kubeadm: fix the bug that 'kubeadm upgrade' hangs in single node cluster

Author: SataQiu

cmd/kubeadm/app/phases/upgrade/BUILD

@@ -40,6 +40,7 @@ go_library(
         "//staging/src/k8s.io/api/core/v1:go_default_library",
         "//staging/src/k8s.io/apimachinery/pkg/api/errors:go_default_library",
         "//staging/src/k8s.io/apimachinery/pkg/apis/meta/v1:go_default_library",
+        "//staging/src/k8s.io/apimachinery/pkg/fields:go_default_library",
         "//staging/src/k8s.io/apimachinery/pkg/labels:go_default_library",
         "//staging/src/k8s.io/apimachinery/pkg/util/errors:go_default_library",
         "//staging/src/k8s.io/apimachinery/pkg/util/sets:go_default_library",

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

@@ -24,6 +24,7 @@ import (
 
 	apierrors "k8s.io/apimachinery/pkg/api/errors"
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/apimachinery/pkg/fields"
 	errorsutil "k8s.io/apimachinery/pkg/util/errors"
 	"k8s.io/apimachinery/pkg/util/version"
 	clientset "k8s.io/client-go/kubernetes"
@@ -119,8 +120,15 @@ func removeOldDNSDeploymentIfAnotherDNSIsUsed(cfg *kubeadmapi.ClusterConfigurati
 			deploymentToDelete = kubeadmconstants.KubeDNSDeploymentName
 		}
 
-		// If we're dry-running, we don't need to wait for the new DNS addon to become ready
-		if !dryRun {
+		nodes, err := client.CoreV1().Nodes().List(context.TODO(), metav1.ListOptions{
+			FieldSelector: fields.Set{"spec.unschedulable": "false"}.AsSelector().String(),
+		})
+		if err != nil {
+			return err
+		}
+
+		// If we're dry-running or there are no scheduable nodes available, we don't need to wait for the new DNS addon to become ready
+		if !dryRun && len(nodes.Items) != 0 {
 			dnsDeployment, err := client.AppsV1().Deployments(metav1.NamespaceSystem).Get(context.TODO(), installedDeploymentName, metav1.GetOptions{})
 			if err != nil {
 				return err
@@ -132,7 +140,7 @@ func removeOldDNSDeploymentIfAnotherDNSIsUsed(cfg *kubeadmapi.ClusterConfigurati
 
 		// We don't want to wait for the DNS deployment above to become ready when dryrunning (as it never will)
 		// but here we should execute the DELETE command against the dryrun clientset, as it will only be logged
-		err := apiclient.DeleteDeploymentForeground(client, metav1.NamespaceSystem, deploymentToDelete)
+		err = apiclient.DeleteDeploymentForeground(client, metav1.NamespaceSystem, deploymentToDelete)
 		if err != nil && !apierrors.IsNotFound(err) {
 			return err
 		}