Re: Logical Replication WIP

On 04/11/16 13:07, Andres Freund wrote:
>
> Hm. I think I have to agree a bit with Peter here.  Overloading
> MyReplicationSlot this way seems ugly, and I think there's a bunch of
> bugs around it too.
>
>
> Sounds what we really want is a) two different lifetimes for ephemeral
> slots, session and "command" b) have a number of slots that are released
> either after a failed transaction / command or at session end.   The
> easiest way for that appears to have a list of slots to be checked at
> end-of-xact and backend shutdown.
>

Ok so how about attached? It adds temp slots as new type of persistence.
It does not really touch the behavior of any of the existing API or
persistence settings.

The temp slots are just cleaned up on backend exit or error, other than
that they are not special. I don't use any specific backend local list
to track them, instead they have active_pid always set and just cleanup
everything that has that set at the end of the session. This has nice
property that it forbids other backends for acquiring them.

It does not do any locking while searching for the slots to cleanup (see
ReplicationSlotCleanup), mainly because it complicates the interaction
with ReplicationSlotDropPtr and it seems to me that locking there is not
really needed there as other backends will never change active_pid to
our backend pid and then the ReplicationSlotDropPtr does exclusive lock
when resetting it.

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