Reduce pg_listener lock taken by NOTIFY et al from AccessExclusiveLock

to ExclusiveLock.  This still serializes the operations of this module,
but doesn't conflict with concurrent ANALYZE operations.  Per trouble
report from Philip Warner a few weeks ago.

src/backend/commands/async.c

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/async.c,v 1.109 2004/02/08 22:28:56 neilc Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/async.c,v 1.110 2004/05/22 21:58:24 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -52,9 +52,10 @@
  *	  transaction, since by assumption it is only called from outside any
  *	  transaction.
  *
- * Although we grab AccessExclusiveLock on pg_listener for any operation,
+ * Although we grab ExclusiveLock on pg_listener for any operation,
  * the lock is never held very long, so it shouldn't cause too much of
- * a performance problem.
+ * a performance problem.  (Previously we used AccessExclusiveLock, but
+ * there's no real reason to forbid concurrent reads.)
  *
  * An application that listens on the same relname it notifies will get
  * NOTIFY messages for its own NOTIFYs.  These can be ignored, if not useful,
@@ -196,7 +197,7 @@ Async_Listen(char *relname, int pid)
 	if (Trace_notify)
 		elog(DEBUG1, "Async_Listen(%s,%d)", relname, pid);
 
-	lRel = heap_openr(ListenerRelationName, AccessExclusiveLock);
+	lRel = heap_openr(ListenerRelationName, ExclusiveLock);
 
 	/* Detect whether we are already listening on this relname */
 	scan = heap_beginscan(lRel, SnapshotNow, 0, NULL);
@@ -216,7 +217,7 @@ Async_Listen(char *relname, int pid)
 
 	if (alreadyListener)
 	{
-		heap_close(lRel, AccessExclusiveLock);
+		heap_close(lRel, ExclusiveLock);
 		return;
 	}
 
@@ -244,7 +245,7 @@ Async_Listen(char *relname, int pid)
 
 	heap_freetuple(tuple);
 
-	heap_close(lRel, AccessExclusiveLock);
+	heap_close(lRel, ExclusiveLock);
 
 	/*
 	 * now that we are listening, make sure we will unlisten before dying.
@@ -290,7 +291,7 @@ Async_Unlisten(char *relname, int pid)
 	if (Trace_notify)
 		elog(DEBUG1, "Async_Unlisten(%s,%d)", relname, pid);
 
-	lRel = heap_openr(ListenerRelationName, AccessExclusiveLock);
+	lRel = heap_openr(ListenerRelationName, ExclusiveLock);
 
 	scan = heap_beginscan(lRel, SnapshotNow, 0, NULL);
 	while ((tuple = heap_getnext(scan, ForwardScanDirection)) != NULL)
@@ -312,7 +313,7 @@ Async_Unlisten(char *relname, int pid)
 	}
 	heap_endscan(scan);
 
-	heap_close(lRel, AccessExclusiveLock);
+	heap_close(lRel, ExclusiveLock);
 
 	/*
 	 * We do not complain about unlistening something not being listened;
@@ -348,7 +349,7 @@ Async_UnlistenAll(void)
 	if (Trace_notify)
 		elog(DEBUG1, "Async_UnlistenAll");
 
-	lRel = heap_openr(ListenerRelationName, AccessExclusiveLock);
+	lRel = heap_openr(ListenerRelationName, ExclusiveLock);
 	tdesc = RelationGetDescr(lRel);
 
 	/* Find and delete all entries with my listenerPID */
@@ -362,7 +363,7 @@ Async_UnlistenAll(void)
 		simple_heap_delete(lRel, &lTuple->t_self);
 
 	heap_endscan(scan);
-	heap_close(lRel, AccessExclusiveLock);
+	heap_close(lRel, ExclusiveLock);
 }
 
 /*
@@ -457,7 +458,7 @@ AtCommit_Notify(void)
 	value[0] = value[1] = value[2] = (Datum) 0;
 	value[Anum_pg_listener_notify - 1] = Int32GetDatum(MyProcPid);
 
-	lRel = heap_openr(ListenerRelationName, AccessExclusiveLock);
+	lRel = heap_openr(ListenerRelationName, ExclusiveLock);
 	tdesc = RelationGetDescr(lRel);
 	scan = heap_beginscan(lRel, SnapshotNow, 0, NULL);
 
@@ -812,7 +813,7 @@ ProcessIncomingNotify(void)
 
 	StartTransactionCommand();
 
-	lRel = heap_openr(ListenerRelationName, AccessExclusiveLock);
+	lRel = heap_openr(ListenerRelationName, ExclusiveLock);
 	tdesc = RelationGetDescr(lRel);
 
 	/* Scan only entries with my listenerPID */