Walreceiver fsyncs excessively

While testing Fujii-san's patch to flush any already-written WAL on
error in walreceiver, I added a debugging elog to XLogWalRcvFlush() to
print out how far it has written and flushed.

I saw an unexpected pattern while the standby catches up with the master:

LOG: streaming replication successfully connected to primary
LOG: flushing flush=0/0 write=0/1E020000
LOG: flushing flush=0/1E020000 write=0/1E040000
LOG: flushing flush=0/1E040000 write=0/1E060000
LOG: flushing flush=0/1E060000 write=0/1E080000
LOG: flushing flush=0/1E080000 write=0/1E0A0000
LOG: flushing flush=0/1E0A0000 write=0/1E0C0000
LOG: flushing flush=0/1E0C0000 write=0/1E0E0000
LOG: flushing flush=0/1E0E0000 write=0/1E100000
LOG: flushing flush=0/1E100000 write=0/1E120000
LOG: flushing flush=0/1E120000 write=0/1E140000

The master sends the WAL at full-speed, but walreceiver always fsyncs it
in 128 kB chunks. That's excessive, it should be able to read and write
to disk the whole WAL segment before flushing.

There's a little flaw in the walreceiver logic that tries to read all
the available WAL before flushing and sleeping. The way libpqrcv_receive
is written, when it's called with timeout==0 it will not call
PQconsumeInput. So what happens is that when walreceiver main loop calls
libpqrcv_receive() in a loop to fetch all WAL that's available without
blocking, it actually only reads the WAL that's in the libpq receive
buffer - it will not read the WAL that's in the TCP read buffer.

Attached patch fixes libpqrcv_receive() so that it calls
PQconsumeInput() before concluding that there's no data available. The
excessive fsyncing can lead to very bad performance, so this needs to be
appled to 9.0 too.

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