As it turned out, this particular feature of POSIX pthreads was quite tricky to implement correctly, and the broader C library community does not have a definitive answer for how to correctly approach the problem

I think that’s not true. The definitive answer is: do not use this feature.

Congrats on all of the work, it sounds like you’ve worked through a lot of terrifying corner cases. At the same time, I hope no one ever uses it.