feat:多跳（B->D->C->A）功能

cmd/internal/peer/client.go

@@ -25,6 +25,7 @@ type clientOptions struct {
 	udpLinuxTimestamping    bool
 	bindIP                  string
 	bindDevice              string
+	kcpDialAddress          string
 }
 
 // Option 用于配置 Client 的可选行为，例如时延日志。
@@ -73,6 +74,13 @@ func WithBindDevice(device string) Option {
 	}
 }
 
+// WithKCPDialAddress 指定 KCP 实际拨号使用的 UDP 地址，可用于通过 relay 连接逻辑上的 server。
+func WithKCPDialAddress(addr string) Option {
+	return func(options *clientOptions) {
+		options.kcpDialAddress = addr
+	}
+}
+
 // WithUDPLinuxTimestamping controls whether UDP clients enable Linux timestamping.
 func WithUDPLinuxTimestamping(enabled bool) Option {
 	return func(options *clientOptions) {

cmd/internal/peer/kcp_client.go

@@ -32,8 +32,13 @@ func DialKCP(serverAddr, peerID string, opts ...Option) (*KCPClient, error) {
 		options.logger = latencylog.NoopLogger{}
 	}
 
+	dialAddr := serverAddr
+	if options.kcpDialAddress != "" {
+		dialAddr = options.kcpDialAddress
+	}
+
 	session, err := transport.DialKCPSession(
-		serverAddr,
+		dialAddr,
 		options.bindIP,
 		options.bindDevice,
 		options.kcpPacketDebugLogger,

cmd/internal/peer/kcp_client_test.go

@@ -267,6 +267,96 @@ func TestKCPClientsExchangeMessagesAcrossRelayedServers(t *testing.T) {
 	}
 }
 
+func TestKCPClientsExchangeMessagesViaUDPRelayToSingleHub(t *testing.T) {
+	hub := server.NewKCPHub()
+	serverAddr, cleanupHub := startRealKCPHubServer(t, hub)
+	defer cleanupHub()
+
+	remoteAddr, err := net.ResolveUDPAddr("udp", serverAddr)
+	if err != nil {
+		t.Fatalf("ResolveUDPAddr(server) error = %v", err)
+	}
+
+	baseRelayConn, err := net.ListenPacket("udp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("ListenPacket(relay) error = %v", err)
+	}
+	relayConn := &countingPacketConn{PacketConn: baseRelayConn}
+
+	relay, err := server.NewUDPRelay(relayConn, remoteAddr)
+	if err != nil {
+		_ = relayConn.Close()
+		t.Fatalf("NewUDPRelay() error = %v", err)
+	}
+
+	var relayWG sync.WaitGroup
+	relayWG.Add(1)
+	go func() {
+		defer relayWG.Done()
+		if serveErr := relay.Serve(); serveErr != nil {
+			t.Errorf("relay.Serve() error = %v", serveErr)
+		}
+	}()
+	defer func() {
+		_ = relayConn.Close()
+		relayWG.Wait()
+	}()
+
+	peerA, err := DialKCP(serverAddr, "peer-a", WithKCPDialAddress(relayConn.LocalAddr().String()))
+	if err != nil {
+		t.Fatalf("DialKCP(peer-a via relay) error = %v", err)
+	}
+	defer func() { _ = peerA.Close() }()
+
+	peerB, err := DialKCP(serverAddr, "peer-b")
+	if err != nil {
+		t.Fatalf("DialKCP(peer-b direct) error = %v", err)
+	}
+	defer func() { _ = peerB.Close() }()
+
+	waitFor(t, func() bool { return hub.HasPeer("peer-a") && hub.HasPeer("peer-b") }, "both peers to be registered on the single hub")
+
+	if err := peerB.SendText("peer-a", "hello via udp relay"); err != nil {
+		t.Fatalf("peerB.SendText() error = %v", err)
+	}
+	gotAtA, err := peerA.Receive()
+	if err != nil {
+		t.Fatalf("peerA.Receive() error = %v", err)
+	}
+	wantAtA := protocol.Message{
+		Type: protocol.MessageTypeText,
+		ID:   1,
+		From: "peer-b",
+		To:   "peer-a",
+		Body: []byte("hello via udp relay"),
+	}
+	if !reflect.DeepEqual(gotAtA, wantAtA) {
+		t.Fatalf("peerA received %+v, want %+v", gotAtA, wantAtA)
+	}
+
+	if err := peerA.SendText("peer-b", "hello back through relay"); err != nil {
+		t.Fatalf("peerA.SendText() error = %v", err)
+	}
+	gotAtB, err := peerB.Receive()
+	if err != nil {
+		t.Fatalf("peerB.Receive() error = %v", err)
+	}
+	wantAtB := protocol.Message{
+		Type: protocol.MessageTypeText,
+		ID:   1,
+		From: "peer-a",
+		To:   "peer-b",
+		Body: []byte("hello back through relay"),
+	}
+	if !reflect.DeepEqual(gotAtB, wantAtB) {
+		t.Fatalf("peerB received %+v, want %+v", gotAtB, wantAtB)
+	}
+
+	if got := relayConn.WriteCount(); got == 0 {
+		t.Fatal("relay should have forwarded packets for peer-a session")
+	}
+}
+
 func TestKCPHubPrefersLocalPeerBeforeRelay(t *testing.T) {
 	fixture := startRelayedKCPHubs(t)
 	defer fixture.cleanup()