Don't allow immediate interrupts during authentication anymore.

We used to handle authentication_timeout by setting
ImmediateInterruptOK to true during large parts of the authentication
phase of a new connection.  While that happens to work acceptably in
practice, it's not particularly nice and has ugly corner cases.

Previous commits converted the FE/BE communication to use latches and
implemented support for interrupt handling during both
send/recv. Building on top of that work we can get rid of
ImmediateInterruptOK during authentication, by immediately treating
timeouts during authentication as a reason to die. As die interrupts
are handled immediately during client communication that provides a
sensibly quick reaction time to authentication timeout.

Additionally add a few CHECK_FOR_INTERRUPTS() to some more complex
authentication methods. More could be added, but this already should
provides a reasonable coverage.

While it this overall increases the maximum time till a timeout is
reacted to, it greatly reduces complexity and increases
reliability. That seems like a overall win. If the increase proves to
be noticeable we can deal with those cases by moving to nonblocking
network code and add interrupt checking there.

Reviewed-By: Heikki Linnakangas

src/backend/utils/init/postinit.c

@@ -1099,18 +1099,24 @@ ShutdownPostgres(int code, Datum arg)
 static void
 StatementTimeoutHandler(void)
 {
+	int sig = SIGINT;
+
+	/*
+	 * During authentication the timeout is used to deal with
+	 * authentication_timeout - we want to quit in response to such timeouts.
+	 */
+	if (ClientAuthInProgress)
+		sig = SIGTERM;
+
 #ifdef HAVE_SETSID
 	/* try to signal whole process group */
-	kill(-MyProcPid, SIGINT);
+	kill(-MyProcPid, sig);
 #endif
-	kill(MyProcPid, SIGINT);
+	kill(MyProcPid, sig);
 }
 
 /*
  * LOCK_TIMEOUT handler: trigger a query-cancel interrupt.
- *
- * This is identical to StatementTimeoutHandler, but since it's so short,
- * we might as well keep the two functions separate for clarity.
  */
 static void
 LockTimeoutHandler(void)