Mysterious logwarning warnings only happen with xdist under Pytest 4.1

[Zac-HD]

Here's a CI log from HypothesisWorks/hypothesis#1782. To reproduce this problem, clone Hypothesis (and for posterity, check out the current master d1b0df5).

pip install -r requirements/test.txt
pip install -e hypothesis-python/
pip install pytest==4.1
pytest hypothesis-python/tests/cover/test_testdecorators.py       # quick subset, passes
pytest hypothesis-python/tests/cover/test_testdecorators.py -n 2  # fails thanks to -Werror

So somewhere, our dependencies must be using a pytest_logwarning, right? Except that I can't find such a location in any of them, and we're not using any pytest plugins aside from xdist itself!

Do you have any idea what's happening here, or how I might find out? It's been blocking our move to Pytest 4.1 for weeks now 😞

[RonnyPfannschmidt]

the culprit is https://github.com/pytest-dev/pytest-xdist/blob/master/xdist/remote.py#L119-L128
in combination with https://github.com/pytest-dev/pytest/blob/5119abe4984a1b21e238b242fe233a250d2cf410/src/_pytest/hookspec.py#L501-L502

CC @nicoddemus

[RonnyPfannschmidt]

@Zac-HD as a workaround i propose ignoring that specific warning

but i notice that -Werror is quite a pain as it hides passed on location information

[Zac-HD]

Thanks for tracking that down! I'd seen both chunks of code but assumed that using the latest versions of each would not emit warnings on the default configuration.

Is there a way to handle this upstream in pytest-xdist? I'd prefer to fix it for everyone and minimize the number of workarounds we have to maintain in Hypothesis.

[RonnyPfannschmidt]

pytest-xdist already conditionally imports the hook, i believe simply not implementing it when the warning is there suffices

https://github.com/pytest-dev/pluggy/blob/master/pluggy/hooks.py#L37 suggests if we not only check for the existence, but also for (untested) pytest_spec.get("warn_on_impl") things should be fine

[nicoddemus]

I think we can simply stop implementing it, given that we will remove it anyway in the future.

[RonnyPfannschmidt]

@nicoddemus we should take a look at the support matrix then just in case

[nicoddemus]

@RonnyPfannschmidt went ahead with your suggestion given that it is really simple anyway. 👍

[Zac-HD]

Two hours from report to merged fix... you guys are legends 😍