fetchGit produces confusing error message for some (invalid) URLs

[sheijk]

Describe the bug

When passing the following expression containing an error (a missing / at the end of the url):

let emacsOverlay = (import (builtins.fetchGit {
      url = "https://github.com/nix-community/emacs-overlay";
      ref = "master";
      rev = "47acbeca3e13a03131a0a7a9a3bd3443e1760d71";
    }));
in
# [...]

This is the error messages I'm seeing

warning: refname ‘47acbeca3e13a03131a0a7a9a3bd3443e1760d71’ is ambiguous. Git normally never creates a ref that ends with 40 hex characters because it will be ignored when you just specify 40-hex. These refs may be created by mistake. For example,

    git switch -c $br $(git rev-parse …)

where “$br” is somehow empty and a 40-hex ref is created. Please examine these refs and maybe delete them. Turn this message off by running “git config advice.objectNameWarning false”

Steps To Reproduce

• Put above into a default.nix
• Call nix-shell

Expected behavior

A more clear error message hinting at the real issue. Maybe even have a look if there is a bug lurking as it looks as if the rev value is treated as a branch name

nix-env --version output
nix-env (Nix) 2.3.10

Using 20.09 channel

[sheijk]

Another confusing error message with this default.nix. The error here is that ref should be "refs/tags/1.0.4" instead of "1.0.4"

with import <nixpkgs> { };

let unused = builtins.fetchGit {
    url = "https://github.com/craigmbooth/nix-visualize.git/";
    # rev = "9efca80d83431a47024d363d270abff66a9491b6";
    ref = "1.0.4";
  };
in
unused

producing

error: cannot update ref 'refs/heads/1.0.4': trying to write non-commit object a6b1577ad37c77947a33302cdf4d9cf617ad27ce to branch 'refs/heads/1.0.4'
error: program 'git' failed with exit code 1

This only happens if a tag with this name exists. If the tag is completely wrong I get the much clearer error message

fatal: couldn't find remote ref 999
error: program 'git' failed with exit code 128

so it seems like there is some error check missing. Maybe it was introduced when fetchGit started automatically adding prefix to rev?

[milahu]

warning: refname ... is ambiguous.

these warnings come from git/object-name.c, may be a bug in builtins.fetchGit

[stale]

I marked this as stale due to inactivity. → More info