Fallback to db for resource show

This change adds a Stack.resource_get which is used by
describe_stack_resource (stack resource show). It has a different
behaviour when the resource is missing from the stack resources dict,
attempting to return a resource from the DB record in the current
stack.

This allows "stack resource show" to show a resource in a convergence
stack while the resource is being deleted.

Change-Id: I975778ac36ae31fe5bd3ebdbcd7c48a6fcc60757
Partial-Bug: #1301320
Related-Bug: #1523748

heat/engine/service.py

@@ -1732,12 +1732,12 @@ class EngineService(service.Service):
                 LOG.warning(_LW("Access denied to resource %s"), resource_name)
                 raise exception.Forbidden()
 
-        if resource_name not in stack:
+        resource = stack.resource_get(resource_name)
+        if not resource:
             raise exception.ResourceNotFound(resource_name=resource_name,
                                              stack_name=stack.name)
 
-        return api.format_stack_resource(stack[resource_name],
-                                         with_attr=with_attr)
+        return api.format_stack_resource(resource, with_attr=with_attr)
 
     @context.request_context
     def resource_signal(self, cnxt, stack_identity, resource_name, details,

heat/engine/stack.py

@@ -358,6 +358,29 @@ class Stack(collections.Mapping):
             self._db_resources = _db_resources
         return self._db_resources.get(name)
 
+    def _resource_from_db_resource(self, db_res):
+        tid = db_res.current_template_id
+        if tid == self.t.id:
+            t = self.t
+        else:
+            t = tmpl.Template.load(self.context, tid)
+
+        res_defn = t.resource_definitions(self)[db_res.name]
+        return resource.Resource(db_res.name, res_defn, self)
+
+    def resource_get(self, name):
+        """Return a stack resource, even if not in the current template."""
+        res = self.resources.get(name)
+        if res:
+            return res
+
+        # fall back to getting the resource from the database
+        db_res = self.db_resource_get(name)
+        if db_res:
+            return self._resource_from_db_resource(db_res)
+
+        return None
+
     @property
     def dependencies(self):
         if self._dependencies is None:

heat/tests/test_stack.py

@@ -224,6 +224,46 @@ class StackTest(common.HeatTestCase):
         self.assertEqual(1, self.stack.total_resources(self.stack.id))
         self.assertEqual(1, self.stack.total_resources())
 
+    def test_resource_get(self):
+        tpl = {'HeatTemplateFormatVersion': '2012-12-12',
+               'Resources':
+               {'A': {'Type': 'GenericResourceType'}}}
+        self.stack = stack.Stack(self.ctx, 'test_stack',
+                                 template.Template(tpl),
+                                 status_reason='blarg')
+        self.stack.store()
+        self.assertEqual('A', self.stack.resource_get('A').name)
+        self.assertEqual(self.stack['A'], self.stack.resource_get('A'))
+        self.assertIsNone(self.stack.resource_get('B'))
+
+    @mock.patch.object(resource_objects.Resource, 'get_all_by_stack')
+    def test_resource_get_db_fallback(self, gabs):
+        tpl = {'HeatTemplateFormatVersion': '2012-12-12',
+               'Resources':
+               {'A': {'Type': 'GenericResourceType'}}}
+        self.stack = stack.Stack(self.ctx, 'test_stack',
+                                 template.Template(tpl),
+                                 status_reason='blarg')
+        self.stack.store()
+        tpl2 = {'HeatTemplateFormatVersion': '2012-12-12',
+                'Resources':
+                {'A': {'Type': 'GenericResourceType'},
+                 'B': {'Type': 'GenericResourceType'}}}
+        t2 = template.Template(tpl2)
+        t2.store(self.ctx)
+
+        db_resources = {
+            'A': mock.MagicMock(),
+            'B': mock.MagicMock(current_template_id=t2.id)
+        }
+        db_resources['A'].name = 'A'
+        db_resources['B'].name = 'B'
+        gabs.return_value = db_resources
+
+        self.assertEqual('A', self.stack.resource_get('A').name)
+        self.assertEqual('B', self.stack.resource_get('B').name)
+        self.assertIsNone(self.stack.resource_get('C'))
+
     def test_iter_resources_with_nested(self):
         tpl = {'HeatTemplateFormatVersion': '2012-12-12',
                'Resources':