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Commit07f32fc

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Fix race condition in snapshot caching when 2PC is used.
When preparing a transaction xactCompletionCount needs to beincremented, even though the transaction has not committedyet. Otherwise the snapshot used within the transaction otherwise canget reused outside of the prepared transaction. As GetSnapshotData()does not include the current xid when building a snapshot, reuse wouldnot be correct.Somewhat surprisingly the regression tests only rarely show incorrectresults without the fix. The reason for that is that often thesnapshot's xmax will be >= the backend xid, yielding a snapshot thatis correct, despite the bug.I'm working on a reliable test for the bug, but it seems worth seeingwhether this fixes all the BF failures while I do.Author: Andres Freund <andres@anarazel.de>Discussion:https://postgr.es/m/E1k7tGP-0005V0-5k@gemulon.postgresql.org
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‎src/backend/storage/ipc/procarray.c

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@@ -860,6 +860,15 @@ ProcArrayClearTransaction(PGPROC *proc)
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Assert(!(proc->vacuumFlags&PROC_VACUUM_STATE_MASK));
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Assert(!proc->delayChkpt);
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/*
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* Need to increment completion count even though transaction hasn't
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* really committed yet. The reason for that is that GetSnapshotData()
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* omits the xid of the current transaction, thus without the increment we
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* otherwise could end up reusing the snapshot later. Which would be bad,
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* because it might not count the prepared transaction as running.
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*/
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ShmemVariableCache->xactCompletionCount++;
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/* Clear the subtransaction-XID cache too */
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Assert(ProcGlobal->subxidStates[pgxactoff].count==proc->subxidStatus.count&&
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ProcGlobal->subxidStates[pgxactoff].overflowed==proc->subxidStatus.overflowed);

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