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.

New changeset 4c07b9c49b75 by Richard Oudkerk in branch '2.7':
Issue #15101: Make pool finalizer avoid joining current thread
http://hg.python.org/cpython/rev/4c07b9c49b75

New changeset e1cd1f430ff1 by Richard Oudkerk in branch '3.2':
Issue #15101: Make pool finalizer avoid joining current thread.
http://hg.python.org/cpython/rev/e1cd1f430ff1

New changeset 59e0a51c5fc3 by Richard Oudkerk in branch 'default':
Issue #15101: Make pool finalizer avoid joining current thread.
http://hg.python.org/cpython/rev/59e0a51c5fc3