test check_delay > 0 and call callback immediately

 - add test for positive check_delay

in production environment check_delay often set to a positive value, the
tornado API is changed so the PeriodicCallback no longer recieve event
loop as parameter.

 - use concurrent.future_add_done_callback which invoke callback immediately

When the future is already finished, we expect that the callback will be
executed immediately, but concurrent.add_done_callback did not make this
check and only add it to the event loop, while future_add_done_callback did this check.

circus/tests/test_arbiter.py

@@ -633,6 +633,21 @@ def callback(*args):
 
         self.assertEqual(callee.call_count, 1)
 
+    def test_start_with_callback_delay(self):
+        controller = "tcp://127.0.0.1:%d" % get_available_port()
+        sub = "tcp://127.0.0.1:%d" % get_available_port()
+        arbiter = Arbiter([], controller, sub, check_delay=1)
+
+        callee = mock.MagicMock()
+
+        def callback(*args):
+            callee()
+            arbiter.stop()
+
+        arbiter.start(cb=callback)
+
+        self.assertEqual(callee.call_count, 1)
+
     @tornado.testing.gen_test
     def test_start_with_callback_and_given_loop(self):
         controller = "tcp://127.0.0.1:%d" % get_available_port()

circus/util.py

@@ -1032,7 +1032,7 @@ def wrapper(self, *args, **kwargs):
             finally:
                 if isinstance(resp, concurrent.Future):
                     cb = functools.partial(_synchronized_cb, arbiter)
-                    resp.add_done_callback(cb)
+                    concurrent.future_add_done_callback(resp, cb)
                 else:
                     if arbiter is not None:
                         arbiter._exclusive_running_command = None