New submission from OscaTutenchamon:
I've experienced that queue can be deadlocked for reading when:
1. The "reader" process is using `get` with timeout > 0:
self.queue.get(timeout=3)
2. "reader" dies while `get` is blocking due to timeout.
After that queue's `_rlock` is never being released so reading becomes
impossible.
Application demonstrating the problem.
I create two child processes "Worker" (putting into queue) and "Receiver"
(getting from queue). Also parent process periodically checks if his children
are alive and starts new child if needed.
#!/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()
Colored version here: http://pastebin.com/YifY63vE
Console output.
$ 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
System information.
$ uname -sr
Linux 3.11.8-200.fc19.x86_64
$ python -V
Python 2.7.5
In [3]: multiprocessing.__version__
Out[3]: '0.70a1'
----------
components: Library (Lib)
messages: 210369
nosy: OscaTutenchamon
priority: normal
severity: normal
status: open
title: multiprocessing.Queue deadlocks after “reader” process death
type: behavior
versions: Python 2.7
_______________________________________
Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue20527>
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