BUG #4089: When available disk space is low pg_stop_backup() fails, as do subsequent recovery attempts.

The following bug has been logged online:

Bug reference:      4089
Logged by:          John Smith
Email address:      sodgodofall@gmail.com
PostgreSQL version: 8.3.0
Operating system:   Linux 2.6.20-gentoo-r8
Description:        When available disk space is low pg_stop_backup() fails,
as do subsequent recovery attempts.
Details:

Steps to reproduce:
 -- start with a running PG instance with WAL archiving enabled
 -- select pg_start_backup('test');
 -- Fill up the disk on which the data directory is present
 -- select pg_stop_backup();
    -- fails with: ERROR:  could not write file
"pg_xlog/000000010000000000000000.004989E8.backup": No space left on device
    -- at this point there is a 0-byte file
pg_xlog/000000010000000000000000.004989E8.backup present on disk
 -- stop and start PG
 -- recovery fails with: FATAL:  invalid data in file
"000000010000000000000000.004989E8.backup"
 -- NOTE: At this point removing 000000010000000000000000.004989E8.backup
allows PG to start successfully