add Repo.set_channel()

[jaimergp]

I am trying to use libmambapy with already downloaded repodatas from conda side. I just want libmamba to load a given JSON for a given channel.

I've tried setting context.offline = True and other tricks so SubdirData finds the downloaded JSON just fine, but there's some disagreement in how caching works right now, and actually at that point we already know that there's no download needed, and I have the direct path anyway.

As a result, I am trying to load it use libmamba.Repo directly, but the Python-exposed constructors differ from the ones SubdirData is using.

• SubdirData uses the constructor that relies on the RepoMetadata helper. But this is not exposed in the Python bindings.
• We can do almost the same by using this other constructor (only strings), but this one doesn't set p_channel. (Check what happens after read_file(index).

I am assuming this is the reason why I can't use api.Repo directly with channel containing MatchSpec like conda-forge::python: libsolv will say "unsupported request" for these!

So if I can set a channel separately, then things should work. This PR adds a couple bindings in libmambapy, hoping that's it :D

[jaimergp]

This seems to fix the issue I was seeing in conda/conda-libmamba-solver#65!

[wolfv]

Looks good to me!

[JohanMabille]

Hey, sorry for the late review. Would you consider binding the constructor used by SubdirData instead of adding a setter for the channel? I think it makes more sense to fully initialize an object within its constructor rather than partially initiliaze it in the constructor and later call a setter to finish the job. Also, there is a chance that we change the API of the constructor accepting strings only (or even that we remove it), since it does not fully initialize the repo object.

[jaimergp]

Makes sense! I can give it a try; thanks for the review @JohanMabille.

That said, that means we also need to expose RepoMetadata. And in a way, the str-based constructor was incomplete to begin with (the channel is missing). Should we maybe set the channel internally in the str-constructor by creating a Channel out of the url right away? Something like:

    MRepo::MRepo(MPool& pool, const std::string& name, const std::string& index, const std::string& url)
        : m_url(url)
    {
        m_repo = repo_create(pool, name.c_str());
        m_repo->appdata = this;
        read_file(index);
        // not true C++, just me making it up
        const Channel channel = Channel(url);
        p_channel = &channel;
    }

[AntoinePrv]

@jaimergp I'm working on changes that will make possible to use a Pool and Solver directly from a repodata.json without interacting with the rest of Mamba (downloads etc). Would that be of interest?

[jaimergp]

Hi! Yes, that's exactly the use case we are after. We are directly instantiating Repo and Solver, but in that API the channel info is missing. Hence this PR.

[AntoinePrv]

What is you timeline? We're about to to a Mamba release so we could try to get this one in. Otherwise the larger refactoring will be in another release where we will also break libmamba API (maybe a couple months).

[jaimergp]

Ideally we can get this one in now 🙏

[wolfv]

maybe we can make a quikc follow up release? cc @JohanMabille @AntoinePrv

[JohanMabille]

Closing this one in favor of #2398 which is definitely simpler and safer.