When map is called, a MapResult object is created, which adds itself to the Pool's result cache.
When the pool is shut down, the result handler thread waits until the cache drains (while cache and thread._state != TERMINATE). But since no result is posted to the result queue (since the iterable is empty), the result handler never receives any task, and never gets to drain the cache. It thus waits forever on the recv on the result queue.