pgsql: Make pg_promote() detect postmaster death while waiting forprom

Make pg_promote() detect postmaster death while waiting for promotion to end.

Previously even if postmaster died and WaitLatch() woke up with that event
while pg_promote() was waiting for the standby promotion to finish,
pg_promote() did nothing special and kept waiting until timeout occurred.
This could cause a busy loop.

This patch make pg_promote() return false immediately when postmaster
dies, to avoid such a busy loop.

Back-patch to v12 where pg_promote() was added.

Author: Fujii Masao
Reviewed-by: Michael Paquier
Discussion: https://postgr.es/m/CAHGQGwEs9ROgSp+QF+YdDU+xP8W=CY1k-_Ov-d_Z3JY+to3eXA@mail.gmail.com

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7aa815a7aadd0dcc0515dfc3a88677deb0ca11ff

Modified Files
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src/backend/access/transam/xlogfuncs.c | 17 +++++++++++++----
1 file changed, 13 insertions(+), 4 deletions(-)