pgsql: Fix NOTIFY to cope with I/O problems, such as out-of-disk-space.

Fix NOTIFY to cope with I/O problems, such as out-of-disk-space.

The LISTEN/NOTIFY subsystem got confused if SimpleLruZeroPage failed,
which would typically happen as a result of a write() failure while
attempting to dump a dirty pg_notify page out of memory.  Subsequently,
all attempts to send more NOTIFY messages would fail with messages like
"Could not read from file "pg_notify/nnnn" at offset nnnnn: Success".
Only restarting the server would clear this condition.  Per reports from
Kevin Grittner and Christoph Berg.

Back-patch to 9.0, where the problem was introduced during the
LISTEN/NOTIFY rewrite.

Branch
------
REL9_2_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/f374098e8c5f58b95e03b923c8a808cdeeea93e7

Modified Files
--------------
src/backend/commands/async.c |   27 ++++++++++++++++++++++-----
1 files changed, 22 insertions(+), 5 deletions(-)