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The root cause for the reference leak is the global set threading._shutdown_locks. It contains Thread._tstate_lock locks of non-daemon threads. If a non-daemon thread terminates and no other thread joins the terminated thread, the _tstate_lock remains in threading._shutdown_locks forever.
I could imagine that a long running server could accumulate many locks in threading._shutdown_locks over time. Therefore the leak should be fixed.
There are probably several ways to deal with this issue. A straight forward approach is to discard the lock from within `tstate->on_delete` hook, that is function "void release_sentinel(void *)" in _threadmodule.c. Pull request (GH-15175) implements this idea. Eventually I should add another C-Python specific test-case to the PR.