Re: BUG #19484: Segmentation fault triggered by FDW

Hi,

On Wed, May 20, 2026 at 5:37 AM Ayush Tiwari
 wrote:
> On Wed, 20 May 2026 at 03:59, PG Bug reporting form  wrote:
>> I found the following test case triggers a segmentation fault:

[snip]

> Thanks for the very precise repro, that made this easy to track down.
>
> I reproduced the crash on master.  The plan EXPLAIN under
> force_generic_plan shows runtime pruning is in effect:
>
>   Update on pt
>     Foreign Update on pt_p2 pt_2
>     ->  Append
>           Subplans Removed: 1
>           ->  Foreign Update on pt_p2 pt_2
>
> The SEGV happens inside postgresBeginForeignModify() because
> ExecInitModifyTable() builds re-indexed "kept" copies of several
> parallel per-result-relation lists after dropping pruned relations -
> withCheckOptionLists, returningLists, updateColnosLists,
> mergeActionLists and mergeJoinConditions, however two members were
> missed:
>
>   - node->fdwPrivLists, read with list_nth(node->fdwPrivLists, i) when
>     BeginForeignModify() is called, and
>   - node->fdwDirectModifyPlans, checked with bms_is_member(i, ...) when
>     setting ri_usesFdwDirectModify.
>
> Both were still indexed against the original (pre-pruning) positions
> while the surrounding loop's "i" is now the kept position.  When the
> foreign partition's kept-index no longer matched its original index,
> BeginForeignModify() got the wrong fdw_private and crashed.
>
> Attached patch builds re-indexed kept copies for these two arrays in
> the same loop as the other parallel lists, and uses them at the two
> call sites.

Thanks Chi for the report, and Ayush for the analysis and patch!  Will review.

Best regards,
Etsuro Fujita