On MacOS X, and apparently also on other BSD-derived systems, attaching
a debugger causes getppid() to return the pid of the debugging process
rather than the actual parent PID. As a result, debugging the
autovacuum launcher, startup process, or WAL sender on such systems
causes it to exit, because the previous coding of PostmasterIsAlive()
detects postmaster death by testing whether getppid() == PostmasterPid.
Work around that behavior by checking the return value of getppid()
more carefully. If it's PostmasterPid, the postmaster must be alive;
if it's 1, assume the postmaster is dead. If it's any other value,
assume we've been debugged and fall through to the less-reliable
kill() test.
Review by Tom Lane.
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