Missing template file on update fails silently

When the template file for a template/facade/provider resource is
not found in the new files list when doing a stack update, Heat
silently falls back to using the existing template (i.e. so no
change will be made).
The cause is that the update code shares the same code path for
obtaining the template as the init code. The init code needs to
have a fallback, so that loading the (parent) stack never fails,
however in the update case it is undesirable.

Change-Id: Id12b63d9f04caba8e9325aed0ee2acf51ffe0c4c
Closes-Bug: #1452520

heat/engine/resources/template_resource.py

@@ -181,6 +181,8 @@ class TemplateResource(stack_resource.StackResource):
                 t_data = self.get_template_file(self.template_name,
                                                 self.allowed_schemes)
             except exception.TemplateNotFound as err:
+                if self.action == self.UPDATE:
+                    raise
                 reported_excp = err
 
         if t_data is None:

heat/tests/test_provider_template.py

@@ -813,6 +813,54 @@ class ProviderTemplateTest(common.HeatTestCase):
         self.m.VerifyAll()
 
 
+class TemplateDataTest(common.HeatTestCase):
+
+    def setUp(self):
+        super(TemplateDataTest, self).setUp()
+        files = {}
+        self.ctx = utils.dummy_context()
+
+        class DummyResource(object):
+            support_status = support.SupportStatus()
+            properties_schema = {"Foo":
+                                 properties.Schema(properties.Schema.STRING,
+                                                   required=True)}
+            attributes_schema = {}
+
+        env = environment.Environment()
+        resource._register_class('DummyResource', DummyResource)
+        env.load({'resource_registry':
+                  {'DummyResource': 'test_resource.template'}})
+
+        self.stack = parser.Stack(self.ctx, 'test_stack',
+                                  template.Template(empty_template,
+                                                    files=files,
+                                                    env=env),
+                                  stack_id=str(uuid.uuid4()))
+
+        self.defn = rsrc_defn.ResourceDefinition('test_t_res',
+                                                 "DummyResource",
+                                                 {"Foo": "bar"})
+        self.res = template_resource.TemplateResource('test_t_res',
+                                                      self.defn, self.stack)
+
+    def test_template_data_in_update_without_template_file(self):
+        self.res.action = self.res.UPDATE
+        self.res.nested = mock.MagicMock()
+        self.res.get_template_file = mock.Mock(
+            side_effect=exception.TemplateNotFound(
+                message='test_resource.template'))
+        self.assertRaises(exception.TemplateNotFound, self.res.template_data)
+
+    def test_template_data_in_create_without_template_file(self):
+        self.res.action = self.res.CREATE
+        self.res.nested = mock.MagicMock()
+        self.res.get_template_file = mock.Mock(
+            side_effect=exception.TemplateNotFound(
+                message='test_resource.template'))
+        self.assertEqual(self.res.template_data(), '{}')
+
+
 class TemplateResourceCrudTest(common.HeatTestCase):
     provider = {
         'HeatTemplateFormatVersion': '2012-12-12',