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# Copyright 2009 Brian Quinlan. All Rights Reserved.
# Licensed to PSF under a Contributor Agreement.
"""Implements ThreadPoolExecutor."""
from __future__ import with_statement
import atexit
import threading
import weakref
import sys
from concurrent.futures import _base
try:
import queue
except ImportError:
import Queue as queue
__author__ = 'Brian Quinlan (brian@sweetapp.com)'
# Workers are created as daemon threads. This is done to allow the interpreter
# to exit when there are still idle threads in a ThreadPoolExecutor's thread
# pool (i.e. shutdown() was not called). However, allowing workers to die with
# the interpreter has two undesirable properties:
# - The workers would still be running during interpretor shutdown,
# meaning that they would fail in unpredictable ways.
# - The workers could be killed while evaluating a work item, which could
# be bad if the callable being evaluated has external side-effects e.g.
# writing to a file.
#
# To work around this problem, an exit handler is installed which tells the
# workers to exit when their work queues are empty and then waits until the
# threads finish.
_thread_references = set()
_shutdown = False
def _python_exit():
global _shutdown
_shutdown = True
for thread_reference in _thread_references:
thread = thread_reference()
if thread is not None:
thread.join()
def _remove_dead_thread_references():
"""Remove inactive threads from _thread_references.
Should be called periodically to prevent memory leaks in scenarios such as:
>>> while True:
... t = ThreadPoolExecutor(max_workers=5)
... t.map(int, ['1', '2', '3', '4', '5'])
"""
for thread_reference in set(_thread_references):
if thread_reference() is None:
_thread_references.discard(thread_reference)
atexit.register(_python_exit)
class _WorkItem(object):
def __init__(self, future, fn, args, kwargs):
self.future = future
self.fn = fn
self.args = args
self.kwargs = kwargs
def run(self):
if not self.future.set_running_or_notify_cancel():
return
try:
result = self.fn(*self.args, **self.kwargs)
except BaseException:
e = sys.exc_info()[1]
self.future.set_exception(e)
else:
self.future.set_result(result)
def _worker(executor_reference, work_queue):
try:
while True:
try:
work_item = work_queue.get(block=True, timeout=0.1)
except queue.Empty:
executor = executor_reference()
# Exit if:
# - The interpreter is shutting down OR
# - The executor that owns the worker has been collected OR
# - The executor that owns the worker has been shutdown.
if _shutdown or executor is None or executor._shutdown:
return
del executor
else:
work_item.run()
except BaseException:
_base.LOGGER.critical('Exception in worker', exc_info=True)
class ThreadPoolExecutor(_base.Executor):
def __init__(self, max_workers):
"""Initializes a new ThreadPoolExecutor instance.
Args:
max_workers: The maximum number of threads that can be used to
execute the given calls.
"""
_remove_dead_thread_references()
self._max_workers = max_workers
self._work_queue = queue.Queue()
self._threads = set()
self._shutdown = False
self._shutdown_lock = threading.Lock()
def submit(self, fn, *args, **kwargs):
with self._shutdown_lock:
if self._shutdown:
raise RuntimeError('cannot schedule new futures after shutdown')
f = _base.Future()
w = _WorkItem(f, fn, args, kwargs)
self._work_queue.put(w)
self._adjust_thread_count()
return f
submit.__doc__ = _base.Executor.submit.__doc__
def _adjust_thread_count(self):
# TODO(bquinlan): Should avoid creating new threads if there are more
# idle threads than items in the work queue.
if len(self._threads) < self._max_workers:
t = threading.Thread(target=_worker,
args=(weakref.ref(self), self._work_queue))
t.daemon = True
t.start()
self._threads.add(t)
_thread_references.add(weakref.ref(t))
def shutdown(self, wait=True):
with self._shutdown_lock:
self._shutdown = True
if wait:
for t in self._threads:
t.join()
shutdown.__doc__ = _base.Executor.shutdown.__doc__
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