The design of ProcessPoolExecutor contains some possible race conditions that may freeze the interpreter due to deadlocks. This is notably the case with pickling and unpickling errors for a submitted job and returned results. This makes it hard to reuse a launched executor. We propose in the joint PR to fix some of those situations to make the ProcessPoolExecutor more robust to failure in the different threads and worker.
The deadlocks I refer to in this issue are fixed by the PR #3895. Subsequent failures (like the fact that the Executor is set in a broken state when there is an unpickling error) are tracked in other issues so I think it is safe to close this one.
