I'm getting a headache now - LOL ;-)  Thanks for the explanation!

What I still don't understand:  the new lock is an internal implementation detail.  How would it gain a weakref with a callback?  Users aren't going to mess with this lock, and if you want to stop Python maintainers from giving it a weakref with a callback, simply say they shouldn't do that (in the code comments) - you could even add code verifying it doesn't have any weakrefs outstanding (although that would likely be a waste of time and code:  no maintainer is going to _want_ to make a weakref to it, let alone a weakref with a callback).

My concern is the bulk and obscurity of this code, all to plug such a minor hole.  I call it "minor" because it's been reported once in the history of the project, and Tamas wormed around it with a 1-liner (added a sleep).

Granted, it's much harder to fix "for real" and when most of the interpreter has been destroyed ;-)