Return timer after adding it to internal list

In some cases, its useful to selectively stop
some timers and not all. This fix allows a way
for client applications to maintain a handle
on timers and be able to stop some of them
selectively.

This is needed by
https://review.openstack.org/#/c/190842/

Change-Id: Ib258c07d6dd12ef7cc9f4a6c8cbde885048ceef8
Closes-Bug: 1466661

oslo_service/tests/test_threadgroup.py

@@ -79,3 +79,34 @@ class ThreadGroupTestCase(test_base.BaseTestCase):
         self.assertEqual(1, len(self.tg.timers))
         self.tg.stop_timers()
         self.assertEqual(0, len(self.tg.timers))
+
+    def test_add_and_remove_timer(self):
+
+        def foo(*args, **kwargs):
+            pass
+
+        timer = self.tg.add_timer('1234', foo)
+        self.assertEqual(1, len(self.tg.timers))
+        timer.stop()
+        self.assertEqual(1, len(self.tg.timers))
+
+        self.tg.timer_done(timer)
+        self.assertEqual(0, len(self.tg.timers))
+
+    def test_add_and_remove_dynamic_timer(self):
+
+        def foo(*args, **kwargs):
+            pass
+        initial_delay = 1
+        periodic_interval_max = 2
+        timer = self.tg.add_dynamic_timer(foo, initial_delay,
+                                          periodic_interval_max)
+
+        self.assertEqual(1, len(self.tg.timers))
+        self.assertTrue(timer._running)
+
+        timer.stop()
+        self.assertEqual(1, len(self.tg.timers))
+
+        self.tg.timer_done(timer)
+        self.assertEqual(0, len(self.tg.timers))

oslo_service/threadgroup.py

@@ -69,6 +69,7 @@ class ThreadGroup(object):
         timer.start(initial_delay=initial_delay,
                     periodic_interval_max=periodic_interval_max)
         self.timers.append(timer)
+        return timer
 
     def add_timer(self, interval, callback, initial_delay=None,
                   *args, **kwargs):
@@ -76,6 +77,7 @@ class ThreadGroup(object):
         pulse.start(interval=interval,
                     initial_delay=initial_delay)
         self.timers.append(pulse)
+        return pulse
 
     def add_thread(self, callback, *args, **kwargs):
         gt = self.pool.spawn(callback, *args, **kwargs)
@@ -86,6 +88,9 @@ class ThreadGroup(object):
     def thread_done(self, thread):
         self.threads.remove(thread)
 
+    def timer_done(self, timer):
+        self.timers.remove(timer)
+
     def _stop_threads(self):
         current = threading.current_thread()