pgsql: Re-read standby LSN after recovery ends

Re-read standby LSN after recovery ends

WaitForLSN() samples the current position before checking whether
recovery is still in progress. If recovery reaches the target and ends
between those operations, the promotion path can compare against the
stale sample and incorrectly return NOT_IN_RECOVERY.

Read the position again after observing that recovery has ended, before
deciding whether promotion reached the target.

Author: Xuneng Zhou <xunengzhou(at)gmail(dot)com>
Discussion: https://postgr.es/m/CABPTF7U0gW5%2B-4oL7-qdML-yerZxUb7ku4QXp7JxCYo0qyJ_Tw%40mail.gmail.com
Reviewed-by: Alexander Korotkov <aekorotkov(at)gmail(dot)com>
Backpatch-through: 19

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/8c7a74c3239ce29940582643533a190721b395c0

Modified Files
--------------
src/backend/access/transam/xlogwait.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)