1# Copyright 2009 Brian Quinlan. All Rights Reserved.
2# Licensed to PSF under a Contributor Agreement.
3
4"""Implements ThreadPoolExecutor."""
5
6__author__ = 'Brian Quinlan (brian@sweetapp.com)'
7
8import atexit
9from concurrent.futures import _base
10import itertools
11import queue
12import threading
13import weakref
14import os
15
16# Workers are created as daemon threads. This is done to allow the interpreter
17# to exit when there are still idle threads in a ThreadPoolExecutor's thread
18# pool (i.e. shutdown() was not called). However, allowing workers to die with
19# the interpreter has two undesirable properties:
20#   - The workers would still be running during interpreter shutdown,
21#     meaning that they would fail in unpredictable ways.
22#   - The workers could be killed while evaluating a work item, which could
23#     be bad if the callable being evaluated has external side-effects e.g.
24#     writing to a file.
25#
26# To work around this problem, an exit handler is installed which tells the
27# workers to exit when their work queues are empty and then waits until the
28# threads finish.
29
30_threads_queues = weakref.WeakKeyDictionary()
31_shutdown = False
32
33def _python_exit():
34    global _shutdown
35    _shutdown = True
36    items = list(_threads_queues.items())
37    for t, q in items:
38        q.put(None)
39    for t, q in items:
40        t.join()
41
42atexit.register(_python_exit)
43
44
45class _WorkItem(object):
46    def __init__(self, future, fn, args, kwargs):
47        self.future = future
48        self.fn = fn
49        self.args = args
50        self.kwargs = kwargs
51
52    def run(self):
53        if not self.future.set_running_or_notify_cancel():
54            return
55
56        try:
57            result = self.fn(*self.args, **self.kwargs)
58        except BaseException as exc:
59            self.future.set_exception(exc)
60            # Break a reference cycle with the exception 'exc'
61            self = None
62        else:
63            self.future.set_result(result)
64
65
66def _worker(executor_reference, work_queue, initializer, initargs):
67    if initializer is not None:
68        try:
69            initializer(*initargs)
70        except BaseException:
71            _base.LOGGER.critical('Exception in initializer:', exc_info=True)
72            executor = executor_reference()
73            if executor is not None:
74                executor._initializer_failed()
75            return
76    try:
77        while True:
78            work_item = work_queue.get(block=True)
79            if work_item is not None:
80                work_item.run()
81                # Delete references to object. See issue16284
82                del work_item
83
84                # attempt to increment idle count
85                executor = executor_reference()
86                if executor is not None:
87                    executor._idle_semaphore.release()
88                del executor
89                continue
90
91            executor = executor_reference()
92            # Exit if:
93            #   - The interpreter is shutting down OR
94            #   - The executor that owns the worker has been collected OR
95            #   - The executor that owns the worker has been shutdown.
96            if _shutdown or executor is None or executor._shutdown:
97                # Flag the executor as shutting down as early as possible if it
98                # is not gc-ed yet.
99                if executor is not None:
100                    executor._shutdown = True
101                # Notice other workers
102                work_queue.put(None)
103                return
104            del executor
105    except BaseException:
106        _base.LOGGER.critical('Exception in worker', exc_info=True)
107
108
109class BrokenThreadPool(_base.BrokenExecutor):
110    """
111    Raised when a worker thread in a ThreadPoolExecutor failed initializing.
112    """
113
114
115class ThreadPoolExecutor(_base.Executor):
116
117    # Used to assign unique thread names when thread_name_prefix is not supplied.
118    _counter = itertools.count().__next__
119
120    def __init__(self, max_workers=None, thread_name_prefix='',
121                 initializer=None, initargs=()):
122        """Initializes a new ThreadPoolExecutor instance.
123
124        Args:
125            max_workers: The maximum number of threads that can be used to
126                execute the given calls.
127            thread_name_prefix: An optional name prefix to give our threads.
128            initializer: A callable used to initialize worker threads.
129            initargs: A tuple of arguments to pass to the initializer.
130        """
131        if max_workers is None:
132            # ThreadPoolExecutor is often used to:
133            # * CPU bound task which releases GIL
134            # * I/O bound task (which releases GIL, of course)
135            #
136            # We use cpu_count + 4 for both types of tasks.
137            # But we limit it to 32 to avoid consuming surprisingly large resource
138            # on many core machine.
139            max_workers = min(32, (os.cpu_count() or 1) + 4)
140        if max_workers <= 0:
141            raise ValueError("max_workers must be greater than 0")
142
143        if initializer is not None and not callable(initializer):
144            raise TypeError("initializer must be a callable")
145
146        self._max_workers = max_workers
147        self._work_queue = queue.SimpleQueue()
148        self._idle_semaphore = threading.Semaphore(0)
149        self._threads = set()
150        self._broken = False
151        self._shutdown = False
152        self._shutdown_lock = threading.Lock()
153        self._thread_name_prefix = (thread_name_prefix or
154                                    ("ThreadPoolExecutor-%d" % self._counter()))
155        self._initializer = initializer
156        self._initargs = initargs
157
158    def submit(*args, **kwargs):
159        if len(args) >= 2:
160            self, fn, *args = args
161        elif not args:
162            raise TypeError("descriptor 'submit' of 'ThreadPoolExecutor' object "
163                            "needs an argument")
164        elif 'fn' in kwargs:
165            fn = kwargs.pop('fn')
166            self, *args = args
167            import warnings
168            warnings.warn("Passing 'fn' as keyword argument is deprecated",
169                          DeprecationWarning, stacklevel=2)
170        else:
171            raise TypeError('submit expected at least 1 positional argument, '
172                            'got %d' % (len(args)-1))
173
174        with self._shutdown_lock:
175            if self._broken:
176                raise BrokenThreadPool(self._broken)
177
178            if self._shutdown:
179                raise RuntimeError('cannot schedule new futures after shutdown')
180            if _shutdown:
181                raise RuntimeError('cannot schedule new futures after '
182                                   'interpreter shutdown')
183
184            f = _base.Future()
185            w = _WorkItem(f, fn, args, kwargs)
186
187            self._work_queue.put(w)
188            self._adjust_thread_count()
189            return f
190    submit.__text_signature__ = _base.Executor.submit.__text_signature__
191    submit.__doc__ = _base.Executor.submit.__doc__
192
193    def _adjust_thread_count(self):
194        # if idle threads are available, don't spin new threads
195        if self._idle_semaphore.acquire(timeout=0):
196            return
197
198        # When the executor gets lost, the weakref callback will wake up
199        # the worker threads.
200        def weakref_cb(_, q=self._work_queue):
201            q.put(None)
202
203        num_threads = len(self._threads)
204        if num_threads < self._max_workers:
205            thread_name = '%s_%d' % (self._thread_name_prefix or self,
206                                     num_threads)
207            t = threading.Thread(name=thread_name, target=_worker,
208                                 args=(weakref.ref(self, weakref_cb),
209                                       self._work_queue,
210                                       self._initializer,
211                                       self._initargs))
212            t.daemon = True
213            t.start()
214            self._threads.add(t)
215            _threads_queues[t] = self._work_queue
216
217    def _initializer_failed(self):
218        with self._shutdown_lock:
219            self._broken = ('A thread initializer failed, the thread pool '
220                            'is not usable anymore')
221            # Drain work queue and mark pending futures failed
222            while True:
223                try:
224                    work_item = self._work_queue.get_nowait()
225                except queue.Empty:
226                    break
227                if work_item is not None:
228                    work_item.future.set_exception(BrokenThreadPool(self._broken))
229
230    def shutdown(self, wait=True):
231        with self._shutdown_lock:
232            self._shutdown = True
233            self._work_queue.put(None)
234        if wait:
235            for t in self._threads:
236                t.join()
237    shutdown.__doc__ = _base.Executor.shutdown.__doc__
238