support udp in relay if last proxy could udp-over-tcp

adapter/outboundgroup/relay.go

@@ -17,12 +17,7 @@ type Relay struct {
 
 // DialContext implements C.ProxyAdapter
 func (r *Relay) DialContext(ctx context.Context, metadata *C.Metadata, opts ...dialer.Option) (C.Conn, error) {
-	var proxies []C.Proxy
-	for _, proxy := range r.proxies(metadata, true) {
-		if proxy.Type() != C.Direct && proxy.Type() != C.Compatible {
-			proxies = append(proxies, proxy)
-		}
-	}
+	proxies, chainProxies := r.proxies(metadata, true)
 
 	switch len(proxies) {
 	case 0:
@@ -60,7 +55,80 @@ func (r *Relay) DialContext(ctx context.Context, metadata *C.Metadata, opts ...d
 		return nil, fmt.Errorf("%s connect error: %w", last.Addr(), err)
 	}
 
-	return outbound.NewConn(c, r), nil
+	conn := outbound.NewConn(c, last)
+
+	for i := len(chainProxies) - 2; i >= 0; i-- {
+		conn.AppendToChains(chainProxies[i])
+	}
+
+	conn.AppendToChains(r)
+
+	return conn, nil
+}
+
+// ListenPacketContext implements C.ProxyAdapter
+func (r *Relay) ListenPacketContext(ctx context.Context, metadata *C.Metadata, opts ...dialer.Option) (_ C.PacketConn, err error) {
+	proxies, chainProxies := r.proxies(metadata, true)
+
+	switch len(proxies) {
+	case 0:
+		return outbound.NewDirect().ListenPacketContext(ctx, metadata, r.Base.DialOptions(opts...)...)
+	case 1:
+		return proxies[0].ListenPacketContext(ctx, metadata, r.Base.DialOptions(opts...)...)
+	}
+
+	first := proxies[0]
+	last := proxies[len(proxies)-1]
+
+	c, err := dialer.DialContext(ctx, "tcp", first.Addr(), r.Base.DialOptions(opts...)...)
+	if err != nil {
+		return nil, fmt.Errorf("%s connect error: %w", first.Addr(), err)
+	}
+	tcpKeepAlive(c)
+
+	var currentMeta *C.Metadata
+	for _, proxy := range proxies[1:] {
+		currentMeta, err = addrToMetadata(proxy.Addr())
+		if err != nil {
+			return nil, err
+		}
+
+		c, err = first.StreamConn(c, currentMeta)
+		if err != nil {
+			return nil, fmt.Errorf("%s connect error: %w", first.Addr(), err)
+		}
+
+		first = proxy
+	}
+
+	c, err = last.StreamConn(c, metadata)
+	if err != nil {
+		return nil, fmt.Errorf("%s connect error: %w", last.Addr(), err)
+	}
+
+	var pc C.PacketConn
+	pc, err = last.ListenPacketOnStreamConn(c, metadata)
+	if err != nil {
+		return nil, fmt.Errorf("%s connect error: %w", first.Addr(), err)
+	}
+
+	for i := len(chainProxies) - 2; i >= 0; i-- {
+		pc.AppendToChains(chainProxies[i])
+	}
+
+	pc.AppendToChains(r)
+
+	return pc, nil
+}
+
+// SupportUDP implements C.ProxyAdapter
+func (r *Relay) SupportUDP() bool {
+	proxies, _ := r.proxies(nil, false)
+	if len(proxies) == 0 { // C.Direct
+		return true
+	}
+	last := proxies[len(proxies)-1]
+	return last.SupportUDP() && last.SupportUOT()
 }
 
 // MarshalJSON implements C.ProxyAdapter
@@ -75,18 +143,31 @@ func (r *Relay) MarshalJSON() ([]byte, error) {
 	})
 }
 
-func (r *Relay) proxies(metadata *C.Metadata, touch bool) []C.Proxy {
-	proxies := r.GetProxies(touch)
+func (r *Relay) proxies(metadata *C.Metadata, touch bool) ([]C.Proxy, []C.Proxy) {
+	rawProxies := r.GetProxies(touch)
 
-	for n, proxy := range proxies {
+	var proxies []C.Proxy
+	var chainProxies []C.Proxy
+	var targetProxies []C.Proxy
+
+	for n, proxy := range rawProxies {
+		proxies = append(proxies, proxy)
+		chainProxies = append(chainProxies, proxy)
 		subproxy := proxy.Unwrap(metadata)
 		for subproxy != nil {
+			chainProxies = append(chainProxies, subproxy)
 			proxies[n] = subproxy
 			subproxy = subproxy.Unwrap(metadata)
 		}
 	}
 
-	return proxies
+	for _, proxy := range proxies {
+		if proxy.Type() != C.Direct && proxy.Type() != C.Compatible {
+			targetProxies = append(targetProxies, proxy)
+		}
+	}
+
+	return targetProxies, chainProxies
 }
 
 func NewRelay(option *GroupCommonOption, providers []provider.ProxyProvider) *Relay {