TUN-5659: Proxy UDP with zero-byte payload

datagramsession/session.go

@@ -106,6 +106,7 @@ func (s *Session) waitForCloseCondition(ctx context.Context, closeAfterIdle time
 func (s *Session) dstToTransport(buffer []byte) error {
 	n, err := s.dstConn.Read(buffer)
 	s.markActive()
+	// https://pkg.go.dev/io#Reader suggests caller should always process n > 0 bytes
 	if n > 0 {
 		if n <= int(s.transport.MTU()) {
 			err = s.transport.SendTo(s.ID, buffer[:n])
@@ -118,6 +119,10 @@ func (s *Session) dstToTransport(buffer []byte) error {
 				Msg("dropped packet exceeding MTU")
 		}
 	}
+	// Some UDP application might send 0-size payload.
+	if err == nil && n == 0 {
+		err = s.transport.SendTo(s.ID, []byte{})
+	}
 	return err
 }
 

datagramsession/session_test.go

@@ -195,3 +195,40 @@ func TestMarkActiveNotBlocking(t *testing.T) {
 	}
 	wg.Wait()
 }
+
+func TestZeroBytePayload(t *testing.T) {
+	sessionID := uuid.New()
+	cfdConn, originConn := net.Pipe()
+	transport := &mockQUICTransport{
+		reqChan:  newDatagramChannel(1),
+		respChan: newDatagramChannel(1),
+	}
+	log := zerolog.Nop()
+	session := newSession(sessionID, transport, cfdConn, &log)
+
+	ctx, cancel := context.WithCancel(context.Background())
+	errGroup, ctx := errgroup.WithContext(ctx)
+	errGroup.Go(func() error {
+		// Read from underlying conn and send to transport
+		closedByRemote, err := session.Serve(ctx, time.Minute*2)
+		require.Equal(t, context.Canceled, err)
+		require.False(t, closedByRemote)
+		return nil
+	})
+
+	errGroup.Go(func() error {
+		// Write to underlying connection
+		n, err := originConn.Write([]byte{})
+		require.NoError(t, err)
+		require.Equal(t, 0, n)
+		return nil
+	})
+
+	receivedSessionID, payload, err := transport.respChan.Receive(ctx)
+	require.NoError(t, err)
+	require.Len(t, payload, 0)
+	require.Equal(t, sessionID, receivedSessionID)
+
+	cancel()
+	require.NoError(t, errGroup.Wait())
+}