Re: crash with synchronized_standby_slots

On Tue, Dec 3, 2024 at 10:34 PM Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> wrote:
>
> On 2024-Nov-29, Amit Kapila wrote:
>
> BTW it occurs to me that there might well be some sort of thundering
> herd problem if every process needs to run the check_hook when a SIGHUP
> is broadcast, and they'll all be waiting on that particular lwlock and
> run the same validation locally again and again. I bet if you have a
> few thousand backends (hi Jakub! [1]) it's problematic.
>

The lock is taken in shared mode, so, ideally, it shouldn't create a
problem but if it ever creates a problem, we can even skip that check
during validation. The validation will anyway happen later during
replication in StandbySlotsHaveCaughtup(). This check is mostly to
detect the error in GUC early.

> Maybe we need a
> different way to validate the GUC, but I don't know what that might be;
> but doing the validation once and storing the result in shmem might be
> better.
>

What if that particular GUC changes again? We may need some additional
invalidation mechanism.

--
With Regards,
Amit Kapila.