Include "original" and "locked" in nix flake prefetch --json

[andreabedini]

Motivation

For 3rd party tools, it is important to be able to follow the same basic mechanisms behind nix flake. In particular, two things I have struggled with in the past are:

1. a solution to parse the flake uri
2. a solution to resolve flake inputs to their locked version

Efforts like rime make me think I am not the only one trying to do this.

Context

I tried to twist the cli into giving me the right information but the only way seems to be creating an ad-hoc temporary flake with a single input and read the lock file, which is not something I am comfortable building on top of.

With nix flake prefect --json gets us very close:

❯ nix flake prefetch github:input-output-hk/haskell.nix
Downloaded 'github:input-output-hk/haskell.nix/717cf3915d86023c0d029ac1dd27d9ba7dbc43ed' to '/nix/store/vj07b8cx98pbysa5l4mhj6ail8j9fdzh-source' (hash 'sha256-bJyJnFKG3damZ36tbCv5TfQ2xgI8T/j35cMNO+ZgAnk=').

But its structured output does not include the locked url just printed above!

❯ nix flake prefetch github:input-output-hk/haskell.nix --json
{"hash":"sha256-bJyJnFKG3damZ36tbCv5TfQ2xgI8T/j35cMNO+ZgAnk=","storePath":"/nix/store/vj07b8cx98pbysa5l4mhj6ail8j9fdzh-source"}

This (minimal) change adds two fields to the output of nix flake prefetch --json: the original url as parsed by nix flake and its locked version.

nix-env ❯ ./outputs/out/bin/nix flake prefetch github:input-output-hk/haskell.nix --json | jq
{
  "hash": "sha256-bJyJnFKG3damZ36tbCv5TfQ2xgI8T/j35cMNO+ZgAnk=",
  "locked": {
    "lastModified": 1695084617,
    "narHash": "sha256-bJyJnFKG3damZ36tbCv5TfQ2xgI8T/j35cMNO+ZgAnk=",
    "owner": "input-output-hk",
    "repo": "haskell.nix",
    "rev": "717cf3915d86023c0d029ac1dd27d9ba7dbc43ed",
    "type": "github"
  },
  "original": {
    "owner": "input-output-hk",
    "repo": "haskell.nix",
    "type": "github"
  },
  "storePath": "/nix/store/vj07b8cx98pbysa5l4mhj6ail8j9fdzh-source"
}

Note: doing this introduces a duplication of the hash field, now represented both at the top-level and inside locked. I agree this is unoptimal but I didn't want to make start with a breaking change (let me say this glossing over the experimental/unstable discussion). I can remove the top-level hash field if it's deemed acceptable.

Another solution could be obtained by changing builtins.fetchTree to interpret strings as flake uris. Currently it is not possible to call builtins.fetchTree "github:input-output-hk/haskell.nix" but if the uri gets parsed as nix flake would do, then it would turn into

nix-repl> builtins.fetchTree { type = "github"; owner = "input-output-hk"; repo = "haskell.nix"; }
{ lastModified = 1695084617; lastModifiedDate = "20230919005017"; narHash = "sha256-bJyJnFKG3damZ36tbCv5TfQ2xgI8T/j35cMNO+ZgAnk="; outPath = "/nix/store/vj07b8cx98pbysa5l4mhj6ail8j9fdzh-source"; rev = "717cf3915d86023c0d029ac1dd27d9ba7dbc43ed"; shortRev = "717cf39"; }

which exposes all the information I am looking for. This could even be preferable as the information exposed be reused in nix logic while still being accessible from the command line.

I think this make it a great feature for third party tooling since it (almost?) allows one to reimplement flake-locking externally.

Priorities

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[andreabedini]

Oh, I had missed the newly-released parseFlakeRef and flakeRefToString builtins. I haven't tested them yet but they seem to match the alternative implementation I described above.

[tomberek]

This should make it far easier to create tooling that integrates with the ecosystem.

[andreabedini]

<3