Unable to checkout by tag commit id.

[alee-r7]

So I'm trying to checkout by the tag commit id. Basically I get the tag, its hash then do a checkout. Like this.

err = w.Checkout(&git.CheckoutOptions{
		Hash: commitHash,
		Force: true,
	})

I've verified the working directory has the commit, even checked objects folder by unpacking the repo I'm working with. I'm also able to checkout using the command line.

I tracked down the issue to this here. Here is the order

1. Checkout will call Reset which calls ResetSparsely.
2. ResetSparsely will then call getTreeFromCommitHash.

   go-git/worktree.go

   Line 307 in 5d08d3b

   t, err := w.r.getTreeFromCommitHash(opts.Commit)

3. getTreeFromCommitHash will call r.CommitObject which then calls object.GetCommit.
   https://github.com/go-git/go-git/blob/master/repository.go#L1424
4. GetCommit will then call s.EncodedObject. https://github.com/go-git/go-git/blob/master/plumbing/object/commit.go#L70
5. This is where it fails. It seems based off the hash, the object that is retrieved from the pack or objects folder is a TabObject. However, it will fail on the last "IF" check because it expects a commit object. Due to the two types be different it returns an error.

func (s *ObjectStorage) EncodedObject(t plumbing.ObjectType, h plumbing.Hash) (plumbing.EncodedObject, error) {
	var obj plumbing.EncodedObject
	var err error

	if s.index != nil {
		obj, err = s.getFromPackfile(h, false)
		if err == plumbing.ErrObjectNotFound {
			obj, err = s.getFromUnpacked(h)
		}
	} else {
		obj, err = s.getFromUnpacked(h)
		if err == plumbing.ErrObjectNotFound {
			obj, err = s.getFromPackfile(h, false)
		}
	}

	// If the error is still object not found, check if it's a shared object
	// repository.
	if err == plumbing.ErrObjectNotFound {
		dotgits, e := s.dir.Alternates()
		if e == nil {
			// Create a new object storage with the DotGit(s) and check for the
			// required hash object. Skip when not found.
			for _, dg := range dotgits {
				o := NewObjectStorage(dg, s.objectCache)
				enobj, enerr := o.EncodedObject(t, h)
				if enerr != nil {
					continue
				}
				return enobj, nil
			}
		}
	}

	if err != nil {
		return nil, err
	}

	if plumbing.AnyObject != t && obj.Type() != t {
		return nil, plumbing.ErrObjectNotFound
	}

	return obj, nil
}

I'm wondering if this is a bug or if I'm going about the wrong way to checkout by tag.

[alee-r7]

I've submitted a PR related to this issue.

#964

[AriehSchneier]

Documentation for Hash in CheckoutOptions:

type CheckoutOptions struct {
	// Hash is the hash of the commit to be checked out. If used, HEAD will be
	// in detached mode. If Create is not used, Branch and Hash are mutually
	// exclusive.
	Hash plumbing.Hash

Is the problem that you are passing a tag hash rather than the commit hash? Would the correct procedure be to get the commit from the tag first and then pass that to CheckoutOptions?

You could call: tag, err := r.TagObject(hash) to get the tag object and then use tag.Hash