pgsql: Fix an assertion failure related to an exclusive backup.

Fix an assertion failure related to an exclusive backup.

Previously multiple sessions could execute pg_start_backup() and
pg_stop_backup() to start and stop an exclusive backup at the same time.
This could trigger the assertion failure of
"FailedAssertion("!(XLogCtl->Insert.exclusiveBackup)".
This happend because, even while pg_start_backup() was starting
an exclusive backup, other session could run pg_stop_backup()
concurrently and mark the backup as not-in-progress unconditionally.

This patch introduces ExclusiveBackupState indicating the state of
an exclusive backup. This state is used to ensure that there is only
one session running pg_start_backup() or pg_stop_backup() at
the same time, to avoid the assertion failure.

Back-patch to all supported versions.

Author: Michael Paquier
Reviewed-By: Kyotaro Horiguchi and me
Reported-By: Andreas Seltenreich
Discussion: <87mvktojme(dot)fsf(at)credativ(dot)de>

Branch
------
REL9_4_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/9e7f00d861fffacaec49473f89513ee8abca076c

Modified Files
--------------
src/backend/access/transam/xlog.c | 210 +++++++++++++++++++++++++++-----------
1 file changed, 148 insertions(+), 62 deletions(-)