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Commit0dcff75

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Avoid deadlock between concurrent CREATE INDEX CONCURRENTLY commands.
There was a high probability of two or more concurrent C.I.C. commandsdeadlocking just before completion, because each would wait for the othersto release their reference snapshots. Fix by releasing the snapshotbefore waiting for other snapshots to go away.Per report from Paul Hinze. Back-patch to all active branches.
1 parentfe1d8c1 commit0dcff75

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‎src/backend/commands/indexcmds.c

Lines changed: 15 additions & 8 deletions
Original file line numberDiff line numberDiff line change
@@ -141,6 +141,7 @@ DefineIndex(RangeVar *heapRelation,
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int16*coloptions;
142142
IndexInfo*indexInfo;
143143
intnumberOfAttributes;
144+
TransactionIdlimitXmin;
144145
VirtualTransactionId*old_lockholders;
145146
VirtualTransactionId*old_snapshots;
146147
intn_old_snapshots;
@@ -640,6 +641,18 @@ DefineIndex(RangeVar *heapRelation,
640641
*/
641642
validate_index(relationId,indexRelationId,snapshot);
642643

644+
/*
645+
* Drop the reference snapshot. We must do this before waiting out other
646+
* snapshot holders, else we will deadlock against other processes also
647+
* doing CREATE INDEX CONCURRENTLY, which would see our snapshot as one
648+
* they must wait for. But first, save the snapshot's xmin to use as
649+
* limitXmin for GetCurrentVirtualXIDs().
650+
*/
651+
limitXmin=snapshot->xmin;
652+
653+
PopActiveSnapshot();
654+
UnregisterSnapshot(snapshot);
655+
643656
/*
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* The index is now valid in the sense that it contains all currently
645658
* interesting tuples.But since it might not contain tuples deleted just
@@ -672,7 +685,7 @@ DefineIndex(RangeVar *heapRelation,
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* GetCurrentVirtualXIDs. If, during any iteration, a particular vxid
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* doesn't show up in the output, we know we can forget about it.
674687
*/
675-
old_snapshots=GetCurrentVirtualXIDs(snapshot->xmin, true, false,
688+
old_snapshots=GetCurrentVirtualXIDs(limitXmin, true, false,
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PROC_IS_AUTOVACUUM |PROC_IN_VACUUM,
677690
&n_old_snapshots);
678691

@@ -689,7 +702,7 @@ DefineIndex(RangeVar *heapRelation,
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intj;
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intk;
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692-
newer_snapshots=GetCurrentVirtualXIDs(snapshot->xmin,
705+
newer_snapshots=GetCurrentVirtualXIDs(limitXmin,
693706
true, false,
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PROC_IS_AUTOVACUUM |PROC_IN_VACUUM,
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&n_newer_snapshots);
@@ -728,12 +741,6 @@ DefineIndex(RangeVar *heapRelation,
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*/
729742
CacheInvalidateRelcacheByRelid(heaprelid.relId);
730743

731-
/* we can now do away with our active snapshot */
732-
PopActiveSnapshot();
733-
734-
/* And we can remove the validating snapshot too */
735-
UnregisterSnapshot(snapshot);
736-
737744
/*
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* Last thing to do is release the session-level lock on the parent table.
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*/

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