core: split preserve_tty_fp() out into a function

mitogen/core.py

@@ -3680,13 +3680,15 @@ class ExternalContext(object):
                 os.dup2(fd, stdfd)
                 os.close(fd)
 
-    def _setup_stdio(self):
-        # #481: when stderr is a TTY due to being started via
-        # tty_create_child()/hybrid_tty_create_child(), and some privilege
-        # escalation tool like prehistoric versions of sudo exec this process
-        # over the top of itself, there is nothing left to keep the slave PTY
-        # open after we replace our stdio. Therefore if stderr is a TTY, keep
-        # around a permanent dup() to avoid receiving SIGHUP.
+    def _preserve_tty_fp(self):
+        """
+        #481: when stderr is a TTY due to being started via tty_create_child()
+        or hybrid_tty_create_child(), and some privilege escalation tool like
+        prehistoric versions of sudo exec this process over the top of itself,
+        there is nothing left to keep the slave PTY open after we replace our
+        stdio. Therefore if stderr is a TTY, keep around a permanent dup() to
+        avoid receiving SIGHUP.
+        """
         try:
             if os.isatty(2):
                 self.reserve_tty_fp = os.fdopen(os.dup(2), 'r+b', 0)
@@ -3694,6 +3696,8 @@ class ExternalContext(object):
         except OSError:
             pass
 
+    def _setup_stdio(self):
+        self._preserve_tty_fp()
         # When sys.stdout was opened by the runtime, overwriting it will not
         # close FD 1. However when forking from a child that previously used
         # fdopen(), overwriting it /will/ close FD 1. So we must swallow the