nix <cmd> --update-input ... fails in detached git worktrees

[spacefrogg]

Describe the bug

nix <cmd> --update-input ... fails when the following conditions come together:

1. when called in a git worktree, i.e. when .git is a file (pointing to the real repository) instead of a directory.
2. when the worktree is clean

It reports the worktree as a shallow clone and that it needs a deep clone to update the inputs.

error: '/path/to/repo' is a shallow Git repository, but a non-shallow repository is needed

       … while fetching the input 'git+file:///path/to/repo'

Steps To Reproduce

1. Enter a git repo with a flake that has inputs.
2. Create a worktree with git worktree add ../workree branch
3. cd ../worktree
4. nix flake lock --update-input <input> or any other command that accepts the --update-input option

Expected behavior

Updates the input.

nix-env --version output

2.4

Workaround

1. Run nix <cmd> --update-input <input> path:., which forces nix to interpret the flake URL as a path and avoids the git+file:// codepath.
2. Modify any tracked file (not necessarily flake.lock) to make the tree dirty, also solves the issue.

[spacefrogg]

Update

It is even worse, I am unable to do anything inside a clean ckeckout as described above. Even a simple nix build .#<package> errors.

[rollf]

This is still valid and doesn't work as of 2.13.6:

[I] rollf@nixos /tmp/
❯ nix-env --version
nix-env (Nix) 2.13.6

# Shallow copy:
[I] rollf@nixos /tmp
❯ git clone --depth 1 https://github.com/NixOS/nixpkgs.git
...
[I] rollf@nixos /tmp
❯ cd nixpkgs/

# "shallow=1" is provided by default:
[I] rollf@nixos /tmp/nixpkgs (master)
❯ nix build -L --show-trace --keep-failed .#hellow
error: flake 'git+file:///tmp/nixpkgs?shallow=1' does not provide attribute 'packages.x86_64-linux.hellow', 'legacyPackages.x86_64-linux.hellow' or 'hellow'
       Did you mean one of hello, below, halloy or jello?

# Works fine in the repo itself:
[N] rollf@nixos /tmp/nixpkgs (master) [1]
❯ nix build -L --show-trace --keep-failed .#hello
[I] rollf@nixos /tmp/nixpkgs (master)
❯ ./result/bin/hello
Hello, world!

# Doesn't work in a worktree:
[I] rollf@nixos /tmp/nixpkgs (master)
❯ git worktree add ../feature-x
...
[I] rollf@nixos /tmp/nixpkgs (master)
❯ cd ../feature-x/
[N] rollf@nixos /tmp/feature-x (feature-x)
❯ nix build -L --show-trace --keep-failed .#hello
error:
       … while fetching the input 'git+file:///tmp/feature-x'

       error: '/tmp/feature-x' is a shallow Git repository, but shallow repositories are only allowed when `shallow = true;` is specified.

# Works fine with the workaround mentioned in ths PR:
[I] rollf@nixos /tmp/feature-x (feature-x) [1]
❯ nix build -L --show-trace --keep-failed path:.#hello
[I] rollf@nixos /tmp/nixpkgs (master)
❯ ./result/bin/hello
Hello, world!

Relevant issue is #3756 which seems to have fixed in with shallow copies per se, but not in worktrees