code: def test_timer_calls_background_thread_at_regular_intervals(
      self,
  ) -> None:
      timer_begin_time: float
  
      # Call our function every 200ms.
      call_interval = 0.2
  
      # Keep the log of intervals between each consecutive call.
      actual_call_intervals: List[float] = []
  
      # Keep the number of times the function was called.
      call_count = 0
  
      # In order to prevent a flaky test instead of asserting that the
      # function was called an exact number of times we use a lower bound
      # that is guaranteed to be true for a correct implementation.
      min_required_call_count = 4
  
      timer_stop_event = threading.Event()
  
      def log_call(self):
          nonlocal timer_begin_time, call_count
  
          actual_call_intervals.append(
              time.monotonic() - timer_begin_time
          )
  
          call_count += 1
          if call_count == min_required_call_count:
              timer_stop_event.set()
  
          timer_begin_time = time.monotonic()
  
      timer = _PeriodicTimer(
          timedelta(seconds=call_interval), log_call, self
      )
  
      timer_begin_time = time.monotonic()
  
      timer.start()
  
      # Although this is theoretically non-deterministic, if our timer, which
      # has a 200ms call interval, does not get called 4 times in 60 seconds,
      # there is very likely something else going on.
      timer_stop_event.wait(60)
  
      timer.cancel()
  
      self.longMessage = False
  
      self.assertGreaterEqual(
          call_count,
          min_required_call_count,
          f"The function has been called {call_count} time(s) but expected to be called at least "
          f"{min_required_call_count} time(s).",
      )
  
      for actual_call_interval in actual_call_intervals:
          self.assertGreaterEqual(
              actual_call_interval,
              call_interval,
              f"The interval between two function calls was {actual_call_interval} second(s) but "
              f"expected to be at least {call_interval} second(s).",
          )
  
imports: import threading
  import time
  import socket
  from datetime import timedelta
  from typing import List
  from unittest import TestCase
  from unittest.mock import patch
  from torch.distributed.elastic.rendezvous.utils import _PeriodicTimer, _delay, _matches_machine_hostname, _parse_rendezvous_config, _try_parse_port, parse_rendezvous_endpoint
label_desc: Use image node_id 6 for calling the PeriodicTimerTest obj's underlying member method code with example usage: obj.test_timer_calls_background_thread_at_regular_intervals() without return types