Convergence: Allow creating stacks of empty templates

Change-Id: I25449a69a4565c2433832845b70cf32bc1cf53a9

heat/engine/stack.py

@@ -991,15 +991,19 @@ class Stack(collections.Mapping):
                       self.convergence_dependencies.graph().edges()]}
         self.store()
 
-        for rsrc_id, is_update in self.convergence_dependencies.leaves():
-            LOG.info(_LI("Triggering resource %(rsrc_id)s "
-                         "for %(is_update)s update"),
-                     {'rsrc_id': rsrc_id, 'is_update': is_update})
-            input_data = {'input_data': {},
-                          'adopt_stack_data': self.adopt_stack_data}
-            self.worker_client.check_resource(self.context, rsrc_id,
-                                              self.current_traversal,
-                                              input_data, is_update)
+        leaves = set(self.convergence_dependencies.leaves())
+        if not any(leaves):
+            self.mark_complete(self.current_traversal)
+        else:
+            for rsrc_id, is_update in self.convergence_dependencies.leaves():
+                LOG.info(_LI("Triggering resource %(rsrc_id)s "
+                             "for %(is_update)s update"),
+                         {'rsrc_id': rsrc_id, 'is_update': is_update})
+                input_data = {'input_data': {},
+                              'adopt_stack_data': self.adopt_stack_data}
+                self.worker_client.check_resource(self.context, rsrc_id,
+                                                  self.current_traversal,
+                                                  input_data, is_update)
 
     def _get_best_existing_rsrc_db(self, rsrc_name):
         candidate = None

heat/tests/test_engine_service.py

@@ -189,6 +189,16 @@ class StackConvergenceCreateUpdateDeleteTest(common.HeatTestCase):
         super(StackConvergenceCreateUpdateDeleteTest, self).setUp()
         cfg.CONF.set_override('convergence_engine', True)
 
+    @mock.patch.object(parser.Stack, 'mark_complete')
+    def test_converge_empty_template(self, mock_mc, mock_cr):
+        empty_tmpl = templatem.Template.create_empty_template()
+        stack = parser.Stack(utils.dummy_context(), 'empty_tmpl_stack',
+                             empty_tmpl, convergence=True)
+        stack.store()
+        stack.converge_stack(template=stack.t, action=stack.CREATE)
+        self.assertFalse(mock_cr.called)
+        mock_mc.assert_called_once_with(stack.current_traversal)
+
     def test_conv_wordpress_single_instance_stack_create(self, mock_cr):
         stack = tools.get_stack('test_stack', utils.dummy_context(),
                                 convergence=True)