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Commit240e0db

Browse files
author
Amit Kapila
committed
Add additional checks while creating the initial decoding snapshot.
As per one of the CI reports, there is an assertion failure whichindicates that we were trying to use an unenforced xmin horizon fordecoding snapshots. Though, we couldn't figure out the reason forassertion failure these checks would help us in finding the reason if theproblem happens again in the future.Author: Amit Kapila based on suggestions by Andres FreundReviewd by: Andres FreundDiscussion:https://postgr.es/m/CAA4eK1L8wYcyTPxNzPGkhuO52WBGoOZbT0A73Le=ZUWYAYmdfw@mail.gmail.com
1 parenta4adc31 commit240e0db

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2 files changed

+23
-11
lines changed

2 files changed

+23
-11
lines changed

‎src/backend/replication/logical/snapbuild.c

Lines changed: 18 additions & 11 deletions
Original file line numberDiff line numberDiff line change
@@ -566,11 +566,18 @@ SnapBuildInitialSnapshot(SnapBuild *builder)
566566
{
567567
Snapshotsnap;
568568
TransactionIdxid;
569+
TransactionIdsafeXid;
569570
TransactionId*newxip;
570571
intnewxcnt=0;
571572

572-
Assert(!FirstSnapshotSet);
573573
Assert(XactIsoLevel==XACT_REPEATABLE_READ);
574+
Assert(builder->building_full_snapshot);
575+
576+
/* don't allow older snapshots */
577+
InvalidateCatalogSnapshot();/* about to overwrite MyProc->xmin */
578+
if (HaveRegisteredOrActiveSnapshot())
579+
elog(ERROR,"cannot build an initial slot snapshot when snapshots exist");
580+
Assert(!HistoricSnapshotActive());
574581

575582
if (builder->state!=SNAPBUILD_CONSISTENT)
576583
elog(ERROR,"cannot build an initial slot snapshot before reaching a consistent state");
@@ -588,18 +595,18 @@ SnapBuildInitialSnapshot(SnapBuild *builder)
588595
* We know that snap->xmin is alive, enforced by the logical xmin
589596
* mechanism. Due to that we can do this without locks, we're only
590597
* changing our own value.
598+
*
599+
* Building an initial snapshot is expensive and an unenforced xmin
600+
* horizon would have bad consequences, therefore always double-check that
601+
* the horizon is enforced.
591602
*/
592-
#ifdefUSE_ASSERT_CHECKING
593-
{
594-
TransactionIdsafeXid;
603+
LWLockAcquire(ProcArrayLock,LW_SHARED);
604+
safeXid=GetOldestSafeDecodingTransactionId(false);
605+
LWLockRelease(ProcArrayLock);
595606

596-
LWLockAcquire(ProcArrayLock,LW_SHARED);
597-
safeXid=GetOldestSafeDecodingTransactionId(false);
598-
LWLockRelease(ProcArrayLock);
599-
600-
Assert(TransactionIdPrecedesOrEquals(safeXid,snap->xmin));
601-
}
602-
#endif
607+
if (TransactionIdFollows(safeXid,snap->xmin))
608+
elog(ERROR,"cannot build an initial slot snapshot as oldest safe xid %u follows snapshot's xmin %u",
609+
safeXid,snap->xmin);
603610

604611
MyProc->xmin=snap->xmin;
605612

‎src/backend/replication/walsender.c

Lines changed: 5 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -1099,6 +1099,11 @@ CreateReplicationSlot(CreateReplicationSlotCmd *cmd)
10991099
/*- translator: %s is a CREATE_REPLICATION_SLOT statement */
11001100
(errmsg("%s must be called in REPEATABLE READ isolation mode transaction",
11011101
"CREATE_REPLICATION_SLOT ... (SNAPSHOT 'use')")));
1102+
if (!XactReadOnly)
1103+
ereport(ERROR,
1104+
/*- translator: %s is a CREATE_REPLICATION_SLOT statement */
1105+
(errmsg("%s must be called in a read only transaction",
1106+
"CREATE_REPLICATION_SLOT ... (SNAPSHOT 'use')")));
11021107

11031108
if (FirstSnapshotSet)
11041109
ereport(ERROR,

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