Re: Replication slot stats misgivings

On Mon, Mar 22, 2021 at 1:25 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Sat, Mar 20, 2021 at 3:52 AM Andres Freund <andres(at)anarazel(dot)de> wrote:
> >
> > - If max_replication_slots was lowered between a restart,
> > pgstat_read_statfile() will happily write beyond the end of
> > replSlotStats.
>
> I think we cannot restart the server after lowering
> max_replication_slots to a value less than the number of replication
> slots actually created on the server. No?

This problem happens in the case where max_replication_slots is
lowered and there still are stats for a slot.

I understood the risk of running out of replSlotStats. If we use the
index in replSlotStats instead, IIUC we need to somehow synchronize
the indexes in between replSlotStats and
ReplicationSlotCtl->replication_slots. The order of replSlotStats is
preserved across restarting whereas the order of
ReplicationSlotCtl->replication_slots isn’t (readdir() that is used by
StartupReplicationSlots() doesn’t guarantee the order of the returned
entries in the directory). Maybe we can compare the slot name in the
received message to the name in the element of replSlotStats. If they
don’t match, we swap entries in replSlotStats to synchronize the index
of the replication slot in ReplicationSlotCtl->replication_slots and
replSlotStats. If we cannot find the entry in replSlotStats that has
the name in the received message, it probably means either it's a new
slot or the previous create message is dropped, we can create the new
stats for the slot. Is that what you mean, Andres?

Regards,

--
Masahiko Sawada
EDB: https://www.enterprisedb.com/