Environment variable to specify number of workers for "auto"

[encukou]

Would you accept a PR to read an environment variable when determining what -n auto means?

Concrete use case:
We're trying to make testing easier in Fedora RPM builds, where one should ideally use a somewhat obscure macro (%{_smp_build_ncpus}) for the best number of processes on a builder machine, but many packagers simply reuse upstream projects' command lines, with -n auto. It would be nice if we could set e.g. PYTEST_XDIST_AUTO_NUM_WORKERS in our default pytest environment, and document -n auto as best practice for pytest-xdist.
Doing this in the plugin hook (pytest_xdist_auto_num_workers) would probably be possible, but very cumbersome compared to an environment variable.
I assume there are other use cases where this would be useful.

I'm thinking the pytest_xdist_auto_num_workers hook should have priority if both it and the env var are specified: for having it the other way. it's fairly easy to check the env var from a plugin.

[nicoddemus]

Sounds like a good idea, we would love a PR.

I'm thinking the pytest_xdist_auto_num_workers hook should have priority if both it and the env var are specified

Agreed, the env var has the least precedence.

[hroncok]

Hey Petr, have you ever managed to have a work in progress for this? I've only seen #791 and I've been hit by this use case once again in Fedora :)

[encukou]

Sorry, I've dropped the ball here.
Rebased and pushed my branch as #829.