multiprocessing.Process generates FileNotFoundError when argument isn't explicitly referenced
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Description
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
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Reproduce the supplied script on Linux with the spawn start method, then read multiprocessing/spawn.py and multiprocessing/synchronize.py around the traceback. Compare the inline multiprocessing.Value argument with the explicitly referenced value and the fork behavior. Done means the reported FileNotFoundError is resolved or clearly documented, with regression coverage for the reproducer.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- operating-systems
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 45/100