Automatic `CI` profile detection may not work under tox or other test orchestrators

[Liam-DeVoe]

In the passenv docs, CI is not listed as a passthrough envvar, which makes me believe that we are not using the CI profile by default under tox, and potentially under other similar tools. (related: tox-dev/tox#3442)

I'd prefer not to resolve this solely by documenting that users to set passenv = ["CI"] (though we could also do this), since the CI profile is meant to provide good defaults to everyone, including those that don't read the docs from top to bottom.

Is there a more robust way to detect CI runners? Perhaps a file only available under github, gitlab, etc?

(Prompted by https://github.com/Quansight-Labs/pytest-run-parallel/actions/runs/16376546862/job/46366236714?pr=96#step:7:151, which raised DeadlineExceeded in CI under tox, which is not something that should happen if the CI profile was being loaded.)

[Liam-DeVoe]

For github, it looks like checking os.getcwd against /home/runner/work/ and /Users/runner/work/ would work for linux and macos respectively: [1], [2]. Windows is more nondescript: D:\a\hypothesis\hypothesis.

[Zac-HD]

Hmm, checking cwd seems a bit imprecise for my tasks with such nondescript paths 🤔

[Liam-DeVoe]

it would be combined with checking "CI", not replace it, so I think it's ok if there are false negatives as long as there aren't false positives. /home/runner/work/ is very generic sounding, but I think it's pretty unlikely to be used by a human. (counterpoint: local VM or docker container may collide). I'm all ears for better options though 😅

[Liam-DeVoe]

We'll be able to fix this for tox once they cut a release: tox-dev/tox#3442 / tox-dev/tox#3592