On MacOS, bootstrap requires both python and python3

[jyn514]

python: https://github.com/rust-lang/rust/blob/beb480d908f091e7c42c866e13641f26b2ee230b/src/bootstrap/sanity.rs#L100-L106
python3: https://github.com/rust-lang/rust/blob/beb480d908f091e7c42c866e13641f26b2ee230b/src/bootstrap/test.rs#L1382-L1386
It sounds like python3 is strictly required for LLDB, so we should move this check from test.rs to builder.python(), which will only require python3 instead of both commands.

cc @Walther

@rustbot label: +E-easy +E-mentor +E-help-wanted +A-rustbuild +A-contributor-roadblock

[Mark-Simulacrum]

But not everyone will want to run lldb, right? We only mention /usr/bin/python3 there, we don't actually run it unless lldb tests are run, which is pretty rare.

This may still be an OK change, of course.

[jyn514]

Sure - but in that case, we hardly need python at all, only for rustdoc tests, and it seems ok to use the system python.

[pierwill]

I would actually support requiring Python 3 in any case.

[jyn514]

@pierwill let's please keep discussion about 2 vs 3 in #71818 and not relitigate it on each issue that mentions python.

[AlecGoncharow]

@rustbot claim

[AlecGoncharow]

If I understand the issue here correctly, just moving the check from test.rs to the builder.python() doesn't entirely remove the issue of needing the python name defined on MacOS given the call to cmd_finder.must_have("python") in sanity.rs. To resolve this should the check in sanity.rs include a MacOS specific check for /usr/bin/python3 or an additional check for python3?

[jyn514]

To resolve this should the check in sanity.rs include a MacOS specific check for /usr/bin/python3 or an additional check for python3?

The former - it should just use /usr/bin/python3 any time it needs python on MacOS, and remove the existing check for python on that platform.

[AlecGoncharow]

About to raise a PR, but is it even necessary to perform any check in builder.python() if we are always forcing /usr/bin/python3 in sanity.rs? Are there instances where sanity::check won't be called before CompileTest::run?

Also first time using x.py, what would be the correct test to run here?

[jyn514]

Is it even necessary to perform any check in builder.python() if we are always forcing /usr/bin/python3 in sanity.rs?

I don't think it's necessary to check in both places, no, you can return /usr/bin/python3 unconditionally in builder.python(). Whether you can remove the check in sanity.rs is a question for @Mark-Simulacrum - I think probably yes, since you can still run the build without python, but it doesn't necessarily need to go in the same change switching to /bin/python3 consistently.

Also first time using x.py, what would be the correct test to run here?

Basically everything runs the check in sanity.rs, but I think it's only relevant when BOOTSTRAP_PYTHON is unset, so probably you'll need cargo run check to make sure the check executes. If you want to check the version of python used works, you can run cargo run test --stage 1 src/test/rustdoc.

[Mark-Simulacrum]

If we can rely on /usr/bin/python3 always existing on macOS there's no need to check for it, but I would expect that the code in sanity.rs isn't platform-specific and so should continue to look for python in a variety of ways (likely the same ways across all platforms).

It looks like python is currently used for two separate things: lldb tests, and docck. The latter likely works with python 2 and python 3, or at least, so it's likely that we need to leave the discovery and configuration in place. Certainly on e.g. Windows /usr/bin/python3 is not expected to exist, I believe.