Fix empty config error message

Broken by bc819cb, which merged shortly before 8c2b6c1.

Change-Id: I5b95993ee69a2082b3290aafe93643855ff7f56a

rally/task/engine.py

@@ -185,11 +185,6 @@ class TaskEngine(object):
         :param abort_on_sla_failure: True if the execution should be stopped
                                      when some SLA check fails
         """
-        if config is None:
-            msg = _("Input task is empty")
-            task.set_failed(log=msg)
-            raise exceptions.InvalidTaskException(msg)
-
         try:
             self.config = TaskConfig(config)
         except Exception as e:
@@ -456,6 +451,13 @@ class TaskConfig(object):
 
         :param config: Dict with configuration of specified task
         """
+        if config is None:
+            # NOTE(stpierre): This gets reraised as
+            # InvalidTaskException. if we raise it here as
+            # InvalidTaskException, then "Task config is invalid: "
+            # gets prepended to the message twice.
+            raise Exception(_("Input task is empty"))
+
         self.version = self._get_version(config)
         self._validate_version()
         self._validate_json(config)

tests/functional/test_cli_task.py

@@ -164,6 +164,13 @@ class TaskTestCase(unittest.TestCase):
         detailed_iterations_data = rally("task detailed --iterations-data")
         self.assertNotIn("n/a", detailed_iterations_data)
 
+    def test_start_with_empty_config(self):
+        rally = utils.Rally()
+        config = utils.TaskConfig(None)
+        with self.assertRaises(utils.RallyCliError) as err:
+            rally("task start --task %s" % config.filename)
+        self.assertIn("Input task is empty", err.exception.output)
+
     def test_results(self):
         rally = utils.Rally()
         cfg = self._get_sample_task_config()

tests/unit/task/test_engine.py

@@ -44,8 +44,7 @@ class TaskEngineTestCase(test.TestCase):
         self.assertEqual(eng.config, fake_task_instance)
         self.assertEqual(eng.task, task)
 
-    @mock.patch("rally.task.engine.TaskConfig")
-    def test_init_empty_config(self, mock_task_config):
+    def test_init_empty_config(self):
         config = None
         task = mock.Mock()
         exception = self.assertRaises(exceptions.InvalidTaskException,