text: """
  Modified ThreadPoolExecutor to support threads leaving the thread pool
  
  This includes a global `secede` method that a submitted function can call to
  have its thread leave the ThreadPoolExecutor's thread pool.  This allows the
  thread pool to allocate another thread if necessary and so is useful when a
  function realises that it is going to be a long-running job that doesn't want
  to take up space.  When the function finishes its thread will terminate
  gracefully.
  
  This code copies and modifies two functions from the
  `concurrent.futures.thread` module, notably `_worker` and
  ThreadPoolExecutor._adjust_thread_count` to allow for checking against a global
  `threading.local` state.  These functions are subject to the following license,
  which is included as a comment at the end of this file:
  
      https://docs.python.org/3/license.html
  
  ... and are under copyright by the Python Software Foundation
  
     Copyright 2001-2016 Python Software Foundation; All Rights Reserved
  """
  from . import _concurrent_futures_thread as thread
  import os
  import logging
  import queue
  import threading
  import itertools
  
  from .metrics import time
  
  logger = logging.getLogger(__name__)
  
  thread_state = threading.local()
  
  
  def _worker(executor, work_queue):
      thread_state.proceed = True
      thread_state.executor = executor
  
      try:
          while thread_state.proceed:
              with executor._rejoin_lock:
                  if executor._rejoin_list:
                      rejoin_thread, rejoin_event = executor._rejoin_list.pop()
                      executor._threads.add(rejoin_thread)
                      executor._threads.remove(threading.current_thread())
                      rejoin_event.set()
                      break
              try:
                  task = work_queue.get(timeout=1)
              except queue.Empty:
                  continue
              if task is not None:  # sentinel
                  task.run()
                  del task
              elif thread._shutdown or executor is None or executor._shutdown:
                  work_queue.put(None)
                  return
          del executor
      except BaseException:
          logger.critical("Exception in worker", exc_info=True)
      finally:
          del thread_state.proceed
          del thread_state.executor
  
  
  class ThreadPoolExecutor(thread.ThreadPoolExecutor):
      # Used to assign unique thread names
      _counter = itertools.count()
  
      def __init__(self, *args, **kwargs):
          super().__init__(*args, **kwargs)
          self._rejoin_list = []
          self._rejoin_lock = threading.Lock()
          self._thread_name_prefix = kwargs.get(
              "thread_name_prefix", "DaskThreadPoolExecutor"
          )
  
      def _adjust_thread_count(self):
          if len(self._threads) < self._max_workers:
              t = threading.Thread(
                  target=_worker,
                  name=self._thread_name_prefix
                  + "-%d-%d" % (os.getpid(), next(self._counter)),
                  args=(self, self._work_queue),
              )
              t.daemon = True
              self._threads.add(t)
              t.start()
  
      def shutdown(self, wait=True, timeout=None):
          with threads_lock:
              with self._shutdown_lock:
                  self._shutdown = True
                  self._work_queue.put(None)
              if timeout is not None:
                  deadline = time() + timeout
              if wait:
                  for t in self._threads:
                      if timeout is not None:
                          timeout2 = max(deadline - time(), 0)
                      else:
                          timeout2 = None
                      t.join(timeout=timeout2)
  
  
  def secede(adjust=True):
      """Have this thread secede from the ThreadPoolExecutor
  
      See Also
      --------
      rejoin: rejoin the thread pool
      """
      thread_state.proceed = False
      with threads_lock:
          thread_state.executor._threads.remove(threading.current_thread())
          if adjust:
              thread_state.executor._adjust_thread_count()
  
  
  def rejoin():
      """Have this thread rejoin the ThreadPoolExecutor
  
      This will block until a new slot opens up in the executor.  The next thread
      to finish a task will leave the pool to allow this one to join.
  
      See Also
      --------
      secede: leave the thread pool
      """
      thread = threading.current_thread()
      event = threading.Event()
      e = thread_state.executor
      with e._rejoin_lock:
          e._rejoin_list.append((thread, event))
      e.submit(lambda: None)
      event.wait()
      thread_state.proceed = True
  
  
  threads_lock = threading.Lock()
  
  """
  PSF LICENSE AGREEMENT FOR PYTHON 3.5.2
  ======================================
  
  1. This LICENSE AGREEMENT is between the Python Software Foundation ("PSF"), and
     the Individual or Organization ("Licensee") accessing and otherwise using Python
     3.5.2 software in source or binary form and its associated documentation.
  
  2. Subject to the terms and conditions of this License Agreement, PSF hereby
     grants Licensee a nonexclusive, royalty-free, world-wide license to reproduce,
     analyze, test, perform and/or display publicly, prepare derivative works,
     distribute, and otherwise use Python 3.5.2 alone or in any derivative
     version, provided, however, that PSF's License Agreement and PSF's notice of
     copyright, i.e., "Copyright c 2001-2016 Python Software Foundation; All Rights
     Reserved" are retained in Python 3.5.2 alone or in any derivative version
     prepared by Licensee.
  
  3. In the event Licensee prepares a derivative work that is based on or
     incorporates Python 3.5.2 or any part thereof, and wants to make the
     derivative work available to others as provided herein, then Licensee hereby
     agrees to include in any such work a brief summary of the changes made to Python
     3.5.2.
  
  4. PSF is making Python 3.5.2 available to Licensee on an "AS IS" basis.
     PSF MAKES NO REPRESENTATIONS OR WARRANTIES, EXPRESS OR IMPLIED.  BY WAY OF
     EXAMPLE, BUT NOT LIMITATION, PSF MAKES NO AND DISCLAIMS ANY REPRESENTATION OR
     WARRANTY OF MERCHANTABILITY OR FITNESS FOR ANY PARTICULAR PURPOSE OR THAT THE
     USE OF PYTHON 3.5.2 WILL NOT INFRINGE ANY THIRD PARTY RIGHTS.
  
  5. PSF SHALL NOT BE LIABLE TO LICENSEE OR ANY OTHER USERS OF PYTHON 3.5.2
     FOR ANY INCIDENTAL, SPECIAL, OR CONSEQUENTIAL DAMAGES OR LOSS AS A RESULT OF
     MODIFYING, DISTRIBUTING, OR OTHERWISE USING PYTHON 3.5.2, OR ANY DERIVATIVE
     THEREOF, EVEN IF ADVISED OF THE POSSIBILITY THEREOF.
  
  6. This License Agreement will automatically terminate upon a material breach of
     its terms and conditions.
  
  7. Nothing in this License Agreement shall be deemed to create any relationship
     of agency, partnership, or joint venture between PSF and Licensee.  This License
     Agreement does not grant permission to use PSF trademarks or trade name in a
     trademark sense to endorse or promote products or services of Licensee, or any
     third party.
  
  8. By copying, installing or otherwise using Python 3.5.2, Licensee agrees
     to be bound by the terms and conditions of this License Agreement.
  """
  