Suggested patch: https://bitbucket.org/ecatmur/cpython/compare/19096-multiprocessing-race..#diff

Move the worker_handler.join() to immediately after setting the worker handler thread state to TERMINATE.  This is a safe change as nothing in the moved-over code affects the worker handler thread, except by terminating workers which is precisely what we don't want to happen.  In addition, this is near-equivalent behaviour to current close() + join(), which is well-tested.

Also: write tests; and modify Pool.__init__ to refer to its static methods using self rather than class name, to make them overridable for testing purposes.