use index in offline mode when called from conda-build

[jaimergp]

Description

Fixes #386, together with conda/conda#13357.

We need to instantiate IndexHelper in offline mode for compatibility with conda-build. Otherwise the index can get out of sync during long build processes.

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

[jaimergp]

Errors are due to conda/conda#13360

[jaimergp]

@conda/conda-core @conda/conda-libmamba-solver this is ready for review :) 🙏

[jezdez]

The type change from class to function for the index helper is a little miffy, I'd rather see use a subclass and then override context.offline in the _CachedLibMambaIndexHelper.__init__ when calling super.

[jaimergp]

Ok, I like it.

[mbargull]

Makes sense to me.
It would be good to have this in conda-build itself too (i.e., it should request a consistent index for spanning the whole build).

---

I'm not too familiar with the code base to give a proper review yet.
It would be good to test the usual corner cases like multi-output recipes.

[jaimergp]

It would be good to test the usual corner cases like multi-output recipes.

We have some simple tests for these conda-build cases:

../conda-libmamba-solver-src/tests/test_downstream.py::test_build_recipe[baddeps] PASSED [ 15%]
../conda-libmamba-solver-src/tests/test_downstream.py::test_build_recipe[jedi] PASSED [ 17%]
../conda-libmamba-solver-src/tests/test_downstream.py::test_build_recipe[multioutput] PASSED [ 18%]
../conda-libmamba-solver-src/tests/test_downstream.py::test_build_recipe[stackvana] PASSED [ 19%]