backward-combat for ip, port in LocalProcessSpawner subclasses

Subclasses prior to 0.6 may assume return value
of LocalProcessSpawner.start can be ignored
instead of passing it through.

For these cases, keep setting ip/port in the deprecated way
so that it still works with a warning,
rather than failing with the wrong port.

jupyterhub/spawner.py

@@ -495,6 +495,16 @@ class LocalProcessSpawner(Spawner):
             raise
 
         self.pid = self.proc.pid
+
+        if self.__class__ is not LocalProcessSpawner:
+            # subclasses may not pass through return value of super().start,
+            # relying on deprecated 0.6 way of setting ip, port,
+            # so keep a redundant copy here for now.
+            # A deprecation warning will be shown if the subclass
+            # does not return ip, port.
+            if self.ip:
+                self.user.server.ip = self.ip
+            self.user.server.port = self.port
         return (self.ip or '127.0.0.1', self.port)
 
     @gen.coroutine