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I converted the example into attached file mp_exit.py and I added a call to faulthandler to see what is going on.
Output with the master branch of Python:
vstinner@apu$ ~/python/master/python ~/mp_exit.py 
Timeout (0:00:05)!
Thread 0x00007ff40139a700 (most recent call first):
 File "/home/vstinner/python/master/Lib/multiprocessing/connection.py", line 379 in _recv
 File "/home/vstinner/python/master/Lib/multiprocessing/connection.py", line 414 in _recv_bytes
 File "/home/vstinner/python/master/Lib/multiprocessing/connection.py", line 250 in recv
 File "/home/vstinner/python/master/Lib/multiprocessing/pool.py", line 576 in _handle_results
 File "/home/vstinner/python/master/Lib/threading.py", line 882 in run
 File "/home/vstinner/python/master/Lib/threading.py", line 944 in _bootstrap_inner
 File "/home/vstinner/python/master/Lib/threading.py", line 902 in _bootstrap
Thread 0x00007ff401b9b700 (most recent call first):
 File "/home/vstinner/python/master/Lib/multiprocessing/pool.py", line 528 in _handle_tasks
 File "/home/vstinner/python/master/Lib/threading.py", line 882 in run
 File "/home/vstinner/python/master/Lib/threading.py", line 944 in _bootstrap_inner
 File "/home/vstinner/python/master/Lib/threading.py", line 902 in _bootstrap
Thread 0x00007ff40239c700 (most recent call first):
 File "/home/vstinner/python/master/Lib/selectors.py", line 415 in select
 File "/home/vstinner/python/master/Lib/multiprocessing/connection.py", line 930 in wait
 File "/home/vstinner/python/master/Lib/multiprocessing/pool.py", line 499 in _wait_for_updates
 File "/home/vstinner/python/master/Lib/multiprocessing/pool.py", line 519 in _handle_workers
 File "/home/vstinner/python/master/Lib/threading.py", line 882 in run
 File "/home/vstinner/python/master/Lib/threading.py", line 944 in _bootstrap_inner
 File "/home/vstinner/python/master/Lib/threading.py", line 902 in _bootstrap
Thread 0x00007ff4102cf740 (most recent call first):
 File "/home/vstinner/python/master/Lib/threading.py", line 303 in wait
 File "/home/vstinner/python/master/Lib/threading.py", line 565 in wait
 File "/home/vstinner/python/master/Lib/multiprocessing/pool.py", line 759 in wait
 File "/home/vstinner/python/master/Lib/multiprocessing/pool.py", line 762 in get
 File "/home/vstinner/python/master/Lib/multiprocessing/pool.py", line 364 in map
 File "/home/vstinner/mp_exit.py", line 12 in <module>
In the main process, Pool._handle_results() thread is blocked on os.read() which never completes, even if the child process died and so the other end of the pipe should be closed.