Initial implementation of PEP 3148

5 files changed, 1033 insertions, 0 deletions

diff --git a/Lib/concurrent/__init__.py b/Lib/concurrent/__init__.py
new file mode 100644
index 0000000..196d378
--- /dev/null
+++ b/Lib/concurrent/__init__.py
@@ -0,0 +1 @@
+# This directory is a Python package.
diff --git a/Lib/concurrent/futures/__init__.py b/Lib/concurrent/futures/__init__.py
new file mode 100644
index 0000000..b5231f8
--- /dev/null
+++ b/Lib/concurrent/futures/__init__.py
@@ -0,0 +1,18 @@
+# Copyright 2009 Brian Quinlan. All Rights Reserved.
+# Licensed to PSF under a Contributor Agreement.
+
+"""Execute computations asynchronously using threads or processes."""
+
+__author__ = 'Brian Quinlan (brian@sweetapp.com)'
+
+from concurrent.futures._base import (FIRST_COMPLETED,
+                                      FIRST_EXCEPTION,
+                                      ALL_COMPLETED,
+                                      CancelledError,
+                                      TimeoutError,
+                                      Future,
+                                      Executor,
+                                      wait,
+                                      as_completed)
+from concurrent.futures.process import ProcessPoolExecutor
+from concurrent.futures.thread import ThreadPoolExecutor
diff --git a/Lib/concurrent/futures/_base.py b/Lib/concurrent/futures/_base.py
new file mode 100644
index 0000000..ba4cbc7
--- /dev/null
+++ b/Lib/concurrent/futures/_base.py
@@ -0,0 +1,541 @@
+# Copyright 2009 Brian Quinlan. All Rights Reserved.
+# Licensed to PSF under a Contributor Agreement.
+
+__author__ = 'Brian Quinlan (brian@sweetapp.com)'
+
+import collections
+import functools
+import logging
+import threading
+import time
+
+FIRST_COMPLETED = 'FIRST_COMPLETED'
+FIRST_EXCEPTION = 'FIRST_EXCEPTION'
+ALL_COMPLETED = 'ALL_COMPLETED'
+
+# Possible future states (for internal use by the futures package).
+PENDING = 'PENDING'
+RUNNING = 'RUNNING'
+# The future was cancelled by the user...
+CANCELLED = 'CANCELLED'
+# ...and _Waiter.add_cancelled() was called by a worker.
+CANCELLED_AND_NOTIFIED = 'CANCELLED_AND_NOTIFIED'
+FINISHED = 'FINISHED'
+
+_FUTURE_STATES = [
+    PENDING,
+    RUNNING,
+    CANCELLED,
+    CANCELLED_AND_NOTIFIED,
+    FINISHED
+]
+
+_STATE_TO_DESCRIPTION_MAP = {
+    PENDING: "pending",
+    RUNNING: "running",
+    CANCELLED: "cancelled",
+    CANCELLED_AND_NOTIFIED: "cancelled",
+    FINISHED: "finished"
+}
+
+# Logger for internal use by the futures package.
+LOGGER = logging.getLogger("concurrent.futures")
+_handler = logging.StreamHandler()
+LOGGER.addHandler(_handler)
+del _handler
+
+class Error(Exception):
+    """Base class for all future-related exceptions."""
+    pass
+
+class CancelledError(Error):
+    """The Future was cancelled."""
+    pass
+
+class TimeoutError(Error):
+    """The operation exceeded the given deadline."""
+    pass
+
+class _Waiter(object):
+    """Provides the event that wait() and as_completed() block on."""
+    def __init__(self):
+        self.event = threading.Event()
+        self.finished_futures = []
+
+    def add_result(self, future):
+        self.finished_futures.append(future)
+
+    def add_exception(self, future):
+        self.finished_futures.append(future)
+
+    def add_cancelled(self, future):
+        self.finished_futures.append(future)
+
+class _FirstCompletedWaiter(_Waiter):
+    """Used by wait(return_when=FIRST_COMPLETED) and as_completed()."""
+
+    def add_result(self, future):
+        super().add_result(future)
+        self.event.set()
+
+    def add_exception(self, future):
+        super().add_exception(future)
+        self.event.set()
+
+    def add_cancelled(self, future):
+        super().add_cancelled(future)
+        self.event.set()
+
+class _AllCompletedWaiter(_Waiter):
+    """Used by wait(return_when=FIRST_EXCEPTION and ALL_COMPLETED)."""
+
+    def __init__(self, num_pending_calls, stop_on_exception):
+        self.num_pending_calls = num_pending_calls
+        self.stop_on_exception = stop_on_exception
+        super().__init__()
+
+    def _decrement_pending_calls(self):
+        self.num_pending_calls -= 1
+        if not self.num_pending_calls:
+            self.event.set()
+
+    def add_result(self, future):
+        super().add_result(future)
+        self._decrement_pending_calls()
+
+    def add_exception(self, future):
+        super().add_exception(future)
+        if self.stop_on_exception:
+            self.event.set()
+        else:
+            self._decrement_pending_calls()
+
+    def add_cancelled(self, future):
+        super().add_cancelled(future)
+        self._decrement_pending_calls()
+
+class _AcquireFutures(object):
+    """A context manager that does an ordered acquire of Future conditions."""
+
+    def __init__(self, futures):
+        self.futures = sorted(futures, key=id)
+
+    def __enter__(self):
+        for future in self.futures:
+            future._condition.acquire()
+
+    def __exit__(self, *args):
+        for future in self.futures:
+            future._condition.release()
+
+def _create_and_install_waiters(fs, return_when):
+    if return_when == FIRST_COMPLETED:
+        waiter = _FirstCompletedWaiter()
+    else:
+        pending_count = sum(
+                f._state not in [CANCELLED_AND_NOTIFIED, FINISHED] for f in fs)
+
+        if return_when == FIRST_EXCEPTION:
+            waiter = _AllCompletedWaiter(pending_count, stop_on_exception=True)
+        elif return_when == ALL_COMPLETED:
+            waiter = _AllCompletedWaiter(pending_count, stop_on_exception=False)
+        else:
+            raise ValueError("Invalid return condition: %r" % return_when)
+
+    for f in fs:
+        f._waiters.append(waiter)
+
+    return waiter
+
+def as_completed(fs, timeout=None):
+    """An iterator over the given futures that yields each as it completes.
+
+    Args:
+        fs: The sequence of Futures (possibly created by different Executors) to
+            iterate over.
+        timeout: The maximum number of seconds to wait. If None, then there
+            is no limit on the wait time.
+
+    Returns:
+        An iterator that yields the given Futures as they complete (finished or
+        cancelled).
+
+    Raises:
+        TimeoutError: If the entire result iterator could not be generated
+            before the given timeout.
+    """
+    if timeout is not None:
+        end_time = timeout + time.time()
+
+    with _AcquireFutures(fs):
+        finished = set(
+                f for f in fs
+                if f._state in [CANCELLED_AND_NOTIFIED, FINISHED])
+        pending = set(fs) - finished
+        waiter = _create_and_install_waiters(fs, FIRST_COMPLETED)
+
+    try:
+        for future in finished:
+            yield future
+
+        while pending:
+            if timeout is None:
+                wait_timeout = None
+            else:
+                wait_timeout = end_time - time.time()
+                if wait_timeout < 0:
+                    raise TimeoutError(
+                            '%d (of %d) futures unfinished' % (
+                            len(pending), len(fs)))
+
+            waiter.event.wait(timeout)
+
+            for future in waiter.finished_futures[:]:
+                yield future
+                waiter.finished_futures.remove(future)
+                pending.remove(future)
+
+    finally:
+        for f in fs:
+            f._waiters.remove(waiter)
+
+DoneAndNotDoneFutures = collections.namedtuple(
+        'DoneAndNotDoneFutures', 'done not_done')
+def wait(fs, timeout=None, return_when=ALL_COMPLETED):
+    """Wait for the futures in the given sequence to complete.
+
+    Args:
+        fs: The sequence of Futures (possibly created by different Executors) to
+            wait upon.
+        timeout: The maximum number of seconds to wait. If None, then there
+            is no limit on the wait time.
+        return_when: Indicates when this function should return. The options
+            are:
+
+            FIRST_COMPLETED - Return when any future finishes or is
+                              cancelled.
+            FIRST_EXCEPTION - Return when any future finishes by raising an
+                              exception. If no future raises an exception
+                              then it is equivalent to ALL_COMPLETED.
+            ALL_COMPLETED -   Return when all futures finish or are cancelled.
+
+    Returns:
+        A named 2-tuple of sets. The first set, named 'done', contains the
+        futures that completed (is finished or cancelled) before the wait
+        completed. The second set, named 'not_done', contains uncompleted
+        futures.
+    """
+    with _AcquireFutures(fs):
+        done = set(f for f in fs
+                   if f._state in [CANCELLED_AND_NOTIFIED, FINISHED])
+        not_done = set(fs) - done
+
+        if (return_when == FIRST_COMPLETED) and done:
+            return DoneAndNotDoneFutures(done, not_done)
+        elif (return_when == FIRST_EXCEPTION) and done:
+            if any(f for f in done
+                   if not f.cancelled() and f.exception() is not None):
+                return DoneAndNotDoneFutures(done, not_done)
+
+        if len(done) == len(fs):
+            return DoneAndNotDoneFutures(done, not_done)
+
+        waiter = _create_and_install_waiters(fs, return_when)
+
+    waiter.event.wait(timeout)
+    for f in fs:
+        f._waiters.remove(waiter)
+
+    done.update(waiter.finished_futures)
+    return DoneAndNotDoneFutures(done, set(fs) - done)
+
+class Future(object):
+    """Represents the result of an asynchronous computation."""
+
+    def __init__(self):
+        """Initializes the future. Should not be called by clients."""
+        self._condition = threading.Condition()
+        self._state = PENDING
+        self._result = None
+        self._exception = None
+        self._waiters = []
+        self._done_callbacks = []
+
+    def _invoke_callbacks(self):
+        for callback in self._done_callbacks:
+            try:
+                callback(self)
+            except Exception:
+                LOGGER.exception('exception calling callback for %r', self)
+
+    def __repr__(self):
+        with self._condition:
+            if self._state == FINISHED:
+                if self._exception:
+                    return '<Future at %s state=%s raised %s>' % (
+                        hex(id(self)),
+                        _STATE_TO_DESCRIPTION_MAP[self._state],
+                        self._exception.__class__.__name__)
+                else:
+                    return '<Future at %s state=%s returned %s>' % (
+                        hex(id(self)),
+                        _STATE_TO_DESCRIPTION_MAP[self._state],
+                        self._result.__class__.__name__)
+            return '<Future at %s state=%s>' % (
+                    hex(id(self)),
+                   _STATE_TO_DESCRIPTION_MAP[self._state])
+
+    def cancel(self):
+        """Cancel the future if possible.
+
+        Returns True if the future was cancelled, False otherwise. A future
+        cannot be cancelled if it is running or has already completed.
+        """
+        with self._condition:
+            if self._state in [RUNNING, FINISHED]:
+                return False
+
+            if self._state in [CANCELLED, CANCELLED_AND_NOTIFIED]:
+                return True
+
+            self._state = CANCELLED
+            self._condition.notify_all()
+
+        self._invoke_callbacks()
+        return True
+
+    def cancelled(self):
+        """Return True if the future has cancelled."""
+        with self._condition:
+            return self._state in [CANCELLED, CANCELLED_AND_NOTIFIED]
+
+    def running(self):
+        """Return True if the future is currently executing."""
+        with self._condition:
+            return self._state == RUNNING
+
+    def done(self):
+        """Return True of the future was cancelled or finished executing."""
+        with self._condition:
+            return self._state in [CANCELLED, CANCELLED_AND_NOTIFIED, FINISHED]
+
+    def __get_result(self):
+        if self._exception:
+            raise self._exception
+        else:
+            return self._result
+
+    def add_done_callback(self, fn):
+        """Attaches a callable that will be called when the future finishes.
+
+        Args:
+            fn: A callable that will be called with this future as its only
+                argument when the future completes or is cancelled. The callable
+                will always be called by a thread in the same process in which
+                it was added. If the future has already completed or been
+                cancelled then the callable will be called immediately. These
+                callables are called in the order that they were added.
+        """
+        with self._condition:
+            if self._state not in [CANCELLED, CANCELLED_AND_NOTIFIED, FINISHED]:
+                self._done_callbacks.append(fn)
+                return
+        fn(self)
+
+    def result(self, timeout=None):
+        """Return the result of the call that the future represents.
+
+        Args:
+            timeout: The number of seconds to wait for the result if the future
+                isn't done. If None, then there is no limit on the wait time.
+
+        Returns:
+            The result of the call that the future represents.
+
+        Raises:
+            CancelledError: If the future was cancelled.
+            TimeoutError: If the future didn't finish executing before the given
+                timeout.
+            Exception: If the call raised then that exception will be raised.
+        """
+        with self._condition:
+            if self._state in [CANCELLED, CANCELLED_AND_NOTIFIED]:
+                raise CancelledError()
+            elif self._state == FINISHED:
+                return self.__get_result()
+
+            self._condition.wait(timeout)
+
+            if self._state in [CANCELLED, CANCELLED_AND_NOTIFIED]:
+                raise CancelledError()
+            elif self._state == FINISHED:
+                return self.__get_result()
+            else:
+                raise TimeoutError()
+
+    def exception(self, timeout=None):
+        """Return the exception raised by the call that the future represents.
+
+        Args:
+            timeout: The number of seconds to wait for the exception if the
+                future isn't done. If None, then there is no limit on the wait
+                time.
+
+        Returns:
+            The exception raised by the call that the future represents or None
+            if the call completed without raising.
+
+        Raises:
+            CancelledError: If the future was cancelled.
+            TimeoutError: If the future didn't finish executing before the given
+                timeout.
+        """
+
+        with self._condition:
+            if self._state in [CANCELLED, CANCELLED_AND_NOTIFIED]:
+                raise CancelledError()
+            elif self._state == FINISHED:
+                return self._exception
+
+            self._condition.wait(timeout)
+
+            if self._state in [CANCELLED, CANCELLED_AND_NOTIFIED]:
+                raise CancelledError()
+            elif self._state == FINISHED:
+                return self._exception
+            else:
+                raise TimeoutError()
+
+    # The following methods should only be used by Executors and in tests.
+    def set_running_or_notify_cancel(self):
+        """Mark the future as running or process any cancel notifications.
+
+        Should only be used by Executor implementations and unit tests.
+
+        If the future has been cancelled (cancel() was called and returned
+        True) then any threads waiting on the future completing (though calls
+        to as_completed() or wait()) are notified and False is returned.
+
+        If the future was not cancelled then it is put in the running state
+        (future calls to running() will return True) and True is returned.
+
+        This method should be called by Executor implementations before
+        executing the work associated with this future. If this method returns
+        False then the work should not be executed.
+
+        Returns:
+            False if the Future was cancelled, True otherwise.
+
+        Raises:
+            RuntimeError: if this method was already called or if set_result()
+                or set_exception() was called.
+        """
+        with self._condition:
+            if self._state == CANCELLED:
+                self._state = CANCELLED_AND_NOTIFIED
+                for waiter in self._waiters:
+                    waiter.add_cancelled(self)
+                # self._condition.notify_all() is not necessary because
+                # self.cancel() triggers a notification.
+                return False
+            elif self._state == PENDING:
+                self._state = RUNNING
+                return True
+            else:
+                LOGGER.critical('Future %s in unexpected state: %s',
+                                id(self.future),
+                                self.future._state)
+                raise RuntimeError('Future in unexpected state')
+
+    def set_result(self, result):
+        """Sets the return value of work associated with the future.
+
+        Should only be used by Executor implementations and unit tests.
+        """
+        with self._condition:
+            self._result = result
+            self._state = FINISHED
+            for waiter in self._waiters:
+                waiter.add_result(self)
+            self._condition.notify_all()
+        self._invoke_callbacks()
+
+    def set_exception(self, exception):
+        """Sets the result of the future as being the given exception.
+
+        Should only be used by Executor implementations and unit tests.
+        """
+        with self._condition:
+            self._exception = exception
+            self._state = FINISHED
+            for waiter in self._waiters:
+                waiter.add_exception(self)
+            self._condition.notify_all()
+        self._invoke_callbacks()
+
+class Executor(object):
+    """This is an abstract base class for concrete asynchronous executors."""
+
+    def submit(self, fn, *args, **kwargs):
+        """Submits a callable to be executed with the given arguments.
+
+        Schedules the callable to be executed as fn(*args, **kwargs) and returns
+        a Future instance representing the execution of the callable.
+
+        Returns:
+            A Future representing the given call.
+        """
+        raise NotImplementedError()
+
+    def map(self, fn, *iterables, timeout=None):
+        """Returns a iterator equivalent to map(fn, iter).
+
+        Args:
+            fn: A callable that will take 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.
+
+        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 timeout is not None:
+            end_time = timeout + time.time()
+
+        fs = [self.submit(fn, *args) for args in zip(*iterables)]
+
+        try:
+            for future in fs:
+                if timeout is None:
+                    yield future.result()
+                else:
+                    yield future.result(end_time - time.time())
+        finally:
+            for future in fs:
+                future.cancel()
+
+    def shutdown(self, wait=True):
+        """Clean-up the resources associated with the Executor.
+
+        It is safe to call this method several times. Otherwise, no other
+        methods can be called after this one.
+
+        Args:
+            wait: If True then shutdown will not return until all running
+                futures have finished executing and the resources used by the
+                executor have been reclaimed.
+        """
+        pass
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        self.shutdown(wait=True)
+        return False
diff --git a/Lib/concurrent/futures/process.py b/Lib/concurrent/futures/process.py
new file mode 100644
index 0000000..36a40d3
--- /dev/null
+++ b/Lib/concurrent/futures/process.py
@@ -0,0 +1,337 @@
+# 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    | => |         |
+|          |     +----------+       |        |     +-----------+    |         |
+|          |     | ...      |       |        |     | ...       |    |         |
+|          |     | 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 "Request Q"
+"""
+
+__author__ = 'Brian Quinlan (brian@sweetapp.com)'
+
+import atexit
+from concurrent.futures import _base
+import queue
+import multiprocessing
+import threading
+import weakref
+
+# Workers are created as daemon threads and processes. This is done to allow the
+# interpreter to exit when there are still idle processes in a
+# ProcessPoolExecutor's process 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/processes 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)
+
+# 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 _WorkItem(object):
+    def __init__(self, future, fn, args, kwargs):
+        self.future = future
+        self.fn = fn
+        self.args = args
+        self.kwargs = kwargs
+
+class _ResultItem(object):
+    def __init__(self, work_id, exception=None, result=None):
+        self.work_id = work_id
+        self.exception = exception
+        self.result = result
+
+class _CallItem(object):
+    def __init__(self, work_id, fn, args, kwargs):
+        self.work_id = work_id
+        self.fn = fn
+        self.args = args
+        self.kwargs = kwargs
+
+def _process_worker(call_queue, result_queue, shutdown):
+    """Evaluates calls from call_queue and places the results in result_queue.
+
+    This worker is run in a seperate process.
+
+    Args:
+        call_queue: A multiprocessing.Queue of _CallItems that will be read and
+            evaluated by the worker.
+        result_queue: A multiprocessing.Queue of _ResultItems that will written
+            to by the worker.
+        shutdown: A multiprocessing.Event that will be set as a signal to the
+            worker that it should exit when call_queue is empty.
+    """
+    while True:
+        try:
+            call_item = call_queue.get(block=True, timeout=0.1)
+        except queue.Empty:
+            if shutdown.is_set():
+                return
+        else:
+            try:
+                r = call_item.fn(*call_item.args, **call_item.kwargs)
+            except BaseException as e:
+                result_queue.put(_ResultItem(call_item.work_id,
+                                             exception=e))
+            else:
+                result_queue.put(_ResultItem(call_item.work_id,
+                                             result=r))
+
+def _add_call_item_to_queue(pending_work_items,
+                            work_ids,
+                            call_queue):
+    """Fills call_queue with _WorkItems from pending_work_items.
+
+    This function never blocks.
+
+    Args:
+        pending_work_items: A dict mapping work ids to _WorkItems e.g.
+            {5: <_WorkItem...>, 6: <_WorkItem...>, ...}
+        work_ids: A queue.Queue of work ids e.g. Queue([5, 6, ...]). Work ids
+            are consumed and the corresponding _WorkItems from
+            pending_work_items are transformed into _CallItems and put in
+            call_queue.
+        call_queue: A multiprocessing.Queue that will be filled with _CallItems
+            derived from _WorkItems.
+    """
+    while True:
+        if call_queue.full():
+            return
+        try:
+            work_id = work_ids.get(block=False)
+        except queue.Empty:
+            return
+        else:
+            work_item = pending_work_items[work_id]
+
+            if work_item.future.set_running_or_notify_cancel():
+                call_queue.put(_CallItem(work_id,
+                                         work_item.fn,
+                                         work_item.args,
+                                         work_item.kwargs),
+                               block=True)
+            else:
+                del pending_work_items[work_id]
+                continue
+
+def _queue_manangement_worker(executor_reference,
+                              processes,
+                              pending_work_items,
+                              work_ids_queue,
+                              call_queue,
+                              result_queue,
+                              shutdown_process_event):
+    """Manages the communication between this process and the worker processes.
+
+    This function is run in a local thread.
+
+    Args:
+        executor_reference: A weakref.ref to the ProcessPoolExecutor that owns
+            this thread. Used to determine if the ProcessPoolExecutor has been
+            garbage collected and that this function can exit.
+        process: A list of the multiprocessing.Process instances used as
+            workers.
+        pending_work_items: A dict mapping work ids to _WorkItems e.g.
+            {5: <_WorkItem...>, 6: <_WorkItem...>, ...}
+        work_ids_queue: A queue.Queue of work ids e.g. Queue([5, 6, ...]).
+        call_queue: A multiprocessing.Queue that will be filled with _CallItems
+            derived from _WorkItems for processing by the process workers.
+        result_queue: A multiprocessing.Queue of _ResultItems generated by the
+            process workers.
+        shutdown_process_event: A multiprocessing.Event used to signal the
+            process workers that they should exit when their work queue is
+            empty.
+    """
+    while True:
+        _add_call_item_to_queue(pending_work_items,
+                                work_ids_queue,
+                                call_queue)
+
+        try:
+            result_item = result_queue.get(block=True, timeout=0.1)
+        except queue.Empty:
+            executor = executor_reference()
+            # No more work items can be added if:
+            #   - The interpreter is shutting down OR
+            #   - The executor that owns this worker has been collected OR
+            #   - The executor that owns this worker has been shutdown.
+            if _shutdown or executor is None or executor._shutdown_thread:
+                # Since no new work items can be added, it is safe to shutdown
+                # this thread if there are no pending work items.
+                if not pending_work_items:
+                    shutdown_process_event.set()
+
+                    # If .join() is not called on the created processes then
+                    # some multiprocessing.Queue methods may deadlock on Mac OS
+                    # X.
+                    for p in processes:
+                        p.join()
+                    return
+            del executor
+        else:
+            work_item = pending_work_items[result_item.work_id]
+            del pending_work_items[result_item.work_id]
+
+            if result_item.exception:
+                work_item.future.set_exception(result_item.exception)
+            else:
+                work_item.future.set_result(result_item.result)
+
+class ProcessPoolExecutor(_base.Executor):
+    def __init__(self, max_workers=None):
+        """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.
+        """
+        _remove_dead_thread_references()
+
+        if max_workers is None:
+            self._max_workers = multiprocessing.cpu_count()
+        else:
+            self._max_workers = max_workers
+
+        # 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.
+        self._call_queue = multiprocessing.Queue(self._max_workers +
+                                                 EXTRA_QUEUED_CALLS)
+        self._result_queue = multiprocessing.Queue()
+        self._work_ids = queue.Queue()
+        self._queue_management_thread = None
+        self._processes = set()
+
+        # Shutdown is a two-step process.
+        self._shutdown_thread = False
+        self._shutdown_process_event = multiprocessing.Event()
+        self._shutdown_lock = threading.Lock()
+        self._queue_count = 0
+        self._pending_work_items = {}
+
+    def _start_queue_management_thread(self):
+        if self._queue_management_thread is None:
+            self._queue_management_thread = threading.Thread(
+                    target=_queue_manangement_worker,
+                    args=(weakref.ref(self),
+                          self._processes,
+                          self._pending_work_items,
+                          self._work_ids,
+                          self._call_queue,
+                          self._result_queue,
+                          self._shutdown_process_event))
+            self._queue_management_thread.daemon = True
+            self._queue_management_thread.start()
+            _thread_references.add(weakref.ref(self._queue_management_thread))
+
+    def _adjust_process_count(self):
+        for _ in range(len(self._processes), self._max_workers):
+            p = multiprocessing.Process(
+                    target=_process_worker,
+                    args=(self._call_queue,
+                          self._result_queue,
+                          self._shutdown_process_event))
+            p.start()
+            self._processes.add(p)
+
+    def submit(self, fn, *args, **kwargs):
+        with self._shutdown_lock:
+            if self._shutdown_thread:
+                raise RuntimeError('cannot schedule new futures after 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
+
+            self._start_queue_management_thread()
+            self._adjust_process_count()
+            return f
+    submit.__doc__ = _base.Executor.submit.__doc__
+
+    def shutdown(self, wait=True):
+        with self._shutdown_lock:
+            self._shutdown_thread = True
+        if wait:
+            if self._queue_management_thread:
+                self._queue_management_thread.join()
+        # To reduce the risk of openning too many files, remove references to
+        # objects that use file descriptors.
+        self._queue_management_thread = None
+        self._call_queue = None
+        self._result_queue = None
+        self._shutdown_process_event = None
+        self._processes = None
+    shutdown.__doc__ = _base.Executor.shutdown.__doc__
+
+atexit.register(_python_exit)
diff --git a/Lib/concurrent/futures/thread.py b/Lib/concurrent/futures/thread.py
new file mode 100644
index 0000000..15736da
--- /dev/null
+++ b/Lib/concurrent/futures/thread.py
@@ -0,0 +1,136 @@
+# 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 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 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 as e:
+            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 as e:
+        _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__