Re: Re-read subscription state after lock in AlterSubscription

On Thu, Jul 2, 2026 at 5:38 PM Bertrand Drouvot
<bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
>
> Hi hackers,
>
> while playing with the new ALTER SUBSCRIPTION parameter added in a5918fddf10,
> I realized that the subscription is not re-read once we acquire the lock in
> AlterSubscription().
>
> This pre-existing issue is now more visible after a5918fddf10:
>
> 1/ two concurrent ALTER SUBSCRIPTION SET (conflict_log_destination = 'table')
> could result in the second session attempting to create an already-existing
> conflict log table, producing a confusing "relation already exists" error:
>
> ERROR: relation "pg_conflict_log_24614" already exists
>
> It's confusing because ALTER SUBSCRIPTION SET (conflict_log_destination = 'table')
> would not report an error if the conflict table already exists (and no concurrent
> ALTER is running).
>
> 2/ a concurrent DROP followed by the ALTER would emit a NOTICE about creating the
> conflict log table before failing with "referenced subscription was concurrently
> dropped". That sounds like a weird messaging:
>
> NOTICE: created conflict log table "pg_conflict.pg_conflict_log_24620" for subscription "mysub"
> ERROR: referenced subscription was concurrently dropped
>
> The attached fixes it by:
>
> - Re-reading the subscription tuple after LockSharedObject() and refreshing the
> Subscription struct.
> - Moving the local variable assignments to after the re-read.
> - Re-checking the password_required privilege restriction after the re-read.
>
> Remarks:
>
> 1/ not re-checking password_required after the re-read would still produce a
> "tuple concurrently updated" error, but re-checking it allows us to display a
> better error message.
>
> 2/ the ownership check is intentionally not re-done after the lock because
> AlterSubscriptionOwner() does not take AccessExclusiveLock on the subscription
> object: it only takes RowExclusiveLock on the pg_subscription catalog table.
> This means ownership can change regardless of our lock, making a re-check after
> lock acquisition pointless. The existing "tuple concurrently updated" error from
> CatalogTupleUpdate() already provides a protection if ownership changes
> concurrently.
>
> 3/ the "privileges" checks are still also done before the lock acquisition because
> we don't want to lock an object we don't have privileges on.
>

Thanks Bertrand, yeah this seems like a valid issue, and I agree we
need to reread the subscription after acquiring the object lock.

--
Regards,
Dilip Kumar
Google