Replication slots and footguns

All:

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?

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?

If I'm not, that seems like something to fix before 9.4 release.

--
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com