python - 线程和条件 : debugging the acquirement

Question

我的一个程序中有一个奇怪的问题，一个线程获取一个条件，而另一个线程告诉我没有获取该条件。

为了知道线程是否获得了条件，我做了一些调试信息，看起来是的，他做到了。但是另一个线程告诉我条件没有获得。

这是关于我是如何做到的以及我得到的输出的总结(不是很短，对此感到抱歉):

import logging
import time
from threading import Condition, Lock, Thread
logging.basicConfig(level=logging.DEBUG)

class MyClass:

    def __init__(self):

        self._is_alive = False
        self._thread_update = None
        self._conditioned_thread = None
        self._thread_condition = Condition(Lock()) # Or RLock() or nothing, same issue

    def start(self):

        self._is_alive = True
        self._thread_update = Thread(target=self._loop_update, name="Updater")
        self._conditioned_thread = Thread(target=self._loop_conditioned, name="Conditioned")
        self._thread_update.start()
        self._conditioned_thread.start()

    def _loop_conditioned(self):
        logging.debug("Starting conditioned thread")

        with self._thread_condition:
            while self._is_alive:
                logging.debug("Awaiting... Is acquired ? %s", self._thread_condition._is_owned())
                self._thread_condition.wait()
                logging.debug("Success !")

    def _loop_update(self):
        time.sleep(1)
        logging.debug("Notifying ! Is acquired ? %s", self._thread_condition._is_owned())
        self._thread_condition.notify(1)
        # Do some stuff

    def stop(self):

        self._is_alive = False
        self._thread_condition.notify()
        self._thread_update.join()
        self._thread_condition.join()

if __name__ == "__main__":
    c = MyClass()
    c.start()
    time.sleep(4)
    c.stop()

这是输出:

DEBUG:root:Starting conditioned thread
DEBUG:root:Awaiting... Is acquired ? True
DEBUG:root:Notifying ! Is acquired ? False
Exception in thread Updater:
Traceback (most recent call last):
  File "/usr/lib64/python2.6/threading.py", line 532, in __bootstrap_inner
    self.run()
  File "/usr/lib64/python2.6/threading.py", line 484, in run
    self.__target(*self.__args, **self.__kwargs)
  File ".../test.py", line 39, in _loop_update
    self._thread_condition.notify(1)
  File "/usr/lib64/python2.6/threading.py", line 274, in notify
    raise RuntimeError("cannot notify on un-acquired lock")
RuntimeError: cannot notify on un-acquired lock

Traceback (most recent call last):
  File ".../test.py", line 53, in <module>
    c.stop()
  File ".../test.py", line 45, in stop
    self._thread_condition.notify()
  File "/usr/lib64/python2.6/threading.py", line 274, in notify
    raise RuntimeError("cannot notify on un-acquired lock")
RuntimeError: cannot notify on un-acquired lock

问题是为什么我有 RuntimeError，即使我已经获得了条件？

Answer 1

我不是线程专家，但是 documentation有这样的话:

wait(timeout=None)

Wait until notified or until a timeout occurs. Ifthe calling thread has not acquired the lock when this method iscalled, a RuntimeError is raised.

This method releases the underlying lock, and then blocks until it isawakened by a notify() or notify_all() call for the same conditionvariable in another thread, or until the optional timeout occurs. Onceawakened or timed out, it re-acquires the lock and returns.

(强调我的)。

因此，调用 wait() 似乎释放了 _conditioned_thread 线程获取的锁。可能，该锁然后需要以某种方式被另一个线程获取(可能在循环中检查锁是否可用)；然后，另一个线程可以通知(1)另一个线程继续(并重新获取锁，按照上面的最后一句话)。