752 lines
29 KiB
Python
752 lines
29 KiB
Python
# Copyright 2009 Brian Quinlan. All Rights Reserved.
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# Licensed to PSF under a Contributor Agreement.
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"""Implements ProcessPoolExecutor.
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The following diagram and text describe the data-flow through the system:
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|======================= In-process =====================|== Out-of-process ==|
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+----------+ +----------+ +--------+ +-----------+ +---------+
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| | => | Work Ids | | | | Call Q | | Process |
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| | +----------+ | | +-----------+ | Pool |
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| | | ... | | | | ... | +---------+
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| | | 6 | => | | => | 5, call() | => | |
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| | | 7 | | | | ... | | |
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| Process | | ... | | Local | +-----------+ | Process |
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| Pool | +----------+ | Worker | | #1..n |
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| Executor | | Thread | | |
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| | +----------- + | | +-----------+ | |
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| | <=> | Work Items | <=> | | <= | Result Q | <= | |
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| | +------------+ | | +-----------+ | |
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| | | 6: call() | | | | ... | | |
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| | | future | | | | 4, result | | |
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| | | ... | | | | 3, except | | |
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+----------+ +------------+ +--------+ +-----------+ +---------+
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Executor.submit() called:
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- creates a uniquely numbered _WorkItem and adds it to the "Work Items" dict
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- adds the id of the _WorkItem to the "Work Ids" queue
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Local worker thread:
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- reads work ids from the "Work Ids" queue and looks up the corresponding
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WorkItem from the "Work Items" dict: if the work item has been cancelled then
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it is simply removed from the dict, otherwise it is repackaged as a
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_CallItem and put in the "Call Q". New _CallItems are put in the "Call Q"
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until "Call Q" is full. NOTE: the size of the "Call Q" is kept small because
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calls placed in the "Call Q" can no longer be cancelled with Future.cancel().
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- reads _ResultItems from "Result Q", updates the future stored in the
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"Work Items" dict and deletes the dict entry
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Process #1..n:
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- reads _CallItems from "Call Q", executes the calls, and puts the resulting
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_ResultItems in "Result Q"
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"""
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__author__ = 'Brian Quinlan (brian@sweetapp.com)'
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import os
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from concurrent.futures import _base
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import queue
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import multiprocessing as mp
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import multiprocessing.connection
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from multiprocessing.queues import Queue
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import threading
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import weakref
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from functools import partial
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import itertools
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import sys
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import traceback
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_threads_wakeups = weakref.WeakKeyDictionary()
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_global_shutdown = False
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class _ThreadWakeup:
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def __init__(self):
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self._closed = False
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self._reader, self._writer = mp.Pipe(duplex=False)
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def close(self):
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if not self._closed:
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self._closed = True
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self._writer.close()
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self._reader.close()
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def wakeup(self):
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if not self._closed:
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self._writer.send_bytes(b"")
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def clear(self):
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if not self._closed:
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while self._reader.poll():
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self._reader.recv_bytes()
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def _python_exit():
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global _global_shutdown
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_global_shutdown = True
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items = list(_threads_wakeups.items())
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for _, thread_wakeup in items:
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# call not protected by ProcessPoolExecutor._shutdown_lock
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thread_wakeup.wakeup()
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for t, _ in items:
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t.join()
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# Register for `_python_exit()` to be called just before joining all
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# non-daemon threads. This is used instead of `atexit.register()` for
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# compatibility with subinterpreters, which no longer support daemon threads.
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# See bpo-39812 for context.
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threading._register_atexit(_python_exit)
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# Controls how many more calls than processes will be queued in the call queue.
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# A smaller number will mean that processes spend more time idle waiting for
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# work while a larger number will make Future.cancel() succeed less frequently
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# (Futures in the call queue cannot be cancelled).
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EXTRA_QUEUED_CALLS = 1
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# On Windows, WaitForMultipleObjects is used to wait for processes to finish.
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# It can wait on, at most, 63 objects. There is an overhead of two objects:
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# - the result queue reader
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# - the thread wakeup reader
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_MAX_WINDOWS_WORKERS = 63 - 2
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# Hack to embed stringification of remote traceback in local traceback
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class _RemoteTraceback(Exception):
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def __init__(self, tb):
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self.tb = tb
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def __str__(self):
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return self.tb
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class _ExceptionWithTraceback:
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def __init__(self, exc, tb):
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tb = traceback.format_exception(type(exc), exc, tb)
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tb = ''.join(tb)
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self.exc = exc
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self.tb = '\n"""\n%s"""' % tb
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def __reduce__(self):
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return _rebuild_exc, (self.exc, self.tb)
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def _rebuild_exc(exc, tb):
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exc.__cause__ = _RemoteTraceback(tb)
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return exc
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class _WorkItem(object):
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def __init__(self, future, fn, args, kwargs):
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self.future = future
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self.fn = fn
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self.args = args
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self.kwargs = kwargs
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class _ResultItem(object):
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def __init__(self, work_id, exception=None, result=None):
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self.work_id = work_id
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self.exception = exception
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self.result = result
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class _CallItem(object):
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def __init__(self, work_id, fn, args, kwargs):
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self.work_id = work_id
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self.fn = fn
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self.args = args
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self.kwargs = kwargs
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class _SafeQueue(Queue):
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"""Safe Queue set exception to the future object linked to a job"""
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def __init__(self, max_size=0, *, ctx, pending_work_items, shutdown_lock,
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thread_wakeup):
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self.pending_work_items = pending_work_items
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self.shutdown_lock = shutdown_lock
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self.thread_wakeup = thread_wakeup
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super().__init__(max_size, ctx=ctx)
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def _on_queue_feeder_error(self, e, obj):
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if isinstance(obj, _CallItem):
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tb = traceback.format_exception(type(e), e, e.__traceback__)
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e.__cause__ = _RemoteTraceback('\n"""\n{}"""'.format(''.join(tb)))
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work_item = self.pending_work_items.pop(obj.work_id, None)
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with self.shutdown_lock:
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self.thread_wakeup.wakeup()
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# work_item can be None if another process terminated. In this
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# case, the executor_manager_thread fails all work_items
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# with BrokenProcessPool
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if work_item is not None:
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work_item.future.set_exception(e)
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else:
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super()._on_queue_feeder_error(e, obj)
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def _get_chunks(*iterables, chunksize):
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""" Iterates over zip()ed iterables in chunks. """
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it = zip(*iterables)
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while True:
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chunk = tuple(itertools.islice(it, chunksize))
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if not chunk:
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return
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yield chunk
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def _process_chunk(fn, chunk):
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""" Processes a chunk of an iterable passed to map.
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Runs the function passed to map() on a chunk of the
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iterable passed to map.
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This function is run in a separate process.
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"""
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return [fn(*args) for args in chunk]
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def _sendback_result(result_queue, work_id, result=None, exception=None):
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"""Safely send back the given result or exception"""
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try:
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result_queue.put(_ResultItem(work_id, result=result,
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exception=exception))
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except BaseException as e:
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exc = _ExceptionWithTraceback(e, e.__traceback__)
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result_queue.put(_ResultItem(work_id, exception=exc))
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def _process_worker(call_queue, result_queue, initializer, initargs):
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"""Evaluates calls from call_queue and places the results in result_queue.
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This worker is run in a separate process.
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Args:
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call_queue: A ctx.Queue of _CallItems that will be read and
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evaluated by the worker.
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result_queue: A ctx.Queue of _ResultItems that will written
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to by the worker.
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initializer: A callable initializer, or None
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initargs: A tuple of args for the initializer
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"""
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if initializer is not None:
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try:
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initializer(*initargs)
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except BaseException:
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_base.LOGGER.critical('Exception in initializer:', exc_info=True)
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# The parent will notice that the process stopped and
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# mark the pool broken
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return
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while True:
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call_item = call_queue.get(block=True)
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if call_item is None:
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# Wake up queue management thread
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result_queue.put(os.getpid())
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return
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try:
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r = call_item.fn(*call_item.args, **call_item.kwargs)
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except BaseException as e:
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exc = _ExceptionWithTraceback(e, e.__traceback__)
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_sendback_result(result_queue, call_item.work_id, exception=exc)
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else:
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_sendback_result(result_queue, call_item.work_id, result=r)
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del r
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# Liberate the resource as soon as possible, to avoid holding onto
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# open files or shared memory that is not needed anymore
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del call_item
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class _ExecutorManagerThread(threading.Thread):
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"""Manages the communication between this process and the worker processes.
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The manager is run in a local thread.
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Args:
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executor: A reference to the ProcessPoolExecutor that owns
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this thread. A weakref will be own by the manager as well as
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references to internal objects used to introspect the state of
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the executor.
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"""
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def __init__(self, executor):
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# Store references to necessary internals of the executor.
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# A _ThreadWakeup to allow waking up the queue_manager_thread from the
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# main Thread and avoid deadlocks caused by permanently locked queues.
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self.thread_wakeup = executor._executor_manager_thread_wakeup
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self.shutdown_lock = executor._shutdown_lock
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# A weakref.ref to the ProcessPoolExecutor that owns this thread. Used
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# to determine if the ProcessPoolExecutor has been garbage collected
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# and that the manager can exit.
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# When the executor gets garbage collected, the weakref callback
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# will wake up the queue management thread so that it can terminate
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# if there is no pending work item.
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def weakref_cb(_,
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thread_wakeup=self.thread_wakeup,
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shutdown_lock=self.shutdown_lock):
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mp.util.debug('Executor collected: triggering callback for'
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' QueueManager wakeup')
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with shutdown_lock:
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thread_wakeup.wakeup()
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self.executor_reference = weakref.ref(executor, weakref_cb)
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# A list of the ctx.Process instances used as workers.
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self.processes = executor._processes
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# A ctx.Queue that will be filled with _CallItems derived from
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# _WorkItems for processing by the process workers.
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self.call_queue = executor._call_queue
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# A ctx.SimpleQueue of _ResultItems generated by the process workers.
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self.result_queue = executor._result_queue
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# A queue.Queue of work ids e.g. Queue([5, 6, ...]).
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self.work_ids_queue = executor._work_ids
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# A dict mapping work ids to _WorkItems e.g.
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# {5: <_WorkItem...>, 6: <_WorkItem...>, ...}
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self.pending_work_items = executor._pending_work_items
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super().__init__()
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def run(self):
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# Main loop for the executor manager thread.
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while True:
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self.add_call_item_to_queue()
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result_item, is_broken, cause = self.wait_result_broken_or_wakeup()
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if is_broken:
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self.terminate_broken(cause)
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return
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if result_item is not None:
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self.process_result_item(result_item)
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# Delete reference to result_item to avoid keeping references
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# while waiting on new results.
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del result_item
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# attempt to increment idle process count
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executor = self.executor_reference()
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if executor is not None:
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executor._idle_worker_semaphore.release()
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del executor
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if self.is_shutting_down():
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self.flag_executor_shutting_down()
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# Since no new work items can be added, it is safe to shutdown
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# this thread if there are no pending work items.
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if not self.pending_work_items:
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self.join_executor_internals()
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return
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def add_call_item_to_queue(self):
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# Fills call_queue with _WorkItems from pending_work_items.
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# This function never blocks.
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while True:
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if self.call_queue.full():
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return
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try:
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work_id = self.work_ids_queue.get(block=False)
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except queue.Empty:
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return
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else:
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work_item = self.pending_work_items[work_id]
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if work_item.future.set_running_or_notify_cancel():
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self.call_queue.put(_CallItem(work_id,
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work_item.fn,
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work_item.args,
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work_item.kwargs),
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block=True)
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else:
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del self.pending_work_items[work_id]
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continue
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def wait_result_broken_or_wakeup(self):
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# Wait for a result to be ready in the result_queue while checking
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# that all worker processes are still running, or for a wake up
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# signal send. The wake up signals come either from new tasks being
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# submitted, from the executor being shutdown/gc-ed, or from the
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# shutdown of the python interpreter.
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result_reader = self.result_queue._reader
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assert not self.thread_wakeup._closed
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wakeup_reader = self.thread_wakeup._reader
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readers = [result_reader, wakeup_reader]
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worker_sentinels = [p.sentinel for p in self.processes.values()]
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ready = mp.connection.wait(readers + worker_sentinels)
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cause = None
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is_broken = True
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result_item = None
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if result_reader in ready:
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try:
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result_item = result_reader.recv()
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is_broken = False
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except BaseException as e:
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cause = traceback.format_exception(type(e), e, e.__traceback__)
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elif wakeup_reader in ready:
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is_broken = False
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with self.shutdown_lock:
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self.thread_wakeup.clear()
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return result_item, is_broken, cause
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def process_result_item(self, result_item):
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# Process the received a result_item. This can be either the PID of a
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# worker that exited gracefully or a _ResultItem
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if isinstance(result_item, int):
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# Clean shutdown of a worker using its PID
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# (avoids marking the executor broken)
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assert self.is_shutting_down()
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p = self.processes.pop(result_item)
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p.join()
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if not self.processes:
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self.join_executor_internals()
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return
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else:
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# Received a _ResultItem so mark the future as completed.
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work_item = self.pending_work_items.pop(result_item.work_id, None)
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# work_item can be None if another process terminated (see above)
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if work_item is not None:
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if result_item.exception:
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work_item.future.set_exception(result_item.exception)
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else:
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work_item.future.set_result(result_item.result)
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def is_shutting_down(self):
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# Check whether we should start shutting down the executor.
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executor = self.executor_reference()
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# No more work items can be added if:
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# - The interpreter is shutting down OR
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# - The executor that owns this worker has been collected OR
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# - The executor that owns this worker has been shutdown.
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return (_global_shutdown or executor is None
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or executor._shutdown_thread)
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def terminate_broken(self, cause):
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# Terminate the executor because it is in a broken state. The cause
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# argument can be used to display more information on the error that
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# lead the executor into becoming broken.
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# Mark the process pool broken so that submits fail right now.
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executor = self.executor_reference()
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if executor is not None:
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executor._broken = ('A child process terminated '
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'abruptly, the process pool is not '
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'usable anymore')
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executor._shutdown_thread = True
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executor = None
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# All pending tasks are to be marked failed with the following
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# BrokenProcessPool error
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bpe = BrokenProcessPool("A process in the process pool was "
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"terminated abruptly while the future was "
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"running or pending.")
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if cause is not None:
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bpe.__cause__ = _RemoteTraceback(
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f"\n'''\n{''.join(cause)}'''")
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# Mark pending tasks as failed.
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for work_id, work_item in self.pending_work_items.items():
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work_item.future.set_exception(bpe)
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# Delete references to object. See issue16284
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del work_item
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self.pending_work_items.clear()
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# Terminate remaining workers forcibly: the queues or their
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# locks may be in a dirty state and block forever.
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for p in self.processes.values():
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p.terminate()
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# clean up resources
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self.join_executor_internals()
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def flag_executor_shutting_down(self):
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# Flag the executor as shutting down and cancel remaining tasks if
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# requested as early as possible if it is not gc-ed yet.
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executor = self.executor_reference()
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if executor is not None:
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executor._shutdown_thread = True
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# Cancel pending work items if requested.
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if executor._cancel_pending_futures:
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# Cancel all pending futures and update pending_work_items
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# to only have futures that are currently running.
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new_pending_work_items = {}
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for work_id, work_item in self.pending_work_items.items():
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if not work_item.future.cancel():
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new_pending_work_items[work_id] = work_item
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self.pending_work_items = new_pending_work_items
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# Drain work_ids_queue since we no longer need to
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# add items to the call queue.
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while True:
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try:
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self.work_ids_queue.get_nowait()
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except queue.Empty:
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break
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# Make sure we do this only once to not waste time looping
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# on running processes over and over.
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executor._cancel_pending_futures = False
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def shutdown_workers(self):
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n_children_to_stop = self.get_n_children_alive()
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n_sentinels_sent = 0
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# Send the right number of sentinels, to make sure all children are
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# properly terminated.
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while (n_sentinels_sent < n_children_to_stop
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and self.get_n_children_alive() > 0):
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for i in range(n_children_to_stop - n_sentinels_sent):
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try:
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self.call_queue.put_nowait(None)
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n_sentinels_sent += 1
|
|
except queue.Full:
|
|
break
|
|
|
|
def join_executor_internals(self):
|
|
self.shutdown_workers()
|
|
# Release the queue's resources as soon as possible.
|
|
self.call_queue.close()
|
|
self.call_queue.join_thread()
|
|
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 Mac OS X.
|
|
for p in self.processes.values():
|
|
p.join()
|
|
|
|
def get_n_children_alive(self):
|
|
# This is an upper bound on the number of children alive.
|
|
return sum(p.is_alive() for p in self.processes.values())
|
|
|
|
|
|
_system_limits_checked = False
|
|
_system_limited = None
|
|
|
|
|
|
def _check_system_limits():
|
|
global _system_limits_checked, _system_limited
|
|
if _system_limits_checked:
|
|
if _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:
|
|
# indetermined 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 = ("system provides too few semaphores (%d"
|
|
" available, 256 necessary)" % nsems_max)
|
|
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()
|
|
|
|
|
|
class BrokenProcessPool(_base.BrokenExecutor):
|
|
"""
|
|
Raised when a process in a ProcessPoolExecutor terminated abruptly
|
|
while a future was in the running state.
|
|
"""
|
|
|
|
|
|
class ProcessPoolExecutor(_base.Executor):
|
|
def __init__(self, max_workers=None, mp_context=None,
|
|
initializer=None, initargs=()):
|
|
"""Initializes a new ProcessPoolExecutor instance.
|
|
|
|
Args:
|
|
max_workers: 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 machine has processors.
|
|
mp_context: A multiprocessing context to launch the workers. This
|
|
object should provide SimpleQueue, Queue and Process.
|
|
initializer: A callable used to initialize worker processes.
|
|
initargs: A tuple of arguments to pass to the initializer.
|
|
"""
|
|
_check_system_limits()
|
|
|
|
if max_workers is None:
|
|
self._max_workers = os.cpu_count() or 1
|
|
if sys.platform == 'win32':
|
|
self._max_workers = min(_MAX_WINDOWS_WORKERS,
|
|
self._max_workers)
|
|
else:
|
|
if max_workers <= 0:
|
|
raise ValueError("max_workers must be greater than 0")
|
|
elif (sys.platform == 'win32' and
|
|
max_workers > _MAX_WINDOWS_WORKERS):
|
|
raise ValueError(
|
|
f"max_workers must be <= {_MAX_WINDOWS_WORKERS}")
|
|
|
|
self._max_workers = max_workers
|
|
|
|
if mp_context is None:
|
|
mp_context = mp.get_context()
|
|
self._mp_context = mp_context
|
|
|
|
if initializer is not None and not callable(initializer):
|
|
raise TypeError("initializer must be a callable")
|
|
self._initializer = initializer
|
|
self._initargs = initargs
|
|
|
|
# Management thread
|
|
self._executor_manager_thread = None
|
|
|
|
# Map of pids to processes
|
|
self._processes = {}
|
|
|
|
# Shutdown is a two-step process.
|
|
self._shutdown_thread = False
|
|
self._shutdown_lock = threading.Lock()
|
|
self._idle_worker_semaphore = threading.Semaphore(0)
|
|
self._broken = False
|
|
self._queue_count = 0
|
|
self._pending_work_items = {}
|
|
self._cancel_pending_futures = False
|
|
|
|
# _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.
|
|
self._executor_manager_thread_wakeup = _ThreadWakeup()
|
|
|
|
# Create communication channels for the executor
|
|
# 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.
|
|
queue_size = self._max_workers + EXTRA_QUEUED_CALLS
|
|
self._call_queue = _SafeQueue(
|
|
max_size=queue_size, ctx=self._mp_context,
|
|
pending_work_items=self._pending_work_items,
|
|
shutdown_lock=self._shutdown_lock,
|
|
thread_wakeup=self._executor_manager_thread_wakeup)
|
|
# 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 = mp_context.SimpleQueue()
|
|
self._work_ids = queue.Queue()
|
|
|
|
def _start_executor_manager_thread(self):
|
|
if self._executor_manager_thread is None:
|
|
# Start the processes so that their sentinels are known.
|
|
self._executor_manager_thread = _ExecutorManagerThread(self)
|
|
self._executor_manager_thread.start()
|
|
_threads_wakeups[self._executor_manager_thread] = \
|
|
self._executor_manager_thread_wakeup
|
|
|
|
def _adjust_process_count(self):
|
|
# if there's an idle process, we don't need to spawn a new one.
|
|
if self._idle_worker_semaphore.acquire(blocking=False):
|
|
return
|
|
|
|
process_count = len(self._processes)
|
|
if process_count < self._max_workers:
|
|
p = self._mp_context.Process(
|
|
target=_process_worker,
|
|
args=(self._call_queue,
|
|
self._result_queue,
|
|
self._initializer,
|
|
self._initargs))
|
|
p.start()
|
|
self._processes[p.pid] = p
|
|
|
|
def submit(self, fn, /, *args, **kwargs):
|
|
with self._shutdown_lock:
|
|
if self._broken:
|
|
raise BrokenProcessPool(self._broken)
|
|
if self._shutdown_thread:
|
|
raise RuntimeError('cannot schedule new futures after shutdown')
|
|
if _global_shutdown:
|
|
raise RuntimeError('cannot schedule new futures after '
|
|
'interpreter shutdown')
|
|
|
|
f = _base.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._adjust_process_count()
|
|
self._start_executor_manager_thread()
|
|
return f
|
|
submit.__doc__ = _base.Executor.submit.__doc__
|
|
|
|
def map(self, fn, *iterables, timeout=None, chunksize=1):
|
|
"""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.
|
|
"""
|
|
if chunksize < 1:
|
|
raise ValueError("chunksize must be >= 1.")
|
|
|
|
results = super().map(partial(_process_chunk, fn),
|
|
_get_chunks(*iterables, chunksize=chunksize),
|
|
timeout=timeout)
|
|
return _chain_from_iterable_of_lists(results)
|
|
|
|
def shutdown(self, wait=True, *, cancel_futures=False):
|
|
with self._shutdown_lock:
|
|
self._cancel_pending_futures = cancel_futures
|
|
self._shutdown_thread = True
|
|
if self._executor_manager_thread_wakeup is not None:
|
|
# Wake up queue management thread
|
|
self._executor_manager_thread_wakeup.wakeup()
|
|
|
|
if self._executor_manager_thread is not None and wait:
|
|
self._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._call_queue = None
|
|
if self._result_queue is not None and wait:
|
|
self._result_queue.close()
|
|
self._result_queue = None
|
|
self._processes = None
|
|
self._executor_manager_thread_wakeup = None
|
|
|
|
shutdown.__doc__ = _base.Executor.shutdown.__doc__
|