Document tmp_path fixture does not support concurrent runs of the same test by default

[faph]

The tmp_path fixture documentation (https://docs.pytest.org/en/7.1.x/how-to/tmp_path.html#the-tmp-path-fixture) states that temporary directories are unique for each "test invocation". This is correct in the sense that each test function is associated by a single unique temporary directory.

However, this is not correct when considering running the same test function concurrently by distinct pytest invocations. We are doing this all the time using Tox where tests are executed from multiple virtualenvs (different Python versions), concurrently.

In that scenario, the tmp_path generated directory is shared/clashes between multiple invocations.

I appreciate that the solution is to uniqueify the temp path prefix, similar to what the pytest-xdist extension does. With Tox, that is trivial to achieve. Reading the Pytest docs, I do not necessarily know that I need that solution.

[faph]

This is related, but distinct, from #11789

[RonnyPfannschmidt]

multiple concurrent pytest processes should always have different basetemp directories

[faph]

@RonnyPfannschmidt Thanks, yes I understand that now. But I would NOT know by reading the how-to docs, right?

edit: inserted "NOT"

[RonnyPfannschmidt]

indeed, the wording probably should use "unique for the current test" as the implication is a bit of a hidden rabbit hole

[faph]

@nicoddemus | @RonnyPfannschmidt Could I suggest the docs to be more explicit about concurrent execution? I agree that the #11800 more accurately reflects reality. However, I feel it is still somewhat ambigious in what "current test" means...

[nicoddemus]

@faph Sure, feel free to open a PR suggesting a change you feel clarifies things. 👍

[faph]

@nicoddemus What about something like #11830?