Use git url fragment to specify reference and dir context.

Signed-off-by: David Calavera <david.calavera@gmail.com>

utils/git.go

@@ -4,7 +4,10 @@ import (
 	"fmt"
 	"io/ioutil"
 	"net/http"
+	"net/url"
+	"os"
 	"os/exec"
+	"path/filepath"
 	"strings"
 
 	"github.com/docker/docker/pkg/urlutil"
@@ -19,20 +22,26 @@ func GitClone(remoteURL string) (string, error) {
 		return "", err
 	}
 
-	clone := cloneArgs(remoteURL, root)
+	u, err := url.Parse(remoteURL)
+	if err != nil {
+		return "", err
+	}
 
-	if output, err := exec.Command("git", clone...).CombinedOutput(); err != nil {
+	fragment := u.Fragment
+	clone := cloneArgs(u, root)
+
+	if output, err := git(clone...); err != nil {
 		return "", fmt.Errorf("Error trying to use git: %s (%s)", err, output)
 	}
 
-	return root, nil
+	return checkoutGit(fragment, root)
 }
 
-func cloneArgs(remoteURL, root string) []string {
+func cloneArgs(remoteURL *url.URL, root string) []string {
 	args := []string{"clone", "--recursive"}
-	shallow := true
+	shallow := len(remoteURL.Fragment) == 0
 
-	if strings.HasPrefix(remoteURL, "http") {
+	if shallow && strings.HasPrefix(remoteURL.Scheme, "http") {
 		res, err := http.Head(fmt.Sprintf("%s/info/refs?service=git-upload-pack", remoteURL))
 		if err != nil || res.Header.Get("Content-Type") != "application/x-git-upload-pack-advertisement" {
 			shallow = false
@@ -43,5 +52,42 @@ func cloneArgs(remoteURL, root string) []string {
 		args = append(args, "--depth", "1")
 	}
 
-	return append(args, remoteURL, root)
+	if remoteURL.Fragment != "" {
+		remoteURL.Fragment = ""
+	}
+
+	return append(args, remoteURL.String(), root)
+}
+
+func checkoutGit(fragment, root string) (string, error) {
+	refAndDir := strings.SplitN(fragment, ":", 2)
+
+	if len(refAndDir[0]) != 0 {
+		if output, err := gitWithinDir(root, "checkout", refAndDir[0]); err != nil {
+			return "", fmt.Errorf("Error trying to use git: %s (%s)", err, output)
+		}
+	}
+
+	if len(refAndDir) > 1 && len(refAndDir[1]) != 0 {
+		newCtx := filepath.Join(root, refAndDir[1])
+		fi, err := os.Stat(newCtx)
+		if err != nil {
+			return "", err
+		}
+		if !fi.IsDir() {
+			return "", fmt.Errorf("Error setting git context, not a directory: %s", newCtx)
+		}
+		root = newCtx
+	}
+
+	return root, nil
+}
+
+func gitWithinDir(dir string, args ...string) ([]byte, error) {
+	a := []string{"--work-tree", dir, "--git-dir", filepath.Join(dir, ".git")}
+	return git(append(a, args...)...)
+}
+
+func git(args ...string) ([]byte, error) {
+	return exec.Command("git", args...).CombinedOutput()
 }