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Fix concurrent update issue with MERGE.

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When executing a MERGE UPDATE action, if there is more than one
concurrent update of the target row, the lock-and-retry code would
sometimes incorrectly identify the latest version of the target tuple,
leading to incorrect results.

This was caused by using the ctid field from the TM_FailureData
returned by table_tuple_lock() in a case where the result was TM_Ok,
which is unsafe because the TM_FailureData struct is not guaranteed to
be fully populated in that case. Instead, it should use the tupleid
passed to (and updated by) table_tuple_lock().

To reduce the chances of similar errors in the future, improve the
commentary for table_tuple_lock() and TM_FailureData to make it
clearer that table_tuple_lock() updates the tid passed to it, and most
fields of TM_FailureData should not be relied on in non-failure cases.
An exception to this is the "traversed" field, which is set in both
success and failure cases.

Reported-by: Dmitry <dsy.075@yandex.ru>
Author: Yugo Nagata <nagata@sraoss.co.jp>
Reviewed-by: Dean Rasheed <dean.a.rasheed@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/1570d30e-2b95-4239-b9c3-f7bf2f2f8556@yandex.ru
Backpatch-through: 15
This commit is contained in:
Dean Rasheed
2025-09-05 08:18:18 +01:00
parent 567d27e8e2
commit 6ede13d1b5
4 changed files with 179 additions and 8 deletions
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9
src/backend/executor/nodeModifyTable.c
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@@ -3402,7 +3402,7 @@ lmerge_matched:
* the tuple moved, and setting our current
* resultRelInfo to that.
*/
if (ItemPointerIndicatesMovedPartitions(&context->tmfd.ctid))
if (ItemPointerIndicatesMovedPartitions(tupleid))
ereport(ERROR,
(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
errmsg("tuple to be merged was already moved to another partition due to concurrent update")));
@@ -3450,12 +3450,13 @@ lmerge_matched:
if (ItemPointerIsValid(&lockedtid))
UnlockTuple(resultRelInfo->ri_RelationDesc, &lockedtid,
InplaceUpdateTupleLock);
LockTuple(resultRelInfo->ri_RelationDesc, &context->tmfd.ctid,
LockTuple(resultRelInfo->ri_RelationDesc, tupleid,
InplaceUpdateTupleLock);
lockedtid = context->tmfd.ctid;
lockedtid = *tupleid;
}
if (!table_tuple_fetch_row_version(resultRelationDesc,
&context->tmfd.ctid,
tupleid,
SnapshotAny,
resultRelInfo->ri_oldTupleSlot))
elog(ERROR, "failed to fetch the target tuple");