Re: FIX: BUG #19687: ALTER SEQUENCE missing lock

On Tue, Sep 15, 2026 at 9:00 AM Ayush Tiwari <ayushtiwari(dot)slg01(at)gmail(dot)com>
wrote:

> Hi,
>
> On Tue, 15 Sept 2026 at 12:19, Alexandre Felipe <
> o(dot)alexandre(dot)felipe(at)gmail(dot)com> wrote:
> >
> > Trying to do something different today.
> >
> > Hunting bugs.
> >
> > Addressing an issue in `ALTER SEQUENCE` commands [1], by adding
> > a missing lock when calling RelationSetNewRelfilenumber and being
> > more accurate about the type of lock required in that function's
> doc-comment.
> >
> > The change is sound, even if it is not the complete fix yet.
>
> I posted about the same bug on pgsql-bugs yesterday [2] (quoted below),
> so maybe it is better to keep this in one thread.
>

Transaction conflict, my bad :(

I tried locking at the RelationSetNewRelfilenumber() call first, but it
> deadlocks. ALTER still holds ShareRowExclusiveLock while it waits for the
> upgrade, and that conflicts with the RowExclusiveLock a scanning
> transaction needs for nextval():
>

Callers elsewhere, even in sequence.c, already lock before calling it,
so acquiring a lock in RelationSetNewRelfilenumber will definitely deadlock.

Regards,
Alexandre

>