process.spawn: Use multiprocessing.Process for returnproc

[zmedico]

Use multiprocessing.Process for returnproc, so that fork will stop being used when python makes "spawn" the default multiprocessing start method.

Continue to use _start_fork when returnproc is not enabled, for backward compatibility. Ultimately, it can be removed at the same time as the returnpid parameter.

Bug: https://bugs.gentoo.org/916566

[thesamesam]

btw it might be worth us adding a single impl (3.12 maybe) to CI w/ a hack to use spawn

[zmedico]

btw it might be worth us adding a single impl (3.12 maybe) to CI w/ a hack to use spawn

Yeah, that's a necessity. I suppose we could use an environment variable to trigger multiprocessing.set_start_method("spawn", force=True) at the beginning of all portage programs. Then there's this little script I use to launch pytest:

import multiprocessing
import sys
import pytest

if __name__ == "__main__":
    multiprocessing.set_start_method("spawn", force=True)
    sys.exit(pytest.console_main())

Maybe it's easier to use as a one-liner:

python -c 'import multiprocessing, sys, pytest; multiprocessing.set_start_method("spawn", force=True); sys.exit(pytest.console_main())'