Streaming replication status

I've gone through the patch in detail now. Here's my list of remaining
issues:

* If there's no WAL to send, walsender doesn't notice if the client has
closed connection already. This is the issue Fujii reported already.
We'll need to add a select() call to the walsender main loop to check if
the socket has been closed.

* I removed the feature that archiver was started during recovery. The
idea of that was to enable archiving from a standby server, to relieve
the master server of that duty, but I found it annoying because it
causes trouble if the standby and master are configured to archive to
the same location; they will fight over which copies the file to the
archive first. Frankly the feature doesn't seem very useful as the patch
stands, because you still have to configure archiving in the master in
practice; you can't take an online base backup otherwise, and you have
the risk of standby falling too much behind and having to restore from
base backup whenever the standby is disconnected for any reason. Let's
revisit this later when it's truly useful.

* We still have a related issue, though: if standby is configured to
archive to the same location as master (as it always is on my laptop,
where I use the postgresql.conf of the master unmodified in the server),
right after failover the standby server will try to archive all the old
WAL files that were streamed from the master; but they exist already in
the archive, as the master archived them already. I'm not sure if this
is a pilot error, or if we should do something in the server to tell
apart WAL segments streamed from master and those generated in the
standby server after failover. Maybe we should immediately create a
.done file for every file received from master?

* I don't think we should require superuser rights for replication.
Although you see all WAL and potentially all data in the system through
that, a standby doesn't need any write access to the master, so it would
be good practice to create a dedicated account with limited privileges
for replication.

* A standby that connects to master, initiates streaming, and then sits
idle without stalls recycling of old WAL files in the master. That will
eventually lead to a full disk in master. Do we need some kind of a
emergency valve on that?

* Do we really need REPLICATION_DEBUG_ENABLED? The output doesn't seem
very useful to me.

* Need to add comments somewhere to note that ReadRecord depends on the
fact that a WAL record is always send as whole, never split across two
messages.

* Do we really need to split the sleep in walsender to NAPTIME_PER_CYCLE
increments?

* Walreceiver should flush less aggresively than after each received
piece of WAL as noted by XXX comment.

* Consider renaming PREPARE_REPLICATION to IDENTIFY_SYSTEM or something.

* What's the change in bgwriter.c for?

* ReadRecord/FetchRecord is a bit of mess. I earlier tried to refactor
it into something simpler a couple of times, but failed. So I'm going to
leave it as it is, but if someone else wants to give it a shot, that
would be good.

* Documentation. The patch used to move around some sections, but I
think that has been partially reverted so that it now just duplicates
them. It probably needs other work too, I haven't looked at the docs in
any detail.

These are all the issues I know of right now. Assuming no new issues
crop up (which often does happen), the patch is ready for committing
after those have been addressed.

Attached is my latest version as a patch, also available in my git
repository.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com