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Commitb5b418b

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Fix MVCC bug with prepared xact with subxacts on standby
We did not recover the subtransaction IDs of prepared transactionswhen starting a hot standby from a shutdown checkpoint. As a result,such subtransactions were considered as aborted, rather thanin-progress. That would lead to hint bits being set incorrectly, andthe subtransactions suddenly becoming visible to old snapshots whenthe prepared transaction was committed.To fix, update pg_subtrans with prepared transactions's subxids whenstarting hot standby from a shutdown checkpoint. The snapshots takenfrom that state need to be marked as "suboverflowed", so that we alsocheck the pg_subtrans.Backport to all supported versions.Discussion:https://www.postgresql.org/message-id/6b852e98-2d49-4ca1-9e95-db419a2696e0@iki.fi
1 parente00a9a4 commitb5b418b

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7 files changed

+85
-17
lines changed

7 files changed

+85
-17
lines changed

‎src/backend/access/transam/twophase.c‎

Lines changed: 3 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -2011,9 +2011,8 @@ PrescanPreparedTransactions(TransactionId **xids_p, int *nxids_p)
20112011
* This is never called at the end of recovery - we use
20122012
* RecoverPreparedTransactions() at that point.
20132013
*
2014-
* The lack of calls to SubTransSetParent() calls here is by design;
2015-
* those calls are made by RecoverPreparedTransactions() at the end of recovery
2016-
* for those xacts that need this.
2014+
* This updates pg_subtrans, so that any subtransactions will be correctly
2015+
* seen as in-progress in snapshots taken during recovery.
20172016
*/
20182017
void
20192018
StandbyRecoverPreparedTransactions(void)
@@ -2033,7 +2032,7 @@ StandbyRecoverPreparedTransactions(void)
20332032

20342033
buf=ProcessTwoPhaseBuffer(xid,
20352034
gxact->prepare_start_lsn,
2036-
gxact->ondisk,false, false);
2035+
gxact->ondisk,true, false);
20372036
if (buf!=NULL)
20382037
pfree(buf);
20392038
}

‎src/backend/access/transam/xlog.c‎

Lines changed: 8 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -5420,6 +5420,9 @@ StartupXLOG(void)
54205420
RunningTransactionsDatarunning;
54215421
TransactionIdlatestCompletedXid;
54225422

5423+
/* Update pg_subtrans entries for any prepared transactions */
5424+
StandbyRecoverPreparedTransactions();
5425+
54235426
/*
54245427
* Construct a RunningTransactions snapshot representing a
54255428
* shut down server, with only prepared transactions still
@@ -5428,7 +5431,7 @@ StartupXLOG(void)
54285431
*/
54295432
running.xcnt=nxids;
54305433
running.subxcnt=0;
5431-
running.subxid_overflow=false;
5434+
running.subxid_status=SUBXIDS_IN_SUBTRANS;
54325435
running.nextXid=XidFromFullTransactionId(checkPoint.nextXid);
54335436
running.oldestRunningXid=oldestActiveXID;
54345437
latestCompletedXid=XidFromFullTransactionId(checkPoint.nextXid);
@@ -5438,8 +5441,6 @@ StartupXLOG(void)
54385441
running.xids=xids;
54395442

54405443
ProcArrayApplyRecoveryInfo(&running);
5441-
5442-
StandbyRecoverPreparedTransactions();
54435444
}
54445445
}
54455446

@@ -7808,6 +7809,9 @@ xlog_redo(XLogReaderState *record)
78087809

78097810
oldestActiveXID=PrescanPreparedTransactions(&xids,&nxids);
78107811

7812+
/* Update pg_subtrans entries for any prepared transactions */
7813+
StandbyRecoverPreparedTransactions();
7814+
78117815
/*
78127816
* Construct a RunningTransactions snapshot representing a shut
78137817
* down server, with only prepared transactions still alive. We're
@@ -7816,7 +7820,7 @@ xlog_redo(XLogReaderState *record)
78167820
*/
78177821
running.xcnt=nxids;
78187822
running.subxcnt=0;
7819-
running.subxid_overflow=false;
7823+
running.subxid_status=SUBXIDS_IN_SUBTRANS;
78207824
running.nextXid=XidFromFullTransactionId(checkPoint.nextXid);
78217825
running.oldestRunningXid=oldestActiveXID;
78227826
latestCompletedXid=XidFromFullTransactionId(checkPoint.nextXid);
@@ -7826,8 +7830,6 @@ xlog_redo(XLogReaderState *record)
78267830
running.xids=xids;
78277831

78287832
ProcArrayApplyRecoveryInfo(&running);
7829-
7830-
StandbyRecoverPreparedTransactions();
78317833
}
78327834

78337835
/* ControlFile->checkPointCopy always tracks the latest ckpt XID */

‎src/backend/storage/ipc/procarray.c‎

Lines changed: 15 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -1097,7 +1097,7 @@ ProcArrayApplyRecoveryInfo(RunningTransactions running)
10971097
* If the snapshot isn't overflowed or if its empty we can reset our
10981098
* pending state and use this snapshot instead.
10991099
*/
1100-
if (!running->subxid_overflow||running->xcnt==0)
1100+
if (running->subxid_status!=SUBXIDS_MISSING||running->xcnt==0)
11011101
{
11021102
/*
11031103
* If we have already collected known assigned xids, we need to
@@ -1249,7 +1249,7 @@ ProcArrayApplyRecoveryInfo(RunningTransactions running)
12491249
* missing, so conservatively assume the last one is latestObservedXid.
12501250
* ----------
12511251
*/
1252-
if (running->subxid_overflow)
1252+
if (running->subxid_status==SUBXIDS_MISSING)
12531253
{
12541254
standbyState=STANDBY_SNAPSHOT_PENDING;
12551255

@@ -1261,6 +1261,18 @@ ProcArrayApplyRecoveryInfo(RunningTransactions running)
12611261
standbyState=STANDBY_SNAPSHOT_READY;
12621262

12631263
standbySnapshotPendingXmin=InvalidTransactionId;
1264+
1265+
/*
1266+
* If the 'xids' array didn't include all subtransactions, we have to
1267+
* mark any snapshots taken as overflowed.
1268+
*/
1269+
if (running->subxid_status==SUBXIDS_IN_SUBTRANS)
1270+
procArray->lastOverflowedXid=latestObservedXid;
1271+
else
1272+
{
1273+
Assert(running->subxid_status==SUBXIDS_IN_ARRAY);
1274+
procArray->lastOverflowedXid=InvalidTransactionId;
1275+
}
12641276
}
12651277

12661278
/*
@@ -2842,7 +2854,7 @@ GetRunningTransactionData(void)
28422854

28432855
CurrentRunningXacts->xcnt=count-subcount;
28442856
CurrentRunningXacts->subxcnt=subcount;
2845-
CurrentRunningXacts->subxid_overflow=suboverflowed;
2857+
CurrentRunningXacts->subxid_status=suboverflowed ?SUBXIDS_IN_SUBTRANS :SUBXIDS_IN_ARRAY;
28462858
CurrentRunningXacts->nextXid=XidFromFullTransactionId(ShmemVariableCache->nextXid);
28472859
CurrentRunningXacts->oldestRunningXid=oldestRunningXid;
28482860
CurrentRunningXacts->latestCompletedXid=latestCompletedXid;

‎src/backend/storage/ipc/standby.c‎

Lines changed: 3 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -1186,7 +1186,7 @@ standby_redo(XLogReaderState *record)
11861186

11871187
running.xcnt=xlrec->xcnt;
11881188
running.subxcnt=xlrec->subxcnt;
1189-
running.subxid_overflow=xlrec->subxid_overflow;
1189+
running.subxid_status=xlrec->subxid_overflow ?SUBXIDS_MISSING :SUBXIDS_IN_ARRAY;
11901190
running.nextXid=xlrec->nextXid;
11911191
running.latestCompletedXid=xlrec->latestCompletedXid;
11921192
running.oldestRunningXid=xlrec->oldestRunningXid;
@@ -1351,7 +1351,7 @@ LogCurrentRunningXacts(RunningTransactions CurrRunningXacts)
13511351

13521352
xlrec.xcnt=CurrRunningXacts->xcnt;
13531353
xlrec.subxcnt=CurrRunningXacts->subxcnt;
1354-
xlrec.subxid_overflow=CurrRunningXacts->subxid_overflow;
1354+
xlrec.subxid_overflow=(CurrRunningXacts->subxid_status!=SUBXIDS_IN_ARRAY);
13551355
xlrec.nextXid=CurrRunningXacts->nextXid;
13561356
xlrec.oldestRunningXid=CurrRunningXacts->oldestRunningXid;
13571357
xlrec.latestCompletedXid=CurrRunningXacts->latestCompletedXid;
@@ -1368,7 +1368,7 @@ LogCurrentRunningXacts(RunningTransactions CurrRunningXacts)
13681368

13691369
recptr=XLogInsert(RM_STANDBY_ID,XLOG_RUNNING_XACTS);
13701370

1371-
if (CurrRunningXacts->subxid_overflow)
1371+
if (xlrec.subxid_overflow)
13721372
elog(trace_recovery(DEBUG2),
13731373
"snapshot of %d running transactions overflowed (lsn %X/%X oldest xid %u latest complete %u next xid %u)",
13741374
CurrRunningXacts->xcnt,

‎src/include/storage/standby.h‎

Lines changed: 9 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -75,11 +75,19 @@ extern void StandbyReleaseOldLocks(TransactionId oldxid);
7575
* almost immediately see the data we need to begin executing queries.
7676
*/
7777

78+
typedefenum
79+
{
80+
SUBXIDS_IN_ARRAY,/* xids array includes all running subxids */
81+
SUBXIDS_MISSING,/* snapshot overflowed, subxids are missing */
82+
SUBXIDS_IN_SUBTRANS,/* subxids are not included in 'xids', but
83+
* pg_subtrans is fully up-to-date */
84+
}subxids_array_status;
85+
7886
typedefstructRunningTransactionsData
7987
{
8088
intxcnt;/* # of xact ids in xids[] */
8189
intsubxcnt;/* # of subxact ids in xids[] */
82-
boolsubxid_overflow;/* snapshot overflowed, subxids missing */
90+
subxids_array_statussubxid_status;
8391
TransactionIdnextXid;/* xid from ShmemVariableCache->nextXid */
8492
TransactionIdoldestRunningXid;/* *not* oldestXmin */
8593
TransactionIdlatestCompletedXid;/* so we can set xmax */

‎src/test/recovery/t/009_twophase.pl‎

Lines changed: 46 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -308,6 +308,52 @@ sub configure_and_reload
308308

309309
$cur_primary->psql('postgres',"COMMIT PREPARED 'xact_009_12'");
310310

311+
###############################################################################
312+
# Check visibility of prepared transactions in standby after a restart while
313+
# primary is down.
314+
###############################################################################
315+
316+
$cur_primary->psql(
317+
'postgres',"
318+
CREATE TABLE t_009_tbl_standby_mvcc (id int, msg text);
319+
BEGIN;
320+
INSERT INTO t_009_tbl_standby_mvcc VALUES (1, 'issued to${cur_primary_name}');
321+
SAVEPOINT s1;
322+
INSERT INTO t_009_tbl_standby_mvcc VALUES (2, 'issued to${cur_primary_name}');
323+
PREPARE TRANSACTION 'xact_009_standby_mvcc';
324+
");
325+
$cur_primary->stop;
326+
$cur_standby->restart;
327+
328+
# Acquire a snapshot in standby, before we commit the prepared transaction
329+
my$standby_session =$cur_standby->background_psql('postgres',on_error_die=> 1);
330+
$standby_session->query_safe("BEGIN ISOLATION LEVEL REPEATABLE READ");
331+
$psql_out =$standby_session->query_safe(
332+
"SELECT count(*) FROM t_009_tbl_standby_mvcc");
333+
is($psql_out,'0',
334+
"Prepared transaction not visible in standby before commit");
335+
336+
# Commit the transaction in primary
337+
$cur_primary->start;
338+
$cur_primary->psql('postgres',"
339+
SET synchronous_commit='remote_apply'; -- To ensure the standby is caught up
340+
COMMIT PREPARED 'xact_009_standby_mvcc';
341+
");
342+
343+
# Still not visible to the old snapshot
344+
$psql_out =$standby_session->query_safe(
345+
"SELECT count(*) FROM t_009_tbl_standby_mvcc");
346+
is($psql_out,'0',
347+
"Committed prepared transaction not visible to old snapshot in standby");
348+
349+
# Is visible to a new snapshot
350+
$standby_session->query_safe("COMMIT");
351+
$psql_out =$standby_session->query_safe(
352+
"SELECT count(*) FROM t_009_tbl_standby_mvcc");
353+
is($psql_out,'2',
354+
"Committed prepared transaction is visible to new snapshot in standby");
355+
$standby_session->quit;
356+
311357
###############################################################################
312358
# Check for a lock conflict between prepared transaction with DDL inside and
313359
# replay of XLOG_STANDBY_LOCK wal record.

‎src/tools/pgindent/typedefs.list‎

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -3769,6 +3769,7 @@ string
37693769
substitute_actual_parameters_context
37703770
substitute_actual_srf_parameters_context
37713771
substitute_phv_relids_context
3772+
subxids_array_status
37723773
symbol
37733774
tablespaceinfo
37743775
teSection

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