multiprocessing forkserver main contains dead main_path= handling code #126631
Comments
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It did silently break something: import multiprocessing
import os
print(f"importing {__name__} ({__file__}) pid={os.getpid()}")
def runner():
print(f"running in {os.getpid()}")
if __name__ == '__main__':
multiprocessing.set_start_method('forkserver')
multiprocessing.set_forkserver_preload(['__main__'])
N=4
procs = [multiprocessing.Process(target=runner) for _ in range(N)]
for p in procs:
p.start()
for p in procs:
p.join()Currently: If we fix it (and presumably before 3.4, although I haven't tested): Having said that, no-one noticing for eleven years suggests it isn't actually required. Perhaps it would be better to simplify things and remove the code? I'll post a quick and dirty fix, for reference. |
duaneg
added a commit
to duaneg/cpython
that referenced
this issue
Jun 9, 2025
The `main_path` parameter was renamed `init_main_from_name`, update the forkserver code accordingly.
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Bug report
Bug description:
The
multiprocessingmodule's forkserver start methodmultiprocessing.forkserver.mainhas two arguments that are values passed from the parent process to the forkserver when first launching it.main_path=andsys_path=. Via code inspection while fixing and testing #126538:There is no possible way for
main_path=to be set on the multiprocessing's forkserver.main() call since 2013's 9a76735 for #64145 as shipped in 3.4 (which also introduced the forkserver feature).the
forkserver.main(...)call is constructed in Lib/multiprocessing/forkserver.py getting its two possible allowed named args fromspawn.get_preparation_data()inLib/multiprocessing/spawn.pywhich was changed to set"init_main_from_name"in the kwargs dict it returns instead of"main_path"in the above change. Effectively making themain_path=handling code dead inforkserver.main.We should either remove the dead code, or determine if it was intended to support something that has been silently broken when using the forkserver start method for the past 11 years, and if so, add an explicit test for that and adapt it to use
"init_main_from_name"as the spawn code does.CPython versions tested on:
3.12, 3.13, 3.14, CPython main branch
Operating systems tested on:
Linux
Linked PRs
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