pgsql: Make WL_POSTMASTER_DEATH level-triggered on kqueue builds.

Make WL_POSTMASTER_DEATH level-triggered on kqueue builds.

If WaitEventSetWait() reports that the postmaster has gone away, later
calls to WaitEventSetWait() should continue to report that.  Otherwise
further waits that occur in the proc_exit() path after we already
noticed the postmaster's demise could block forever.

Back-patch to 13, where the kqueue support landed.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/3624029.1602701929%40sss.pgh.pa.us

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e0950135ae5d50140feecc7bc87c018019c6e406

Modified Files
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src/backend/storage/ipc/latch.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)