Implement support for symbolic labels in --use-kubernetes-version

Now, defaults can be pointing to "stable" and users will always get
latest available stable build of Kubernetes via kubeadm.
There is no need anymore to hardcode version string inside kubeadm
binary.

It is also possible to use labels like "latest" or point to exact
branch: "stable-1.4"

cmd/kubeadm/app/apis/kubeadm/v1alpha1/defaults.go

@@ -23,7 +23,7 @@ import (
 const (
 	DefaultServiceDNSDomain  = "cluster.local"
 	DefaultServicesSubnet    = "10.96.0.0/12"
-	DefaultKubernetesVersion = "v1.4.4"
+	DefaultKubernetesVersion = "stable"
 	DefaultAPIBindPort       = 6443
 	DefaultDiscoveryBindPort = 9898
 )

cmd/kubeadm/app/cmd/init.go

@@ -193,6 +193,14 @@ func NewInit(cfgPath string, cfg *kubeadmapi.MasterConfiguration, skipPreFlight
 		fmt.Println("Skipping pre-flight checks")
 	}
 
+	// validate version argument
+	ver, err := kubeadmutil.KubernetesReleaseVersion(cfg.KubernetesVersion)
+	if err != nil {
+		return nil, err
+	}
+	cfg.KubernetesVersion = ver
+	fmt.Println("Using Kubernetes version:", ver)
+
 	// TODO(phase1+) create a custom flag
 	if cfg.CloudProvider != "" {
 		if cloudprovider.IsCloudProvider(cfg.CloudProvider) {

cmd/kubeadm/app/util/BUILD

@@ -16,6 +16,7 @@ go_library(
         "error.go",
         "kubeconfig.go",
         "tokens.go",
+        "version.go",
     ],
     tags = ["automanaged"],
     deps = [

cmd/kubeadm/app/util/version.go

@@ -0,0 +1,71 @@
+/*
+Copyright 2016 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package util
+
+import (
+	"fmt"
+	"io/ioutil"
+	"net/http"
+	"regexp"
+	"strings"
+)
+
+var (
+	kubeReleaseBucketURL  = "https://storage.googleapis.com/kubernetes-release/release"
+	kubeReleaseRegex      = regexp.MustCompile(`^v(0|[1-9][0-9]*)\.(0|[1-9][0-9]*)\.(0|[1-9][0-9]*)([-0-9a-zA-Z_\.+]*)?$`)
+	kubeReleaseLabelRegex = regexp.MustCompile(`^[[:lower:]]+(-[-\w_\.]+)?$`)
+)
+
+// KubernetesReleaseVersion is helper function that can fetch
+// available version information from release servers based on
+// label names, like "stable" or "latest".
+//
+// If argument is already semantic version string, it
+// will return same string.
+//
+// In case of labels, it tries to fetch from release
+// servers and then return actual semantic version.
+//
+// Available names on release servers:
+//  stable      (latest stable release)
+//  stable-1    (latest stable release in 1.x)
+//  stable-1.0  (and similarly 1.1, 1.2, 1.3, ...)
+//  latest      (latest release, including alpha/beta)
+//  latest-1    (latest release in 1.x, including alpha/beta)
+//  latest-1.0  (and similarly 1.1, 1.2, 1.3, ...)
+func KubernetesReleaseVersion(version string) (string, error) {
+	if kubeReleaseRegex.MatchString(version) {
+		return version, nil
+	} else if kubeReleaseLabelRegex.MatchString(version) {
+		url := fmt.Sprintf("%s/%s.txt", kubeReleaseBucketURL, version)
+		resp, err := http.Get(url)
+		if err != nil {
+			return "", fmt.Errorf("Error: unable to get URL %q: %s", url, err.Error())
+		}
+		defer resp.Body.Close()
+		if resp.StatusCode != http.StatusOK {
+			return "", fmt.Errorf("Error: unable to fetch release information. URL: %q Status: %v", url, resp.Status)
+		}
+		body, err := ioutil.ReadAll(resp.Body)
+		if err != nil {
+			return "", fmt.Errorf("Error: unable to read content of URL %q: %s", url, err.Error())
+		}
+		// Re-validate received version and return.
+		return KubernetesReleaseVersion(strings.Trim(string(body), " \t\n"))
+	}
+	return "", fmt.Errorf("Error: version %q doesn't match patterns for neither semantic version nor labels (stable, latest, ...)", version)
+}