Regression with pixi-build-python 0.4.2

[glemaitre]

I'm building scikit-learn from source in a workspace: https://github.com/glemaitre/python-stack/blob/main/scikit-learn/pixi.toml

I assume that together with @lucascolley, we used a bit of bleeding edge version by using the channel "https://prefix.dev/pixi-build-backends".

Recently, I got the 0.4.2 release and it seems that there is a regression because I get the following error then:

> pixi shell -e dev --verbose
 INFO pixi_core::lock_file::update: the lock-file is up-to-date
 INFO source-build{source=. package=osx-arm64/scikit-learn=1.8.0.dev0=hbf21a9e_0}: pixi_command_dispatcher::command_dispatcher::instantiate_backend: Instantiated backend pixi-build-python@0.4.2, negotiated API version 2
Error:   × failed to build 'scikit-learn' from '.'
  ╰─▶   × The build backend does not provide the requested output: osx-arm64/scikit-learn=1.8.0.dev0=hbf21a9e_0.

Not a big deal because I revert to using the conda-forge channel but just thought that it might be cool to report.

[baszalmstra]

Thanks for reporting that is definitely always appreciated!

This issue is caused by us properly dealing with variants. This changes the build strings of the outputs.

The bulld backends identify outputs to build by the build atrings which is recorded in the lock-file. But if the backend can no longer produce an output with a particular build string there is no way to build the package again. This is quite annoying.

In a next version of pixi we record the variants themselves in the lock-file and no longer rely on the build string which should fix this issue completely.

For now, if you run into this issue, the easiest thing is to run:

pixi update scikit-learn

This will update the recorded build string in the lockfile.