Re: postgres_fdw: transaction mode inheritance corner cases

On Sat, Oct 17, 2026 at 12:57 AM Matheus Alcantara
<matheusssilv97(at)gmail(dot)com> wrote:
> On Fri Oct 16, 2026 at 9:10 AM -03, Etsuro Fujita wrote:
> >> 2: stale ForeignScanHash entries after a failed portal

> > I couldn't reproduce this in my environment. Could you provide the
> > reproducer in more detail including the foreign table definition, the
> > postgres_fdw options, ...?

> create extension postgres_fdw;
> create server loopback foreign data wrapper postgres_fdw options (dbname 'postgres', port '5432');
> create user mapping for current_user server loopback;
> create table t (a int);
> insert into t values (1), (2), (3), (4);
> create foreign table ft (a int) server loopback options (table_name 't');
>
> begin;
> declare c cursor for select 1 / (a - 3) from ft order by a;
> fetch 1 from c;
> savepoint s2;
> fetch 5 from c;
> rollback to s2;
> commit;
>
> After the COMMIT command it produces:
>
> WARNING: 01000: ForeignScan node 0x106019368 still existing
> LOCATION: AtEOXact_ForeignScan, postgres_fdw.c:4381
>
> The details that matter are that the first "fetch 1" happens before the
> savepoint, so IIUC the scan and its cursor belong to the outer level,
> and that the error happens while fetching inside the savepoint. The
> expression 1 / (a - 3) is not pushed down, so the division by zero is
> raised locally while the scan returns rows, and the portal is left in
> the failed state. If I add this after "rollback to s2" and before
> "commit":
>
> close c;
> savepoint s3;
> select 1;
> release s3;
>
> the backend crashes in the subtransaction callback.

Thanks for the detailed information! I could reproduce this. I was
testing using the wrong patch, sorry.

> > Thanks for working on the fix! It would be great if you share it.

> I'm attaching the v4 patch. It is your v3-0001 plus the fixes from my
> previous email that you agree. Since you said you'd remove the
> restrictions of 3 and 4, and add the comments of 6, I went ahead and did
> them in this version.

Thanks for the patch! Will review.

I think the patch is master-only, because 1) there is no field report
about the series of open-cursor issues while they exist since the
introduction of postgres_fdw, and 2) the patch is large/invasive, so I
will return to this somewhere in the v20 development cycle after
working on some other stuff.

Best regards,
Etsuro Fujita