Comment 3 for bug 413335

Description of the situation:

Prior to Python 2.5.1, the atexit handlers were executed when the *main* thread exits. However, http://bugs.python.org/issue1566280 was then filed, the essence of which is that the atexit handlers could tear down cross-thread resources that were still in use by other threads.

For this reason, the shutdown procedure was modified in Python 2.5.1 - now, when the main thread exits, it first makes a private call to threading._shutdown(), which waits until all non-daemon threads have exited, and *only then* proceeds to run the atexit handlers.

Herein lies the problem. zope.sendmail.delivery.QueueProcessorThread attempts to use atexit to notify itself when it should shut down. However, the Python runtime >= 2.5.1 will wait for the QueueProcessorThread to exit before it calls the atexit handlers!

Potential solutions:

(1) To gain behaviour most similar to earlier Python versions, an evil monkeypatch into Python internals like this: http://bazaar.launchpad.net/~maxb/zope3/launchpad-3.4-py2.5/revision/9 works.

(2) Make the QueueProcessorThread a daemon thread. It will be terminated without notification when the interpreter exits, but the current QueueProcessorThread does not attempt to ensure its queue is completely flushed before shutdown anyway, so that should not matter.

I will attach a small Python demo program useful for illustrating and exploring the issue.