Allow an autovacuum worker to be interrupted automatically when it is found

to be locking another process (except when it's working to prevent Xid
wraparound problems).

Author: alvherre

src/backend/postmaster/autovacuum.c

@@ -55,7 +55,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/postmaster/autovacuum.c,v 1.65 2007/10/25 19:13:37 alvherre Exp $
+ *	  $PostgreSQL: pgsql/src/backend/postmaster/autovacuum.c,v 1.66 2007/10/26 20:45:10 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -2120,6 +2120,14 @@ do_autovacuum(void)
 									  tab->at_doanalyze,
 									  tab->at_freeze_min_age,
 									  bstrategy);
+
+			/*
+			 * Clear a possible query-cancel signal, to avoid a late reaction
+			 * to an automatically-sent signal because of vacuuming the current
+			 * table (we're done with it, so it would make no sense to cancel
+			 * at this point.)
+			 */
+			QueryCancelPending = false;
 		}
 		PG_CATCH();
 		{

src/backend/storage/lmgr/README

@@ -1,4 +1,4 @@
-$PostgreSQL: pgsql/src/backend/storage/lmgr/README,v 1.21 2006/09/18 22:40:36 tgl Exp $
+$PostgreSQL: pgsql/src/backend/storage/lmgr/README,v 1.22 2007/10/26 20:45:10 alvherre Exp $
 
 
 LOCKING OVERVIEW
@@ -487,6 +487,13 @@ seems a safer approach than trying to allocate workspace on the fly; we
 don't want to risk having the deadlock detector run out of memory, else
 we really have no guarantees at all that deadlock will be detected.
 
+6. We abuse the deadlock detector to implement autovacuum cancellation.
+When we run the detector and we find that there's an autovacuum worker
+involved in the waits-for graph, we store a pointer to its PGPROC, and
+return a special return code (unless a hard deadlock has been detected).
+The caller can then send a cancellation signal.  This implements the
+principle that autovacuum has a low locking priority (eg it must not block
+DDL on the table).
 
 USER LOCKS
 

src/backend/storage/lmgr/deadlock.c

@@ -12,7 +12,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/storage/lmgr/deadlock.c,v 1.48 2007/06/19 20:13:21 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/storage/lmgr/deadlock.c,v 1.49 2007/10/26 20:45:10 alvherre Exp $
  *
  *	Interface:
  *
@@ -109,6 +109,9 @@ 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
@@ -206,6 +209,9 @@ DeadLockCheck(PGPROC *proc)
 	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))
 	{
@@ -255,10 +261,28 @@ DeadLockCheck(PGPROC *proc)
 	/* 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
  *
@@ -497,6 +521,25 @@ FindLockCycleRecurse(PGPROC *checkProc,
 				if ((proclock->holdMask & LOCKBIT_ON(lm)) &&
 					(conflictMask & LOCKBIT_ON(lm)))
 				{
+					/*
+					 * Look for a blocking autovacuum. There can be more than
+					 * one in the deadlock cycle, in which case we just pick a
+					 * random one.  We stash the autovacuum worker's PGPROC so
+					 * that the caller can send a cancel signal to it, if
+					 * appropriate.
+					 *
+					 * 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 cancelling an
+					 * autovacuum that is working for preventing Xid wraparound
+					 * problems (which needs to read a different vacuumFlag
+					 * bit), but we don't do that here to avoid grabbing
+					 * ProcArrayLock.
+					 */
+					if (proc->vacuumFlags & PROC_IS_AUTOVACUUM)
+						blocking_autovacuum_proc = proc;
+
 					/* This proc hard-blocks checkProc */
 					if (FindLockCycleRecurse(proc, depth + 1,
 											 softEdges, nSoftEdges))

src/backend/storage/lmgr/proc.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/storage/lmgr/proc.c,v 1.195 2007/10/24 20:55:36 alvherre Exp $
+ *	  $PostgreSQL: pgsql/src/backend/storage/lmgr/proc.c,v 1.196 2007/10/26 20:45:10 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -734,6 +734,7 @@ ProcSleep(LOCALLOCK *locallock, LockMethod lockMethodTable)
 	PROC_QUEUE *waitQueue = &(lock->waitProcs);
 	LOCKMASK	myHeldLocks = MyProc->heldLocks;
 	bool		early_deadlock = false;
+	bool 		allow_autovacuum_cancel = true;
 	int			myWaitStatus;
 	PGPROC	   *proc;
 	int			i;
@@ -893,6 +894,48 @@ ProcSleep(LOCALLOCK *locallock, LockMethod lockMethodTable)
 		 */
 		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.
@@ -1189,13 +1232,16 @@ CheckDeadLock(void)
 		 * RemoveFromWaitQueue took care of waking up any such processes.
 		 */
 	}
-	else if (log_lock_waits)
+	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);
 	}

src/include/storage/lock.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/storage/lock.h,v 1.107 2007/09/05 18:10:48 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/storage/lock.h,v 1.108 2007/10/26 20:45:10 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -442,7 +442,9 @@ typedef enum
 	DS_NOT_YET_CHECKED,			/* no deadlock check has run yet */
 	DS_NO_DEADLOCK,				/* no deadlock detected */
 	DS_SOFT_DEADLOCK,			/* deadlock avoided by queue rearrangement */
-	DS_HARD_DEADLOCK			/* deadlock, no way out but ERROR */
+	DS_HARD_DEADLOCK,			/* deadlock, no way out but ERROR */
+	DS_BLOCKED_BY_AUTOVACUUM	/* no deadlock; queue blocked by autovacuum
+								   worker */
 } DeadLockState;
 
 
@@ -495,6 +497,7 @@ extern void lock_twophase_postabort(TransactionId xid, uint16 info,
 						void *recdata, uint32 len);
 
 extern DeadLockState DeadLockCheck(PGPROC *proc);
+extern PGPROC *GetBlockingAutoVacuumPgproc(void);
 extern void DeadLockReport(void);
 extern void RememberSimpleDeadLock(PGPROC *proc1,
 					   LOCKMODE lockmode,