# Source code for distributed.threadpoolexecutor

    """
    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 __future__ import annotations

    import itertools
    import logging
    import os
    import queue
    import threading

    from distributed import _concurrent_futures_thread as thread
    from distributed.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)
            raise
        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()

    """
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       the Individual or Organization ("Licensee") accessing and otherwise using Python
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