git context: fix support for empty git ref with subdir

[crazy-max]

fixes docker/buildx#1604

Git context detection in the frontend

buildkit/frontend/dockerfile/builder/build.go

Lines 700 to 703 in ecc5937

	g, err := gitutil.ParseGitRef(ref)
	if err != nil {
	return nil, false
	}

does not match myrepo.git#:myfolder but should be supported.

Also adds more test cases to be sure we don't miss anything else.

Looking at our docs in https://docs.docker.com/build/building/context/#git-repositories

@dvdksn "Commit Used" for myrepo.git and myrepo.git#:myfolder should be instead default branch imo.

I also wonder if we should mutualize the logic from

buildkit/source/gitidentifier.go

Lines 23 to 54 in ecc5937

	func NewGitIdentifier(remoteURL string) (*GitIdentifier, error) {
	repo := GitIdentifier{}
	if !isGitTransport(remoteURL) {
	remoteURL = "https://" + remoteURL
	}
	var fragment string
	if sshutil.IsImplicitSSHTransport(remoteURL) {
	// implicit ssh urls such as "git@.." are not actually a URL, so cannot be parsed as URL
	parts := strings.SplitN(remoteURL, "#", 2)
	repo.Remote = parts[0]
	if len(parts) == 2 {
	fragment = parts[1]
	}
	repo.Ref, repo.Subdir = getRefAndSubdir(fragment)
	} else {
	u, err := url.Parse(remoteURL)
	if err != nil {
	return nil, err
	}
	repo.Ref, repo.Subdir = getRefAndSubdir(u.Fragment)
	u.Fragment = ""
	repo.Remote = u.String()
	}
	if sd := path.Clean(repo.Subdir); sd == "/" || sd == "." {
	repo.Subdir = ""
	}
	return &repo, nil
	}

with

buildkit/util/gitutil/git_ref.go

Lines 51 to 95 in ecc5937

	func ParseGitRef(ref string) (*GitRef, error) {
	res := &GitRef{}
	if strings.HasPrefix(ref, "github.com/") {
	res.IndistinguishableFromLocal = true // Deprecated
	} else {
	_, proto := ParseProtocol(ref)
	switch proto {
	case UnknownProtocol:
	return nil, errdefs.ErrInvalidArgument
	}
	switch proto {
	case HTTPProtocol, GitProtocol:
	res.UnencryptedTCP = true // Discouraged, but not deprecated
	}
	switch proto {
	// An HTTP(S) URL is considered to be a valid git ref only when it has the ".git[...]" suffix.
	case HTTPProtocol, HTTPSProtocol:
	var gitURLPathWithFragmentSuffix = regexp.MustCompile(`\.git(?:#.+)?$`)
	if !gitURLPathWithFragmentSuffix.MatchString(ref) {
	return nil, errdefs.ErrInvalidArgument
	}
	}
	}
	refSplitBySharp := strings.SplitN(ref, "#", 2)
	res.Remote = refSplitBySharp[0]
	if len(res.Remote) == 0 {
	return res, errdefs.ErrInvalidArgument
	}
	if len(refSplitBySharp) > 1 {
	refSplitBySharpSplitByColon := strings.SplitN(refSplitBySharp[1], ":", 2)
	res.Commit = refSplitBySharpSplitByColon[0]
	if len(res.Commit) == 0 {
	return res, errdefs.ErrInvalidArgument
	}
	if len(refSplitBySharpSplitByColon) > 1 {
	res.SubDir = refSplitBySharpSplitByColon[1]
	}
	}
	repoSplitBySlash := strings.Split(res.Remote, "/")
	res.ShortName = strings.TrimSuffix(repoSplitBySlash[len(repoSplitBySlash)-1], ".git")
	return res, nil
	}