docstr: Ensures worker and preemption threads are closed.
code: def _ensure_threads_closed(self):
      """Ensures worker and preemption threads are closed."""
      # Worker and preemption threads should exist before releasing
      # ClusterCoordinator.
      running_threads = test_util.get_running_threads()
      self.assertTrue(
          test_util.has_thread(_WORKER_THREAD_PREFIX, running_threads)
      )
      self.assertIn(_WORKER_PREEMPTION_THREAD_NAME, running_threads)
  
      # Print object graph if ClusterCoordinator may leak.
      if sys.getrefcount(self.cluster_coord) > 2:
          try:
              test_util.show_backref(self.cluster_coord)
          except:  # pylint: disable=bare-except
              pass
  
      # Wait for threads to close.
      self.cluster_coord = None
      self.strategy = None
      gc.collect()
      time.sleep(1)
  
      # Verify thread names.
      running_threads = test_util.get_running_threads()
      self.assertNotIn(_WORKER_PREEMPTION_THREAD_NAME, running_threads)
      self.assertFalse(
          test_util.has_thread(_WORKER_THREAD_PREFIX, running_threads),
          "Worker thread is not stopped properly.",
      )
  
imports: import gc
  import os
  import sys
  import threading
  import time
  from tensorflow.python.data.ops import dataset_ops
  from tensorflow.python.distribute import multi_worker_test_base
  from tensorflow.python.distribute import parameter_server_strategy_v2
  from tensorflow.python.distribute import test_util
  from tensorflow.python.distribute.cluster_resolver.cluster_resolver import SimpleClusterResolver
  from tensorflow.python.distribute.coordinator import cluster_coordinator
  from tensorflow.python.eager import context
  from tensorflow.python.eager import def_function
  from tensorflow.python.framework import dtypes
  from tensorflow.python.framework import errors
  from tensorflow.python.framework import ops
  from tensorflow.python.framework import tensor
  from tensorflow.python.ops import array_ops
  from tensorflow.python.ops import check_ops
  from tensorflow.python.ops import math_ops
  from tensorflow.python.ops import random_ops
  from tensorflow.python.ops import variables
  from tensorflow.python.platform import tf_logging
  from tensorflow.python.training import coordinator
  from tensorflow.python.training import server_lib
label_desc: Use image node_id 5 for calling the BaseFaultToleranceTest obj's underlying member method code with example usage: obj._ensure_threads_closed() without return types