Re: BUG #19484: Segmentation fault triggered by FDW

On Wed, Jun 24, 2026 at 10:53 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Amit Langote <amitlangote09(at)gmail(dot)com> writes:
> > crake is now red on REL_18:
> > https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=crake&dt=2026-06-24%2000%3A02%3A03
>
> > It reports sizeof(ModifyTableState) grew by 8 bytes.
>
> > That’s from adding mt_fdwPrivLists at the end of the struct. Placing it
> > last keeps existing field offsets stable, which is what extensions reading
> > the node rely on, but it does grow the struct, which is what the checker
> > flags. I believe we’ve added trailing struct members in back branches
> > before for exactly this reason. Is a flagged sizeof increase acceptable
> > here under that precedent, or do we now want to avoid any flagged ABI
> > change on a back branch?
>
> I don't believe that the addition of ABI checking was meant to change
> our back-patching policies, just to make sure we don't make
> unintentional ABI changes. As you say, we've added trailing fields
> before. The critical question is whether it's likely that any
> extensions create their own ModifyTableState nodes (and might make
> them too small and/or fail to fill the new field correctly).
>
> I was about to say that that seems unlikely to me, but a check of
> Debian Code Search immediately found pg_rewrite doing it. So yeah,
> looks like we'd better adopt the no-ABI-break solution.

Ah, ok, will post the patch to do so in the REL_18_STABLE branch.

Thanks for the comment.

--
Thanks, Amit Langote