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Commit326b922

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Fix corner case in cleanup of transactions using SSI.
When the only remaining active transactions are READ ONLY, we do a "partialcleanup" of committed transactions because certain types of conflictsaren't possible anymore. For committed r/w transactions, we release theSIREAD locks but keep the SERIALIZABLEXACT. However, for committed r/otransactions, we can go further and release the SERIALIZABLEXACT too. Theproblem was with the latter case: we were returning the SERIALIZABLEXACT tothe free list without removing it from the finished list.The only real change in the patch is the SHMQueueDelete line, but I alsoreworked some of the surrounding code to make it obvious that r/o and r/wtransactions are handled differently -- the existing code felt a bit tooclever.Dan Ports
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‎src/backend/storage/lmgr/predicate.c

Lines changed: 23 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -3528,10 +3528,29 @@ ClearOldPredicateLocks(void)
35283528
elseif (finishedSxact->commitSeqNo>PredXact->HavePartialClearedThrough
35293529
&&finishedSxact->commitSeqNo <=PredXact->CanPartialClearThrough)
35303530
{
3531+
/*
3532+
* Any active transactions that took their snapshot before this
3533+
* transaction committed are read-only, so we can clear part of
3534+
* its state.
3535+
*/
35313536
LWLockRelease(SerializableXactHashLock);
3532-
ReleaseOneSerializableXact(finishedSxact,
3533-
!SxactIsReadOnly(finishedSxact),
3534-
false);
3537+
3538+
if (SxactIsReadOnly(finishedSxact))
3539+
{
3540+
/* A read-only transaction can be removed entirely */
3541+
SHMQueueDelete(&(finishedSxact->finishedLink));
3542+
ReleaseOneSerializableXact(finishedSxact, false, false);
3543+
}
3544+
else
3545+
{
3546+
/*
3547+
* A read-write transaction can only be partially
3548+
* cleared. We need to keep the SERIALIZABLEXACT but
3549+
* can release the SIREAD locks and conflicts in.
3550+
*/
3551+
ReleaseOneSerializableXact(finishedSxact, true, false);
3552+
}
3553+
35353554
PredXact->HavePartialClearedThrough=finishedSxact->commitSeqNo;
35363555
LWLockAcquire(SerializableXactHashLock,LW_SHARED);
35373556
}
@@ -3637,6 +3656,7 @@ ReleaseOneSerializableXact(SERIALIZABLEXACT *sxact, bool partial,
36373656

36383657
Assert(sxact!=NULL);
36393658
Assert(SxactIsRolledBack(sxact)||SxactIsCommitted(sxact));
3659+
Assert(partial|| !SxactIsOnFinishedList(sxact));
36403660
Assert(LWLockHeldByMe(SerializableFinishedListLock));
36413661

36423662
/*

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