git clone specific tag

[milosgajdos]

When I try to clone a specific tag, I run something like this:

git clone --branch v1.0.0 https://github.com/ministryofjustice/supervisor-formula.git vendor/supervisor

The above command succeeds without any hiccups.
I figured I could write a go func like below.

func CloneRepo(url string, tag string, path string) error {
    opts := git.CloneOptions{
        CheckoutBranch: tag,
    }

    repo, err := git.Clone(url, path, &opts)
    if err != nil {
        return err
    }

    fmt.Println(repo)
    return nil
}

However, I'm getting the following error:

Failed to clone https://github.com/ministryofjustice/supervisor-formula.git Reference 'refs/remotes/origin/v1.0.0' not found

I'm guessing this is a lack of knowledge on my side, but I'm wondering whether I can accomplish something like this using the git package ?

[carlosmn]

The thing here is that the option is called "--branch" so that's what we implemented, checking out a particular branch. But as it happens, what git means here is more like "meh, I'll take whatever", which is a behaviour which we haven't implemented.

Until we do accept any string to DWIM, you'd have to perform the switch to the commit in that tag via CheckoutTree() and SetHeadDetached().

[milosgajdos]

Thanks for the hints! I've eventually done something like below:

ref, err := iter.Next()
    for err == nil {
        if ref.IsTag() {
            tagName := strings.TrimPrefix(ref.Name(), "refs/tags/")
            if tagName == "my_tag" {
                fmt.Println("Found the tag", "my_tag")
                obj, err := ref.Peel(git.ObjectTag)
                if err != nil {
                    return fmt.Errorf("Unable to peel the ref: %s\n", err)
                }

                if err := repo.SetHeadDetached(obj.Id(), nil, ""); err != nil {
                    return fmt.Errorf("Checking out tag %s failed: %s", tag, err)
                }

                return nil
            }
        }

        ref, err = iter.Next()
    }

However, I don't understand why I can't Peel the tag reference ? Every time I call ref.Peel(git.ObjectTag) I get the following error:

The git_object of id 'abe8b968b0607f92ddeac31b062208b7ee5fd475' can not be successfully peeled into a tag (git_otype=4).

I had the look at the actual tag hash and it's the same as reported by the error above:

$ cat repo/.git/refs/tags/my_tag
abe8b968b0607f92ddeac31b062208b7ee5fd475

So the tag actually exists. Am I missing something important here ?

Thanks!

[carlosmn]

The tag reference exists, that doesn't mean there's a tag object underneath. If you look at that ref with git show or git cat-file -t, it should show that there is no tag object there.

But you're not interested in a tag object being there to perform be branch switch. What you need is a commit, so that's what you should be looking for.

You also might be interested in Repository.DwimReference() which will perform this shortname-to-reference conversion that you're doing manually in this code.

[milosgajdos]

Thanks @carlosmn Eventually I went with something like this:

    ref, err := repo.DwimReference(tag)
    if err != nil {
        return fmt.Errorf("Could not find the %s ref: %s\n", ref.Name(), err)
    }

    if err := repo.SetHeadDetached(ref.Target(), nil, ""); err != nil {
        return fmt.Errorf("Checking out tag %s failed: %s", tag, err)
    }

I'm not sure if that's the best way to approach this, but it does seem to do the trick 👍