rabbit: fix timeout timer when duration is None

When the duration of the timeout timer used in the rabbit driver.is
None and we want that the timer return a maximum of N secs it return None
(infinite) instead of N.

This change fixes that.

Closes-bug: #1408370

Change-Id: I7f4cb3075f776c63aa7dc497173677f92b68c16d

oslo/messaging/_drivers/common.py

@@ -343,13 +343,13 @@ class DecayingTimer(object):
         return self
 
     def check_return(self, timeout_callback, *args, **kwargs):
+        maximum = kwargs.pop('maximum', None)
         if self._duration is None:
-            return None
+            return None if maximum is None else maximum
         if self._ends_at is None:
             raise RuntimeError("Can not check/return a timeout from a timer"
                                " that has not been started")
 
-        maximum = kwargs.pop('maximum', None)
         left = self._ends_at - time.time()
         if left <= 0:
             timeout_callback(*args, **kwargs)

tests/test_utils.py

@@ -13,6 +13,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+from oslo.messaging._drivers import common
 from oslo.messaging import _utils as utils
 from tests import utils as test_utils
 
@@ -47,3 +48,17 @@ class VersionIsCompatibleTestCase(test_utils.BaseTestCase):
 
     def test_version_is_compatible_no_rev_is_zero(self):
         self.assertTrue(utils.version_is_compatible('1.23.0', '1.23'))
+
+
+class TimerTestCase(test_utils.BaseTestCase):
+    def test_duration_is_none(self):
+        t = common.DecayingTimer()
+        t.start()
+        remaining = t.check_return(None)
+        self.assertEqual(None, remaining)
+
+    def test_duration_is_none_and_maximun_set(self):
+        t = common.DecayingTimer()
+        t.start()
+        remaining = t.check_return(None, maximum=2)
+        self.assertEqual(2, remaining)