When a function is executed by a multiprocessing.Process and uses sys.exit,
the actual exit code reported by multiprocessing is different than would be
expected given the Python interpreter behavior and documentation. For example,
given:

    from functools import partial
    from multiprocessing import get_context
    import sys
    
    
    def run(ctx, fn):
        p = ctx.Process(target=fn)
        p.start()
        p.join()
        return p.exitcode
    
    
    if __name__ == '__main__':
        ctx = get_context('fork')
        print(run(ctx, partial(sys.exit, 2)))
        print(run(ctx, partial(sys.exit, None)))
        print(run(ctx, sys.exit))
    
        ctx = get_context('spawn')
        print(run(ctx, partial(sys.exit, 2)))
        print(run(ctx, partial(sys.exit, None)))
        print(run(ctx, sys.exit))
    
        ctx = get_context('forkserver')
        print(run(ctx, partial(sys.exit, 2)))
        print(run(ctx, partial(sys.exit, None)))
        print(run(ctx, sys.exit))

when executed results in

    $ python exit.py
    2
    1
    1
    2
    1
    1
    2
    1
    1

but when Python itself is executed we see different behavior

    $ for arg in 2 None ''; do python -c "import sys; sys.exit($arg)"; echo $?; done
    2
    0
    0

The documentation states

sys.exit([arg])
...
The optional argument arg can be an integer giving the exit status
(defaulting to zero), or another type of object.

The relevant line in multiprocessing (https://github.com/python/cpython/blame/1cffd0eed313011c0c2bb071c8affeb4a7ed05c7/Lib/multiprocessing/process.py#L307)
seems to be from the original pyprocessing module itself, and I could
not locate an active site that maintains the repository to see if there
was any justification for the behavior.

The same behavior on 3.8 and 3.5

I believe the mentality behind multiprocessing.Process triggering an exit code of 1 when sys.exit() is invoked inside its process is to indicate a non-standard exit out of its execution. There may yet be other side effects that could be triggered by having a sys.exit(0) translate into an exit code of 0 from the Process's process -- and we might not notice them with the current tests.

Was there a particular use case that motivates this suggested change?

I believe the mentality behind multiprocessing.Process triggering an exit code of 1 when sys.exit() is invoked inside its process is to indicate a non-standard exit out of its execution.

Can I ask what this is based on? I did a pretty good amount of digging but didn't find any justification for it. It just seems like a simple oversight to me.

There may yet be other side effects that could be triggered by having a sys.exit(0) translate into an exit code of 0 from the Process's process -- and we might not notice them with the current tests.

This is definitely a behavior change and will break any code that currently relies on sys.exit(None) or sys.exit() exiting with a non-zero exit code from a multiprocessing.Process. The fact that all documentation indicates that sys.exit(None) or sys.exit() results in a 0 exit code in normal Python (with no documentation on it related to multiprocessing) makes me think that any code relying on this behavior is subtly broken, however. Any impacted user can update their code and explicitly pass 1 to sys.exit, which should be forward and backwards compatible.

Was there a particular use case that motivates this suggested change?

I have a wrapper library that invokes arbitrary user code and attempts to behave as if that code was executed in a vanilla Python process, to include propagating the correct exit code.

Currently I have a workaround here: https://github.com/chrahunt/quicken/blob/2dd00a5f024d7b114b211aad8a2618ec8f101956/quicken/_internal/server.py#L344-L353, but it would be nice to get rid of it in 5-6 years if this fix gets in and the non-conformant Python versions fall out of support. :)

Any other concerns here?

New changeset c2ac4cf by Christopher Hunt in branch 'master':
bpo-35727: Use exit code 0 on sys.exit() in multiprocessing.Process. (GH-11538)
c2ac4cf

Sorry for the delay. I've now merged the PR for 3.9. Since this is a slight behaviour change, and the original issue was easy to workaround, I won't backport it.