Re: Replication slots and footguns

On 2014-03-12 15:18:04 -0400, Robert Haas wrote:
> On Wed, Mar 12, 2014 at 3:03 PM, Andres Freund  wrote:
> > Hi,
> >
> > On 2014-03-12 12:00:25 -0700, Josh Berkus wrote:
> >> I was just reading Michael's explanation of replication slots
> >> (http://michael.otacoo.com/postgresql-2/postgres-9-4-feature-highlight-replication-slots/)
> >> and realized there was something which had completely escaped me in the
> >> pre-commit discussion:
> >>
> >> select pg_drop_replication_slot('slot_1');
> >> ERROR:  55006: replication slot "slot_1" is already active
> >> LOCATION:  ReplicationSlotAcquire, slot.c:339
> >>
> >> What defines an "active" slot?
> >
> > One with a connected walsender.
> >
> >> It seems like there's no way for a DBA to drop slots from the master if
> >> it's rapidly running out of disk WAL space without doing a restart, and
> >> there's no way to drop the slot for a replica which the DBA knows is
> >> permanently offline but was connected earlier.  Am I missing something?
> >
> > It's sufficient to terminate the walsender and then drop the slot. That
> > seems ok for now?
> 
> Urgh.  That error message looks susceptible to improvement.  How about:
> 
> replication slot "%s" cannot be dropped because it is currently in use

I think that'd require duplicating some code between acquire and drop,
but how about "replication slot "%s" is in use by another backend"?

Greetings,

Andres Freund

-- Andres Freund	                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services