Add NOWAIT option to SELECT FOR UPDATE/SHARE.

Original patch by Hans-Juergen Schoenig, revisions by Karel Zak
and Tom Lane.

src/backend/access/heap/heapam.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/heap/heapam.c,v 1.195 2005/06/20 18:37:01 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/heap/heapam.c,v 1.196 2005/08/01 20:31:05 tgl Exp $
  *
  *
  * INTERFACE ROUTINES
@@ -1945,7 +1945,7 @@ simple_heap_update(Relation relation, ItemPointer otid, HeapTuple tup)
  */
 HTSU_Result
 heap_lock_tuple(Relation relation, HeapTuple tuple, Buffer *buffer,
-				 CommandId cid, LockTupleMode mode)
+				 CommandId cid, LockTupleMode mode, bool nowait)
 {
 	HTSU_Result	result;
 	ItemPointer tid = &(tuple->t_self);
@@ -1998,7 +1998,16 @@ l3:
 		 */
 		if (!have_tuple_lock)
 		{
-			LockTuple(relation, tid, tuple_lock_type);
+			if (nowait)
+			{
+				if (!ConditionalLockTuple(relation, tid, tuple_lock_type))
+					ereport(ERROR,
+							(errcode(ERRCODE_LOCK_NOT_AVAILABLE),
+							 errmsg("could not obtain lock on row in relation \"%s\"",
+									RelationGetRelationName(relation))));
+			}
+			else
+				LockTuple(relation, tid, tuple_lock_type);
 			have_tuple_lock = true;
 		}
 
@@ -2020,7 +2029,17 @@ l3:
 		else if (infomask & HEAP_XMAX_IS_MULTI)
 		{
 			/* wait for multixact to end */
-			MultiXactIdWait((MultiXactId) xwait);
+			if (nowait)
+			{
+				if (!ConditionalMultiXactIdWait((MultiXactId) xwait))
+					ereport(ERROR,
+							(errcode(ERRCODE_LOCK_NOT_AVAILABLE),
+							 errmsg("could not obtain lock on row in relation \"%s\"",
+									RelationGetRelationName(relation))));
+			}
+			else
+				MultiXactIdWait((MultiXactId) xwait);
+
 			LockBuffer(*buffer, BUFFER_LOCK_EXCLUSIVE);
 
 			/*
@@ -2045,7 +2064,17 @@ l3:
 		else
 		{
 			/* wait for regular transaction to end */
-			XactLockTableWait(xwait);
+			if (nowait)
+			{
+				if (!ConditionalXactLockTableWait(xwait))
+					ereport(ERROR,
+							(errcode(ERRCODE_LOCK_NOT_AVAILABLE),
+							 errmsg("could not obtain lock on row in relation \"%s\"",
+									RelationGetRelationName(relation))));
+			}
+			else
+				XactLockTableWait(xwait);
+
 			LockBuffer(*buffer, BUFFER_LOCK_EXCLUSIVE);
 
 			/*

src/backend/access/transam/multixact.c

@@ -42,7 +42,7 @@
  * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/backend/access/transam/multixact.c,v 1.5 2005/06/08 15:50:25 tgl Exp $
+ * $PostgreSQL: pgsql/src/backend/access/transam/multixact.c,v 1.6 2005/08/01 20:31:06 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -558,6 +558,43 @@ MultiXactIdWait(MultiXactId multi)
 	}
 }
 
+/*
+ * ConditionalMultiXactIdWait
+ *		As above, but only lock if we can get the lock without blocking.
+ */
+bool
+ConditionalMultiXactIdWait(MultiXactId multi)
+{
+	bool	result = true;
+	TransactionId *members;
+	int		nmembers;
+
+	nmembers = GetMultiXactIdMembers(multi, &members);
+
+	if (nmembers >= 0)
+	{
+		int		i;
+
+		for (i = 0; i < nmembers; i++)
+		{
+			TransactionId	member = members[i];
+
+			debug_elog4(DEBUG2, "ConditionalMultiXactIdWait: trying %d (%u)",
+						i, member);
+			if (!TransactionIdIsCurrentTransactionId(member))
+			{
+				result = ConditionalXactLockTableWait(member);
+				if (!result)
+					break;
+			}
+		}
+
+		pfree(members);
+	}
+
+	return result;
+}
+
 /*
  * CreateMultiXactId
  * 		Make a new MultiXactId
@@ -590,7 +627,7 @@ CreateMultiXactId(int nxids, TransactionId *xids)
 	 */
 	multi = mXactCacheGetBySet(nxids, xids);
 	if (MultiXactIdIsValid(multi))
-	{	
+	{
 		debug_elog2(DEBUG2, "Create: in cache!");
 		return multi;
 	}