Checkpoint gets stuck in mdsync

Now that the CheckpointStartLock starvation has been taken care of, I'm
seeing another problem with checkpoints in my test run: mdsync never
finishes.

Here's what's happening:
1. checkpoint calls mdsync
2. mdsync start processing pending fsyncs from pendingOpsTable
(at this point, normal backends have to start doing writes themselves,
because bgwriter is busy checkpointing and isn't keeping buffers clean)
3. after fsyncing 10 files, it calls AbsorbFsyncRequests
4. AbsorbFsyncRequests puts back entries into pendingOpsTable for those
files that were already fsynced.
5. mdsync starts over, goto 2.

The loop doesn't end until the test run is over, mdsync keeps fsyncing
the same over and over again.

My proposed fix is to make a copy of pendingOpsTable before entering the
loop. AbsorbFsyncRequest will put new requests to a fresh new
pendingOpsTable, while the mdsync loop will drain the copy. I'll write a
patch along those lines if there's no better ideas.

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