[New-bugs-announce] [issue21913] Possible deadlock in threading.Condition.wait() in Python 2.7.7

[Sangeeth Saravanaraj]

New submission from Sangeeth Saravanaraj:

Python version 2.7.7
Mac OS Darwin Kernel Version 13.2.0

I have the following code which when executed waits to be interrupted by SIGINT, SIGTERM or SIGQUIT. When an object is initialized, it creates a threading.Condition() and acquires() it! The program then registers the signal handlers where notify() and release() is called when the above mentioned signals are received. After registering the signal handlers, it calls wait() on the condition variable and block.

When I tried to stop the program with Ctrl-C, its did not respond. IOW, the _signal_handler() method did not get called.  

# start

from signal import signal, SIGINT, SIGTERM, SIGQUIT
from threading import Condition

class A:
    def __init__(self):
        self._termination_signal = Condition()
        self._termination_signal.acquire(blocking=0)

    def _signal_handler(self, signum, frame):
        print "Received terminate request - signal = {0}".format(signum)
        del frame
        self._termination_signal.notify()
        self._termination_signal.release()
        return

    def register_and_wait(self):
        signal(SIGINT, self._signal_handler)
        signal(SIGTERM, self._signal_handler)
        signal(SIGQUIT, self._signal_handler)
        print "Waiting to be interrupted!"
        self._termination_signal.wait()      # control blocks here!
        print "Notified!!"

def main():
    a = A()
    a.register_and_wait()

if __name__ == "__main__":
    main()

# end


When the same code was tried in Python 3.4, it threw a "RuntimeError: cannot notify on un-acquired lock".

More information is available in this conversation in python-list mailer - https://mail.python.org/pipermail/python-list/2014-July/674350.html

----------
components: Library (Lib)
messages: 222250
nosy: pitrou, sangeeth
priority: normal
severity: normal
status: open
title: Possible deadlock in threading.Condition.wait() in Python 2.7.7
type: behavior
versions: Python 2.7

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue21913>
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