Daemon threads keep running until they finish or until finalization starts. For the latter, there is a check right after the thread acquires the GIL which causes the thread to exit if runtime finalization has started. [1] However, there are functions in the C-API that facilitate acquiring the GIL, but do not cause the thread to exit during finalization: PyEval_AcquireLock() PyEval_AcquireThread() Daemon threads that acquire the GIL through these can cause a deadlock during finalization. (See issue #36469.) They should probably be updated to match what PyEval_RestoreThread() does. [1] see PyEval_RestoreThread() and the eval loop, in PyEval_EvalFrameEx()
I am not comfortable to backport this change to Python 3.7. It's too early to know how it will impact applications and how many complains we will get :-) If someone really wants to backport this scary change to 3.7, I would suggest to wait for 1 month after Python 3.8.0 final release. I close the issue. See bpo-36479 for the follow-up.
