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Commit65e388d

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Fix race in SERIALIZABLE READ ONLY.
Commitbdaabb9 started skipping doomed transactions when building thelist of possible conflicts for SERIALIZABLE READ ONLY. That makessense, because doomed transactions won't commit, but a couple of subtlethings broke:1. If all uncommitted r/w transactions are doomed, a READ ONLYtransaction would arbitrarily not benefit from the safe snapshotoptimization. It would not be taken immediately, and yet no othertransaction would set SXACT_FLAG_RO_SAFE later.2. In the same circumstances but with DEFERRABLE, GetSafeSnapshot()would correctly exit its wait loop without sleeping and then take theoptimization in non-assert builds, but assert builds would fail a sanitycheck that SXACT_FLAG_RO_SAFE had been set by another transaction.This is similar to the case for PredXact->WritableSxactCount == 0. Weshould opt out immediately if our possibleUnsafeConflicts list is emptyafter filtering.The code to maintain the serializable global xmin is moved down belowthe new opt out site, because otherwise we'd have to reverse its effectsbefore returning.Back-patch to all supported releases. Bug #17368.Reported-by: Alexander Lakhin <exclusion@gmail.com>Discussion:https://postgr.es/m/17116-d6ca217acc180e30%40postgresql.orgDiscussion:https://postgr.es/m/20110707212159.GF76634%40csail.mit.edu
1 parent8bf8265 commit65e388d

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+31
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lines changed

1 file changed

+31
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lines changed

‎src/backend/storage/lmgr/predicate.c‎

Lines changed: 31 additions & 18 deletions
Original file line numberDiff line numberDiff line change
@@ -1782,24 +1782,6 @@ GetSerializableTransactionSnapshotInt(Snapshot snapshot,
17821782
returnsnapshot;
17831783
}
17841784

1785-
/* Maintain serializable global xmin info. */
1786-
if (!TransactionIdIsValid(PredXact->SxactGlobalXmin))
1787-
{
1788-
Assert(PredXact->SxactGlobalXminCount==0);
1789-
PredXact->SxactGlobalXmin=snapshot->xmin;
1790-
PredXact->SxactGlobalXminCount=1;
1791-
SerialSetActiveSerXmin(snapshot->xmin);
1792-
}
1793-
elseif (TransactionIdEquals(snapshot->xmin,PredXact->SxactGlobalXmin))
1794-
{
1795-
Assert(PredXact->SxactGlobalXminCount>0);
1796-
PredXact->SxactGlobalXminCount++;
1797-
}
1798-
else
1799-
{
1800-
Assert(TransactionIdFollows(snapshot->xmin,PredXact->SxactGlobalXmin));
1801-
}
1802-
18031785
/* Initialize the structure. */
18041786
sxact->vxid=vxid;
18051787
sxact->SeqNo.lastCommitBeforeSnapshot=PredXact->LastSxactCommitSeqNo;
@@ -1839,6 +1821,19 @@ GetSerializableTransactionSnapshotInt(Snapshot snapshot,
18391821
SetPossibleUnsafeConflict(sxact,othersxact);
18401822
}
18411823
}
1824+
1825+
/*
1826+
* If we didn't find any possibly unsafe conflicts because every
1827+
* uncommitted writable transaction turned out to be doomed, then we
1828+
* can "opt out" immediately. See comments above the earlier check for
1829+
* PredXact->WritableSxactCount == 0.
1830+
*/
1831+
if (dlist_is_empty(&sxact->possibleUnsafeConflicts))
1832+
{
1833+
ReleasePredXact(sxact);
1834+
LWLockRelease(SerializableXactHashLock);
1835+
returnsnapshot;
1836+
}
18421837
}
18431838
else
18441839
{
@@ -1847,6 +1842,24 @@ GetSerializableTransactionSnapshotInt(Snapshot snapshot,
18471842
(MaxBackends+max_prepared_xacts));
18481843
}
18491844

1845+
/* Maintain serializable global xmin info. */
1846+
if (!TransactionIdIsValid(PredXact->SxactGlobalXmin))
1847+
{
1848+
Assert(PredXact->SxactGlobalXminCount==0);
1849+
PredXact->SxactGlobalXmin=snapshot->xmin;
1850+
PredXact->SxactGlobalXminCount=1;
1851+
SerialSetActiveSerXmin(snapshot->xmin);
1852+
}
1853+
elseif (TransactionIdEquals(snapshot->xmin,PredXact->SxactGlobalXmin))
1854+
{
1855+
Assert(PredXact->SxactGlobalXminCount>0);
1856+
PredXact->SxactGlobalXminCount++;
1857+
}
1858+
else
1859+
{
1860+
Assert(TransactionIdFollows(snapshot->xmin,PredXact->SxactGlobalXmin));
1861+
}
1862+
18501863
MySerializableXact=sxact;
18511864
MyXactDidWrite= false;/* haven't written anything yet */
18521865

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