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Commita493f93

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Fix nextXid tracking bug on standbys (9.5-11 only).
RecordKnownAssignedTransactionIds() should never movenextXid backwards. Before this commit, that could happenif some other code path had advanced it without advancinglatestObservedXid.One consequence is that a well timed XLOG_CHECKPOINT_ONLINEcould cause hot standby feedback messages to get confusedand report an xmin from a future epoch, potentially allowingvacuum to run too soon on the primary.Repair, by making sure RecordKnownAssignedTransactionIds()can only move nextXid forwards.In release 12 and master, this was already done by commit2fc7af5, which consolidated similar code and straightenedout this bug. Back-patch to supported releases before that.Author: Eka Palamadai <ekanatha@amazon.com>Discussion:https://postgr.es/m/98BB4805-D0A2-48E1-96F4-15014313EADC@amazon.com
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‎src/backend/storage/ipc/procarray.c

Lines changed: 2 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -3256,7 +3256,8 @@ RecordKnownAssignedTransactionIds(TransactionId xid)
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next_expected_xid=latestObservedXid;
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TransactionIdAdvance(next_expected_xid);
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LWLockAcquire(XidGenLock,LW_EXCLUSIVE);
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ShmemVariableCache->nextXid=next_expected_xid;
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if (TransactionIdFollows(next_expected_xid,ShmemVariableCache->nextXid))
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ShmemVariableCache->nextXid=next_expected_xid;
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LWLockRelease(XidGenLock);
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}
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}

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