python - multiprocessing.Queue deadlocks after "reader"进程死亡

Question

我一直在玩 multiprocessing 包并注意到在以下情况下队列可能会因读取而死锁:

1. “阅读器”进程正在使用 get 超时> 0:

   self.queue.get(timeout=3)
   

2. “读者”在 get 时死亡由于超时而阻塞。

在该队列被永久锁定之后。

演示问题的应用

我创建了两个子进程“Worker”(放入队列)和“Receiver”(从队列中取出)。父进程也定期检查他的 child 是否are alive如果需要，开始新的 child 。

#!/usr/bin/env python
# -*- coding: utf-8 -*-

import multiprocessing
import procname
import time

class Receiver(multiprocessing.Process):
    ''' Reads from queue with 3 secs timeout '''

    def __init__(self, queue):
        multiprocessing.Process.__init__(self)
        self.queue = queue

    def run(self):
        procname.setprocname('Receiver')
        while True:
            try:
                msg = self.queue.get(timeout=3)
                print '<<< `{}`, queue rlock: {}'.format(
                    msg, self.queue._rlock)
            except multiprocessing.queues.Empty:
                print '<<< EMPTY, Queue rlock: {}'.format(
                    self.queue._rlock)
                pass


class Worker(multiprocessing.Process):
    ''' Puts into queue with 1 sec sleep '''

    def __init__(self, queue):
        multiprocessing.Process.__init__(self)
        self.queue = queue

    def run(self):
        procname.setprocname('Worker')
        while True:
            time.sleep(1)
            print 'Worker: putting msg, Queue size: ~{}'.format(
                self.queue.qsize())
            self.queue.put('msg from Worker')


if __name__ == '__main__':
    queue = multiprocessing.Queue()

    worker = Worker(queue)
    worker.start()

    receiver = Receiver(queue)
    receiver.start()

    while True:
        time.sleep(1)
        if not worker.is_alive():
            print 'Restarting worker'
            worker = Worker(queue)
            worker.start()
        if not receiver.is_alive():
            print 'Restarting receiver'
            receiver = Receiver(queue)
            receiver.start()

进程树在 ps 中的样子

bash
 \_ python queuetest.py
     \_ Worker
     \_ Receiver

控制台输出

$ python queuetest.py
Worker: putting msg, Queue size: ~0
<<< `msg from Worker`, queue rlock: <Lock(owner=None)>
Worker: putting msg, Queue size: ~0
<<< `msg from Worker`, queue rlock: <Lock(owner=None)>
Restarting receiver                        <-- killed Receiver with SIGTERM
Worker: putting msg, Queue size: ~0
Worker: putting msg, Queue size: ~1
Worker: putting msg, Queue size: ~2
<<< EMPTY, Queue rlock: <Lock(owner=SomeOtherProcess)>
Worker: putting msg, Queue size: ~3
Worker: putting msg, Queue size: ~4
Worker: putting msg, Queue size: ~5
<<< EMPTY, Queue rlock: <Lock(owner=SomeOtherProcess)>
Worker: putting msg, Queue size: ~6
Worker: putting msg, Queue size: ~7

有什么办法可以绕过这个吗？使用get_nowait与 sleep 相结合似乎是某种解决方法，但它不会“按原样”读取数据。

系统信息

$ uname -sr
Linux 3.11.8-200.fc19.x86_64

$ python -V
Python 2.7.5

In [3]: multiprocessing.__version__
Out[3]: '0.70a1'

“一切正常”的解决方案

在写这个问题时，我对 Receiver 类进行了一些愚蠢的修改:

class Receiver(multiprocessing.Process):

    def __init__(self, queue):
        multiprocessing.Process.__init__(self)
        self.queue = queue

    def run(self):
        procname.setprocname('Receiver')
        while True:
            time.sleep(1)
            while True:
                try:
                    msg = self.queue.get_nowait()
                    print '<<< `{}`, queue rlock: {}'.format(
                        msg, self.queue._rlock)
                except multiprocessing.queues.Empty:
                    print '<<< EMPTY, Queue rlock: {}'.format(
                        self.queue._rlock)
                    break

但是我觉得不太好。

Answer 1

这可能是因为 Queue.get() 中的 *not_empty.release()* 从未发生过(进程已经被杀死)。您是否尝试在 Receiver 中捕获 TERM 信号并在退出前释放 Queue mutex？