fairy: Cache build results from already ran tasks

[johannwagner]

We run into issues, where fairy is missing important outputs from previous builds and has to build a lot of recently used binaries again.

We want to implement a solution for this, where the fairy caches the results from the last n of days and the build results from the last run of this specific nix run task.

[Kek5chen]

I'd just like to ask about this for a moment, because this seems XY. The issue sounds kind of specific to the use case. For example, if you spin off a task to build "nixpkgs#firefox" you're going get it built once, and then served from cache aka. the nix store.

So I'm wondering:
What caching is missing that you desperately depend on?
Is this something we should actually be supporting here in vicky; midstream? (It sounds more downstream than that)

It seems more like a flake-level issue at first sight. Nix should already be doing the caching that we should be taking care of regarding vicky itself imho.

[yuyuyureka]

The problem is that it gets garbage collected every night because there are no gcrooots added from the nix build. The solution is installing the build results into a profile, which part of gcroots and rotated after 7 days.

[Kek5chen]

Thanks for explaining, I also think we should support a cache like this.