Re: Missing LWLock protection in pgstat_reset_replslot()

On Thu, Mar 7, 2024 at 11:12 AM Michael Paquier <michael@paquier.xyz> wrote:
>

> Yeah, it is possible that what you retrieve from
> pgstat_fetch_replslot() does not refer exactly to the slot's content
> under concurrent activity, but you cannot protect a single scan of
> pg_stat_replication_slots as of an effect of its design:
> pg_stat_get_replication_slot() has to be called multiple times.  The
> patch at least makes sure that the copy of the slot's stats retrieved
> by pgstat_fetch_entry() is slightly more consistent

Right, I understand that.

, but you cannot do
> better than that except if the data retrieved from
> pg_replication_slots and its stats are fetched in the same context
> function call, holding the replslot LWLock for the whole scan
> duration.

Yes, agreed.

>
> > Do you feel that the lock in pgstat_fetch_replslot() should be moved
> > to its caller when we are done copying the results of that slot? This
> > will improve the protection slightly.
>
> I don't see what extra protection this would offer, as
> pg_stat_get_replication_slot() is called once for each slot.  Feel
> free to correct me if I'm missing something.

It slightly improves the chances.  pgstat_fetch_replslot is only
called from pg_stat_get_replication_slot(), I thought it might be
better to acquire lock before we call pgstat_fetch_replslot and
release once we are done copying the results for that particular slot.
But  I also understand that it will not prevent someone from dropping
that slot at a later stage when the rest of the calls of
pg_stat_get_replication_slot() are still pending. So I am okay with
the current one.

thanks
Shveta