Method checkReferenceAndTruncate

storage/filesystem/dotgit/dotgit.go:682–706

checkReferenceAndTruncate reads the reference from the given file, or the `pack-refs` file if the file was empty. Then it checks that the old reference matches the stored reference and truncates the file.

(f billy.File, old *plumbing.Reference)

// the file was empty. Then it checks that the old reference matches the stored reference and
// truncates the file.
func (d *DotGit) checkReferenceAndTruncate(f billy.File, old *plumbing.Reference) error {
	if old == nil {
		return nil
	}

	ref, err := d.readReferenceFrom(f, old.Name().String())
	if errors.Is(err, ErrEmptyRefFile) {
		// This may happen if the reference is being read from a newly created file.
		// In that case, try getting the reference from the packed refs file.
		ref, err = d.packedRef(old.Name())
	}

	if err != nil {
		return err
	}

	if ref.Hash() != old.Hash() {
		return storage.ErrReferenceHasChanged
	}
	_, err = f.Seek(0, io.SeekStart)
	if err != nil {
		return err
	}
	return f.Truncate(0)
}

func (d *DotGit) SetRef(r, old *plumbing.Reference) error {
	var content string