Reading Tom's posting here:
http://archives.postgresql.org/pgsql-general/2006-04/msg00499.php
I just realised we don't seem to mention this in the docs anywhere. I
propose adding a short paragraph to 23.3.1
http://www.postgresql.org/docs/8.1/static/backup-online.html#BACKUP-ONLINE-CAVEATS
After "Except in the case of retrying a failure, it will be called only
once for any given file name."
To identify the current, partially-filled WAL segment, sort first by
mtime and second by file name. That is, take the latest mtime among the
properly-named files, breaking ties by taking the higher filename.
-- Richard Huxton Archonet Ltd
