pkg/cloudlaunch: added latestStableCoreOSImage

To get the latest stable CoreOS image URL for GCE.

Change-Id: I608b4c8d6d1628046cf5f03440afcf158a3ee609

pkg/cloudlaunch/cloudlaunch.go

@@ -19,13 +19,17 @@ limitations under the License.
 package cloudlaunch
 
 import (
+	"encoding/json"
+	"errors"
 	"flag"
 	"io/ioutil"
 	"log"
+	"net/http"
 	"os"
 	"path/filepath"
 	"runtime"
 	"strings"
+	"time"
 
 	"golang.org/x/oauth2"
 	"golang.org/x/oauth2/google"
@@ -300,3 +304,44 @@ func instanceDisk(svc *compute.Service) *compute.AttachedDisk {
 	}
 }
 */
+
+type coreOSImage struct {
+	SelfLink          string
+	CreationTimestamp time.Time
+	Name              string
+}
+
+type coreOSImageList struct {
+	Items []coreOSImage
+}
+
+func latestStableCoreOSImage(cl *http.Client) (string, error) {
+	resp, err := cl.Get("https://www.googleapis.com/compute/v1/projects/coreos-cloud/global/images")
+	if err != nil {
+		return "", err
+	}
+	defer resp.Body.Close()
+	imageList := &coreOSImageList{}
+	if err := json.NewDecoder(resp.Body).Decode(imageList); err != nil {
+		return "", err
+	}
+	if imageList == nil || len(imageList.Items) == 0 {
+		return "", errors.New("no images list in response")
+	}
+
+	imageURL := ""
+	var max time.Time // latest stable image creation time
+	for _, v := range imageList.Items {
+		if !strings.HasPrefix(v.Name, "coreos-stable") {
+			continue
+		}
+		if v.CreationTimestamp.After(max) {
+			max = v.CreationTimestamp
+			imageURL = v.SelfLink
+		}
+	}
+	if imageURL == "" {
+		return "", errors.New("no stable coreOS image found")
+	}
+	return imageURL, nil
+}