```
from concurrent.futures import ProcessPoolExecutor
import time

t = ProcessPoolExecutor(max_workers=3)
t.map(time.sleep, [1,2,3])
t.shutdown(wait=False)
```

Results in this exception and then a hang (i.e. Python doesn't terminate):
```
Exception in thread QueueManagerThread:
Traceback (most recent call last):
  File "/usr/local/google/home/bquinlan/cpython/Lib/threading.py", line 944, in _bootstrap_inner
    self.run()
  File "/usr/local/google/home/bquinlan/cpython/Lib/threading.py", line 882, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/local/google/home/bquinlan/cpython/Lib/concurrent/futures/process.py", line 352, in _queue_management_worker
    _add_call_item_to_queue(pending_work_items,
  File "/usr/local/google/home/bquinlan/cpython/Lib/concurrent/futures/process.py", line 280, in _add_call_item_to_queue
    call_queue.put(_CallItem(work_id,
  File "/usr/local/google/home/bquinlan/cpython/Lib/multiprocessing/queues.py", line 82, in put
    raise ValueError(f"Queue {self!r} is closed")
ValueError: Queue <concurrent.futures.process._SafeQueue object at 0x7f371c4ae7f0> is closed
```

New changeset 884eb89d4a5cc8e023deaa65001dfa74a436694c by Brian Quinlan in branch 'master':
bpo-39205: Tests that highlight a hang on ProcessPoolExecutor shutdown (#18221)
https://github.com/python/cpython/commit/884eb89d4a5cc8e023deaa65001dfa74a436694c