feat: bridge mixed wire protocol SUDP payloads (#5347)

SUDP payload codec follows transport wireProtocol; same-protocol v1/v1 and v2/v2 keep raw join; only mixed proxy/visitor protocols use message-aware bridge; no new capability/selection field.

server/proxy/sudp_test.go

@@ -0,0 +1,141 @@
+// Copyright 2026 The frp Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package proxy
+
+import (
+	"bufio"
+	"bytes"
+	"encoding/binary"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/fatedier/frp/pkg/msg"
+	"github.com/fatedier/frp/pkg/proto/wire"
+)
+
+func TestSUDPBridgeTranscodesProxyV1ToVisitorV2(t *testing.T) {
+	var in, out bytes.Buffer
+	writeSUDPBridgeMsg(t, &in, wire.ProtocolV1, &msg.UDPPacket{Content: []byte("proxy-to-visitor")})
+
+	var count int64
+	err := bridgeSUDPProxyToVisitor(
+		msg.NewReadWriter(&in, wire.ProtocolV1),
+		msg.NewReadWriter(&out, wire.ProtocolV2),
+		&count,
+		nil,
+	)
+	require.NoError(t, err)
+	require.Equal(t, int64(len("proxy-to-visitor")), count)
+
+	frame, err := wire.NewConn(&out).ReadFrame()
+	require.NoError(t, err)
+	require.Equal(t, wire.FrameTypeMessage, frame.Type)
+	require.GreaterOrEqual(t, len(frame.Payload), 2)
+	require.Equal(t, msg.V2TypeUDPPacket, binary.BigEndian.Uint16(frame.Payload[:2]))
+
+	var got msg.UDPPacket
+	require.NoError(t, msg.DecodeV2MessageFrameInto(frame, &got))
+	require.Equal(t, []byte("proxy-to-visitor"), got.Content)
+}
+
+func TestSUDPBridgeTranscodesVisitorV2ToProxyV1(t *testing.T) {
+	var in, out bytes.Buffer
+	writeSUDPBridgeMsg(t, &in, wire.ProtocolV2, &msg.UDPPacket{Content: []byte("visitor-to-proxy")})
+
+	var count int64
+	err := bridgeSUDPVisitorToProxy(
+		msg.NewReadWriter(&in, wire.ProtocolV2),
+		msg.NewReadWriter(&out, wire.ProtocolV1),
+		&count,
+		nil,
+	)
+	require.NoError(t, err)
+	require.Equal(t, int64(len("visitor-to-proxy")), count)
+
+	reader := bufio.NewReader(&out)
+	typeByte, err := reader.ReadByte()
+	require.NoError(t, err)
+	require.Equal(t, msg.TypeUDPPacket, typeByte)
+	require.NoError(t, reader.UnreadByte())
+
+	var got msg.UDPPacket
+	require.NoError(t, msg.ReadMsgInto(reader, &got))
+	require.Equal(t, []byte("visitor-to-proxy"), got.Content)
+}
+
+func TestSUDPBridgeForwardsProxyPing(t *testing.T) {
+	var in, out bytes.Buffer
+	writeSUDPBridgeMsg(t, &in, wire.ProtocolV1, &msg.Ping{})
+
+	var count int64
+	err := bridgeSUDPProxyToVisitor(
+		msg.NewReadWriter(&in, wire.ProtocolV1),
+		msg.NewReadWriter(&out, wire.ProtocolV2),
+		&count,
+		nil,
+	)
+	require.NoError(t, err)
+	require.Zero(t, count)
+
+	rawMsg, err := msg.NewReadWriter(&out, wire.ProtocolV2).ReadMsg()
+	require.NoError(t, err)
+	require.IsType(t, &msg.Ping{}, rawMsg)
+}
+
+func TestSUDPBridgeDropsVisitorPing(t *testing.T) {
+	var in, out bytes.Buffer
+	writeSUDPBridgeMsg(t, &in, wire.ProtocolV2, &msg.Ping{})
+
+	var count int64
+	err := bridgeSUDPVisitorToProxy(
+		msg.NewReadWriter(&in, wire.ProtocolV2),
+		msg.NewReadWriter(&out, wire.ProtocolV1),
+		&count,
+		nil,
+	)
+	require.NoError(t, err)
+	require.Zero(t, count)
+	require.Empty(t, out.Bytes())
+}
+
+func TestSUDPBridgeRejectsUnknownVisitorMessage(t *testing.T) {
+	var in, out bytes.Buffer
+	writeSUDPBridgeMsg(t, &in, wire.ProtocolV2, &msg.Pong{})
+
+	var count int64
+	err := bridgeSUDPVisitorToProxy(
+		msg.NewReadWriter(&in, wire.ProtocolV2),
+		msg.NewReadWriter(&out, wire.ProtocolV1),
+		&count,
+		nil,
+	)
+	require.ErrorContains(t, err, "unexpected SUDP visitor message *msg.Pong")
+	require.Zero(t, count)
+	require.Empty(t, out.Bytes())
+}
+
+func TestSUDPBridgeDetectsMixedWireProtocol(t *testing.T) {
+	require.False(t, isMixedWireProtocol("", wire.ProtocolV1))
+	require.False(t, isMixedWireProtocol(wire.ProtocolV2, wire.ProtocolV2))
+	require.True(t, isMixedWireProtocol("", wire.ProtocolV2))
+	require.True(t, isMixedWireProtocol(wire.ProtocolV2, wire.ProtocolV1))
+}
+
+func writeSUDPBridgeMsg(t *testing.T, buf *bytes.Buffer, wireProtocol string, m msg.Message) {
+	t.Helper()
+
+	require.NoError(t, msg.NewReadWriter(buf, wireProtocol).WriteMsg(m))
+}