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# Copyright 2009 Brian Quinlan. All Rights Reserved. See LICENSE file.
"""Implements ThreadPoolExecutor."""
__author__ = 'Brian Quinlan (brian@sweetapp.com)'
from futures._base import (PENDING, RUNNING, CANCELLED,
CANCELLED_AND_NOTIFIED, FINISHED,
ALL_COMPLETED,
LOGGER,
set_future_exception, set_future_result,
Executor, Future, FutureList, ThreadEventSink)
import atexit
import queue
import threading
import weakref
# 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 function 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_threads=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, call, future, completion_tracker):
self.call = call
self.future = future
self.completion_tracker = completion_tracker
def run(self):
with self.future._condition:
if self.future._state == PENDING:
self.future._state = RUNNING
elif self.future._state == CANCELLED:
with self.completion_tracker._condition:
self.future._state = CANCELLED_AND_NOTIFIED
self.completion_tracker.add_cancelled()
return
else:
LOGGER.critical('Future %s in unexpected state: %d',
id(self.future),
self.future._state)
return
try:
result = self.call()
except BaseException as e:
set_future_exception(self.future, self.completion_tracker, e)
else:
set_future_result(self.future, self.completion_tracker, 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.
# - The executor that owns the worker has been collected.
# - 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 as e:
LOGGER.critical('Exception in worker', exc_info=True)
class ThreadPoolExecutor(Executor):
def __init__(self, max_threads):
self._max_threads = max_threads
self._work_queue = queue.Queue()
self._threads = set()
self._shutdown = False
self._shutdown_lock = threading.Lock()
def _adjust_thread_count(self):
for _ in range(len(self._threads),
min(self._max_threads, self._work_queue.qsize())):
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 run_to_futures(self, calls, timeout=None, return_when=ALL_COMPLETED):
with self._shutdown_lock:
if self._shutdown:
raise RuntimeError('cannot run new futures after shutdown')
futures = []
event_sink = ThreadEventSink()
for index, call in enumerate(calls):
f = Future(index)
w = _WorkItem(call, f, event_sink)
self._work_queue.put(w)
futures.append(f)
self._adjust_thread_count()
fl = FutureList(futures, event_sink)
fl.wait(timeout=timeout, return_when=return_when)
return fl
def shutdown(self):
with self._shutdown_lock:
self._shutdown = True
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