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# Copyright 2009 Brian Quinlan. All Rights Reserved.
# Licensed to PSF under a Contributor Agreement.
"""Implements ThreadPoolExecutor."""
__author__ = 'Brian Quinlan (brian@sweetapp.com)'
import atexit
from concurrent.futures import _base
import itertools
import queue
import threading
import weakref
import os
# 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 interpreter 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.
_threads_queues = weakref.WeakKeyDictionary()
_shutdown = False
def _python_exit():
global _shutdown
_shutdown = True
items = list(_threads_queues.items())
for t, q in items:
q.put(None)
for t, q in items:
t.join()
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 as exc:
self.future.set_exception(exc)
# Break a reference cycle with the exception 'exc'
self = None
else:
self.future.set_result(result)
def _worker(executor_reference, work_queue, initializer, initargs):
if initializer is not None:
try:
initializer(*initargs)
except BaseException:
_base.LOGGER.critical('Exception in initializer:', exc_info=True)
executor = executor_reference()
if executor is not None:
executor._initializer_failed()
return
try:
while True:
work_item = work_queue.get(block=True)
if work_item is not None:
work_item.run()
# Delete references to object. See issue16284
del work_item
# attempt to increment idle count
executor = executor_reference()
if executor is not None:
executor._idle_semaphore.release()
del executor
continue
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:
# Flag the executor as shutting down as early as possible if it
# is not gc-ed yet.
if executor is not None:
executor._shutdown = True
# Notice other workers
work_queue.put(None)
return
del executor
except BaseException:
_base.LOGGER.critical('Exception in worker', exc_info=True)
class BrokenThreadPool(_base.BrokenExecutor):
"""
Raised when a worker thread in a ThreadPoolExecutor failed initializing.
"""
class ThreadPoolExecutor(_base.Executor):
# Used to assign unique thread names when thread_name_prefix is not supplied.
_counter = itertools.count().__next__
def __init__(self, max_workers=None, thread_name_prefix='',
initializer=None, initargs=()):
"""Initializes a new ThreadPoolExecutor instance.
Args:
max_workers: The maximum number of threads that can be used to
execute the given calls.
thread_name_prefix: An optional name prefix to give our threads.
initializer: A callable used to initialize worker threads.
initargs: A tuple of arguments to pass to the initializer.
"""
if max_workers is None:
# ThreadPoolExecutor is often used to:
# * CPU bound task which releases GIL
# * I/O bound task (which releases GIL, of course)
#
# We use cpu_count + 4 for both types of tasks.
# But we limit it to 32 to avoid consuming surprisingly large resource
# on many core machine.
max_workers = min(32, (os.cpu_count() or 1) + 4)
if max_workers <= 0:
raise ValueError("max_workers must be greater than 0")
if initializer is not None and not callable(initializer):
raise TypeError("initializer must be a callable")
self._max_workers = max_workers
self._work_queue = queue.SimpleQueue()
self._idle_semaphore = threading.Semaphore(0)
self._threads = set()
self._broken = False
self._shutdown = False
self._shutdown_lock = threading.Lock()
self._thread_name_prefix = (thread_name_prefix or
("ThreadPoolExecutor-%d" % self._counter()))
self._initializer = initializer
self._initargs = initargs
def submit(*args, **kwargs):
if len(args) >= 2:
self, fn, *args = args
elif not args:
raise TypeError("descriptor 'submit' of 'ThreadPoolExecutor' object "
"needs an argument")
elif 'fn' in kwargs:
fn = kwargs.pop('fn')
self, *args = args
import warnings
warnings.warn("Passing 'fn' as keyword argument is deprecated",
DeprecationWarning, stacklevel=2)
else:
raise TypeError('submit expected at least 1 positional argument, '
'got %d' % (len(args)-1))
with self._shutdown_lock:
if self._broken:
raise BrokenThreadPool(self._broken)
if self._shutdown:
raise RuntimeError('cannot schedule new futures after shutdown')
if _shutdown:
raise RuntimeError('cannot schedule new futures after '
'interpreter shutdown')
f = _base.Future()
w = _WorkItem(f, fn, args, kwargs)
self._work_queue.put(w)
self._adjust_thread_count()
return f
submit.__text_signature__ = _base.Executor.submit.__text_signature__
submit.__doc__ = _base.Executor.submit.__doc__
def _adjust_thread_count(self):
# if idle threads are available, don't spin new threads
if self._idle_semaphore.acquire(timeout=0):
return
# When the executor gets lost, the weakref callback will wake up
# the worker threads.
def weakref_cb(_, q=self._work_queue):
q.put(None)
num_threads = len(self._threads)
if num_threads < self._max_workers:
thread_name = '%s_%d' % (self._thread_name_prefix or self,
num_threads)
t = threading.Thread(name=thread_name, target=_worker,
args=(weakref.ref(self, weakref_cb),
self._work_queue,
self._initializer,
self._initargs))
t.daemon = True
t.start()
self._threads.add(t)
_threads_queues[t] = self._work_queue
def _initializer_failed(self):
with self._shutdown_lock:
self._broken = ('A thread initializer failed, the thread pool '
'is not usable anymore')
# Drain work queue and mark pending futures failed
while True:
try:
work_item = self._work_queue.get_nowait()
except queue.Empty:
break
if work_item is not None:
work_item.future.set_exception(BrokenThreadPool(self._broken))
def shutdown(self, wait=True):
with self._shutdown_lock:
self._shutdown = True
self._work_queue.put(None)
if wait:
for t in self._threads:
t.join()
shutdown.__doc__ = _base.Executor.shutdown.__doc__