Fix None comparision when sorting by `updated_at`

When sorting resource candidates in `_get_best_existing_rsrc_db`,
resources with the same score are sorted by `updated_at`, which can be
`None`. If that is the case, use `created_at` instead.

Task: 43815
Story: 2009653
Change-Id: Ic0265fcf7ceb811803cdebaa8932fe80dc59a627
(cherry picked from commit 403fa55fe9)

heat/engine/stack.py

@@ -1549,7 +1549,10 @@ class Stack(collections.abc.Mapping):
                     # Rolling back to previous resource
                     score += 10
 
-                return score, ext_rsrc.updated_at
+                last_changed_at = ext_rsrc.updated_at
+                if last_changed_at is None:
+                    last_changed_at = ext_rsrc.created_at
+                return score, last_changed_at
 
             candidates = sorted((r for r in self.ext_rsrcs_db.values()
                                  if r.name == rsrc_name),

heat/tests/test_convg_stack.py

@@ -11,6 +11,8 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+from datetime import datetime
+from datetime import timedelta
 from unittest import mock
 
 from oslo_config import cfg
@@ -429,22 +431,32 @@ class StackConvergenceCreateUpdateDeleteTest(common.HeatTestCase):
         stack.prev_raw_template_id = 2
         stack.t.id = 3
 
-        def db_resource(current_template_id):
+        def db_resource(current_template_id,
+                        created_at=None,
+                        updated_at=None):
             db_res = resource_objects.Resource(stack.context)
             db_res['id'] = current_template_id
             db_res['name'] = 'A'
             db_res['current_template_id'] = current_template_id
-            db_res['action'] = 'CREATE'
+            db_res['action'] = 'UPDATE' if updated_at else 'CREATE'
             db_res['status'] = 'COMPLETE'
-            db_res['updated_at'] = None
+            db_res['updated_at'] = updated_at
+            db_res['created_at'] = created_at
             db_res['replaced_by'] = None
             return db_res
 
+        start_time = datetime.utcfromtimestamp(0)
+
+        def t(minutes):
+            return start_time + timedelta(minutes=minutes)
+
         a_res_2 = db_resource(2)
         a_res_3 = db_resource(3)
-        a_res_1 = db_resource(1)
+        a_res_0 = db_resource(0, created_at=t(0), updated_at=t(1))
+        a_res_1 = db_resource(1, created_at=t(2))
         existing_res = {a_res_2.id: a_res_2,
                         a_res_3.id: a_res_3,
+                        a_res_0.id: a_res_0,
                         a_res_1.id: a_res_1}
         stack.ext_rsrcs_db = existing_res
         best_res = stack._get_best_existing_rsrc_db('A')
@@ -460,9 +472,14 @@ class StackConvergenceCreateUpdateDeleteTest(common.HeatTestCase):
         # no resource with current template id as 3 or 2
         del existing_res[2]
         best_res = stack._get_best_existing_rsrc_db('A')
-        # should return resource with template id 1 existing in DB
+        # should return resource with template id 1 which is the newest
         self.assertEqual(a_res_1.id, best_res.id)
 
+        del existing_res[1]
+        best_res = stack._get_best_existing_rsrc_db('A')
+        # should return resource with template id 0 existing in the db
+        self.assertEqual(a_res_0.id, best_res.id)
+
     @mock.patch.object(parser.Stack, '_converge_create_or_update')
     def test_updated_time_stack_create(self, mock_ccu, mock_cr):
         stack = parser.Stack(utils.dummy_context(), 'convg_updated_time_test',