Improve error message for projected tokens when API is not enabled

Author: liggitt

pkg/kubelet/token/BUILD

@@ -21,6 +21,7 @@ go_library(
     visibility = ["//visibility:public"],
     deps = [
         "//staging/src/k8s.io/api/authentication/v1:go_default_library",
+        "//staging/src/k8s.io/apimachinery/pkg/api/errors:go_default_library",
         "//staging/src/k8s.io/apimachinery/pkg/types:go_default_library",
         "//staging/src/k8s.io/apimachinery/pkg/util/clock:go_default_library",
         "//staging/src/k8s.io/apimachinery/pkg/util/wait:go_default_library",

pkg/kubelet/token/token_manager.go

@@ -25,6 +25,7 @@ import (
 	"time"
 
 	authenticationv1 "k8s.io/api/authentication/v1"
+	apierrors "k8s.io/apimachinery/pkg/api/errors"
 	"k8s.io/apimachinery/pkg/types"
 	"k8s.io/apimachinery/pkg/util/clock"
 	"k8s.io/apimachinery/pkg/util/wait"
@@ -39,12 +40,35 @@ const (
 
 // NewManager returns a new token manager.
 func NewManager(c clientset.Interface) *Manager {
+	// check whether the server supports token requests so we can give a more helpful error message
+	supported := false
+	once := &sync.Once{}
+	tokenRequestsSupported := func() bool {
+		once.Do(func() {
+			resources, err := c.Discovery().ServerResourcesForGroupVersion("v1")
+			if err != nil {
+				return
+			}
+			for _, resource := range resources.APIResources {
+				if resource.Name == "serviceaccounts/token" {
+					supported = true
+					return
+				}
+			}
+		})
+		return supported
+	}
+
 	m := &Manager{
 		getToken: func(name, namespace string, tr *authenticationv1.TokenRequest) (*authenticationv1.TokenRequest, error) {
 			if c == nil {
 				return nil, errors.New("cannot use TokenManager when kubelet is in standalone mode")
 			}
-			return c.CoreV1().ServiceAccounts(namespace).CreateToken(name, tr)
+			tokenRequest, err := c.CoreV1().ServiceAccounts(namespace).CreateToken(name, tr)
+			if apierrors.IsNotFound(err) && !tokenRequestsSupported() {
+				return nil, fmt.Errorf("the API server does not have TokenRequest endpoints enabled")
+			}
+			return tokenRequest, err
 		},
 		cache: make(map[string]*authenticationv1.TokenRequest),
 		clock: clock.RealClock{},