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Commitfbcf4b9

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Fix possible "tuple concurrently updated" error in ALTER TABLE.
When adding an inheritance parent to a table, an AccessShareLock on theparent isn't strong enough to prevent trouble, so takeShareUpdateExclusiveLock instead. Since this is a behavior change,albeit a fairly unobtrusive one, and since we have only one reportfrom the field, no back-patch.Report by Jon Nelson, analysis by Alvaro Herrera, fix by me.
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‎src/backend/commands/tablecmds.c

Lines changed: 11 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -1345,7 +1345,14 @@ MergeAttributes(List *schema, List *supers, char relpersistence,
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AttrNumber*newattno;
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AttrNumberparent_attno;
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1348-
relation=heap_openrv(parent,AccessShareLock);
1348+
/*
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* A self-exclusive lock is needed here. If two backends attempt to
1350+
* add children to the same parent simultaneously, and that parent has
1351+
* no pre-existing children, then both will attempt to update the
1352+
* parent's relhassubclass field, leading to a "tuple concurrently
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* updated" error.
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*/
1355+
relation=heap_openrv(parent,ShareUpdateExclusiveLock);
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if (relation->rd_rel->relkind!=RELKIND_RELATION)
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ereport(ERROR,
@@ -7942,10 +7949,10 @@ ATExecAddInherit(Relation child_rel, RangeVar *parent, LOCKMODE lockmode)
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List*children;
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/*
7945-
*AccessShareLock on the parentiswhat's obtained during normal CREATE
7946-
*TABLE ... INHERITS ..., so should be enough here.
7952+
*A self-exclusive lockisneeded here. See the similar case in
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*MergeAttributes() for a full explanation.
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*/
7948-
parent_rel=heap_openrv(parent,AccessShareLock);
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parent_rel=heap_openrv(parent,ShareUpdateExclusiveLock);
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/*
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* Must be owner of both parent and child -- child was checked by

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