Fix FOR UPDATE NOWAIT on updated tuple chains

If SELECT FOR UPDATE NOWAIT tries to lock a tuple that is concurrently
being updated, it might fail to honor its NOWAIT specification and block
instead of raising an error.

Fix by adding a no-wait flag to EvalPlanQualFetch which it can pass down
to heap_lock_tuple; also use it in EvalPlanQualFetch itself to avoid
blocking while waiting for a concurrent transaction.

Authors: Craig Ringer and Thomas Munro, tweaked by Álvaro
http://www.postgresql.org/message-id/51FB6703.9090801@2ndquadrant.com

Per Thomas Munro in the course of his SKIP LOCKED feature submission,
who also provided one of the isolation test specs.

Backpatch to 9.4, because that's as far back as it applies without
conflicts (although the bug goes all the way back).  To that branch also
backpatch Thomas Munro's new NOWAIT test cases, committed in master by
Heikki as commit 9ee16b49 .

src/backend/executor/execMain.c

@@ -1863,7 +1863,7 @@ EvalPlanQual(EState *estate, EPQState *epqstate,
 	/*
 	 * Get and lock the updated version of the row; if fail, return NULL.
 	 */
-	copyTuple = EvalPlanQualFetch(estate, relation, lockmode,
+	copyTuple = EvalPlanQualFetch(estate, relation, lockmode, false /* wait */,
 								  tid, priorXmax);
 
 	if (copyTuple == NULL)
@@ -1922,6 +1922,7 @@ EvalPlanQual(EState *estate, EPQState *epqstate,
  *	estate - executor state data
  *	relation - table containing tuple
  *	lockmode - requested tuple lock mode
+ *	noWait - wait mode to pass to heap_lock_tuple
  *	*tid - t_ctid from the outdated tuple (ie, next updated version)
  *	priorXmax - t_xmax from the outdated tuple
  *
@@ -1934,7 +1935,7 @@ EvalPlanQual(EState *estate, EPQState *epqstate,
  * but we use "int" to avoid having to include heapam.h in executor.h.
  */
 HeapTuple
-EvalPlanQualFetch(EState *estate, Relation relation, int lockmode,
+EvalPlanQualFetch(EState *estate, Relation relation, int lockmode, bool noWait,
 				  ItemPointer tid, TransactionId priorXmax)
 {
 	HeapTuple	copyTuple = NULL;
@@ -1978,14 +1979,23 @@ EvalPlanQualFetch(EState *estate, Relation relation, int lockmode,
 
 			/*
 			 * If tuple is being updated by other transaction then we have to
-			 * wait for its commit/abort.
+			 * wait for its commit/abort, or die trying.
 			 */
 			if (TransactionIdIsValid(SnapshotDirty.xmax))
 			{
 				ReleaseBuffer(buffer);
-				XactLockTableWait(SnapshotDirty.xmax,
-								  relation, &tuple.t_data->t_ctid,
-								  XLTW_FetchUpdated);
+				if (noWait)
+				{
+					if (!ConditionalXactLockTableWait(SnapshotDirty.xmax))
+						ereport(ERROR,
+								(errcode(ERRCODE_LOCK_NOT_AVAILABLE),
+								 errmsg("could not obtain lock on row in relation \"%s\"",
+										RelationGetRelationName(relation))));
+				}
+				else
+					XactLockTableWait(SnapshotDirty.xmax,
+									  relation, &tuple.t_data->t_ctid,
+									  XLTW_FetchUpdated);
 				continue;		/* loop back to repeat heap_fetch */
 			}
 
@@ -2012,7 +2022,7 @@ EvalPlanQualFetch(EState *estate, Relation relation, int lockmode,
 			 */
 			test = heap_lock_tuple(relation, &tuple,
 								   estate->es_output_cid,
-								   lockmode, false /* wait */ ,
+								   lockmode, noWait,
 								   false, &buffer, &hufd);
 			/* We now have two pins on the buffer, get rid of one */
 			ReleaseBuffer(buffer);

src/backend/executor/nodeLockRows.c

@@ -170,7 +170,7 @@ lnext:
 				}
 
 				/* updated, so fetch and lock the updated version */
-				copyTuple = EvalPlanQualFetch(estate, erm->relation, lockmode,
+				copyTuple = EvalPlanQualFetch(estate, erm->relation, lockmode, erm->noWait,
 											  &hufd.ctid, hufd.xmax);
 
 				if (copyTuple == NULL)

src/include/executor/executor.h

@@ -199,7 +199,7 @@ extern TupleTableSlot *EvalPlanQual(EState *estate, EPQState *epqstate,
 			 Relation relation, Index rti, int lockmode,
 			 ItemPointer tid, TransactionId priorXmax);
 extern HeapTuple EvalPlanQualFetch(EState *estate, Relation relation,
-				  int lockmode, ItemPointer tid, TransactionId priorXmax);
+				  int lockmode, bool noWait, ItemPointer tid, TransactionId priorXmax);
 extern void EvalPlanQualInit(EPQState *epqstate, EState *estate,
 				 Plan *subplan, List *auxrowmarks, int epqParam);
 extern void EvalPlanQualSetPlan(EPQState *epqstate,

src/test/isolation/expected/nowait-4.out

+Parsed test spec with 2 sessions
+
+starting permutation: s2a s1a s2b s2c s2d s2e s1b s2f
+step s2a: SELECT pg_advisory_lock(0);
+pg_advisory_lock
+
+               
+step s1a: SELECT * FROM foo WHERE pg_advisory_lock(0) IS NOT NULL FOR UPDATE NOWAIT; <waiting ...>
+step s2b: UPDATE foo SET data = data;
+step s2c: BEGIN;
+step s2d: UPDATE foo SET data = data;
+step s2e: SELECT pg_advisory_unlock(0);
+pg_advisory_unlock
+
+t              
+step s1a: <... completed>
+error in steps s2e s1a: ERROR:  could not obtain lock on row in relation "foo"
+step s1b: COMMIT;
+step s2f: COMMIT;

src/test/isolation/expected/nowait-4_1.out

+Parsed test spec with 2 sessions
+
+starting permutation: s2a s1a s2b s2c s2d s2e s1b s2f
+step s2a: SELECT pg_advisory_lock(0);
+pg_advisory_lock
+
+               
+step s1a: SELECT * FROM foo WHERE pg_advisory_lock(0) IS NOT NULL FOR UPDATE NOWAIT; <waiting ...>
+step s2b: UPDATE foo SET data = data;
+step s2c: BEGIN;
+step s2d: UPDATE foo SET data = data;
+step s2e: SELECT pg_advisory_unlock(0);
+pg_advisory_unlock
+
+t              
+step s1a: <... completed>
+error in steps s2e s1a: ERROR:  could not serialize access due to concurrent update
+step s1b: COMMIT;
+step s2f: COMMIT;

src/test/isolation/expected/nowait-5.out

+Parsed test spec with 3 sessions
+
+starting permutation: sl1_prep upd_getlock sl1_exec upd_doupdate lk1_doforshare upd_releaselock
+step sl1_prep: 
+	PREPARE sl1_run AS SELECT id FROM test_nowait WHERE pg_advisory_lock(0) is not null FOR UPDATE NOWAIT;
+
+step upd_getlock: 
+	SELECT pg_advisory_lock(0);
+
+pg_advisory_lock
+
+               
+step sl1_exec: 
+	BEGIN ISOLATION LEVEL READ COMMITTED;
+	EXECUTE sl1_run;
+	SELECT xmin, xmax, ctid, * FROM test_nowait;
+ <waiting ...>
+step upd_doupdate: 
+	BEGIN ISOLATION LEVEL READ COMMITTED;
+	UPDATE test_nowait SET value = value WHERE id % 2 = 0;
+	COMMIT;
+
+step lk1_doforshare: 
+	BEGIN ISOLATION LEVEL READ COMMITTED;
+	SELECT id FROM test_nowait WHERE id % 2 = 0 FOR SHARE;
+
+id             
+
+2              
+step upd_releaselock: 
+	SELECT pg_advisory_unlock(0);
+
+pg_advisory_unlock
+
+t              
+step sl1_exec: <... completed>
+error in steps upd_releaselock sl1_exec: ERROR:  could not obtain lock on row in relation "test_nowait"

src/test/isolation/isolation_schedule

@@ -22,9 +22,11 @@ test: aborted-keyrevoke
 test: multixact-no-deadlock
 test: multixact-no-forget
 test: propagate-lock-delete
-test: drop-index-concurrently-1
-test: alter-table-1
-test: timeouts
 test: nowait
 test: nowait-2
 test: nowait-3
+test: nowait-4
+test: nowait-5
+test: drop-index-concurrently-1
+test: alter-table-1
+test: timeouts

src/test/isolation/specs/nowait-4.spec

+# Test NOWAIT with an updated tuple chain.
+
+setup
+{
+  CREATE TABLE foo (
+	id int PRIMARY KEY,
+	data text NOT NULL
+  );
+  INSERT INTO foo VALUES (1, 'x');
+}
+
+teardown
+{
+  DROP TABLE foo;
+}
+
+session "s1"
+setup		{ BEGIN; }
+step "s1a"	{ SELECT * FROM foo WHERE pg_advisory_lock(0) IS NOT NULL FOR UPDATE NOWAIT; }
+step "s1b"	{ COMMIT; }
+
+session "s2"
+step "s2a"	{ SELECT pg_advisory_lock(0); }
+step "s2b"	{ UPDATE foo SET data = data; }
+step "s2c"	{ BEGIN; }
+step "s2d"	{ UPDATE foo SET data = data; }
+step "s2e"	{ SELECT pg_advisory_unlock(0); }
+step "s2f"	{ COMMIT; }
+
+permutation "s2a" "s1a" "s2b" "s2c" "s2d" "s2e" "s1b" "s2f"

src/test/isolation/specs/nowait-5.spec

+# Test NOWAIT on an updated tuple chain
+
+setup
+{
+
+  DROP TABLE IF EXISTS test_nowait;
+  CREATE TABLE test_nowait (
+	id integer PRIMARY KEY,
+	value integer not null
+  );
+
+  INSERT INTO test_nowait
+  SELECT x,x FROM generate_series(1,2) x;
+}
+
+teardown
+{
+  DROP TABLE test_nowait;
+}
+
+session "sl1"
+step "sl1_prep" {
+	PREPARE sl1_run AS SELECT id FROM test_nowait WHERE pg_advisory_lock(0) is not null FOR UPDATE NOWAIT;
+}
+step "sl1_exec" {
+	BEGIN ISOLATION LEVEL READ COMMITTED;
+	EXECUTE sl1_run;
+	SELECT xmin, xmax, ctid, * FROM test_nowait;
+}
+teardown { COMMIT; }
+
+# A session that's used for an UPDATE of the rows to be locked, for when we're testing ctid
+# chain following.
+session "upd"
+step "upd_getlock" {
+	SELECT pg_advisory_lock(0);
+}
+step "upd_doupdate" {
+	BEGIN ISOLATION LEVEL READ COMMITTED;
+	UPDATE test_nowait SET value = value WHERE id % 2 = 0;
+	COMMIT;
+}
+step "upd_releaselock" {
+	SELECT pg_advisory_unlock(0);
+}
+
+# A session that acquires locks that sl1 is supposed to avoid blocking on
+session "lk1"
+step "lk1_doforshare" {
+	BEGIN ISOLATION LEVEL READ COMMITTED;
+	SELECT id FROM test_nowait WHERE id % 2 = 0 FOR SHARE;
+}
+teardown {
+	COMMIT;
+}
+
+permutation "sl1_prep" "upd_getlock" "sl1_exec" "upd_doupdate" "lk1_doforshare" "upd_releaselock"