process_executor.py

###############################################################################
# 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 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
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.

# 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.

_threads_wakeups = weakref.WeakKeyDictionary()
_global_shutdown = False


def _python_exit():
    global _global_shutdown
    _global_shutdown = True
    items = list(_threads_wakeups.items())
    if len(items) > 0:
        mp.util.debug("Interpreter shutting down. Waking up "
                      f"executor_manager_thread {items}")
    for _, (shutdown_lock, thread_wakeup) in items:
        with shutdown_lock:
            thread_wakeup.wakeup()
    for thread, _ in items:
        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


class _ExceptionWithTraceback(BaseException):

    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, reducers=None):
        self.thread_wakeup = thread_wakeup
        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
            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 _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}')
    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}"
            )

        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 > 10.0:
                        raise e
                    mp.util.info(
                        "full call_queue prevented to send all sentinels at "
                        "once, waiting..."
                    )
                    sleep(cooldown_time)
                    cooldown_time *= 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):
    """Raised when a process try 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 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,
            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:
            executor_manager_thread.join()

        # 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__