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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 ;-)