Implement SKIP LOCKED for row-level locks

This clause changes the behavior of SELECT locking clauses in the
presence of locked rows: instead of causing a process to block waiting
for the locks held by other processes (or raise an error, with NOWAIT),
SKIP LOCKED makes the new reader skip over such rows.  While this is not
appropriate behavior for general purposes, there are some cases in which
it is useful, such as queue-like tables.

Catalog version bumped because this patch changes the representation of
stored rules.

Reviewed by Craig Ringer (based on a previous attempt at an
implementation by Simon Riggs, who also provided input on the syntax
used in the current patch), David Rowley, and Álvaro Herrera.

Author: Thomas Munro

src/backend/executor/nodeLockRows.c

@@ -133,11 +133,15 @@ lnext:
 
 		test = heap_lock_tuple(erm->relation, &tuple,
 							   estate->es_output_cid,
-							   lockmode, erm->noWait, true,
+							   lockmode, erm->waitPolicy, true,
 							   &buffer, &hufd);
 		ReleaseBuffer(buffer);
 		switch (test)
 		{
+			case HeapTupleWouldBlock:
+				/* couldn't lock tuple in SKIP LOCKED mode */
+				goto lnext;
+
 			case HeapTupleSelfUpdated:
 
 				/*
@@ -170,12 +174,15 @@ lnext:
 				}
 
 				/* updated, so fetch and lock the updated version */
-				copyTuple = EvalPlanQualFetch(estate, erm->relation, lockmode, erm->noWait,
-											  &hufd.ctid, hufd.xmax);
+				copyTuple = EvalPlanQualFetch(estate, erm->relation, lockmode,
+											  erm->waitPolicy, &hufd.ctid, hufd.xmax);
 
 				if (copyTuple == NULL)
 				{
-					/* Tuple was deleted, so don't return it */
+					/*
+					 * Tuple was deleted; or it's locked and we're under SKIP
+					 * LOCKED policy, so don't return it
+					 */
 					goto lnext;
 				}
 				/* remember the actually locked tuple's TID */