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

src/backend/storage/ipc/latch.c

@@ -1094,9 +1094,18 @@ WaitEventAdjustKqueue(WaitEventSet *set, WaitEvent *event, int old_events)
 					 errmsg("%s failed: %m",
 							"kevent()")));
 	}
-	else if (event->events == WL_POSTMASTER_DEATH && PostmasterPid != getppid())
+	else if (event->events == WL_POSTMASTER_DEATH &&
+			 PostmasterPid != getppid() &&
+			 !PostmasterIsAlive())
+	{
+		/*
+		 * The extra PostmasterIsAliveInternal() check prevents false alarms on
+		 * systems that give a different value for getppid() while being traced
+		 * by a debugger.
+		 */
 		set->report_postmaster_not_running = true;
 	}
+}
 
 #endif