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alvherrealvherre
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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 9ee16b4 .
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+186
-12
lines changed

9 files changed

+186
-12
lines changed

‎src/backend/executor/execMain.c

Lines changed: 17 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -1863,7 +1863,7 @@ EvalPlanQual(EState *estate, EPQState *epqstate,
18631863
/*
18641864
* Get and lock the updated version of the row; if fail, return NULL.
18651865
*/
1866-
copyTuple = EvalPlanQualFetch(estate, relation, lockmode,
1866+
copyTuple = EvalPlanQualFetch(estate, relation, lockmode, false /* wait */,
18671867
tid, priorXmax);
18681868

18691869
if (copyTuple == NULL)
@@ -1922,6 +1922,7 @@ EvalPlanQual(EState *estate, EPQState *epqstate,
19221922
* estate - executor state data
19231923
* relation - table containing tuple
19241924
* lockmode - requested tuple lock mode
1925+
* noWait - wait mode to pass to heap_lock_tuple
19251926
* *tid - t_ctid from the outdated tuple (ie, next updated version)
19261927
* priorXmax - t_xmax from the outdated tuple
19271928
*
@@ -1934,7 +1935,7 @@ EvalPlanQual(EState *estate, EPQState *epqstate,
19341935
* but we use "int" to avoid having to include heapam.h in executor.h.
19351936
*/
19361937
HeapTuple
1937-
EvalPlanQualFetch(EState *estate, Relation relation, int lockmode,
1938+
EvalPlanQualFetch(EState *estate, Relation relation, int lockmode, bool noWait,
19381939
ItemPointer tid, TransactionId priorXmax)
19391940
{
19401941
HeapTuple copyTuple = NULL;
@@ -1978,14 +1979,23 @@ EvalPlanQualFetch(EState *estate, Relation relation, int lockmode,
19781979

19791980
/*
19801981
* If tuple is being updated by other transaction then we have to
1981-
* wait for its commit/abort.
1982+
* wait for its commit/abort, or die trying.
19821983
*/
19831984
if (TransactionIdIsValid(SnapshotDirty.xmax))
19841985
{
19851986
ReleaseBuffer(buffer);
1986-
XactLockTableWait(SnapshotDirty.xmax,
1987-
relation, &tuple.t_data->t_ctid,
1988-
XLTW_FetchUpdated);
1987+
if (noWait)
1988+
{
1989+
if (!ConditionalXactLockTableWait(SnapshotDirty.xmax))
1990+
ereport(ERROR,
1991+
(errcode(ERRCODE_LOCK_NOT_AVAILABLE),
1992+
errmsg("could not obtain lock on row in relation \"%s\"",
1993+
RelationGetRelationName(relation))));
1994+
}
1995+
else
1996+
XactLockTableWait(SnapshotDirty.xmax,
1997+
relation, &tuple.t_data->t_ctid,
1998+
XLTW_FetchUpdated);
19891999
continue; /* loop back to repeat heap_fetch */
19902000
}
19912001

@@ -2012,7 +2022,7 @@ EvalPlanQualFetch(EState *estate, Relation relation, int lockmode,
20122022
*/
20132023
test = heap_lock_tuple(relation, &tuple,
20142024
estate->es_output_cid,
2015-
lockmode, false /* wait */ ,
2025+
lockmode, noWait,
20162026
false, &buffer, &hufd);
20172027
/* We now have two pins on the buffer, get rid of one */
20182028
ReleaseBuffer(buffer);

‎src/backend/executor/nodeLockRows.c

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -170,7 +170,7 @@ ExecLockRows(LockRowsState *node)
170170
}
171171

172172
/* updated, so fetch and lock the updated version */
173-
copyTuple = EvalPlanQualFetch(estate, erm->relation, lockmode,
173+
copyTuple = EvalPlanQualFetch(estate, erm->relation, lockmode, erm->noWait,
174174
&hufd.ctid, hufd.xmax);
175175

176176
if (copyTuple == NULL)

‎src/include/executor/executor.h

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -199,7 +199,7 @@ extern TupleTableSlot *EvalPlanQual(EState *estate, EPQState *epqstate,
199199
Relation relation, Index rti, int lockmode,
200200
ItemPointer tid, TransactionId priorXmax);
201201
extern HeapTuple EvalPlanQualFetch(EState *estate, Relation relation,
202-
int lockmode, ItemPointer tid, TransactionId priorXmax);
202+
int lockmode, bool noWait, ItemPointer tid, TransactionId priorXmax);
203203
extern void EvalPlanQualInit(EPQState *epqstate, EState *estate,
204204
Plan *subplan, List *auxrowmarks, int epqParam);
205205
extern void EvalPlanQualSetPlan(EPQState *epqstate,

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

Lines changed: 19 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,19 @@
1+
Parsed test spec with 2 sessions
2+
3+
starting permutation: s2a s1a s2b s2c s2d s2e s1b s2f
4+
step s2a: SELECT pg_advisory_lock(0);
5+
pg_advisory_lock
6+
7+
8+
step s1a: SELECT * FROM foo WHERE pg_advisory_lock(0) IS NOT NULL FOR UPDATE NOWAIT; <waiting ...>
9+
step s2b: UPDATE foo SET data = data;
10+
step s2c: BEGIN;
11+
step s2d: UPDATE foo SET data = data;
12+
step s2e: SELECT pg_advisory_unlock(0);
13+
pg_advisory_unlock
14+
15+
t
16+
step s1a: <... completed>
17+
error in steps s2e s1a: ERROR: could not obtain lock on row in relation "foo"
18+
step s1b: COMMIT;
19+
step s2f: COMMIT;

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

Lines changed: 19 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,19 @@
1+
Parsed test spec with 2 sessions
2+
3+
starting permutation: s2a s1a s2b s2c s2d s2e s1b s2f
4+
step s2a: SELECT pg_advisory_lock(0);
5+
pg_advisory_lock
6+
7+
8+
step s1a: SELECT * FROM foo WHERE pg_advisory_lock(0) IS NOT NULL FOR UPDATE NOWAIT; <waiting ...>
9+
step s2b: UPDATE foo SET data = data;
10+
step s2c: BEGIN;
11+
step s2d: UPDATE foo SET data = data;
12+
step s2e: SELECT pg_advisory_unlock(0);
13+
pg_advisory_unlock
14+
15+
t
16+
step s1a: <... completed>
17+
error in steps s2e s1a: ERROR: could not serialize access due to concurrent update
18+
step s1b: COMMIT;
19+
step s2f: COMMIT;

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

Lines changed: 37 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,37 @@
1+
Parsed test spec with 3 sessions
2+
3+
starting permutation: sl1_prep upd_getlock sl1_exec upd_doupdate lk1_doforshare upd_releaselock
4+
step sl1_prep:
5+
PREPARE sl1_run AS SELECT id FROM test_nowait WHERE pg_advisory_lock(0) is not null FOR UPDATE NOWAIT;
6+
7+
step upd_getlock:
8+
SELECT pg_advisory_lock(0);
9+
10+
pg_advisory_lock
11+
12+
13+
step sl1_exec:
14+
BEGIN ISOLATION LEVEL READ COMMITTED;
15+
EXECUTE sl1_run;
16+
SELECT xmin, xmax, ctid, * FROM test_nowait;
17+
<waiting ...>
18+
step upd_doupdate:
19+
BEGIN ISOLATION LEVEL READ COMMITTED;
20+
UPDATE test_nowait SET value = value WHERE id % 2 = 0;
21+
COMMIT;
22+
23+
step lk1_doforshare:
24+
BEGIN ISOLATION LEVEL READ COMMITTED;
25+
SELECT id FROM test_nowait WHERE id % 2 = 0 FOR SHARE;
26+
27+
id
28+
29+
2
30+
step upd_releaselock:
31+
SELECT pg_advisory_unlock(0);
32+
33+
pg_advisory_unlock
34+
35+
t
36+
step sl1_exec: <... completed>
37+
error in steps upd_releaselock sl1_exec: ERROR: could not obtain lock on row in relation "test_nowait"

‎src/test/isolation/isolation_schedule

Lines changed: 5 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -22,9 +22,11 @@ test: aborted-keyrevoke
2222
test: multixact-no-deadlock
2323
test: multixact-no-forget
2424
test: propagate-lock-delete
25-
test: drop-index-concurrently-1
26-
test: alter-table-1
27-
test: timeouts
2825
test: nowait
2926
test: nowait-2
3027
test: nowait-3
28+
test: nowait-4
29+
test: nowait-5
30+
test: drop-index-concurrently-1
31+
test: alter-table-1
32+
test: timeouts

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

Lines changed: 30 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,30 @@
1+
# Test NOWAIT with an updated tuple chain.
2+
3+
setup
4+
{
5+
CREATE TABLE foo (
6+
id int PRIMARY KEY,
7+
data text NOT NULL
8+
);
9+
INSERT INTO foo VALUES (1, 'x');
10+
}
11+
12+
teardown
13+
{
14+
DROP TABLE foo;
15+
}
16+
17+
session "s1"
18+
setup { BEGIN; }
19+
step "s1a" { SELECT * FROM foo WHERE pg_advisory_lock(0) IS NOT NULL FOR UPDATE NOWAIT; }
20+
step "s1b" { COMMIT; }
21+
22+
session "s2"
23+
step "s2a" { SELECT pg_advisory_lock(0); }
24+
step "s2b" { UPDATE foo SET data = data; }
25+
step "s2c" { BEGIN; }
26+
step "s2d" { UPDATE foo SET data = data; }
27+
step "s2e" { SELECT pg_advisory_unlock(0); }
28+
step "s2f" { COMMIT; }
29+
30+
permutation "s2a" "s1a" "s2b" "s2c" "s2d" "s2e" "s1b" "s2f"

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

Lines changed: 57 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,57 @@
1+
# Test NOWAIT on an updated tuple chain
2+
3+
setup
4+
{
5+
6+
DROP TABLE IF EXISTS test_nowait;
7+
CREATE TABLE test_nowait (
8+
id integer PRIMARY KEY,
9+
value integer not null
10+
);
11+
12+
INSERT INTO test_nowait
13+
SELECT x,x FROM generate_series(1,2) x;
14+
}
15+
16+
teardown
17+
{
18+
DROP TABLE test_nowait;
19+
}
20+
21+
session "sl1"
22+
step "sl1_prep" {
23+
PREPARE sl1_run AS SELECT id FROM test_nowait WHERE pg_advisory_lock(0) is not null FOR UPDATE NOWAIT;
24+
}
25+
step "sl1_exec" {
26+
BEGIN ISOLATION LEVEL READ COMMITTED;
27+
EXECUTE sl1_run;
28+
SELECT xmin, xmax, ctid, * FROM test_nowait;
29+
}
30+
teardown { COMMIT; }
31+
32+
# A session that's used for an UPDATE of the rows to be locked, for when we're testing ctid
33+
# chain following.
34+
session "upd"
35+
step "upd_getlock" {
36+
SELECT pg_advisory_lock(0);
37+
}
38+
step "upd_doupdate" {
39+
BEGIN ISOLATION LEVEL READ COMMITTED;
40+
UPDATE test_nowait SET value = value WHERE id % 2 = 0;
41+
COMMIT;
42+
}
43+
step "upd_releaselock" {
44+
SELECT pg_advisory_unlock(0);
45+
}
46+
47+
# A session that acquires locks that sl1 is supposed to avoid blocking on
48+
session "lk1"
49+
step "lk1_doforshare" {
50+
BEGIN ISOLATION LEVEL READ COMMITTED;
51+
SELECT id FROM test_nowait WHERE id % 2 = 0 FOR SHARE;
52+
}
53+
teardown {
54+
COMMIT;
55+
}
56+
57+
permutation "sl1_prep" "upd_getlock" "sl1_exec" "upd_doupdate" "lk1_doforshare" "upd_releaselock"

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