fix(mongo): Negative age reported in stats

This patch includes a one-line fix, along with a regression test
to verify the same.

Change-Id: Iffb3cb7c243296b1caacc95e1edd3c6f63c34a4b
Closes-Bug: #1237620

marconi/queues/storage/mongodb/utils.py

@@ -128,7 +128,7 @@ def stat_message(message, now):
     """Creates a stat document from the given message, relative to now."""
     oid = message['_id']
     created = oid_ts(oid)
-    age = created - now
+    age = now - created
 
     return {
         'id': str(oid),

marconi/tests/queues/storage/base.py

@@ -132,7 +132,7 @@ class QueueControllerTest(ControllerBaseTest):
         # NOTE(kgriffs): We can't get around doing this, because
         # we don't know how the storage drive may be calculating
         # message timestamps (and may not be monkey-patchable).
-        time.sleep(1)
+        time.sleep(1.2)
 
         _insert_fixtures(self.message_controller, 'test',
                          project=self.project, client_uuid=client_uuid,
@@ -150,6 +150,9 @@ class QueueControllerTest(ControllerBaseTest):
 
         self.assertNotEqual(oldest, newest)
 
+        age = oldest['age']
+        self.assertThat(age, matchers.GreaterThan(0))
+
         # NOTE(kgriffs): Ensure is different enough
         # for the next comparison to work.
         soon = timeutils.utcnow() + datetime.timedelta(seconds=60)