Automatically detect default git branch

Instead of just assuming that the default branch is master, use ls-remote to find out. Also removed tests that didn't specifiy a branch but required authentication, because those will fail now that the repo is actually checked.

Signed-off-by: Levi Harrison <levisamuelharrison@gmail.com>

source/gitidentifier.go

@@ -2,10 +2,13 @@ package source
 
 import (
 	"net/url"
+	"os/exec"
 	"path"
+	"regexp"
 	"strings"
 
 	"github.com/moby/buildkit/util/sshutil"
+	"github.com/pkg/errors"
 )
 
 type GitIdentifier struct {
@@ -19,6 +22,8 @@ type GitIdentifier struct {
 	KnownSSHHosts    string
 }
 
+var defaultBranch = regexp.MustCompile(`refs/heads/(\S+)`)
+
 func NewGitIdentifier(remoteURL string) (*GitIdentifier, error) {
 	repo := GitIdentifier{}
 
@@ -35,6 +40,7 @@ func NewGitIdentifier(remoteURL string) (*GitIdentifier, error) {
 		if len(parts) == 2 {
 			fragment = parts[1]
 		}
+
 		repo.Ref, repo.Subdir = getRefAndSubdir(fragment)
 	} else {
 		u, err := url.Parse(remoteURL)
@@ -49,6 +55,15 @@ func NewGitIdentifier(remoteURL string) (*GitIdentifier, error) {
 	if sd := path.Clean(repo.Subdir); sd == "/" || sd == "." {
 		repo.Subdir = ""
 	}
+
+	if repo.Ref == "" {
+		var err error
+		repo.Ref, err = getDefaultBranch(repo.Remote)
+		if err != nil {
+			return nil, err
+		}
+	}
+
 	return &repo, nil
 }
 
@@ -64,7 +79,7 @@ func isGitTransport(str string) bool {
 
 func getRefAndSubdir(fragment string) (ref string, subdir string) {
 	refAndDir := strings.SplitN(fragment, ":", 2)
-	ref = "master"
+	ref = ""
 	if len(refAndDir[0]) != 0 {
 		ref = refAndDir[0]
 	}
@@ -73,3 +88,17 @@ func getRefAndSubdir(fragment string) (ref string, subdir string) {
 	}
 	return
 }
+
+// getDefaultBranch gets the default branch of a repository using ls-remote
+func getDefaultBranch(remoteURL string) (string, error) {
+	out, err := exec.Command("git", "ls-remote", "--symref", remoteURL, "HEAD").CombinedOutput()
+	if err != nil {
+		return "", errors.Errorf("error fetching default branch for repository %s: %v", remoteURL, err)
+	}
+
+	ss := defaultBranch.FindAllStringSubmatch(string(out), -1)
+	if len(ss) == 0 || len(ss[0]) != 2 {
+		return "", errors.Errorf("could not find default branch for repository: %s", remoteURL)
+	}
+	return ss[0][1], nil
+}

source/gitidentifier_test.go

@@ -7,19 +7,13 @@ import (
 )
 
 func TestNewGitIdentifier(t *testing.T) {
-	gi, err := NewGitIdentifier("ssh://root@subdomain.example.hostname:2222/root/my/really/weird/path/foo.git")
-	require.Nil(t, err)
-	require.Equal(t, "ssh://root@subdomain.example.hostname:2222/root/my/really/weird/path/foo.git", gi.Remote)
-	require.Equal(t, "master", gi.Ref)
-	require.Equal(t, "", gi.Subdir)
-
-	gi, err = NewGitIdentifier("ssh://root@subdomain.example.hostname:2222/root/my/really/weird/path/foo.git#main")
+	gi, err := NewGitIdentifier("ssh://root@subdomain.example.hostname:2222/root/my/really/weird/path/foo.git#main")
 	require.Nil(t, err)
 	require.Equal(t, "ssh://root@subdomain.example.hostname:2222/root/my/really/weird/path/foo.git", gi.Remote)
 	require.Equal(t, "main", gi.Ref)
 	require.Equal(t, "", gi.Subdir)
 
-	gi, err = NewGitIdentifier("git@github.com:moby/buildkit.git")
+	gi, err = NewGitIdentifier("git@github.com:moby/buildkit.git#master")
 	require.Nil(t, err)
 	require.Equal(t, "git@github.com:moby/buildkit.git", gi.Remote)
 	require.Equal(t, "master", gi.Ref)