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Multiprocessing's process pool originally used a finalizer to shutdown the pool when the pool object is garbage collected.
Since the maxtasksperchild feature was added, the worker_handler thread holds a reference to the pool, preventing garbage collection at least until that thread finished.
In some cases the last reference to the pool is owned by the worker_handler thread, and as it exits, the finalizer is triggered. Since the finalizer tries to join the worker_handler thread, an error will be raised since a thread cannot join itself.
It would have been better if pools had used the context manager protocol rather than using RAII -- see Issue #15064.
Unless/until RAII usage is fixed, a quick fix would be to just make the finalizer skip trying to join the current thread.