Comments on Custom RMGRs

New rmgr stuff looks interesting. I've had a detailed look through it
and tried to think about how it might be used in practice.

Spotted a minor comment that needs adjustment for new methods...
[PATCH: rmgr_001.v1.patch]

I notice rm_startup() and rm_cleanup() presume that this only works in
recovery. If recovery is "not needed", there is no way to run anything
at all, which seems wrong because how do we know that? I would prefer
it if rm_startup() and rm_cleanup() were executed in all cases. Only 4
builtin index rmgrs have these anyway, and they are all quick, so I
suggest we run them always. This allows a greater range of startup
behavior for rmgrs.
[PATCH: rmgr_002.v1.patch]

It occurs to me that any use of WAL presumes that Checkpoints exist
and do something useful. However, the custom rmgr interface doesn't
allow you to specify any actions on checkpoint, so ends up being
limited in scope. So I think we also need an rm_checkpoint() call -
which would be a no-op for existing rmgrs.
[PATCH: rmgr_003.v1.patch]

The above turns out to be fairly simple, but extends the API to
something truly flexible.

Please let me know what you think?

--
Simon Riggs http://www.EnterpriseDB.com/