Introduce a pipe between postmaster and each backend, which can be used to

detect postmaster death. Postmaster keeps the write-end of the pipe open,
so when it dies, children get EOF in the read-end. That can conveniently
be waited for in select(), which allows eliminating some of the polling
loops that check for postmaster death. This patch doesn't yet change all
the loops to use the new mechanism, expect a follow-on patch to do that.

This changes the interface to WaitLatch, so that it takes as argument a
bitmask of events that it waits for. Possible events are latch set, timeout,
postmaster death, and socket becoming readable or writeable.

The pipe method behaves slightly differently from the kill() method
previously used in PostmasterIsAlive() in the case that postmaster has died,
but its parent has not yet read its exit code with waitpid(). The pipe
returns EOF as soon as the process dies, but kill() continues to return
true until waitpid() has been called (IOW while the process is a zombie).
Because of that, change PostmasterIsAlive() to use the pipe too, otherwise
WaitLatch() would return immediately with WL_POSTMASTER_DEATH, while
PostmasterIsAlive() would claim it's still alive. That could easily lead to
busy-waiting while postmaster is in zombie state.

Peter Geoghegan with further changes by me, reviewed by Fujii Masao and
Florian Pflug.

src/backend/storage/ipc/pmsignal.c

@@ -267,42 +267,27 @@ MarkPostmasterChildInactive(void)
 
 /*
  * PostmasterIsAlive - check whether postmaster process is still alive
- *
- * amDirectChild should be passed as "true" by code that knows it is
- * executing in a direct child process of the postmaster; pass "false"
- * if an indirect child or not sure.  The "true" case uses a faster and
- * more reliable test, so use it when possible.
  */
 bool
-PostmasterIsAlive(bool amDirectChild)
+PostmasterIsAlive(void)
 {
 #ifndef WIN32
-	if (amDirectChild)
+	char c;
+	ssize_t rc;
+
+	rc = read(postmaster_alive_fds[POSTMASTER_FD_WATCH], &c, 1);
+	if (rc < 0)
 	{
-		pid_t		ppid = getppid();
-
-		/* If the postmaster is still our parent, it must be alive. */
-		if (ppid == PostmasterPid)
+		if (errno == EAGAIN || errno == EWOULDBLOCK)
 			return true;
-
-		/* If the init process is our parent, postmaster must be dead. */
-		if (ppid == 1)
-			return false;
-
-		/*
-		 * If we get here, our parent process is neither the postmaster nor
-		 * init.  This can occur on BSD and MacOS systems if a debugger has
-		 * been attached.  We fall through to the less-reliable kill() method.
-		 */
+		else
+			elog(FATAL, "read on postmaster death monitoring pipe failed: %m");
 	}
+	else if (rc > 0)
+		elog(FATAL, "unexpected data in postmaster death monitoring pipe");
+
+	return false;
 
-	/*
-	 * Use kill() to see if the postmaster is still alive.	This can sometimes
-	 * give a false positive result, since the postmaster's PID may get
-	 * recycled, but it is good enough for existing uses by indirect children
-	 * and in debugging environments.
-	 */
-	return (kill(PostmasterPid, 0) == 0);
 #else							/* WIN32 */
 	return (WaitForSingleObject(PostmasterHandle, 0) == WAIT_TIMEOUT);
 #endif   /* WIN32 */