Flake fetching of git repos by URL with a specified ref= parameter does not allow a tag as a ref

[IreneKnapp]

Describe the bug

A flake.nix along the lines of:

{
  inputs = {
    upstream = {
      url = "git+file:///tmp/reproduction/foo?ref=some-tag";
    };
  };

  outputs = { self }: { };
}

... will never succeed with nix flake lock or nix flake update, instead printing a message along the lines of:

fatal: couldn't find remote ref refs/heads/some-tag
error: program 'git' failed with exit code 128
(use '--show-trace' to show detailed location information)

What's happening there is that it's assuming ref= refers to a branch (hence prefixing it with refs/heads/), but it refers to a tag.

Steps To Reproduce

Create a local git repository, with at least one commit, and a tag named some-tag pointing to that commit. The repository also needs to contain a flake.nix; the minimal one is { outputs = { self }: { }; }.

In a separate directory, create a flake.nix which references that repository as an input, using a URL in the form git+file:///path/to/repo?ref=some-tag.

Attempt to do nix flake lock to generate a flake.lock.

Expected behavior

The repository should be retrieved, the tag ref should be found, and the flake.lock should be created.

nix-env --version output

nix-env (Nix) 2.13.3

Additional context

This works fine if you use ref=refs/tags/some-tag instead. However, that may defy user expectations, since the git command-line tools accept both branch and tag names anywhere that they ask for a "ref".

I discovered this in a much more complicated situation, so I was greatly relieved that none of the nuances I dug into turned out to be relevant.

This appears to be caused by

nix/src/libfetchers/git.cc

Line 565 in 635df5e

: "refs/heads/" + *ref;

I've spent enough time on this that I want to try my hand at writing a patch, but it's not clear to me why nix thinks it needs to prefix the ref name in the first place, and I don't want to cause a regression by simply removing that logic. Does anybody have context on that?

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[figsoda]

might be duplicate of #5291

[IreneKnapp]

ah yep I believe it is, thanks for that