message: prevent restart of dispatcher w/o fault-tolerance
diff: @@ -25,6 +25,7 @@ from tensorflow.python.data.kernel_tests import test_base
  from tensorflow.python.data.ops import dataset_ops
  from tensorflow.python.framework import combinations
  from tensorflow.python.platform import googletest
  +from tensorflow.python.framework import errors
  # This will be resolved to a tmp directory by `start_dispatch_server`.
  TMP_WORK_DIR = "tmp_work_dir_placeholder"
  @@ -115,7 +116,16 @@ class TestCluster(object):
  # pylint: disable=protected-access
  def restart_dispatcher(self):
  - """Stops `dispatcher` and creates a new dispatcher with the same port."""
  + """Stops `dispatcher` and creates a new dispatcher with the same port.
  +
  + When the dispatcher is restarted with `fault-tolerant_mode=False`,
  + the new dispatcher is unable to find the `dataset_id` of the already
  + created dataset and thus perpetually throws a warn message until it
  + times out. Thus, the test bed prevents this scenario.
  + """
  + if not self.dispatcher._config.fault_tolerant_mode:
  + raise ValueError(
  + "Trying to restart the dispatcher without fault-tolerance.")
  port = int(self.dispatcher_address().split(":")[1])
  self.dispatcher._stop()
  self.dispatcher = server_lib.DispatchServer(
  