Improve coredns upgrade path

cmd/kubeadm/app/phases/addons/dns/dns.go

@@ -205,8 +205,8 @@ func createCoreDNSAddon(deploymentBytes, serviceBytes, configBytes []byte, clien
 		return fmt.Errorf("unable to decode CoreDNS configmap %v", err)
 	}
 
-	// Create the ConfigMap for CoreDNS or update it in case it already exists
-	if err := apiclient.CreateOrUpdateConfigMap(client, coreDNSConfigMap); err != nil {
+	// Create the ConfigMap for CoreDNS or retain it in case it already exists
+	if err := apiclient.CreateOrRetainConfigMap(client, coreDNSConfigMap, kubeadmconstants.CoreDNS); err != nil {
 		return err
 	}
 

cmd/kubeadm/app/phases/upgrade/postupgrade.go

@@ -103,13 +103,13 @@ func PerformPostUpgradeTasks(client clientset.Interface, cfg *kubeadmapi.MasterC
 		}
 	}
 
-	// Upgrade kube-dns and kube-proxy
+	// Upgrade kube-dns/CoreDNS and kube-proxy
 	if err := dns.EnsureDNSAddon(cfg, client); err != nil {
 		errs = append(errs, err)
 	}
-	// Remove the old kube-dns deployment if coredns is now used
+	// Remove the old DNS deployment if a new DNS service is now used (kube-dns to CoreDNS or vice versa)
 	if !dryRun {
-		if err := removeOldKubeDNSDeploymentIfCoreDNSIsUsed(cfg, client); err != nil {
+		if err := removeOldDNSDeploymentIfAnotherDNSIsUsed(cfg, client); err != nil {
 			errs = append(errs, err)
 		}
 	}
@@ -120,24 +120,28 @@ func PerformPostUpgradeTasks(client clientset.Interface, cfg *kubeadmapi.MasterC
 	return errors.NewAggregate(errs)
 }
 
-func removeOldKubeDNSDeploymentIfCoreDNSIsUsed(cfg *kubeadmapi.MasterConfiguration, client clientset.Interface) error {
-	if features.Enabled(cfg.FeatureGates, features.CoreDNS) {
+func removeOldDNSDeploymentIfAnotherDNSIsUsed(cfg *kubeadmapi.MasterConfiguration, client clientset.Interface) error {
 	return apiclient.TryRunCommand(func() error {
-			coreDNSDeployment, err := client.AppsV1().Deployments(metav1.NamespaceSystem).Get(kubeadmconstants.CoreDNS, metav1.GetOptions{})
+		installedDeploymentName := kubeadmconstants.KubeDNS
+		deploymentToDelete := kubeadmconstants.CoreDNS
+
+		if features.Enabled(cfg.FeatureGates, features.CoreDNS) {
+			installedDeploymentName = kubeadmconstants.CoreDNS
+			deploymentToDelete = kubeadmconstants.KubeDNS
+		}
+		dnsDeployment, err := client.AppsV1().Deployments(metav1.NamespaceSystem).Get(installedDeploymentName, metav1.GetOptions{})
 		if err != nil {
 			return err
 		}
-			if coreDNSDeployment.Status.ReadyReplicas == 0 {
-				return fmt.Errorf("the CoreDNS deployment isn't ready yet")
+		if dnsDeployment.Status.ReadyReplicas == 0 {
+			return fmt.Errorf("the DNS deployment isn't ready yet")
 		}
-			err = apiclient.DeleteDeploymentForeground(client, metav1.NamespaceSystem, kubeadmconstants.KubeDNS)
+		err = apiclient.DeleteDeploymentForeground(client, metav1.NamespaceSystem, deploymentToDelete)
 		if err != nil && !apierrors.IsNotFound(err) {
 			return err
 		}
 		return nil
 	}, 10)
-	}
-	return nil
 }
 
 func upgradeToSelfHosting(client clientset.Interface, cfg *kubeadmapi.MasterConfiguration, newK8sVer *version.Version, dryRun bool) error {

cmd/kubeadm/app/util/apiclient/idempotency.go

@@ -44,6 +44,21 @@ func CreateOrUpdateConfigMap(client clientset.Interface, cm *v1.ConfigMap) error
 	return nil
 }
 
+// CreateOrRetainConfigMap creates a ConfigMap if the target resource doesn't exist. If the resource exists already, this function will retain the resource instead.
+func CreateOrRetainConfigMap(client clientset.Interface, cm *v1.ConfigMap, configMapName string) error {
+	if _, err := client.CoreV1().ConfigMaps(cm.ObjectMeta.Namespace).Get(configMapName, metav1.GetOptions{}); err != nil {
+		if !apierrors.IsNotFound(err) {
+			return nil
+		}
+		if _, err := client.CoreV1().ConfigMaps(cm.ObjectMeta.Namespace).Create(cm); err != nil {
+			if !apierrors.IsAlreadyExists(err) {
+				return fmt.Errorf("unable to create configmap: %v", err)
+			}
+		}
+	}
+	return nil
+}
+
 // CreateOrUpdateSecret creates a Secret if the target resource doesn't exist. If the resource exists already, this function will update the resource instead.
 func CreateOrUpdateSecret(client clientset.Interface, secret *v1.Secret) error {
 	if _, err := client.CoreV1().Secrets(secret.ObjectMeta.Namespace).Create(secret); err != nil {