multiprocessing.Process generates FileNotFoundError when argument isn't explicitly referenced

Reproduisez le script fourni sous Linux avec la méthode de démarrage spawn, puis lisez multiprocessing/spawn.py et multiprocessing/synchronize.py autour de la traceback. Comparez l’argument multiprocessing.Value inline avec la valeur référencée explicitement et le comportement de fork. Le travail est considéré comme terminé lorsque le FileNotFoundError signalé est résolu ou clairement documenté, avec une couverture de régression pour le cas reproductible.

Bug report
This is a continuation for the possible bug mentioned in issue https://github.com/python/cpython/issues/82236 which was closed because DonnyBrown, the submitter, didn't provide enough information.

DonnyBrown was getting a FileNotFoundError when starting a process with multiprocessing.Process that uses an argument that doesn't have an explicit reference. I'm able to reproduce the same error using the test code DonnyBrown provided in that issue on Ubuntu Desktop LTS 22.04 x86-64 with CPython 3.10.4. @iritkatriel mentioned that they were unable to reproduce the error on Windows 10 with Python 3.10.

I can also reproduce the error using this slightly modified/simpler version of DonnyBrown's test code that I have been testing:

import multiprocessing

def demo(argument):
    print(argument)

if __name__=="__main__":
    multiprocessing.set_start_method("spawn") # Changing this to "fork" (on platforms where it is
                                              # available) can also cause the below code to work.


    process=multiprocessing.Process(target=demo, args=[multiprocessing.Value("i", 0)]) # FAILS

    #process=multiprocessing.Process(target=demo, args=[0])                            # WORKS

    #reference_To_Number=multiprocessing.Value("i", 0)                                 # WORKS
    #process=multiprocessing.Process(target=demo, args=[reference_To_Number])


    process.start()
    process.join()

The traceback I get with the above code is:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/lib/python3.10/multiprocessing/spawn.py", line 116, in spawn_main
    exitcode = _main(fd, parent_sentinel)
  File "/usr/lib/python3.10/multiprocessing/spawn.py", line 126, in _main
    self = reduction.pickle.load(from_parent)
  File "/usr/lib/python3.10/multiprocessing/synchronize.py", line 110, in __setstate__
    self._semlock = _multiprocessing.SemLock._rebuild(*state)
FileNotFoundError: [Errno 2] No such file or directory

The above code can be made to work on my test system by making any of the following changes:

• Change the process start method to "fork" instead.
• Change the process argument to a simple integer instead of a multiprocessing.Value.
• Assign the multiprocessing.Value to a variable and change the process argument to use the variable.

I'm not a Python expert so maybe this is the expected behavior when spawning a process directly with a multiprocessing.Value but it does seem odd that making any of the above mentioned changes causes the code to work or that (based on @iritkatriel's success with DonnyBrown's test code) running it on Windows 10 (which uses the "spawn" start method) will probably cause the code to work.

Your environment

• CPython versions tested on: 3.10.4
• Operating system and architecture: Linux, Ubuntu Desktop LTS 22.04, x86-64