Do not attempt deletion of a DELETE_COMPLETE stack in service api

A stack may be in transient state where it is DELETE_COMPLETE, but has
has not actually been soft-deleted yet. For the purposes of
delete_stack in service.py, consider a DELETE_COMPLETE stack as
equivalent to a soft-deleted one (it soon will be), thereby avoiding a
race where we would have attempted to update the stack, running into a
foreign-key constraint issue for a non-existing user_cred.

Change-Id: Iec021e6a0df262d447fdf9ee1789603c7a1c55f8
Closes-Bug: #1626173
Closes-Bug: #1626107

heat/engine/service.py

@@ -1358,6 +1358,10 @@ class EngineService(service.Service):
         """
 
         st = self._get_stack(cnxt, stack_identity)
+        if (st.status == parser.Stack.COMPLETE and
+                st.action == parser.Stack.DELETE):
+            raise exception.EntityNotFound(entity='Stack', name=st.name)
+
         LOG.info(_LI('Deleting stack %s'), st.name)
         stack = parser.Stack.load(cnxt, stack=st)
         self.resource_enforcer.enforce_stack(stack)

heat/tests/test_engine_service.py

@@ -975,6 +975,17 @@ class StackServiceTest(common.HeatTestCase):
         outputs = self.eng.list_outputs(self.ctx, mock.ANY)
         self.assertEqual([], outputs)
 
+    def test_stack_delete_complete_is_not_found(self):
+        mock_get_stack = self.patchobject(self.eng, '_get_stack')
+        mock_get_stack.return_value = mock.MagicMock()
+        mock_get_stack.return_value.status = parser.Stack.COMPLETE
+        mock_get_stack.return_value.action = parser.Stack.DELETE
+        ex = self.assertRaises(dispatcher.ExpectedException,
+                               self.eng.delete_stack,
+                               'irrelevant',
+                               'irrelevant')
+        self.assertEqual(exception.EntityNotFound, ex.exc_info[0])
+
     def test_get_environment(self):
         # Setup
         t = template_format.parse(tools.wp_template)