subprocess._USE_VFORK doesn't change usage of vfork

[cmaloney]

Bug report

Bug description:

Discussion: https://discuss.python.org/t/subprocess-use-vfork-escape-hatch-broken-fix-or-remove/56915

This flag was added as an escape hatch in gh-91401 and backported to Python 3.10. The flag broke at some point between its addition and now. As there is currently no publicly known environments that require this, planning to remove it rather than work on fixing it.

I found that the test for this was broken while working on tests for reducing system calls in other cases (GH-120754). The strace based test isn't currently being run by any CI bots. Have a PR which both adds tests around read and fixes the strace tests to run on some of the bots: #121143

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-121381 Remove subprocess._USE_VFORK escape hatch #121383

[hauntsaninja]

Sanity checking the results of https://grep.app/search?q=_USE_VFORK&case=true&filter[lang][0]=Python and https://github.com/search?q=_USE_VFORK+language%3APython+&type=code , I think it would be fine to remove. It's clearly a private API. The billiard code would break a little unfortunately, but they're reading it unchecked despite the documentation explicitly saying not to do so, they're using internals and they have a bunch of version specific code anyway. The gevent hit just looks like a fork of test_subprocess.py for each minor version of Python.