly blocking
                     * us; it's up to the direct blockee to take action.  This
                     * rule simplifies understanding the behavior and ensures
                     * that an autovacuum won't be canceled with less than
                     * deadlock_timeout grace period.
                     *
                     * Note we read vacuumFlags without any locking.  This is
                     * OK only for checking the PROC_IS_AUTOVACUUM flag,
                     * because that flag is set at process start and never
                     * reset.  There is logic elsewhere to avoid canceling an
                     * autovacuum that is working to prevent XID wraparound
                     * problems (which needs to read a different vacuumFlag
                     * bit), but we don't do that here to avoid grabbing
                     * ProcArrayLock.
                     */
                    if (checkProc == MyProc &&
                        pgxact->vacuumFlags & PROC_IS_AUTOVACUUM)
                        blocking_autovacuum_proc = proc;

                    /* We're done looking at this proclock */
                    break;
                }
            }
        }

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

    /*
     * Scan for procs that are ahead of this one in the lock's wait queue.
     * Those that have conflicting requests soft-block this one.  This must be
     * done after the hard-block search, since if another proc both hard- and
     * soft-blocks this one, we want to call it a hard edge.
     *
     * If there is a proposed re-ordering of the lock's wait order, use that
     * rather than the current wait order.
     */
    for (i = 0; i < nWaitOrders; i++)
    {
        if (waitOrders[i].lock == lock)
            break;
    }

    if (i < nWaitOrders)
    {
        /* Use the given hypothetical wait queue order */
        PGPROC    **procs = waitOrders[i].procs;

        queue_size = waitOrders[i].nProcs;

        for (i = 0; i < queue_size; i++)
        {
            proc = procs[i];

            /* Done when we reach the target proc */
            if (proc == checkProc)
                break;

            /* Is there a conflict with this guy's request? */
            if (((1 << proc->waitLockMode) & conflictMask) != 0)
            {
                /* This proc soft-blocks checkProc */
                if (FindLockCycleRecurse(proc, depth + 1,
                                         softEdges, nSoftEdges))
                {
                    /* fill deadlockDetails[] */
                    DEADLOCK_INFO *info = &deadlockDetails[depth];

                    info->locktag = lock->tag;
                    info->lockmode = checkProc->waitLockMode;
                    info->pid = checkProc->pid;

                    /*
                     * Add this edge to the list of soft edges in the cycle
                     */
                    Assert(*nSoftEdges < MaxBackends);
                    softEdges[*nSoftEdges].waiter = checkProc;
                    softEdges[*nSoftEdges].blocker = proc;
                    (*nSoftEdges)++;
                    return true;
                }
            }
        }
    }
    else
    {
        /* Use the true lock wait queue order */
        waitQueue = &(lock->waitProcs);
        queue_size = waitQueue->size;

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

        while (queue_size-- > 0)
        {
            /* Done when we reach the target proc */
            if (proc == checkProc)
                break;

            /* Is there a conflict with this guy's request? */
            if (((1 << proc->waitLockMode) & conflictMask) != 0)
            {
                /* This proc soft-blocks checkProc */
                if (FindLockCycleRecurse(proc, depth + 1,
                                         softEdges, nSoftEdges))
                {
                    /* fill deadlockDetails[] */
                    DEADLOCK_INFO *info = &deadlockDetails[depth];

                    info->locktag = lock->tag;
                    info->lockmode = checkProc->waitLockMode;
                    info->pid = checkProc->pid;

                    /*
                     * Add this edge to the list of soft edges in the cycle
                     */
                    Assert(*nSoftEdges < MaxBackends);
                    softEdges[*nSoftEdges].waiter = checkProc;
                    softEdges[*nSoftEdges].blocker = proc;
                    (*nSoftEdges)++;
                    return true;
                }
            }

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

    /*
     * No conflict detected here.
     */
    return false;
}


/*
 * ExpandConstraints -- expand a list of constraints into a set of
 *      specific new orderings for affected wait queues
 *
 * Input is a list of soft edges to be reversed.  The output is a list
 * of nWaitOrders WAIT_ORDER structs in waitOrders[], with PGPROC array
 * workspace in waitOrderProcs[].
 *
 * Returns TRUE if able to build an ordering that satisfies all the
 * constraints, FALSE if not (there are contradictory constraints).
 */
static bool
ExpandConstraints(EDGE *constraints,
                  int nConstraints)
{
    int         nWaitOrderProcs = 0;
    int         i,
                j;

    nWaitOrders = 0;

    /*
     * Scan constraint list backwards.  This is because the last-added
     * constraint is the only one that could fail, and so we want to test it
     * for inconsistency first.
     */
    for (i = nConstraints; --i >= 0;)
    {
        PGPROC     *proc = constraints[i].waiter;
        LOCK       *lock = proc->waitLock;

        /* Did we already make a list for this lock? */
        for (j = nWaitOrders; --j >= 0;)
        {
            if (waitOrders[j].lock == lock)
                break;
        }
        if (j >= 0)
            continue;
        /* No, so allocate a new list */
        waitOrders[nWaitOrders].lock = lock;
        waitOrders[nWaitOrders].procs = waitOrderProcs + nWaitOrderProcs;
        waitOrders[nWaitOrders].nProcs = lock->waitProcs.size;
        nWaitOrderProcs += lock->waitProcs.size;
        Assert(nWaitOrderProcs <= MaxBackends);

        /*
         * Do the topo sort.  TopoSort need not examine constraints after this
         * one, since they must be for different locks.
         */
        if (!TopoSort(lock, constraints, i + 1,
                      waitOrders[nWaitOrders].procs))
            return false;
        nWaitOrders++;
    }
    return true;
}


/*
 * TopoSort -- topological sort of a wait queue
 *
 * Generate a re-ordering of a lock's wait queue that satisfies given
 * constraints about certain procs preceding others.  (Each such constraint
 * is a fact of a partial ordering.)  Minimize rearrangement of the queue
 * not needed to achieve the partial ordering.
 *
 * This is a lot simpler and slower than, for example, the topological sort
 * algorithm shown in Knuth's Volume 1.  However, Knuth's method doesn't
 * try to minimize the damage to the existing order.  In practice we are
 * not likely to be working with more than a few constraints, so the apparent
 * slowness of the algorithm won't really matter.
 *
 * The initial queue ordering is taken directly from the lock's wait queue.
 * The output is an array of PGPROC pointers, of length equal to the lock's
 * wait queue length (the caller is responsible for providing this space).
 * The partial order is specified by an array of EDGE structs.  Each EDGE
 * is one that we need to reverse, therefore the "waiter" must appear before
 * the "blocker" in the output array.  The EDGE array may well contain
 * edges associated with other locks; these should be ignored.
 *
 * Returns TRUE if able to build an ordering that satisfies all the
 * constraints, FALSE if not (there are contradictory constraints).
 */
static bool
TopoSort(LOCK *lock,
         EDGE *constraints,
         int nConstraints,
         PGPROC **ordering)     /* output argument */
{
    PROC_QUEUE *waitQueue = &(lock->waitProcs);
    int         queue_size = waitQueue->size;
    PGPROC     *proc;
    int         i,
                j,
                k,
                last;

    /* First, fill topoProcs[] array with the procs in their current order */
    proc = (PGPROC *) waitQueue->links.next;
    for (i = 0; i < queue_size; i++)
    {
        topoProcs[i] = proc;
        proc = (PGPROC *) proc->links.next;
    }

    /*
     * Scan the constraints, and for each proc in the array, generate a count
     * of the number of constraints that say it must be before something else,
     * plus a list of the constraints that say it must be after something
     * else. The count for the j'th proc is stored in beforeConstraints[j],
     * and the head of its list in afterConstraints[j].  Each constraint
     * stores its list link in constraints[i].link (note any constraint will
     * be in just one list). The array index for the before-proc of the i'th
     * constraint is remembered in constraints[i].pred.
     */
    MemSet(beforeConstraints, 0, queue_size * sizeof(int));
    MemSet(afterConstraints, 0, queue_size * sizeof(int));
    for (i = 0; i < nConstraints; i++)
    {
        proc = constraints[i].waiter;
        /* Ignore constraint if not for this lock */
        if (proc->waitLock != lock)
            continue;
        /* Find the waiter proc in the array */
        for (j = queue_size; --j >= 0;)
        {
            if (topoProcs[j] == proc)
                break;
        }
        Assert(j >= 0);         /* should have found a match */
        /* Find the blocker proc in the array */
        proc = constraints[i].blocker;
        for (k = queue_size; --k >= 0;)
        {
            if (topoProcs[k] == proc)
                break;
        }
        Assert(k >= 0);         /* should have found a match */
        beforeConstraints[j]++; /* waiter must come before */
        /* add this constraint to list of after-constraints for blocker */
        constraints[i].pred = j;
        constraints[i].link = afterConstraints[k];
        afterConstraints[k] = i + 1;
    }
    /*--------------------
     * Now scan the topoProcs array backwards.  At each step, output the
     * last proc that has no remaining before-constraints, and decrease
     * the beforeConstraints count of each of the procs it was constrained
     * against.
     * i = index of ordering[] entry we want to output this time
     * j = search index for topoProcs[]
     * k = temp for scanning constraint list for proc j
     * last = last non-null index in topoProcs (avoid redundant searches)
     *--------------------
     */
    last = queue_size - 1;
    for (i = queue_size; --i >= 0;)
    {
        /* Find next candidate to output */
        while (topoProcs[last] == NULL)
            last--;
        for (j = last; j >= 0; j--)
        {
            if (topoProcs[j] != NULL && beforeConstraints[j] == 0)
                break;
        }
        /* If no available candidate, topological sort fails */
        if (j < 0)
            return false;
        /* Output candidate, and mark it done by zeroing topoProcs[] entry */
        ordering[i] = topoProcs[j];
        topoProcs[j] = NULL;
        /* Update beforeConstraints counts of its predecessors */
        for (k = afterConstraints[j]; k > 0; k = constraints[k - 1].link)
            beforeConstraints[constraints[k - 1].pred]--;
    }

    /* Done */
    return true;
}

#ifdef DEBUG_DEADLOCK
static void
PrintLockQueue(LOCK *lock, const char *info)
{
    PROC_QUEUE *waitQueue = &(lock->waitProcs);
    int         queue_size = waitQueue->size;
    PGPROC     *proc;
    int         i;

    printf("%s lock %p queue ", info, lock);
    proc = (PGPROC *) waitQueue->links.next;
    for (i = 0; i < queue_size; i++)
    {
        printf(" %d", proc->pid);
        proc = (PGPROC *) proc->links.next;
    }
    printf("\n");
    fflush(stdout);
}
#endif

/*
 * Report a detected deadlock, with available details.
 */
void
DeadLockReport(void)
{
    StringInfoData clientbuf;   /* errdetail for client */
    StringInfoData logbuf;      /* errdetail for server log */
    StringInfoData locktagbuf;
    int         i;

    initStringInfo(&clientbuf);
    initStringInfo(&logbuf);
    initStringInfo(&locktagbuf);

    /* Generate the "waits for" lines sent to the client */
    for (i = 0; i < nDeadlockDetails; i++)
    {
        DEADLOCK_INFO *info = &deadlockDetails[i];
        int         nextpid;

        /* The last proc waits for the first one... */
        if (i < nDeadlockDetails - 1)
            nextpid = info[1].pid;
        else
            nextpid = deadlockDetails[0].pid;

        /* reset locktagbuf to hold next object description */
        resetStringInfo(&locktagbuf);

        DescribeLockTag(&locktagbuf, &info->locktag);

        if (i > 0)
            appendStringInfoChar(&clientbuf, '\n');

        appendStringInfo(&clientbuf,
                  _("Process %d waits for %s on %s; blocked by process %d."),
                         info->pid,
                         GetLockmodeName(info->locktag.locktag_lockmethodid,
                                         info->lockmode),
                         locktagbuf.data,
                         nextpid);
    }

    /* Duplicate all the above for the server ... */
    appendStringInfoString(&logbuf, clientbuf.data);

    /* ... and add info about query strings */
    for (i = 0; i < nDeadlockDetails; i++)
    {
        DEADLOCK_INFO *info = &deadlockDetails[i];

        appendStringInfoChar(&logbuf, '\n');

        appendStringInfo(&logbuf,
                         _("Process %d: %s"),
                         info->pid,
                      pgstat_get_backend_current_activity(info->pid, false));
    }

    pgstat_report_deadlock();

    ereport(ERROR,
            (errcode(ERRCODE_T_R_DEADLOCK_DETECTED),
             errmsg("deadlock detected"),
             errdetail_internal("%s", clientbuf.data),
             errdetail_log("%s", logbuf.data),
             errhint("See server log for query details.")));
}

/*
 * RememberSimpleDeadLock: set up info for DeadLockReport when ProcSleep
 * detects a trivial (two-way) deadlock.  proc1 wants to block for lockmode
 * on lock, but proc2 is already waiting and would be blocked by proc1.
 */
void
RememberSimpleDeadLock(PGPROC *proc1,
                       LOCKMODE lockmode,
                       LOCK *lock,
                       PGPROC *proc2)
{
    DEADLOCK_INFO *info = &deadlockDetails[0];

    info->locktag = lock->tag;
    info->lockmode = lockmode;
    info->pid = proc1->pid;
    info++;
    info->locktag = proc2->waitLock->tag;
    info->lockmode = proc2->waitLockMode;
    info->pid = proc2->pid;
    nDeadlockDetails = 2;
}