kubeadm: optimize the upgrade path from ClusterStatus to annotations

When doing the very first upgrade from a cluster that contains the
source of truth in the ClusterStatus struct, the new kubeadm logic
will try to retrieve this information from annotations.

This changeset adds to both etcd and apiserver endpoint retrieval the
special case in which they won't retry if we are in such cases. The
logic will retry if we find any unknown error, but will not retry in
the following cases:

- etcd annotations do not contain etcd endpoints, but the overall list
  of etcd pods is greater than 0. This means that we listed at least
  one etcd pod, but they are missing the annotation.

- API server annotation is not found on the api server pod for a given
  node name, but no errors aside from that one were found. This means
  that the API server pod is present, but is missing the annotation.

In both cases there is no point in retrying, and so, this speeds up the
upgrade path when coming from a previous existing cluster.

cmd/kubeadm/app/util/config/cluster.go

@@ -41,6 +41,25 @@ import (
 	"k8s.io/kubernetes/cmd/kubeadm/app/util/apiclient"
 )
 
+// unretriableError is an error used temporarily while we are migrating from the
+// ClusterStatus struct to an annotation Pod based information. When performing
+// the upgrade of all control plane nodes with `kubeadm upgrade apply` and
+// `kubeadm upgrade node` we don't want to retry as if we were hitting connectivity
+// issues when the pod annotation is missing on the API server pods. This error will
+// be used in such scenario, for failing fast, and falling back to the ClusterStatus
+// retrieval in those cases.
+type unretriableError struct {
+	err error
+}
+
+func newUnretriableError(err error) *unretriableError {
+	return &unretriableError{err: err}
+}
+
+func (ue *unretriableError) Error() string {
+	return fmt.Sprintf("unretriable error: %s", ue.err.Error())
+}
+
 // FetchInitConfigurationFromCluster fetches configuration from a ConfigMap in the cluster
 func FetchInitConfigurationFromCluster(client clientset.Interface, w io.Writer, logPrefix string, newControlPlane bool) (*kubeadmapi.InitConfiguration, error) {
 	fmt.Fprintf(w, "[%s] Reading configuration from the cluster...\n", logPrefix)
@@ -216,6 +235,13 @@ func getAPIEndpointFromPodAnnotation(client clientset.Interface, nodeName string
 	// static pods were not yet mirrored into the API server we want to wait for this propagation.
 	err := wait.ExponentialBackoff(backoff, func() (bool, error) {
 		rawAPIEndpoint, lastErr = getRawAPIEndpointFromPodAnnotationWithoutRetry(client, nodeName)
+		// TODO (ereslibre): this logic will need tweaking once that we get rid of the ClusterStatus, since we won't have
+		// the ClusterStatus safety net, we will want to remove the UnretriableError and not make the distinction here
+		// anymore.
+		if _, ok := lastErr.(*unretriableError); ok {
+			// Fail fast scenario, to be removed once we get rid of the ClusterStatus
+			return true, errors.Wrapf(lastErr, "API server Pods exist, but no API endpoint annotations were found")
+		}
 		return lastErr == nil, nil
 	})
 	if err != nil {
@@ -246,7 +272,7 @@ func getRawAPIEndpointFromPodAnnotationWithoutRetry(client clientset.Interface,
 	if apiServerEndpoint, ok := podList.Items[0].Annotations[constants.KubeAPIServerAdvertiseAddressEndpointAnnotationKey]; ok {
 		return apiServerEndpoint, nil
 	}
-	return "", errors.Errorf("API server pod for node name %q hasn't got a %q annotation, cannot retrieve API endpoint", nodeName, constants.KubeAPIServerAdvertiseAddressEndpointAnnotationKey)
+	return "", newUnretriableError(errors.Errorf("API server pod for node name %q hasn't got a %q annotation, cannot retrieve API endpoint", nodeName, constants.KubeAPIServerAdvertiseAddressEndpointAnnotationKey))
 }
 
 // TODO: remove after 1.20, when the ClusterStatus struct is removed from the kubeadm-config ConfigMap.

cmd/kubeadm/app/util/etcd/etcd.go

@@ -147,12 +147,20 @@ func getRawEtcdEndpointsFromPodAnnotation(client clientset.Interface, backoff wa
 	// Let's tolerate some unexpected transient failures from the API server or load balancers. Also, if
 	// static pods were not yet mirrored into the API server we want to wait for this propagation.
 	err := wait.ExponentialBackoff(backoff, func() (bool, error) {
-		if etcdEndpoints, lastErr = getRawEtcdEndpointsFromPodAnnotationWithoutRetry(client); lastErr != nil {
+		var overallEtcdPodCount int
+		if etcdEndpoints, overallEtcdPodCount, lastErr = getRawEtcdEndpointsFromPodAnnotationWithoutRetry(client); lastErr != nil {
 			return false, nil
 		}
-		// If the list of etcd endpoints is empty we want to retry: this can happen if joining a secondary
-		// control plane while the primary control plane didn't mirror its static pods yet.
-		return len(etcdEndpoints) > 0, nil
+		// TODO (ereslibre): this logic will need tweaking once that we get rid of the ClusterStatus, since we won't have
+		// the ClusterStatus safety net we will have to retry in both cases.
+		if len(etcdEndpoints) == 0 {
+			if overallEtcdPodCount == 0 {
+				return false, nil
+			}
+			// Fail fast scenario, to be removed once we get rid of the ClusterStatus
+			return true, errors.New("etcd Pods exist, but no etcd endpoint annotations were found")
+		}
+		return true, nil
 	})
 	if err != nil {
 		if lastErr != nil {
@@ -163,7 +171,10 @@ func getRawEtcdEndpointsFromPodAnnotation(client clientset.Interface, backoff wa
 	return etcdEndpoints, nil
 }
 
-func getRawEtcdEndpointsFromPodAnnotationWithoutRetry(client clientset.Interface) ([]string, error) {
+// getRawEtcdEndpointsFromPodAnnotationWithoutRetry returns the list of etcd endpoints as reported by etcd Pod annotations,
+// along with the number of global etcd pods. This allows for callers to tell the difference between "no endpoints found",
+// and "no endpoints found and pods were listed", so they can skip retrying.
+func getRawEtcdEndpointsFromPodAnnotationWithoutRetry(client clientset.Interface) ([]string, int, error) {
 	klog.V(3).Infof("retrieving etcd endpoints from %q annotation in etcd Pods", constants.EtcdAdvertiseClientUrlsAnnotationKey)
 	podList, err := client.CoreV1().Pods(metav1.NamespaceSystem).List(
 		context.TODO(),
@@ -172,17 +183,18 @@ func getRawEtcdEndpointsFromPodAnnotationWithoutRetry(client clientset.Interface
 		},
 	)
 	if err != nil {
-		return []string{}, err
+		return []string{}, 0, err
 	}
 	etcdEndpoints := []string{}
 	for _, pod := range podList.Items {
 		etcdEndpoint, ok := pod.ObjectMeta.Annotations[constants.EtcdAdvertiseClientUrlsAnnotationKey]
 		if !ok {
-			return []string{}, errors.Errorf("etcd Pod %q is missing the %q annotation; cannot infer etcd advertise client URL", pod.ObjectMeta.Name, constants.EtcdAdvertiseClientUrlsAnnotationKey)
+			klog.V(3).Infof("etcd Pod %q is missing the %q annotation; cannot infer etcd advertise client URL using the Pod annotation", pod.ObjectMeta.Name, constants.EtcdAdvertiseClientUrlsAnnotationKey)
+			continue
 		}
 		etcdEndpoints = append(etcdEndpoints, etcdEndpoint)
 	}
-	return etcdEndpoints, nil
+	return etcdEndpoints, len(podList.Items), nil
 }
 
 // TODO: remove after 1.20, when the ClusterStatus struct is removed from the kubeadm-config ConfigMap.

cmd/kubeadm/app/util/etcd/etcd_test.go

@@ -312,7 +312,7 @@ func TestGetRawEtcdEndpointsFromPodAnnotationWithoutRetry(t *testing.T) {
 			if rt.clientSetup != nil {
 				rt.clientSetup(client)
 			}
-			endpoints, err := getRawEtcdEndpointsFromPodAnnotationWithoutRetry(client)
+			endpoints, _, err := getRawEtcdEndpointsFromPodAnnotationWithoutRetry(client)
 			if err != nil && !rt.expectedErr {
 				t.Errorf("got error %v, but wasn't expecting any error", err)
 				return