Support sharing Nix flakes eval cache

[matthewbauer]

Apologies if there's a already a ticket for this. But, from the blog post it mentions the ability to share evaluation caches like we share store paths:

Currently, the evaluation cache is only created and used locally. However, Nix could automatically download precomputed caches, similar to how it has a binary cache for the contents of store paths. That is, if we need a cache like 302043eedf….sqlite, we could first check if it’s available on cache.nixos.org and if so fetch it from there. In this way, when we run a command such as nix shell nixpkgs#firefox, we could even avoid the need to fetch the actual source of the flake!

https://www.tweag.io/blog/2020-06-25-eval-cache/

My thinking is that each eval cache could be treated as a Nix store path. So, you'd check for something like /nix/store/am64vg72qgqpmcag2sr0b11nxm3z2hmq-302043eedfbce13ecd8169612849f6ce789c26365c9aa0e6cfd3a772d746e3ba.sqlite in the store. If it doesn't exist, we'd check the substituters.

[andir]

Doesn't the content of the cache depend on what expressions where actually eval'ed? So we would have to take that actual attribute into account or only upload caches that eagerly went through all the attributes?

[edolstra]

Doesn't the content of the cache depend on what expressions where actually eval'ed?

Yes. So the uploaded caches should be populated enough to contain the attributes necessary for operations like nix search.

[stale]

I marked this as stale due to inactivity. → More info