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Commit9dbf8ab

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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 parent34221af commit9dbf8ab

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7 files changed

+86
-18
lines changed

7 files changed

+86
-18
lines changed

‎src/backend/access/transam/twophase.c

Lines changed: 3 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -1986,9 +1986,8 @@ PrescanPreparedTransactions(TransactionId **xids_p, int *nxids_p)
19861986
* This is never called at the end of recovery - we use
19871987
* RecoverPreparedTransactions() at that point.
19881988
*
1989-
* The lack of calls to SubTransSetParent() calls here is by design;
1990-
* those calls are made by RecoverPreparedTransactions() at the end of recovery
1991-
* for those xacts that need this.
1989+
* This updates pg_subtrans, so that any subtransactions will be correctly
1990+
* seen as in-progress in snapshots taken during recovery.
19921991
*/
19931992
void
19941993
StandbyRecoverPreparedTransactions(void)
@@ -2008,7 +2007,7 @@ StandbyRecoverPreparedTransactions(void)
20082007

20092008
buf=ProcessTwoPhaseBuffer(xid,
20102009
gxact->prepare_start_lsn,
2011-
gxact->ondisk,false, false);
2010+
gxact->ondisk,true, false);
20122011
if (buf!=NULL)
20132012
pfree(buf);
20142013
}

‎src/backend/access/transam/xlog.c

Lines changed: 8 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -7283,6 +7283,9 @@ StartupXLOG(void)
72837283
RunningTransactionsDatarunning;
72847284
TransactionIdlatestCompletedXid;
72857285

7286+
/* Update pg_subtrans entries for any prepared transactions */
7287+
StandbyRecoverPreparedTransactions();
7288+
72867289
/*
72877290
* Construct a RunningTransactions snapshot representing a
72887291
* shut down server, with only prepared transactions still
@@ -7291,7 +7294,7 @@ StartupXLOG(void)
72917294
*/
72927295
running.xcnt=nxids;
72937296
running.subxcnt=0;
7294-
running.subxid_overflow=false;
7297+
running.subxid_status=SUBXIDS_IN_SUBTRANS;
72957298
running.nextXid=XidFromFullTransactionId(checkPoint.nextXid);
72967299
running.oldestRunningXid=oldestActiveXID;
72977300
latestCompletedXid=XidFromFullTransactionId(checkPoint.nextXid);
@@ -7301,8 +7304,6 @@ StartupXLOG(void)
73017304
running.xids=xids;
73027305

73037306
ProcArrayApplyRecoveryInfo(&running);
7304-
7305-
StandbyRecoverPreparedTransactions();
73067307
}
73077308
}
73087309

@@ -10424,6 +10425,9 @@ xlog_redo(XLogReaderState *record)
1042410425

1042510426
oldestActiveXID=PrescanPreparedTransactions(&xids,&nxids);
1042610427

10428+
/* Update pg_subtrans entries for any prepared transactions */
10429+
StandbyRecoverPreparedTransactions();
10430+
1042710431
/*
1042810432
* Construct a RunningTransactions snapshot representing a shut
1042910433
* down server, with only prepared transactions still alive. We're
@@ -10432,7 +10436,7 @@ xlog_redo(XLogReaderState *record)
1043210436
*/
1043310437
running.xcnt=nxids;
1043410438
running.subxcnt=0;
10435-
running.subxid_overflow=false;
10439+
running.subxid_status=SUBXIDS_IN_SUBTRANS;
1043610440
running.nextXid=XidFromFullTransactionId(checkPoint.nextXid);
1043710441
running.oldestRunningXid=oldestActiveXID;
1043810442
latestCompletedXid=XidFromFullTransactionId(checkPoint.nextXid);
@@ -10442,8 +10446,6 @@ xlog_redo(XLogReaderState *record)
1044210446
running.xids=xids;
1044310447

1044410448
ProcArrayApplyRecoveryInfo(&running);
10445-
10446-
StandbyRecoverPreparedTransactions();
1044710449
}
1044810450

1044910451
/* 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
@@ -1108,7 +1108,7 @@ ProcArrayApplyRecoveryInfo(RunningTransactions running)
11081108
* If the snapshot isn't overflowed or if its empty we can reset our
11091109
* pending state and use this snapshot instead.
11101110
*/
1111-
if (!running->subxid_overflow||running->xcnt==0)
1111+
if (running->subxid_status!=SUBXIDS_MISSING||running->xcnt==0)
11121112
{
11131113
/*
11141114
* If we have already collected known assigned xids, we need to
@@ -1260,7 +1260,7 @@ ProcArrayApplyRecoveryInfo(RunningTransactions running)
12601260
* missing, so conservatively assume the last one is latestObservedXid.
12611261
* ----------
12621262
*/
1263-
if (running->subxid_overflow)
1263+
if (running->subxid_status==SUBXIDS_MISSING)
12641264
{
12651265
standbyState=STANDBY_SNAPSHOT_PENDING;
12661266

@@ -1272,6 +1272,18 @@ ProcArrayApplyRecoveryInfo(RunningTransactions running)
12721272
standbyState=STANDBY_SNAPSHOT_READY;
12731273

12741274
standbySnapshotPendingXmin=InvalidTransactionId;
1275+
1276+
/*
1277+
* If the 'xids' array didn't include all subtransactions, we have to
1278+
* mark any snapshots taken as overflowed.
1279+
*/
1280+
if (running->subxid_status==SUBXIDS_IN_SUBTRANS)
1281+
procArray->lastOverflowedXid=latestObservedXid;
1282+
else
1283+
{
1284+
Assert(running->subxid_status==SUBXIDS_IN_ARRAY);
1285+
procArray->lastOverflowedXid=InvalidTransactionId;
1286+
}
12751287
}
12761288

12771289
/*
@@ -2898,7 +2910,7 @@ GetRunningTransactionData(void)
28982910

28992911
CurrentRunningXacts->xcnt=count-subcount;
29002912
CurrentRunningXacts->subxcnt=subcount;
2901-
CurrentRunningXacts->subxid_overflow=suboverflowed;
2913+
CurrentRunningXacts->subxid_status=suboverflowed ?SUBXIDS_IN_SUBTRANS :SUBXIDS_IN_ARRAY;
29022914
CurrentRunningXacts->nextXid=XidFromFullTransactionId(ShmemVariableCache->nextXid);
29032915
CurrentRunningXacts->oldestRunningXid=oldestRunningXid;
29042916
CurrentRunningXacts->latestCompletedXid=latestCompletedXid;

‎src/backend/storage/ipc/standby.c

Lines changed: 3 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -1130,7 +1130,7 @@ standby_redo(XLogReaderState *record)
11301130

11311131
running.xcnt=xlrec->xcnt;
11321132
running.subxcnt=xlrec->subxcnt;
1133-
running.subxid_overflow=xlrec->subxid_overflow;
1133+
running.subxid_status=xlrec->subxid_overflow ?SUBXIDS_MISSING :SUBXIDS_IN_ARRAY;
11341134
running.nextXid=xlrec->nextXid;
11351135
running.latestCompletedXid=xlrec->latestCompletedXid;
11361136
running.oldestRunningXid=xlrec->oldestRunningXid;
@@ -1286,7 +1286,7 @@ LogCurrentRunningXacts(RunningTransactions CurrRunningXacts)
12861286

12871287
xlrec.xcnt=CurrRunningXacts->xcnt;
12881288
xlrec.subxcnt=CurrRunningXacts->subxcnt;
1289-
xlrec.subxid_overflow=CurrRunningXacts->subxid_overflow;
1289+
xlrec.subxid_overflow=(CurrRunningXacts->subxid_status!=SUBXIDS_IN_ARRAY);
12901290
xlrec.nextXid=CurrRunningXacts->nextXid;
12911291
xlrec.oldestRunningXid=CurrRunningXacts->oldestRunningXid;
12921292
xlrec.latestCompletedXid=CurrRunningXacts->latestCompletedXid;
@@ -1303,7 +1303,7 @@ LogCurrentRunningXacts(RunningTransactions CurrRunningXacts)
13031303

13041304
recptr=XLogInsert(RM_STANDBY_ID,XLOG_RUNNING_XACTS);
13051305

1306-
if (CurrRunningXacts->subxid_overflow)
1306+
if (xlrec.subxid_overflow)
13071307
elog(trace_recovery(DEBUG2),
13081308
"snapshot of %u running transactions overflowed (lsn %X/%X oldest xid %u latest complete %u next xid %u)",
13091309
CurrRunningXacts->xcnt,

‎src/include/storage/standby.h

Lines changed: 9 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -74,11 +74,19 @@ extern void StandbyReleaseOldLocks(TransactionId oldxid);
7474
* almost immediately see the data we need to begin executing queries.
7575
*/
7676

77+
typedefenum
78+
{
79+
SUBXIDS_IN_ARRAY,/* xids array includes all running subxids */
80+
SUBXIDS_MISSING,/* snapshot overflowed, subxids are missing */
81+
SUBXIDS_IN_SUBTRANS,/* subxids are not included in 'xids', but
82+
* pg_subtrans is fully up-to-date */
83+
}subxids_array_status;
84+
7785
typedefstructRunningTransactionsData
7886
{
7987
intxcnt;/* # of xact ids in xids[] */
8088
intsubxcnt;/* # of subxact ids in xids[] */
81-
boolsubxid_overflow;/* snapshot overflowed, subxids missing */
89+
subxids_array_statussubxid_status;
8290
TransactionIdnextXid;/* xid from ShmemVariableCache->nextXid */
8391
TransactionIdoldestRunningXid;/* *not* oldestXmin */
8492
TransactionIdlatestCompletedXid;/* so we can set xmax */

‎src/test/recovery/t/009_twophase.pl

Lines changed: 47 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -7,7 +7,7 @@
77

88
use PostgresNode;
99
use TestLib;
10-
use Test::Moretests=>24;
10+
use Test::Moretests=>27;
1111

1212
my$psql_out ='';
1313
my$psql_rc ='';
@@ -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
@@ -3558,6 +3558,7 @@ string
35583558
substitute_actual_parameters_context
35593559
substitute_actual_srf_parameters_context
35603560
substitute_phv_relids_context
3561+
subxids_array_status
35613562
svtype
35623563
symbol
35633564
tablespaceinfo

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