allow RyuApp start() method returns Greenlet thread

This enables an application to continue. Without this, ryu-manager
finishes right after running applications.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>

ryu/base/app_manager.py

@@ -303,8 +303,12 @@ class AppManager(object):
         self._update_bricks()
         self.report_bricks()
 
+        threads = []
         for app in self.applications.values():
-            app.start()
+            t = app.start()
+            if t is not None:
+                threads.append(t)
+        return threads
 
     @staticmethod
     def _close(app):

ryu/cmd/manager.py

@@ -69,9 +69,8 @@ def main():
     app_mgr = AppManager.get_instance()
     app_mgr.load_apps(app_lists)
     contexts = app_mgr.create_contexts()
-    app_mgr.instantiate_apps(**contexts)
-
     services = []
+    services.extend(app_mgr.instantiate_apps(**contexts))
 
     # TODO: do the following in app_manager's instantiate_apps()
     ofpapp = controller.start_service(app_mgr)

ryu/services/protocols/vrrp/manager.py

@@ -61,8 +61,9 @@ class VRRPManager(app_manager.RyuApp):
         self.shutdown = hub.Queue()
 
     def start(self):
-        self.threads.append(hub.spawn(self._shutdown_loop))
+        t = hub.spawn(self._shutdown_loop)
         super(VRRPManager, self).start()
+        return t
 
     @handler.set_ev_cls(vrrp_event.EventVRRPConfigRequest)
     def config_request_handler(self, ev):