Fix possible "tuple concurrently updated" error in ALTER TABLE.

When adding an inheritance parent to a table, an AccessShareLock on the
parent isn't strong enough to prevent trouble, so take
ShareUpdateExclusiveLock instead.  Since this is a behavior change,
albeit a fairly unobtrusive one, and since we have only one report
from the field, no back-patch.

Report by Jon Nelson, analysis by Alvaro Herrera, fix by me.

diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index f1264bfb66d61595eea92274bd107674f8898397..ee34cfa97e15030678b15ae2c034a9430c960939 100644 (file)
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -1345,7 +1345,14 @@ MergeAttributes(List *schema, List *supers, char relpersistence,
                AttrNumber *newattno;
                AttrNumber      parent_attno;
 
-               relation = heap_openrv(parent, AccessShareLock);
+               /*
+                * A self-exclusive lock is needed here.  If two backends attempt to
+                * add children to the same parent simultaneously, and that parent has
+                * no pre-existing children, then both will attempt to update the
+                * parent's relhassubclass field, leading to a "tuple concurrently
+                * updated" error.
+                */
+               relation = heap_openrv(parent, ShareUpdateExclusiveLock);
 
                if (relation->rd_rel->relkind != RELKIND_RELATION)
                        ereport(ERROR,
@@ -7942,10 +7949,10 @@ ATExecAddInherit(Relation child_rel, RangeVar *parent, LOCKMODE lockmode)
        List       *children;
 
        /*
-        * AccessShareLock on the parent is what's obtained during normal CREATE
-        * TABLE ... INHERITS ..., so should be enough here.
+        * A self-exclusive lock is needed here.  See the similar case in
+        * MergeAttributes() for a full explanation.
         */
-       parent_rel = heap_openrv(parent, AccessShareLock);
+       parent_rel = heap_openrv(parent, ShareUpdateExclusiveLock);
 
        /*
         * Must be owner of both parent and child -- child was checked by