Determining which cache a cached derivation is in

[pwaller]

Problem

I'm trying to populate a private cache while leveraging the public cache ("figure out what to build/copy" from an otherwise fresh/empty nix store). This involves a mix of public and private derivations.

My approach involves using nix-eval-jobs --substituters private-cache and inspecting the resulting cacheStatus. If a derivation is notBuilt, then it is not in the private cache.

However, I encountered an issue when the set of things to builds includes a custom Linux kernel:

• IFD (Import From Derivation) in nixpkgs triggers a build against only the private cache.
  • IFD appears to be used to compute a kernel config against which assertions are tested during eval.
• This causes excessive rebuilding during the "figure out what to build" step.
  • (since the built config is available in the public cache, but not the private one).

This could potentially be worked around by doing two evals; first one with all caches available so that IFD can be resolved, and then a later one to figure out what's not in the private cache. (Aside: This also makes me wonder about the problem how to figure out which paths needed IFD should be copied to the private cache. I suppose this is somewhat resolved (though not exactly efficiently) by doing nix copy --all).

Ideal Behavior

• nix-eval-jobs could report which cache a derivation is available in, rather than requiring separate runs with different cache configurations.
• This would allow IFD to reference the public cache, reducing unnecessary builds, and make it easy to determine what is absent from the private cache.
• Ideally there would be a way to acquire a list of IFD derivations that were used for the eval so that they can also be copied to a private cache.

Workaround / Request for Input

• Is there an existing workflow solving this problem I've missed?
• Would it be possible to enhance nix-eval-jobs to expose which cache has the hit?

Related discussion

Pre-RFC: Implement Dependency Retrieval Primitive - this talks quite a bit about the difficulties of IFD. It seems it is assumed most people should avoid IFD, and it is banned from nixpkgs, but I hit a case where it appears in nixpkgs for a custom kernel build, so I need to handle this situation somehow.

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/pre-rfc-implement-dependency-retrieval-primitive/43418/15

[Mic92]

Nix currently doesn't expose this option to us.