replication_reserved_connections

Hi,

Yesterday an interesting scenario was diagnosed on IRC. If you're
running a synchronous slave and the connection to the slave is lost
momentarily, your backends start naturally waiting for the slave to
reconnect. If then your application keeps trying to create new
connections, it can use all non-reserved connections, thus locking out
the synchronous slave when the connection problem has resolved itself.
This brings the entire cluster into a state where manual intervention is
necessary.

While you could limit the number of connections for non-replication
roles, that's not always possible or desirable. I would like to
introduce a way to reserve connection slots for replication. However,
it's not clear how this would work. I looked at how
superuser_reserved_connections is implented, and with small changes I
could see how to implement two ideas:

1) Reserve a portion of superuser_reserved_connections for replication
connections. For example, with max_connections=10,
superuser_reserved_connections=2 and
replication_reserved_connections=1, at 8 connections either a
replication connection or a superuser connection can be created,
and at 9 connections only a superuser one would be allowed. This
is a bit clumsy as there still aren't guaranteed slots for
replication.
2) A GUC which says "superuser_reserved_connections can be used up by
replication connections", and then limiting the number of
replication connections using per-role limits to make sure
superusers aren't locked out.

Does anyone see a better way to do this? I'm not too satisfied with
either of these ideas.

Regards,
Marko Tiikkaja