Jeffrey Kintscher <websur...@surf2c.net> added the comment:
It looks like the problem is that Thread._tstate_lock doesn't get released until Thread.join() is called. _tstate_lock is of type LockType, which has different definitions when using the threading module or the dummy_threading module. It is defined in Modules/_threadmodule.c when using the threading module, and in Lib/_dummy_thread.py when using the dummy_threading module. The lock is acquired when the new thread starts and is supposed to be released when the thread exits. Thread.is_alive() and Thread.join() both call Thread._wait_for_tstate_lock(), which in turn calls Thread._tstate_lock.acquire(). When Thread._wait_for_tstate_lock() successfully acquires the lock, it calls Thread._stop() to set the Thread._is_stop flag to indicate that the thread has exited. The Thread._is_stop flag is checked by Thread.is_alive() before trying to acquire the lock. Thread.is_alive() passes False to Thread._wait_for_tstate_lock(), while Thread.join() effectively passes True as the default parameter. Thread._wait_for_tstate_lock() then passes the parameter value to Thread._tstate_lock.acquire() to indicate whether to block until the lock is acquired (True) or try to acquire the lock and return immediately (False). The return value of the LockType.acquire() function indicates whether (True) or not (False) the lock was acquired. The function defined in the dummy_threading module always returns True when passed True and False when passed False. Since Thread.is_alive() passes False to Thread._wait_for_tstate_lock() and onwards to Thread._tstate_lock.acquire(), Thread._tstate_lock.acquire() returns False which causes Thread._wait_for_tstate_lock() to skip calling Thread._stop() and the Thread._is_stop flag doesn't get set. Thread.is_alive() returns "not self._is_stop", so it always returns True. Thread.join() passes True, so Thread._tstate_lock.acquire() returns True and Thread._wait_for_tstate_lock() calls Thread._stop to set the Thread._is_stop flag. Subsequent calls to Thread.is_alive() see that Thread._is_stop flag is set and return False (i.e. "not self._is_stop") without checking the lock. In a nutshell, the way the code is written, Thread.is_alive() always returns True until after Thread.join() is called. ---------- versions: +Python 3.7 _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue33777> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
