Oh, I think that I found a deadlock (or something like that):
----------------------------
import concurrent.futures
import faulthandler
import os
import signal
import time

def work(n):
    time.sleep(0.1)

def main():
    faulthandler.register(signal.SIGUSR1)
    print("pid: %s" % os.getpid())
    with concurrent.futures.ProcessPoolExecutor() as executor:
        for number, prime in executor.map(work, range(100)):
            print("shutdown")
            executor.shutdown()
            print("shutdown--")

if __name__ == '__main__':
    main()
----------------------------

Trace:
----------------------------
Thread 0x00007fbfc83bd700:
  File "/home/haypo/prog/HG/cpython/Lib/threading.py", line 237 in wait
  File "/home/haypo/prog/HG/cpython/Lib/multiprocessing/queues.py", line 252 in _feed
  File "/home/haypo/prog/HG/cpython/Lib/threading.py", line 690 in run
  File "/home/haypo/prog/HG/cpython/Lib/threading.py", line 737 in _bootstrap_inner
  File "/home/haypo/prog/HG/cpython/Lib/threading.py", line 710 in _bootstrap

Thread 0x00007fbfc8bbe700:
  File "/home/haypo/prog/HG/cpython/Lib/multiprocessing/queues.py", line 101 in put
  File "/home/haypo/prog/HG/cpython/Lib/concurrent/futures/process.py", line 268 in _queue_management_worker
  File "/home/haypo/prog/HG/cpython/Lib/threading.py", line 690 in run
  File "/home/haypo/prog/HG/cpython/Lib/threading.py", line 737 in _bootstrap_inner
  File "/home/haypo/prog/HG/cpython/Lib/threading.py", line 710 in _bootstrap

Current thread 0x00007fbfcc2e3700:
  File "/home/haypo/prog/HG/cpython/Lib/threading.py", line 237 in wait
  File "/home/haypo/prog/HG/cpython/Lib/threading.py", line 851 in join
  File "/home/haypo/prog/HG/cpython/Lib/concurrent/futures/process.py", line 395 in shutdown
  File "/home/haypo/prog/HG/cpython/Lib/concurrent/futures/_base.py", line 570 in __exit__
  File "y.py", line 17 in main
  File "y.py", line 20 in <module>
----------------------------
There are two child processes, but both are zombies (displayed as "<defunct>" by ps). Send SIGUSR1 signal to the frozen process to display the traceback (thanks to faulthandler).