Re: BUG #19484: Segmentation fault triggered by FDW

On Sat May 30, 2026 at 3:18 AM -03, Rafia Sabih wrote:
> A good catch. However there is one issue that remains here,
> in show_modifytable_info still is using the old index here fdw_private =
> (List *) list_nth(node->fdwPrivLists, j) i.e. the one before pruning.
> In fact I found a scenario where it is causing crash, try this
>
> create table fdw_part_update2 (a int not null, b int) partition by list (a);
> create table fdw_part_update2_p1 partition of fdw_part_update2 for values
> in (1);
> create table fdw_part_update2_remote (a int not null, b int);
> create foreign table fdw_part_update2_p2 partition of fdw_part_update2
>     for values in (2)
>     server loopback options (table_name 'fdw_part_update2_remote');
> insert into fdw_part_update2_p1 values (1, 10);
> insert into fdw_part_update2_remote values (2, 20);
> set plan_cache_mode = force_generic_plan;
>  prepare fdw_part_upd2(int) as
>       update fdw_part_update2 set b = b + random()::int * 0 + 1 where a = $1
>       returning tableoid::regclass, a, b;
> execute fdw_part_upd2(2);
> explain (analyze, verbose, costs off, timing off, summary off)
>     execute fdw_part_upd2(2);
>
> Please find the attached file for the patch to fix this. This patch applies
> over the earlier patch (given by Ayush) in this thread.
>

Thanks for catching this, Rafia. The fix is correct —
show_modifytable_info() was indeed still reading from node->fdwPrivLists
using the post-pruning index j, which causes an out-of-bounds access
when partitions are pruned.

I think both patches should be squashed into a single one since they fix
the same underlying issue. I've done this locally and also ran pg_indent
over the result. Attached is the combined patch.

One minor naming observation: the new fdwPrivLists field in
ModifyTableState doesn't follow the mt_ prefix convention used by the
other re-indexed lists (mt_updateColnosLists, mt_mergeActionLists,
mt_mergeJoinConditions). Should we rename it to mt_fdwPrivLists for
consistency?

--
Matheus Alcantara
EDB: https://www.enterprisedb.com