pgsql: While waiting for a condition variable, detect postmaster death.

While waiting for a condition variable, detect postmaster death.

The general assumption for postmaster child processes is that they
should just exit(1), reasonably promptly, if the postmaster disappears.
condition_variable.c neglected this consideration and could be left
waiting forever, if the counterpart process it is waiting for has
done the right thing and exited.

We had some discussion of adjusting the WaitEventSet API to make it
harder to make this type of mistake in future; but for the moment,
and for v10, let's make this narrow fix.

Discussion: https://postgr.es/m/20412.1515456143@sss.pgh.pa.us

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d56a5f994c216847df3338e148b2ae72f20b2e0f

Modified Files
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src/backend/storage/lmgr/condition_variable.c | 17 ++++++++++++++---
1 file changed, 14 insertions(+), 3 deletions(-)