Fix fallback packet conn

common/bufio/fallback.go

@@ -3,6 +3,7 @@ package bufio
 import (
 	"net"
 
+	"github.com/sagernet/sing/common"
 	"github.com/sagernet/sing/common/buf"
 	M "github.com/sagernet/sing/common/metadata"
 	N "github.com/sagernet/sing/common/network"
@@ -12,13 +13,17 @@ var _ N.NetPacketConn = (*FallbackPacketConn)(nil)
 
 type FallbackPacketConn struct {
 	N.PacketConn
+	writer N.NetPacketWriter
 }
 
 func NewNetPacketConn(conn N.PacketConn) N.NetPacketConn {
 	if packetConn, loaded := conn.(N.NetPacketConn); loaded {
 		return packetConn
 	}
-	return &FallbackPacketConn{PacketConn: conn}
+	return &FallbackPacketConn{
+		PacketConn: conn,
+		writer:     NewNetPacketWriter(conn),
+	}
 }
 
 func (c *FallbackPacketConn) ReadFrom(p []byte) (n int, addr net.Addr, err error) {
@@ -36,11 +41,7 @@ func (c *FallbackPacketConn) ReadFrom(p []byte) (n int, addr net.Addr, err error
 }
 
 func (c *FallbackPacketConn) WriteTo(p []byte, addr net.Addr) (n int, err error) {
-	err = c.WritePacket(buf.As(p), M.SocksaddrFromNet(addr))
-	if err == nil {
-		n = len(p)
-	}
-	return
+	return c.writer.WriteTo(p, addr)
 }
 
 func (c *FallbackPacketConn) ReaderReplaceable() bool {
@@ -54,3 +55,50 @@ func (c *FallbackPacketConn) WriterReplaceable() bool {
 func (c *FallbackPacketConn) Upstream() any {
 	return c.PacketConn
 }
+
+func (c *FallbackPacketConn) UpstreamWriter() any {
+	return c.writer
+}
+
+var _ N.NetPacketWriter = (*FallbackPacketWriter)(nil)
+
+type FallbackPacketWriter struct {
+	N.PacketWriter
+	frontHeadroom int
+	rearHeadroom  int
+}
+
+func NewNetPacketWriter(writer N.PacketWriter) N.NetPacketWriter {
+	if packetWriter, loaded := writer.(N.NetPacketWriter); loaded {
+		return packetWriter
+	}
+	return &FallbackPacketWriter{
+		PacketWriter:  writer,
+		frontHeadroom: N.CalculateFrontHeadroom(writer),
+		rearHeadroom:  N.CalculateRearHeadroom(writer),
+	}
+}
+
+func (c *FallbackPacketWriter) WriteTo(p []byte, addr net.Addr) (n int, err error) {
+	if c.frontHeadroom > 0 || c.rearHeadroom > 0 {
+		buffer := buf.NewSize(len(p) + c.frontHeadroom + c.rearHeadroom)
+		buffer.Resize(c.frontHeadroom, 0)
+		common.Must1(buffer.Write(p))
+		err = c.PacketWriter.WritePacket(buffer, M.SocksaddrFromNet(addr))
+	} else {
+		err = c.PacketWriter.WritePacket(buf.As(p), M.SocksaddrFromNet(addr))
+	}
+	if err != nil {
+		return
+	}
+	n = len(p)
+	return
+}
+
+func (c *FallbackPacketWriter) WriterReplaceable() bool {
+	return true
+}
+
+func (c *FallbackPacketWriter) Upstream() any {
+	return c.PacketWriter
+}

common/bufio/io.go

@@ -37,13 +37,7 @@ func WriteBuffer(writer N.ExtendedWriter, buffer *buf.Buffer) (n int, err error)
 	frontHeadroom := N.CalculateFrontHeadroom(writer)
 	rearHeadroom := N.CalculateRearHeadroom(writer)
 	if frontHeadroom > buffer.Start() || rearHeadroom > buffer.FreeLen() {
-		bufferSize := N.CalculateMTU(nil, writer)
-		if bufferSize > 0 {
-			bufferSize += frontHeadroom + rearHeadroom
-		} else {
-			bufferSize = buf.BufferSize
-		}
-		newBuffer := buf.NewSize(bufferSize)
+		newBuffer := buf.NewSize(buffer.Len() + frontHeadroom + rearHeadroom)
 		newBuffer.Resize(frontHeadroom, 0)
 		common.Must1(newBuffer.Write(buffer.Bytes()))
 		buffer.Release()
@@ -69,13 +63,7 @@ func WritePacketBuffer(writer N.PacketWriter, buffer *buf.Buffer, destination M.
 	frontHeadroom := N.CalculateFrontHeadroom(writer)
 	rearHeadroom := N.CalculateRearHeadroom(writer)
 	if frontHeadroom > buffer.Start() || rearHeadroom > buffer.FreeLen() {
-		bufferSize := N.CalculateMTU(nil, writer)
-		if bufferSize > 0 {
-			bufferSize += frontHeadroom + rearHeadroom
-		} else {
-			bufferSize = buf.BufferSize
-		}
-		newBuffer := buf.NewSize(bufferSize)
+		newBuffer := buf.NewSize(buffer.Len() + frontHeadroom + rearHeadroom)
 		newBuffer.Resize(frontHeadroom, 0)
 		common.Must1(newBuffer.Write(buffer.Bytes()))
 		buffer.Release()