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
eithera     replication connection or a superuser connection can be created,     and at 9 connections only a superuser
onewould be allowed.  This     is a bit clumsy as there still aren't guaranteed slots for     replication.  2) A GUC
whichsays "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