Relative git flake inputs git+file:relative/path

[roberth]

Is your feature request related to a problem?

• • Nix currently resolves git+file:relative/path according to the process working directory instead of the flake.nix base directory. This is unreliable. Evaluation should be invariant under the process working directory, or it would violate the ability to be used as a flake input, among other things.
  • fetchTree does not have a base directory argument (and no implicit argument either, because that'd also be bad)
• Therefore fetchTree does not have the information to resolve relative paths.

Proposed solution

• Disallow git relative path inputs in fetchTree
• Either
  • Resolve git relative path inputs in call-flake.nix (with a primop that does it? - unclear if that's future proof enough for reproducibility)
  • add a baseDirectory argument to fetchTree

Alternative solutions

Forbid altogether. This blocks some users:

• Flake input git+file:./${submodule} no longer works #9708

Additional context

• fetchers/git: make relative path absolute for local repo #12107
• Warn against the use of relative 'git+file:' flake inputs #12277

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[nixos-discourse]

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

https://discourse.nixos.org/t/2024-01-22-nix-team-meeting-minutes-210/59239/1