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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>
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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).