# Source code for loky.process_executor

    ###############################################################################
    # Re-implementation of the ProcessPoolExecutor more robust to faults
    #
    # author: Thomas Moreau and Olivier Grisel
    #
    # adapted from concurrent/futures/process_pool_executor.py (17/02/2017)
    #  * Add an extra management thread to detect executor_manager_thread failures,
    #  * Improve the shutdown process to avoid deadlocks,
    #  * Add timeout for workers,
    #  * More robust pickling process.
    #
    # Copyright 2009 Brian Quinlan. All Rights Reserved.
    # Licensed to PSF under a Contributor Agreement.

    """Implements ProcessPoolExecutor.

    The follow diagram and text describe the data-flow through the system:

    |======================= In-process =====================|== Out-of-process ==|

    +----------+     +----------+       +--------+     +-----------+    +---------+
    |          |  => | Work Ids |       |        |     | Call Q    |    | Process |
    |          |     +----------+       |        |     +-----------+    |  Pool   |
    |          |     | ...      |       |        |     | ...       |    +---------+
    |          |     | 6        |    => |        |  => | 5, call() | => |         |
    |          |     | 7        |       |        |     | ...       |    |         |
    | Process  |     | ...      |       | Local  |     +-----------+    | Process |
    |  Pool    |     +----------+       | Worker |                      |  #1..n  |
    | Executor |                        | Thread |                      |         |
    |          |     +----------- +     |        |     +-----------+    |         |
    |          | <=> | Work Items | <=> |        | <=  | Result Q  | <= |         |
    |          |     +------------+     |        |     +-----------+    |         |
    |          |     | 6: call()  |     |        |     | ...       |    |         |
    |          |     |    future  |     +--------+     | 4, result |    |         |
    |          |     | ...        |                    | 3, except |    |         |
    +----------+     +------------+                    +-----------+    +---------+

    Executor.submit() called:
    - creates a uniquely numbered _WorkItem and adds it to the "Work Items" dict
    - adds the id of the _WorkItem to the "Work Ids" queue

    Local worker thread:
    - reads work ids from the "Work Ids" queue and looks up the corresponding
      WorkItem from the "Work Items" dict: if the work item has been cancelled then
      it is simply removed from the dict, otherwise it is repackaged as a
      _CallItem and put in the "Call Q". New _CallItems are put in the "Call Q"
      until "Call Q" is full. NOTE: the size of the "Call Q" is kept small because
      calls placed in the "Call Q" can no longer be cancelled with Future.cancel().
    - reads _ResultItems from "Result Q", updates the future stored in the
      "Work Items" dict and deletes the dict entry

    Process #1..n:
    - reads _CallItems from "Call Q", executes the calls, and puts the resulting
      _ResultItems in "Result Q"
    """

    __author__ = "Thomas Moreau (thomas.moreau.2010@gmail.com)"

    import faulthandler
    import os
    import gc
    import sys
    import queue
    import struct
    import weakref
    import warnings
    import itertools
    import traceback
    import threading
    from time import time, sleep
    import multiprocessing as mp
    from functools import partial
    from pickle import PicklingError
    from concurrent.futures import Executor
    from concurrent.futures._base import LOGGER
    from concurrent.futures.process import BrokenProcessPool as _BPPException
    from multiprocessing.connection import wait

    from ._base import Future
    from .backend import get_context
    from .backend.context import cpu_count, _MAX_WINDOWS_WORKERS
    from .backend.queues import Queue, SimpleQueue
    from .backend.reduction import set_loky_pickler, get_loky_pickler_name
    from .backend.utils import kill_process_tree, get_exitcodes_terminated_worker
    from .initializers import _prepare_initializer

    # Mechanism to prevent infinite process spawning. When a worker of a
    # ProcessPoolExecutor nested in MAX_DEPTH Executor tries to create a new
    # Executor, a LokyRecursionError is raised
    MAX_DEPTH = int(os.environ.get("LOKY_MAX_DEPTH", 10))
    _CURRENT_DEPTH = 0

    # Minimum time interval between two consecutive memory leak protection checks.
    _MEMORY_LEAK_CHECK_DELAY = 1.0

    # Number of bytes of memory usage allowed over the reference process size.
    _MAX_MEMORY_LEAK_SIZE = int(3e8)

    try:
        from psutil import Process

        _USE_PSUTIL = True

        def _get_memory_usage(pid, force_gc=False):
            if force_gc:
                gc.collect()

            mem_size = Process(pid).memory_info().rss
            mp.util.debug(f"psutil return memory size: {mem_size}")
            return mem_size

    except ImportError:
        _USE_PSUTIL = False

    class _ThreadWakeup:
        def __init__(self):
            self._closed = False
            self._reader, self._writer = mp.Pipe(duplex=False)

        def close(self):
            if not self._closed:
                self._closed = True
                self._writer.close()
                self._reader.close()

        def wakeup(self):
            if not self._closed:
                self._writer.send_bytes(b"")

        def clear(self):
            if not self._closed:
                while self._reader.poll():
                    self._reader.recv_bytes()

    class _ExecutorFlags:
        """necessary references to maintain executor states without preventing gc

        It permits to keep the information needed by executor_manager_thread
        and crash_detection_thread to maintain the pool without preventing the
        garbage collection of unreferenced executors.
        """

        def __init__(self, shutdown_lock):

            self.shutdown = False
            self.broken = None
            self.kill_workers = False
            self.shutdown_lock = shutdown_lock

        def flag_as_shutting_down(self, kill_workers=None):
            with self.shutdown_lock:
                self.shutdown = True
                if kill_workers is not None:
                    self.kill_workers = kill_workers

        def flag_as_broken(self, broken):
            with self.shutdown_lock:
                self.shutdown = True
                self.broken = broken

    # Prior to 3.9, executor_manager_thread is created as daemon thread. This means
    # that it is not joined automatically when the interpreter is shutting down.
    # To work around this problem, an exit handler is installed to tell the
    # thread to exit when the interpreter is shutting down and then waits until
    # it finishes. The thread needs to be daemonized because the atexit hooks are
    # called after all non daemonized threads are joined.
    #
    # Starting 3.9, there exists a specific atexit hook to be called before joining
    # the threads so the executor_manager_thread does not need to be daemonized
    # anymore.
    #
    # The atexit hooks are registered when starting the first ProcessPoolExecutor
    # to avoid import having an effect on the interpreter.

    _global_shutdown = False
    _global_shutdown_lock = threading.Lock()
    _threads_wakeups = weakref.WeakKeyDictionary()

    def _python_exit():
        global _global_shutdown
        _global_shutdown = True

        # Materialize the list of items to avoid error due to iterating over
        # changing size dictionary.
        items = list(_threads_wakeups.items())
        if len(items) > 0:
            mp.util.debug(
                f"Interpreter shutting down. Waking up {len(items)}"
                f"executor_manager_thread:\n{items}"
            )

        # Wake up the executor_manager_thread's so they can detect the interpreter
        # is shutting down and exit.
        for _, (shutdown_lock, thread_wakeup) in items:
            with shutdown_lock:
                thread_wakeup.wakeup()

        # Collect the executor_manager_thread's to make sure we exit cleanly.
        for thread, _ in items:
            # This locks is to prevent situations where an executor is gc'ed in one
            # thread while the atexit finalizer is running in another thread.
            with _global_shutdown_lock:
                thread.join()

    # With the fork context, _thread_wakeups is propagated to children.
    # Clear it after fork to avoid some situation that can cause some
    # freeze when joining the workers.
    mp.util.register_after_fork(_threads_wakeups, lambda obj: obj.clear())

    # Module variable to register the at_exit call
    process_pool_executor_at_exit = None

    # Controls how many more calls than processes will be queued in the call queue.
    # A smaller number will mean that processes spend more time idle waiting for
    # work while a larger number will make Future.cancel() succeed less frequently
    # (Futures in the call queue cannot be cancelled).
    EXTRA_QUEUED_CALLS = 1

    class _RemoteTraceback(Exception):
        """Embed stringification of remote traceback in local traceback"""

        def __init__(self, tb=None):
            self.tb = f'\n"""\n{tb}"""'

        def __str__(self):
            return self.tb

    # Do not inherit from BaseException to mirror
    # concurrent.futures.process._ExceptionWithTraceback
    class _ExceptionWithTraceback:
        def __init__(self, exc):
            tb = getattr(exc, "__traceback__", None)
            if tb is None:
                _, _, tb = sys.exc_info()
            tb = traceback.format_exception(type(exc), exc, tb)
            tb = "".join(tb)
            self.exc = exc
            self.tb = tb

        def __reduce__(self):
            return _rebuild_exc, (self.exc, self.tb)

    def _rebuild_exc(exc, tb):
        exc.__cause__ = _RemoteTraceback(tb)
        return exc

    class _WorkItem:

        __slots__ = ["future", "fn", "args", "kwargs"]

        def __init__(self, future, fn, args, kwargs):
            self.future = future
            self.fn = fn
            self.args = args
            self.kwargs = kwargs

    class _ResultItem:
        def __init__(self, work_id, exception=None, result=None):
            self.work_id = work_id
            self.exception = exception
            self.result = result

    class _CallItem:
        def __init__(self, work_id, fn, args, kwargs):
            self.work_id = work_id
            self.fn = fn
            self.args = args
            self.kwargs = kwargs

            # Store the current loky_pickler so it is correctly set in the worker
            self.loky_pickler = get_loky_pickler_name()

        def __call__(self):
            set_loky_pickler(self.loky_pickler)
            return self.fn(*self.args, **self.kwargs)

        def __repr__(self):
            return (
                f"CallItem({self.work_id}, {self.fn}, {self.args}, {self.kwargs})"
            )

    class _SafeQueue(Queue):
        """Safe Queue set exception to the future object linked to a job"""

        def __init__(
            self,
            max_size=0,
            ctx=None,
            pending_work_items=None,
            running_work_items=None,
            thread_wakeup=None,
            shutdown_lock=None,
            reducers=None,
        ):
            self.thread_wakeup = thread_wakeup
            self.shutdown_lock = shutdown_lock
            self.pending_work_items = pending_work_items
            self.running_work_items = running_work_items
            super().__init__(max_size, reducers=reducers, ctx=ctx)

        def _on_queue_feeder_error(self, e, obj):
            if isinstance(obj, _CallItem):
                # format traceback only works on python3
                if isinstance(e, struct.error):
                    raised_error = RuntimeError(
                        "The task could not be sent to the workers as it is too "
                        "large for `send_bytes`."
                    )
                else:
                    raised_error = PicklingError(
                        "Could not pickle the task to send it to the workers."
                    )
                tb = traceback.format_exception(
                    type(e), e, getattr(e, "__traceback__", None)
                )
                raised_error.__cause__ = _RemoteTraceback("".join(tb))
                work_item = self.pending_work_items.pop(obj.work_id, None)
                self.running_work_items.remove(obj.work_id)
                # work_item can be None if another process terminated. In this
                # case, the executor_manager_thread fails all work_items with
                # BrokenProcessPool
                if work_item is not None:
                    work_item.future.set_exception(raised_error)
                    del work_item
                with self.shutdown_lock:
                    self.thread_wakeup.wakeup()
            else:
                super()._on_queue_feeder_error(e, obj)

    def _get_chunks(chunksize, *iterables):
        """Iterates over zip()ed iterables in chunks."""
        it = zip(*iterables)
        while True:
            chunk = tuple(itertools.islice(it, chunksize))
            if not chunk:
                return
            yield chunk

    def _process_chunk(fn, chunk):
        """Processes a chunk of an iterable passed to map.

        Runs the function passed to map() on a chunk of the
        iterable passed to map.

        This function is run in a separate process.

        """
        return [fn(*args) for args in chunk]

    def _sendback_result(result_queue, work_id, result=None, exception=None):
        """Safely send back the given result or exception"""
        try:
            result_queue.put(
                _ResultItem(work_id, result=result, exception=exception)
            )
        except BaseException as e:
            exc = _ExceptionWithTraceback(e)
            result_queue.put(_ResultItem(work_id, exception=exc))

    def _enable_faulthandler_if_needed():
        if "PYTHONFAULTHANDLER" in os.environ:
            # Respect the environment variable to configure faulthandler. This
            # makes it possible to never enable faulthandler in the loky workers by
            # setting PYTHONFAULTHANDLER=0 explicitly in the environment.
            mp.util.debug(
                f"faulthandler explicitly configured by environment variable: "
                f"PYTHONFAULTHANDLER={os.environ['PYTHONFAULTHANDLER']}."
            )
        else:
            if faulthandler.is_enabled():
                # Fault handler is already enabled, possibly via a custom
                # initializer to customize the behavior.
                mp.util.debug("faulthandler already enabled.")
            else:
                # Enable faulthandler by default with default paramaters otherwise.
                mp.util.debug(
                    "Enabling faulthandler to report tracebacks on worker crashes."
                )
                faulthandler.enable()

    def _process_worker(
        call_queue,
        result_queue,
        initializer,
        initargs,
        processes_management_lock,
        timeout,
        worker_exit_lock,
        current_depth,
    ):
        """Evaluates calls from call_queue and places the results in result_queue.

        This worker is run in a separate process.

        Args:
            call_queue: A ctx.Queue of _CallItems that will be read and
                evaluated by the worker.
            result_queue: A ctx.Queue of _ResultItems that will written
                to by the worker.
            initializer: A callable initializer, or None
            initargs: A tuple of args for the initializer
            processes_management_lock: A ctx.Lock avoiding worker timeout while
                some workers are being spawned.
            timeout: maximum time to wait for a new item in the call_queue. If that
                time is expired, the worker will shutdown.
            worker_exit_lock: Lock to avoid flagging the executor as broken on
                workers timeout.
            current_depth: Nested parallelism level, to avoid infinite spawning.
        """
        if initializer is not None:
            try:
                initializer(*initargs)
            except BaseException:
                LOGGER.critical("Exception in initializer:", exc_info=True)
                # The parent will notice that the process stopped and
                # mark the pool broken
                return

        # set the global _CURRENT_DEPTH mechanism to limit recursive call
        global _CURRENT_DEPTH
        _CURRENT_DEPTH = current_depth
        _process_reference_size = None
        _last_memory_leak_check = None
        pid = os.getpid()

        mp.util.debug(f"Worker started with timeout={timeout}")
        _enable_faulthandler_if_needed()

        while True:
            try:
                call_item = call_queue.get(block=True, timeout=timeout)
                if call_item is None:
                    mp.util.info("Shutting down worker on sentinel")
            except queue.Empty:
                mp.util.info(f"Shutting down worker after timeout {timeout:0.3f}s")
                if processes_management_lock.acquire(block=False):
                    processes_management_lock.release()
                    call_item = None
                else:
                    mp.util.info("Could not acquire processes_management_lock")
                    continue
            except BaseException:
                previous_tb = traceback.format_exc()
                try:
                    result_queue.put(_RemoteTraceback(previous_tb))
                except BaseException:
                    # If we cannot format correctly the exception, at least print
                    # the traceback.
                    print(previous_tb)
                mp.util.debug("Exiting with code 1")
                sys.exit(1)
            if call_item is None:
                # Notify queue management thread about worker shutdown
                result_queue.put(pid)
                is_clean = worker_exit_lock.acquire(True, timeout=30)

                # Early notify any loky executor running in this worker process
                # (nested parallelism) that this process is about to shutdown to
                # avoid a deadlock waiting undifinitely for the worker to finish.
                _python_exit()

                if is_clean:
                    mp.util.debug("Exited cleanly")
                else:
                    mp.util.info("Main process did not release worker_exit")
                return
            try:
                r = call_item()
            except BaseException as e:
                exc = _ExceptionWithTraceback(e)
                result_queue.put(_ResultItem(call_item.work_id, exception=exc))
            else:
                _sendback_result(result_queue, call_item.work_id, result=r)
                del r

            # Free the resource as soon as possible, to avoid holding onto
            # open files or shared memory that is not needed anymore
            del call_item

            if _USE_PSUTIL:
                if _process_reference_size is None:
                    # Make reference measurement after the first call
                    _process_reference_size = _get_memory_usage(pid, force_gc=True)
                    _last_memory_leak_check = time()
                    continue
                if time() - _last_memory_leak_check > _MEMORY_LEAK_CHECK_DELAY:
                    mem_usage = _get_memory_usage(pid)
                    _last_memory_leak_check = time()
                    if mem_usage - _process_reference_size < _MAX_MEMORY_LEAK_SIZE:
                        # Memory usage stays within bounds: everything is fine.
                        continue

                    # Check again memory usage; this time take the measurement
                    # after a forced garbage collection to break any reference
                    # cycles.
                    mem_usage = _get_memory_usage(pid, force_gc=True)
                    _last_memory_leak_check = time()
                    if mem_usage - _process_reference_size < _MAX_MEMORY_LEAK_SIZE:
                        # The GC managed to free the memory: everything is fine.
                        continue

                    # The process is leaking memory: let the master process
                    # know that we need to start a new worker.
                    mp.util.info("Memory leak detected: shutting down worker")
                    result_queue.put(pid)
                    with worker_exit_lock:
                        mp.util.debug("Exit due to memory leak")
                        return
            else:
                # if psutil is not installed, trigger gc.collect events
                # regularly to limit potential memory leaks due to reference cycles
                if _last_memory_leak_check is None or (
                    time() - _last_memory_leak_check > _MEMORY_LEAK_CHECK_DELAY
                ):
                    gc.collect()
                    _last_memory_leak_check = time()

    class _ExecutorManagerThread(threading.Thread):
        """Manages the communication between this process and the worker processes.

        The manager is run in a local thread.

        Args:
            executor: A reference to the ProcessPoolExecutor that owns
                this thread. A weakref will be own by the manager as well as
                references to internal objects used to introspect the state of
                the executor.
        """

        def __init__(self, executor):
            # Store references to necessary internals of the executor.

            # A _ThreadWakeup to allow waking up the executor_manager_thread from
            # the main Thread and avoid deadlocks caused by permanently
            # locked queues.
            self.thread_wakeup = executor._executor_manager_thread_wakeup
            self.shutdown_lock = executor._shutdown_lock

            # A weakref.ref to the ProcessPoolExecutor that owns this thread. Used
            # to determine if the ProcessPoolExecutor has been garbage collected
            # and that the manager can exit.
            # When the executor gets garbage collected, the weakref callback
            # will wake up the queue management thread so that it can terminate
            # if there is no pending work item.
            def weakref_cb(
                _,
                thread_wakeup=self.thread_wakeup,
                shutdown_lock=self.shutdown_lock,
            ):
                if mp is not None:
                    # At this point, the multiprocessing module can already be
                    # garbage collected. We only log debug info when still
                    # possible.
                    mp.util.debug(
                        "Executor collected: triggering callback for"
                        " QueueManager wakeup"
                    )
                with shutdown_lock:
                    thread_wakeup.wakeup()

            self.executor_reference = weakref.ref(executor, weakref_cb)

            # The flags of the executor
            self.executor_flags = executor._flags

            # A list of the ctx.Process instances used as workers.
            self.processes = executor._processes

            # A ctx.Queue that will be filled with _CallItems derived from
            # _WorkItems for processing by the process workers.
            self.call_queue = executor._call_queue

            # A ctx.SimpleQueue of _ResultItems generated by the process workers.
            self.result_queue = executor._result_queue

            # A queue.Queue of work ids e.g. Queue([5, 6, ...]).
            self.work_ids_queue = executor._work_ids

            # A dict mapping work ids to _WorkItems e.g.
            #     {5: <_WorkItem...>, 6: <_WorkItem...>, ...}
            self.pending_work_items = executor._pending_work_items

            # A list of the work_ids that are currently running
            self.running_work_items = executor._running_work_items

            # A lock to avoid concurrent shutdown of workers on timeout and spawn
            # of new processes or shut down
            self.processes_management_lock = executor._processes_management_lock

            super().__init__(name="ExecutorManagerThread")
            if sys.version_info < (3, 9):
                self.daemon = True

        def run(self):
            # Main loop for the executor manager thread.

            while True:
                self.add_call_item_to_queue()

                result_item, is_broken, bpe = self.wait_result_broken_or_wakeup()

                if is_broken:
                    self.terminate_broken(bpe)
                    return
                if result_item is not None:
                    self.process_result_item(result_item)
                    # Delete reference to result_item to avoid keeping references
                    # while waiting on new results.
                    del result_item

                if self.is_shutting_down():
                    self.flag_executor_shutting_down()

                    # Since no new work items can be added, it is safe to shutdown
                    # this thread if there are no pending work items.
                    if not self.pending_work_items:
                        self.join_executor_internals()
                        return

        def add_call_item_to_queue(self):
            # Fills call_queue with _WorkItems from pending_work_items.
            # This function never blocks.
            while True:
                if self.call_queue.full():
                    return
                try:
                    work_id = self.work_ids_queue.get(block=False)
                except queue.Empty:
                    return
                else:
                    work_item = self.pending_work_items[work_id]

                    if work_item.future.set_running_or_notify_cancel():
                        self.running_work_items += [work_id]
                        self.call_queue.put(
                            _CallItem(
                                work_id,
                                work_item.fn,
                                work_item.args,
                                work_item.kwargs,
                            ),
                            block=True,
                        )
                    else:
                        del self.pending_work_items[work_id]
                        continue

        def wait_result_broken_or_wakeup(self):
            # Wait for a result to be ready in the result_queue while checking
            # that all worker processes are still running, or for a wake up
            # signal send. The wake up signals come either from new tasks being
            # submitted, from the executor being shutdown/gc-ed, or from the
            # shutdown of the python interpreter.
            result_reader = self.result_queue._reader
            wakeup_reader = self.thread_wakeup._reader
            readers = [result_reader, wakeup_reader]
            worker_sentinels = [p.sentinel for p in list(self.processes.values())]
            ready = wait(readers + worker_sentinels)

            bpe = None
            is_broken = True
            result_item = None
            if result_reader in ready:
                try:
                    result_item = result_reader.recv()
                    if isinstance(result_item, _RemoteTraceback):
                        bpe = BrokenProcessPool(
                            "A task has failed to un-serialize. Please ensure that"
                            " the arguments of the function are all picklable."
                        )
                        bpe.__cause__ = result_item
                    else:
                        is_broken = False
                except BaseException as e:
                    bpe = BrokenProcessPool(
                        "A result has failed to un-serialize. Please ensure that "
                        "the objects returned by the function are always "
                        "picklable."
                    )
                    tb = traceback.format_exception(
                        type(e), e, getattr(e, "__traceback__", None)
                    )
                    bpe.__cause__ = _RemoteTraceback("".join(tb))

            elif wakeup_reader in ready:
                # This is simply a wake-up event that might either trigger putting
                # more tasks in the queue or trigger the clean up of resources.
                is_broken = False
            else:
                # A worker has terminated and we don't know why, set the state of
                # the executor as broken
                exit_codes = ""
                if sys.platform != "win32":
                    # In Windows, introspecting terminated workers exitcodes seems
                    # unstable, therefore they are not appended in the exception
                    # message.
                    exit_codes = (
                        "\nThe exit codes of the workers are "
                        f"{get_exitcodes_terminated_worker(self.processes)}"
                    )
                mp.util.debug(
                    "A worker unexpectedly terminated. Workers that "
                    "might have caused the breakage: "
                    + str(
                        {
                            p.name: p.exitcode
                            for p in list(self.processes.values())
                            if p is not None and p.sentinel in ready
                        }
                    )
                )
                bpe = TerminatedWorkerError(
                    "A worker process managed by the executor was unexpectedly "
                    "terminated. This could be caused by a segmentation fault "
                    "while calling the function or by an excessive memory usage "
                    "causing the Operating System to kill the worker.\n"
                    f"{exit_codes}\n"
                    "Detailed tracebacks of the workers should have been printed "
                    "to stderr in the executor process if faulthandler was not "
                    "disabled."
                )

            self.thread_wakeup.clear()

            return result_item, is_broken, bpe

        def process_result_item(self, result_item):
            # Process the received a result_item. This can be either the PID of a
            # worker that exited gracefully or a _ResultItem

            if isinstance(result_item, int):
                # Clean shutdown of a worker using its PID, either on request
                # by the executor.shutdown method or by the timeout of the worker
                # itself: we should not mark the executor as broken.
                with self.processes_management_lock:
                    p = self.processes.pop(result_item, None)

                # p can be None if the executor is concurrently shutting down.
                if p is not None:
                    p._worker_exit_lock.release()
                    mp.util.debug(
                        f"joining {p.name} when processing {p.pid} as result_item"
                    )
                    p.join()
                    del p

                # Make sure the executor have the right number of worker, even if a
                # worker timeout while some jobs were submitted. If some work is
                # pending or there is less processes than running items, we need to
                # start a new Process and raise a warning.
                n_pending = len(self.pending_work_items)
                n_running = len(self.running_work_items)
                if n_pending - n_running > 0 or n_running > len(self.processes):
                    executor = self.executor_reference()
                    if (
                        executor is not None
                        and len(self.processes) < executor._max_workers
                    ):
                        warnings.warn(
                            "A worker stopped while some jobs were given to the "
                            "executor. This can be caused by a too short worker "
                            "timeout or by a memory leak.",
                            UserWarning,
                        )
                        with executor._processes_management_lock:
                            executor._adjust_process_count()
                        executor = None
            else:
                # Received a _ResultItem so mark the future as completed.
                work_item = self.pending_work_items.pop(result_item.work_id, None)
                # work_item can be None if another process terminated (see above)
                if work_item is not None:
                    if result_item.exception:
                        work_item.future.set_exception(result_item.exception)
                    else:
                        work_item.future.set_result(result_item.result)
                    self.running_work_items.remove(result_item.work_id)

        def is_shutting_down(self):
            # Check whether we should start shutting down the executor.
            executor = self.executor_reference()
            # No more work items can be added if:
            #   - The interpreter is shutting down OR
            #   - The executor that owns this thread is not broken AND
            #        * The executor that owns this worker has been collected OR
            #        * The executor that owns this worker has been shutdown.
            # If the executor is broken, it should be detected in the next loop.
            return _global_shutdown or (
                (executor is None or self.executor_flags.shutdown)
                and not self.executor_flags.broken
            )

        def terminate_broken(self, bpe):
            # Terminate the executor because it is in a broken state. The bpe
            # argument can be used to display more information on the error that
            # lead the executor into becoming broken.

            # Mark the process pool broken so that submits fail right now.
            self.executor_flags.flag_as_broken(bpe)

            # Mark pending tasks as failed.
            for work_item in self.pending_work_items.values():
                work_item.future.set_exception(bpe)
                # Delete references to object. See issue16284
                del work_item
            self.pending_work_items.clear()

            # Terminate remaining workers forcibly: the queues or their
            # locks may be in a dirty state and block forever.
            self.kill_workers(reason="broken executor")

            # clean up resources
            self.join_executor_internals()

        def flag_executor_shutting_down(self):
            # Flag the executor as shutting down and cancel remaining tasks if
            # requested as early as possible if it is not gc-ed yet.
            self.executor_flags.flag_as_shutting_down()

            # Cancel pending work items if requested.
            if self.executor_flags.kill_workers:
                while self.pending_work_items:
                    _, work_item = self.pending_work_items.popitem()
                    work_item.future.set_exception(
                        ShutdownExecutorError(
                            "The Executor was shutdown with `kill_workers=True` "
                            "before this job could complete."
                        )
                    )
                    del work_item

                # Kill the remaining worker forcibly to no waste time joining them
                self.kill_workers(reason="executor shutting down")

        def kill_workers(self, reason=""):
            # Terminate the remaining workers using SIGKILL. This function also
            # terminates descendant workers of the children in case there is some
            # nested parallelism.
            while self.processes:
                _, p = self.processes.popitem()
                mp.util.debug(f"terminate process {p.name}, reason: {reason}")
                try:
                    kill_process_tree(p)
                except ProcessLookupError:  # pragma: no cover
                    pass

        def shutdown_workers(self):
            # shutdown all workers in self.processes

            # Create a list to avoid RuntimeError due to concurrent modification of
            # processes. nb_children_alive is thus an upper bound. Also release the
            # processes' _worker_exit_lock to accelerate the shutdown procedure, as
            # there is no need for hand-shake here.
            with self.processes_management_lock:
                n_children_to_stop = 0
                for p in list(self.processes.values()):
                    mp.util.debug(f"releasing worker exit lock on {p.name}")
                    p._worker_exit_lock.release()
                    n_children_to_stop += 1

            mp.util.debug(f"found {n_children_to_stop} processes to stop")

            # Send the right number of sentinels, to make sure all children are
            # properly terminated. Do it with a mechanism that avoid hanging on
            # Full queue when all workers have already been shutdown.
            n_sentinels_sent = 0
            cooldown_time = 0.001
            while (
                n_sentinels_sent < n_children_to_stop
                and self.get_n_children_alive() > 0
            ):
                for _ in range(n_children_to_stop - n_sentinels_sent):
                    try:
                        self.call_queue.put_nowait(None)
                        n_sentinels_sent += 1
                    except queue.Full as e:
                        if cooldown_time > 5.0:
                            mp.util.info(
                                "failed to send all sentinels and exit with error."
                                f"\ncall_queue size={self.call_queue._maxsize}; "
                                f" full is {self.call_queue.full()}; "
                            )
                            raise e
                        mp.util.info(
                            "full call_queue prevented to send all sentinels at "
                            "once, waiting..."
                        )
                        sleep(cooldown_time)
                        cooldown_time *= 1.2
                        break

            mp.util.debug(f"sent {n_sentinels_sent} sentinels to the call queue")

        def join_executor_internals(self):
            self.shutdown_workers()

            # Release the queue's resources as soon as possible. Flag the feeder
            # thread for clean exit to avoid having the crash detection thread flag
            # the Executor as broken during the shutdown. This is safe as either:
            #  * We don't need to communicate with the workers anymore
            #  * There is nothing left in the Queue buffer except None sentinels
            mp.util.debug("closing call_queue")
            self.call_queue.close()
            self.call_queue.join_thread()

            # Closing result_queue
            mp.util.debug("closing result_queue")
            self.result_queue.close()

            mp.util.debug("closing thread_wakeup")
            with self.shutdown_lock:
                self.thread_wakeup.close()

            # If .join() is not called on the created processes then
            # some ctx.Queue methods may deadlock on macOS.
            with self.processes_management_lock:
                mp.util.debug(f"joining {len(self.processes)} processes")
                n_joined_processes = 0
                while True:
                    try:
                        pid, p = self.processes.popitem()
                        mp.util.debug(f"joining process {p.name} with pid {pid}")
                        p.join()
                        n_joined_processes += 1
                    except KeyError:
                        break

                mp.util.debug(
                    "executor management thread clean shutdown of "
                    f"{n_joined_processes} workers"
                )

        def get_n_children_alive(self):
            # This is an upper bound on the number of children alive.
            with self.processes_management_lock:
                return sum(p.is_alive() for p in list(self.processes.values()))

    _system_limits_checked = False
    _system_limited = None

    def _check_system_limits():
        global _system_limits_checked, _system_limited
        if _system_limits_checked and _system_limited:
            raise NotImplementedError(_system_limited)
        _system_limits_checked = True
        try:
            nsems_max = os.sysconf("SC_SEM_NSEMS_MAX")
        except (AttributeError, ValueError):
            # sysconf not available or setting not available
            return
        if nsems_max == -1:
            # undetermined limit, assume that limit is determined
            # by available memory only
            return
        if nsems_max >= 256:
            # minimum number of semaphores available
            # according to POSIX
            return
        _system_limited = (
            f"system provides too few semaphores ({nsems_max} available, "
            "256 necessary)"
        )
        raise NotImplementedError(_system_limited)

    def _chain_from_iterable_of_lists(iterable):
        """
        Specialized implementation of itertools.chain.from_iterable.
        Each item in *iterable* should be a list.  This function is
        careful not to keep references to yielded objects.
        """
        for element in iterable:
            element.reverse()
            while element:
                yield element.pop()

    def _check_max_depth(context):
        # Limit the maxmal recursion level
        global _CURRENT_DEPTH
        if context.get_start_method() == "fork" and _CURRENT_DEPTH > 0:
            raise LokyRecursionError(
                "Could not spawn extra nested processes at depth superior to "
                "MAX_DEPTH=1. It is not possible to increase this limit when "
                "using the 'fork' start method."
            )

        if 0 < MAX_DEPTH and _CURRENT_DEPTH + 1 > MAX_DEPTH:
            raise LokyRecursionError(
                "Could not spawn extra nested processes at depth superior to "
                f"MAX_DEPTH={MAX_DEPTH}. If this is intendend, you can change "
                "this limit with the LOKY_MAX_DEPTH environment variable."
            )

    class LokyRecursionError(RuntimeError):
        """A process tries to spawn too many levels of nested processes."""

    class BrokenProcessPool(_BPPException):
        """
        Raised when the executor is broken while a future was in the running state.
        The cause can an error raised when unpickling the task in the worker
        process or when unpickling the result value in the parent process. It can
        also be caused by a worker process being terminated unexpectedly.
        """

    class TerminatedWorkerError(BrokenProcessPool):
        """
        Raised when a process in a ProcessPoolExecutor terminated abruptly
        while a future was in the running state.
        """

    # Alias for backward compat (for code written for loky 1.1.4 and earlier). Do
    # not use in new code.
    BrokenExecutor = BrokenProcessPool

    class ShutdownExecutorError(RuntimeError):
        """
        Raised when a ProcessPoolExecutor is shutdown while a future was in the
        running or pending state.
        """

    class ProcessPoolExecutor(Executor):

        _at_exit = None

        def __init__(
            self,
            max_workers=None,
            job_reducers=None,
            result_reducers=None,
            timeout=None,
            context=None,
            initializer=None,
            initargs=(),
            env=None,
        ):
            """Initializes a new ProcessPoolExecutor instance.

            Args:
                max_workers: int, optional (default: cpu_count())
                    The maximum number of processes that can be used to execute the
                    given calls. If None or not given then as many worker processes
                    will be created as the number of CPUs the current process
                    can use.
                job_reducers, result_reducers: dict(type: reducer_func)
                    Custom reducer for pickling the jobs and the results from the
                    Executor. If only `job_reducers` is provided, `result_reducer`
                    will use the same reducers
                timeout: int, optional (default: None)
                    Idle workers exit after timeout seconds. If a new job is
                    submitted after the timeout, the executor will start enough
                    new Python processes to make sure the pool of workers is full.
                context: A multiprocessing context to launch the workers. This
                    object should provide SimpleQueue, Queue and Process.
                initializer: An callable used to initialize worker processes.
                initargs: A tuple of arguments to pass to the initializer.
                env: A dict of environment variable to overwrite in the child
                    process. The environment variables are set before any module is
                    loaded. Note that this only works with the loky context.
            """
            _check_system_limits()

            if max_workers is None:
                self._max_workers = cpu_count()
            else:
                if max_workers <= 0:
                    raise ValueError("max_workers must be greater than 0")
                self._max_workers = max_workers

            if (
                sys.platform == "win32"
                and self._max_workers > _MAX_WINDOWS_WORKERS
            ):
                warnings.warn(
                    f"On Windows, max_workers cannot exceed {_MAX_WINDOWS_WORKERS} "
                    "due to limitations of the operating system."
                )
                self._max_workers = _MAX_WINDOWS_WORKERS

            if context is None:
                context = get_context()
            self._context = context
            self._env = env

            self._initializer, self._initargs = _prepare_initializer(
                initializer, initargs
            )
            _check_max_depth(self._context)

            if result_reducers is None:
                result_reducers = job_reducers

            # Timeout
            self._timeout = timeout

            # Management thread
            self._executor_manager_thread = None

            # Map of pids to processes
            self._processes = {}

            # Internal variables of the ProcessPoolExecutor
            self._processes = {}
            self._queue_count = 0
            self._pending_work_items = {}
            self._running_work_items = []
            self._work_ids = queue.Queue()
            self._processes_management_lock = self._context.Lock()
            self._executor_manager_thread = None
            self._shutdown_lock = threading.Lock()

            # _ThreadWakeup is a communication channel used to interrupt the wait
            # of the main loop of executor_manager_thread from another thread (e.g.
            # when calling executor.submit or executor.shutdown). We do not use the
            # _result_queue to send wakeup signals to the executor_manager_thread
            # as it could result in a deadlock if a worker process dies with the
            # _result_queue write lock still acquired.
            #
            # _shutdown_lock must be locked to access _ThreadWakeup.wakeup.
            self._executor_manager_thread_wakeup = _ThreadWakeup()

            # Flag to hold the state of the Executor. This permits to introspect
            # the Executor state even once it has been garbage collected.
            self._flags = _ExecutorFlags(self._shutdown_lock)

            # Finally setup the queues for interprocess communication
            self._setup_queues(job_reducers, result_reducers)

            mp.util.debug("ProcessPoolExecutor is setup")

        def _setup_queues(self, job_reducers, result_reducers, queue_size=None):
            # Make the call queue slightly larger than the number of processes to
            # prevent the worker processes from idling. But don't make it too big
            # because futures in the call queue cannot be cancelled.
            if queue_size is None:
                queue_size = 2 * self._max_workers + EXTRA_QUEUED_CALLS
            self._call_queue = _SafeQueue(
                max_size=queue_size,
                pending_work_items=self._pending_work_items,
                running_work_items=self._running_work_items,
                thread_wakeup=self._executor_manager_thread_wakeup,
                shutdown_lock=self._shutdown_lock,
                reducers=job_reducers,
                ctx=self._context,
            )
            # Killed worker processes can produce spurious "broken pipe"
            # tracebacks in the queue's own worker thread. But we detect killed
            # processes anyway, so silence the tracebacks.
            self._call_queue._ignore_epipe = True

            self._result_queue = SimpleQueue(
                reducers=result_reducers, ctx=self._context
            )

        def _start_executor_manager_thread(self):
            if self._executor_manager_thread is None:
                mp.util.debug("_start_executor_manager_thread called")

                # Start the processes so that their sentinels are known.
                self._executor_manager_thread = _ExecutorManagerThread(self)
                self._executor_manager_thread.start()

                # register this executor in a mechanism that ensures it will wakeup
                # when the interpreter is exiting.
                _threads_wakeups[self._executor_manager_thread] = (
                    self._shutdown_lock,
                    self._executor_manager_thread_wakeup,
                )

                global process_pool_executor_at_exit
                if process_pool_executor_at_exit is None:
                    # Ensure that the _python_exit function will be called before
                    # the multiprocessing.Queue._close finalizers which have an
                    # exitpriority of 10.

                    if sys.version_info < (3, 9):
                        process_pool_executor_at_exit = mp.util.Finalize(
                            None, _python_exit, exitpriority=20
                        )
                    else:
                        process_pool_executor_at_exit = threading._register_atexit(
                            _python_exit
                        )

        def _adjust_process_count(self):
            while len(self._processes) < self._max_workers:
                worker_exit_lock = self._context.BoundedSemaphore(1)
                args = (
                    self._call_queue,
                    self._result_queue,
                    self._initializer,
                    self._initargs,
                    self._processes_management_lock,
                    self._timeout,
                    worker_exit_lock,
                    _CURRENT_DEPTH + 1,
                )
                worker_exit_lock.acquire()
                try:
                    # Try to spawn the process with some environment variable to
                    # overwrite but it only works with the loky context for now.
                    p = self._context.Process(
                        target=_process_worker, args=args, env=self._env
                    )
                except TypeError:
                    p = self._context.Process(target=_process_worker, args=args)
                p._worker_exit_lock = worker_exit_lock
                p.start()
                self._processes[p.pid] = p
            mp.util.debug(
                f"Adjusted process count to {self._max_workers}: "
                f"{[(p.name, pid) for pid, p in self._processes.items()]}"
            )

        def _ensure_executor_running(self):
            """ensures all workers and management thread are running"""
            with self._processes_management_lock:
                if len(self._processes) != self._max_workers:
                    self._adjust_process_count()
                self._start_executor_manager_thread()

        def submit(self, fn, *args, **kwargs):
            with self._flags.shutdown_lock:
                if self._flags.broken is not None:
                    raise self._flags.broken
                if self._flags.shutdown:
                    raise ShutdownExecutorError(
                        "cannot schedule new futures after shutdown"
                    )

                # Cannot submit a new calls once the interpreter is shutting down.
                # This check avoids spawning new processes at exit.
                if _global_shutdown:
                    raise RuntimeError(
                        "cannot schedule new futures after interpreter shutdown"
                    )

                f = Future()
                w = _WorkItem(f, fn, args, kwargs)

                self._pending_work_items[self._queue_count] = w
                self._work_ids.put(self._queue_count)
                self._queue_count += 1
                # Wake up queue management thread
                self._executor_manager_thread_wakeup.wakeup()

                self._ensure_executor_running()
                return f

        submit.__doc__ = Executor.submit.__doc__

        def map(self, fn, *iterables, **kwargs):
            """Returns an iterator equivalent to map(fn, iter).

            Args:
                fn: A callable that will take as many arguments as there are
                    passed iterables.
                timeout: The maximum number of seconds to wait. If None, then there
                    is no limit on the wait time.
                chunksize: If greater than one, the iterables will be chopped into
                    chunks of size chunksize and submitted to the process pool.
                    If set to one, the items in the list will be sent one at a
                    time.

            Returns:
                An iterator equivalent to: map(func, *iterables) but the calls may
                be evaluated out-of-order.

            Raises:
                TimeoutError: If the entire result iterator could not be generated
                    before the given timeout.
                Exception: If fn(*args) raises for any values.
            """
            timeout = kwargs.get("timeout", None)
            chunksize = kwargs.get("chunksize", 1)
            if chunksize < 1:
                raise ValueError("chunksize must be >= 1.")

            results = super().map(
                partial(_process_chunk, fn),
                _get_chunks(chunksize, *iterables),
                timeout=timeout,
            )
            return _chain_from_iterable_of_lists(results)

        def shutdown(self, wait=True, kill_workers=False):
            mp.util.debug(f"shutting down executor {self}")

            self._flags.flag_as_shutting_down(kill_workers)
            executor_manager_thread = self._executor_manager_thread
            executor_manager_thread_wakeup = self._executor_manager_thread_wakeup

            if executor_manager_thread_wakeup is not None:
                # Wake up queue management thread
                with self._shutdown_lock:
                    self._executor_manager_thread_wakeup.wakeup()

            if executor_manager_thread is not None and wait:
                # This locks avoids concurrent join if the interpreter
                # is shutting down.
                with _global_shutdown_lock:
                    executor_manager_thread.join()
                    _threads_wakeups.pop(executor_manager_thread, None)

            # To reduce the risk of opening too many files, remove references to
            # objects that use file descriptors.
            self._executor_manager_thread = None
            self._executor_manager_thread_wakeup = None
            self._call_queue = None
            self._result_queue = None
            self._processes_management_lock = None

        shutdown.__doc__ = Executor.shutdown.__doc__