pgsql: Fix kqueue support under debugger on macOS.

Fix kqueue support under debugger on macOS.

While running under a debugger, macOS's getppid() can return the
debugger's PID.  That could cause a backend to exit because it falsely
believed that the postmaster had died, since commit 815c2f09.

Continue to use getppid() as a fast postmaster check after adding the
postmaster's PID to a kqueue, to close a PID-reuse race, but double
check that it actually exited by trying to read the pipe.  The new check
isn't reached in the common case.

Reported-by: Alexander Korotkov <a.korotkov@postgrespro.ru>
Discussion: https://postgr.es/m/CA%2BhUKGKhAxJ8V8RVwCo6zJaeVrdOG1kFBHGZOOjf6DzW_omeMA%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/7bc84a1f304acc9f4705c4178eb362ddce310452

Modified Files
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src/backend/storage/ipc/latch.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)