code: def test_launch_fiber_background_thread(self):
      # reset global variable
      fiber.popen_fiber_spawn._fiber_background_thread = None
  
      fp = io.StringIO()
      handler = logging.StreamHandler(fp)
      logger = logging.getLogger("fiber")
      logger.setLevel(level=logging.DEBUG)
      logger.addHandler(handler)
  
      try:
          procs = []
          for i in range(4):
              p = fiber.Process(
                  target=time.sleep,
                  args=(1,),
                  name="test_launch_fiber_background_thread_{}".format(
                      i
                  ),
              )
              procs.append(p)
          for p in procs:
              p.start()
  
          logs = fp.getvalue()
          times = len(
              list(re.finditer("creating background thread", logs))
          )
          assert times == 1
      finally:
          for p in procs:
              p.join()
          logger.removeHandler(handler)
  
file_path: fiber/tests/test_popen.py
imports: import multiprocessing
  import io
  import re
  import time
  import logging
  import socket
  import fiber
  import fiber.popen_fiber_spawn
  import docker
  import pytest
label_desc: Use image node_id 6 for calling the TestPopen obj's underlying member method code with example usage: obj.test_launch_fiber_background_thread() without return types