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Commit02f377d

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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
1 parentef007e6 commit02f377d

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‎src/backend/storage/lmgr/predicate.c

Lines changed: 23 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -3457,10 +3457,29 @@ ClearOldPredicateLocks(void)
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elseif (finishedSxact->commitSeqNo>PredXact->HavePartialClearedThrough
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&&finishedSxact->commitSeqNo <=PredXact->CanPartialClearThrough)
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{
3460+
/*
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* Any active transactions that took their snapshot before this
3462+
* transaction committed are read-only, so we can clear part of
3463+
* its state.
3464+
*/
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LWLockRelease(SerializableXactHashLock);
3461-
ReleaseOneSerializableXact(finishedSxact,
3462-
!SxactIsReadOnly(finishedSxact),
3463-
false);
3466+
3467+
if (SxactIsReadOnly(finishedSxact))
3468+
{
3469+
/* A read-only transaction can be removed entirely */
3470+
SHMQueueDelete(&(finishedSxact->finishedLink));
3471+
ReleaseOneSerializableXact(finishedSxact, false, false);
3472+
}
3473+
else
3474+
{
3475+
/*
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* A read-write transaction can only be partially
3477+
* cleared. We need to keep the SERIALIZABLEXACT but
3478+
* can release the SIREAD locks and conflicts in.
3479+
*/
3480+
ReleaseOneSerializableXact(finishedSxact, true, false);
3481+
}
3482+
34643483
PredXact->HavePartialClearedThrough=finishedSxact->commitSeqNo;
34653484
LWLockAcquire(SerializableXactHashLock,LW_SHARED);
34663485
}
@@ -3566,6 +3585,7 @@ ReleaseOneSerializableXact(SERIALIZABLEXACT *sxact, bool partial,
35663585

35673586
Assert(sxact!=NULL);
35683587
Assert(SxactIsRolledBack(sxact)||SxactIsCommitted(sxact));
3588+
Assert(partial|| !SxactIsOnFinishedList(sxact));
35693589
Assert(LWLockHeldByMe(SerializableFinishedListLock));
35703590

35713591
/*

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