XUDP protocol: Add Global ID & UoT Migration

The first UoT protocol that supports UoT Migration
Thank @yuhan6665 for testing

common/mux/server.go

@@ -2,6 +2,7 @@ package mux
 
 import (
 	"context"
+	"fmt"
 	"io"
 
 	"github.com/xtls/xray-core/common"
@@ -11,6 +12,7 @@ import (
 	"github.com/xtls/xray-core/common/net"
 	"github.com/xtls/xray-core/common/protocol"
 	"github.com/xtls/xray-core/common/session"
+	"github.com/xtls/xray-core/common/xudp"
 	"github.com/xtls/xray-core/core"
 	"github.com/xtls/xray-core/features/routing"
 	"github.com/xtls/xray-core/transport"
@@ -99,7 +101,7 @@ func handle(ctx context.Context, s *Session, output buf.Writer) {
 	}
 
 	writer.Close()
-	s.Close()
+	s.Close(false)
 }
 
 func (w *ServerWorker) ActiveConnections() uint32 {
@@ -131,6 +133,81 @@ func (w *ServerWorker) handleStatusNew(ctx context.Context, meta *FrameMetadata,
 		}
 		ctx = log.ContextWithAccessMessage(ctx, msg)
 	}
+
+	if meta.GlobalID != [8]byte{} {
+		mb, err := NewPacketReader(reader, &meta.Target).ReadMultiBuffer()
+		if err != nil {
+			return err
+		}
+		XUDPManager.Lock()
+		x := XUDPManager.Map[meta.GlobalID]
+		if x == nil {
+			x = &XUDP{GlobalID: meta.GlobalID}
+			XUDPManager.Map[meta.GlobalID] = x
+			XUDPManager.Unlock()
+		} else {
+			if x.Status == Initializing { // nearly impossible
+				XUDPManager.Unlock()
+				if xudp.Show {
+					fmt.Printf("XUDP hit: %v err: conflict\n", meta.GlobalID)
+				}
+				// It's not a good idea to return an err here, so just let client wait.
+				// Client will receive an End frame after sending a Keep frame.
+				return nil
+			}
+			x.Status = Initializing
+			XUDPManager.Unlock()
+			x.Mux.Close(false) // detach from previous Mux
+			b := buf.New()
+			b.Write(mb[0].Bytes())
+			b.UDP = mb[0].UDP
+			if err = x.Mux.output.WriteMultiBuffer(mb); err != nil {
+				x.Interrupt()
+				mb = buf.MultiBuffer{b}
+			} else {
+				b.Release()
+				mb = nil
+			}
+			if xudp.Show {
+				fmt.Printf("XUDP hit: %v err: %v\n", meta.GlobalID, err)
+			}
+		}
+		if mb != nil {
+			ctx = session.ContextWithTimeoutOnly(ctx, true)
+			// Actually, it won't return an error in Xray-core's implementations.
+			link, err := w.dispatcher.Dispatch(ctx, meta.Target)
+			if err != nil {
+				err = newError("failed to dispatch request to ", meta.Target).Base(err)
+				if xudp.Show {
+					fmt.Printf("XUDP new: %v err: %v\n", meta.GlobalID, err)
+				}
+				return err // it will break the whole Mux connection
+			}
+			link.Writer.WriteMultiBuffer(mb) // it's meaningless to test a new pipe
+			x.Mux = &Session{
+				input:  link.Reader,
+				output: link.Writer,
+			}
+			if xudp.Show {
+				fmt.Printf("XUDP new: %v err: %v\n", meta.GlobalID, err)
+			}
+		}
+		x.Mux = &Session{
+			input:        x.Mux.input,
+			output:       x.Mux.output,
+			parent:       w.sessionManager,
+			ID:           meta.SessionID,
+			transferType: protocol.TransferTypePacket,
+			XUDP:         x,
+		}
+		go handle(ctx, x.Mux, w.link.Writer)
+		x.Status = Active
+		if !w.sessionManager.Add(x.Mux) {
+			x.Mux.Close(false)
+		}
+		return nil
+	}
+
 	link, err := w.dispatcher.Dispatch(ctx, meta.Target)
 	if err != nil {
 		if meta.Option.Has(OptionData) {
@@ -157,8 +234,7 @@ func (w *ServerWorker) handleStatusNew(ctx context.Context, meta *FrameMetadata,
 	rr := s.NewReader(reader, &meta.Target)
 	if err := buf.Copy(rr, s.output); err != nil {
 		buf.Copy(rr, buf.Discard)
-		common.Interrupt(s.input)
-		return s.Close()
+		return s.Close(false)
 	}
 	return nil
 }
@@ -182,15 +258,8 @@ func (w *ServerWorker) handleStatusKeep(meta *FrameMetadata, reader *buf.Buffere
 
 	if err != nil && buf.IsWriteError(err) {
 		newError("failed to write to downstream writer. closing session ", s.ID).Base(err).WriteToLog()
-
-		// Notify remote peer to close this session.
-		closingWriter := NewResponseWriter(meta.SessionID, w.link.Writer, protocol.TransferTypeStream)
-		closingWriter.Close()
-
-		drainErr := buf.Copy(rr, buf.Discard)
-		common.Interrupt(s.input)
-		s.Close()
-		return drainErr
+		s.Close(false)
+		return buf.Copy(rr, buf.Discard)
 	}
 
 	return err
@@ -198,12 +267,7 @@ func (w *ServerWorker) handleStatusKeep(meta *FrameMetadata, reader *buf.Buffere
 
 func (w *ServerWorker) handleStatusEnd(meta *FrameMetadata, reader *buf.BufferedReader) error {
 	if s, found := w.sessionManager.Get(meta.SessionID); found {
-		if meta.Option.Has(OptionError) {
-			common.Interrupt(s.input)
-			common.Interrupt(s.output)
-		}
-		common.Interrupt(s.input)
-		s.Close()
+		s.Close(false)
 	}
 	if meta.Option.Has(OptionData) {
 		return buf.Copy(NewStreamReader(reader), buf.Discard)