kubeadm: do not remove the only remaining etcd member during reset

If this is the only remaining stacked etcd member in the cluster,
calling RemoveMember() is not needed.

cmd/kubeadm/app/phases/etcd/local.go

@@ -103,6 +103,22 @@ func RemoveStackedEtcdMemberFromCluster(client clientset.Interface, cfg *kubeadm
 		return err
 	}
 
+	members, err := etcdClient.ListMembers()
+	if err != nil {
+		return err
+	}
+	// If this is the only remaining stacked etcd member in the cluster, calling RemoveMember()
+	// is not needed.
+	if len(members) == 1 {
+		etcdClientAddress := etcdutil.GetClientURL(&cfg.LocalAPIEndpoint)
+		for _, endpoint := range etcdClient.Endpoints {
+			if endpoint == etcdClientAddress {
+				klog.V(1).Info("[etcd] This is the only remaining etcd member in the etcd cluster, skip removing it")
+				return nil
+			}
+		}
+	}
+
 	// notifies the other members of the etcd cluster about the removing member
 	etcdPeerAddress := etcdutil.GetPeerURL(&cfg.LocalAPIEndpoint)
 
@@ -113,7 +129,7 @@ func RemoveStackedEtcdMemberFromCluster(client clientset.Interface, cfg *kubeadm
 	}
 
 	klog.V(1).Infof("[etcd] removing etcd member: %s, id: %d", etcdPeerAddress, id)
-	members, err := etcdClient.RemoveMember(id)
+	members, err = etcdClient.RemoveMember(id)
 	if err != nil {
 		return err
 	}

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

@@ -246,6 +246,10 @@ func (c fakeTLSEtcdClient) CheckClusterHealth() error {
 
 func (c fakeTLSEtcdClient) Sync() error { return nil }
 
+func (c fakeTLSEtcdClient) ListMembers() ([]etcdutil.Member, error) {
+	return []etcdutil.Member{}, nil
+}
+
 func (c fakeTLSEtcdClient) AddMember(name string, peerAddrs string) ([]etcdutil.Member, error) {
 	return []etcdutil.Member{}, nil
 }
@@ -277,6 +281,10 @@ func (c fakePodManifestEtcdClient) CheckClusterHealth() error {
 
 func (c fakePodManifestEtcdClient) Sync() error { return nil }
 
+func (c fakePodManifestEtcdClient) ListMembers() ([]etcdutil.Member, error) {
+	return []etcdutil.Member{}, nil
+}
+
 func (c fakePodManifestEtcdClient) AddMember(name string, peerAddrs string) ([]etcdutil.Member, error) {
 	return []etcdutil.Member{}, nil
 }

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

@@ -55,6 +55,7 @@ type ClusterInterrogator interface {
 	CheckClusterHealth() error
 	WaitForClusterAvailable(retries int, retryInterval time.Duration) (bool, error)
 	Sync() error
+	ListMembers() ([]Member, error)
 	AddMember(name string, peerAddrs string) ([]Member, error)
 	GetMemberID(peerURL string) (uint64, error)
 	RemoveMember(id uint64) ([]Member, error)
@@ -258,8 +259,7 @@ type Member struct {
 	PeerURL string
 }
 
-// GetMemberID returns the member ID of the given peer URL
+func (c *Client) listMembers() (*clientv3.MemberListResponse, error) {
-func (c *Client) GetMemberID(peerURL string) (uint64, error) {
 	cli, err := clientv3.New(clientv3.Config{
 		Endpoints:   c.Endpoints,
 		DialTimeout: dialTimeout,
@@ -269,7 +269,7 @@ func (c *Client) GetMemberID(peerURL string) (uint64, error) {
 		TLS: c.TLS,
 	})
 	if err != nil {
-		return 0, err
+		return nil, err
 	}
 	defer cli.Close()
 
@@ -288,7 +288,16 @@ func (c *Client) GetMemberID(peerURL string) (uint64, error) {
 		return false, nil
 	})
 	if err != nil {
-		return 0, lastError
+		return nil, lastError
+	}
+	return resp, nil
+}
+
+// GetMemberID returns the member ID of the given peer URL
+func (c *Client) GetMemberID(peerURL string) (uint64, error) {
+	resp, err := c.listMembers()
+	if err != nil {
+		return 0, err
 	}
 
 	for _, member := range resp.Members {
@@ -299,6 +308,20 @@ func (c *Client) GetMemberID(peerURL string) (uint64, error) {
 	return 0, nil
 }
 
+// ListMembers returns the member list.
+func (c *Client) ListMembers() ([]Member, error) {
+	resp, err := c.listMembers()
+	if err != nil {
+		return nil, err
+	}
+
+	ret := make([]Member, 0, len(resp.Members))
+	for _, m := range resp.Members {
+		ret = append(ret, Member{Name: m.Name, PeerURL: m.PeerURLs[0]})
+	}
+	return ret, nil
+}
+
 // RemoveMember notifies an etcd cluster to remove an existing member
 func (c *Client) RemoveMember(id uint64) ([]Member, error) {
 	cli, err := clientv3.New(clientv3.Config{