receive: implement flush semantics

device/receive.go

@@ -482,6 +482,33 @@ func (device *Device) RoutineHandshake() {
 	}
 }
 
+func (peer *Peer) elementStopOrFlush(shouldFlush *bool) (stop bool, elemOk bool, elem *QueueInboundElement) {
+	if !*shouldFlush {
+		select {
+		case <-peer.routines.stop:
+			stop = true
+			return
+		case elem, elemOk = <-peer.queue.inbound:
+			return
+		}
+	} else {
+		select {
+		case <-peer.routines.stop:
+			stop = true
+			return
+		case elem, elemOk = <-peer.queue.inbound:
+			return
+		default:
+			*shouldFlush = false
+			err := peer.device.tun.device.Flush()
+			if err != nil {
+				peer.device.log.Error.Printf("Unable to flush packets: %v", err)
+			}
+			return peer.elementStopOrFlush(shouldFlush)
+		}
+	}
+}
+
 func (peer *Peer) RoutineSequentialReceiver() {
 
 	device := peer.device
@@ -491,6 +518,9 @@ func (peer *Peer) RoutineSequentialReceiver() {
 
 	var elem *QueueInboundElement
 	var ok bool
+	var stop bool
+
+	shouldFlush := false
 
 	defer func() {
 		logDebug.Println(peer, "- Routine: sequential receiver - stopped")
@@ -516,14 +546,8 @@ func (peer *Peer) RoutineSequentialReceiver() {
 			elem = nil
 		}
 
-		select {
-
-		case <-peer.routines.stop:
-			return
-
-		case elem, ok = <-peer.queue.inbound:
-
-			if !ok {
+		stop, ok, elem = peer.elementStopOrFlush(&shouldFlush)
+		if stop || !ok {
 			return
 		}
 
@@ -633,9 +657,11 @@ func (peer *Peer) RoutineSequentialReceiver() {
 		offset := MessageTransportOffsetContent
 		atomic.AddUint64(&peer.stats.rxBytes, uint64(len(elem.packet)))
 		_, err := device.tun.device.Write(elem.buffer[:offset+len(elem.packet)], offset)
+		if err == nil {
+			shouldFlush = true
+		}
 		if err != nil && !device.isClosed.Get() {
 			logError.Println("Failed to write packet to TUN device:", err)
 		}
 	}
-	}
 }

tun/tun.go

@@ -21,6 +21,7 @@ type TUNDevice interface {
 	File() *os.File                 // returns the file descriptor of the device
 	Read([]byte, int) (int, error)  // read a packet from the device (without any additional headers)
 	Write([]byte, int) (int, error) // writes a packet to the device (without any additional headers)
+	Flush() error                   // flush all previous writes to the device
 	MTU() (int, error)              // returns the MTU of the device
 	Name() (string, error)          // fetches and returns the current name
 	Events() chan TUNEvent          // returns a constant channel of events related to the device

tun/tun_darwin.go

@@ -281,6 +281,11 @@ func (tun *NativeTun) Write(buff []byte, offset int) (int, error) {
 	return tun.tunFile.Write(buff)
 }
 
+func (tun *NativeTun) Flush() error {
+	//TODO: can flushing be implemented by buffering and using sendmmsg?
+	return nil
+}
+
 func (tun *NativeTun) Close() error {
 	var err2 error
 	err1 := tun.tunFile.Close()

tun/tun_freebsd.go

@@ -406,6 +406,11 @@ func (tun *NativeTun) Write(buff []byte, offset int) (int, error) {
 	return tun.tunFile.Write(buff)
 }
 
+func (tun *NativeTun) Flush() error {
+	//TODO: can flushing be implemented by buffering and using sendmmsg?
+	return nil
+}
+
 func (tun *NativeTun) Close() error {
 	var err3 error
 	err1 := tun.tunFile.Close()

tun/tun_linux.go

@@ -318,6 +318,11 @@ func (tun *NativeTun) Write(buff []byte, offset int) (int, error) {
 	return tun.tunFile.Write(buff)
 }
 
+func (tun *NativeTun) Flush() error {
+	//TODO: can flushing be implemented by buffering and using sendmmsg?
+	return nil
+}
+
 func (tun *NativeTun) Read(buff []byte, offset int) (int, error) {
 	select {
 	case err := <-tun.errors:

tun/tun_openbsd.go

@@ -237,6 +237,11 @@ func (tun *NativeTun) Write(buff []byte, offset int) (int, error) {
 	return tun.tunFile.Write(buff)
 }
 
+func (tun *NativeTun) Flush() error {
+	//TODO: can flushing be implemented by buffering and using sendmmsg?
+	return nil
+}
+
 func (tun *NativeTun) Close() error {
 	var err2 error
 	err1 := tun.tunFile.Close()

tun/tun_windows.go

@@ -281,7 +281,11 @@ func (tun *NativeTun) Read(buff []byte, offset int) (int, error) {
 
 // Note: flush() and putTunPacket() assume the caller comes only from a single thread; there's no locking.
 
-func (tun *NativeTun) flush() error {
+func (tun *NativeTun) Flush() error {
+	if tun.wrBuff.offset == 0 {
+		return nil
+	}
+
 	// Get TUN data pipe.
 	file, err := tun.getTUN()
 	if err != nil {
@@ -322,7 +326,7 @@ func (tun *NativeTun) putTunPacket(buff []byte) error {
 
 	if tun.wrBuff.packetNum >= packetExchangeMax || tun.wrBuff.offset+pSize >= packetExchangeSize {
 		// Exchange buffer is full -> flush first.
-		err := tun.flush()
+		err := tun.Flush()
 		if err != nil {
 			return err
 		}
@@ -345,9 +349,7 @@ func (tun *NativeTun) Write(buff []byte, offset int) (int, error) {
 	if err != nil {
 		return 0, err
 	}
-
-	// Flush write buffer.
-	return len(buff) - offset, tun.flush()
+	return len(buff) - offset, nil
 }
 
 //