slightly better way for the tcp sender goroutine(s) to block waiting for work

src/yggdrasil/tcp.go

@@ -242,7 +242,7 @@ func (iface *tcpInterface) handler(sock net.Conn, incoming bool) {
 	in := func(bs []byte) {
 		p.handlePacket(bs)
 	}
-	out := make(chan []byte, 32) // TODO? what size makes sense
+	out := make(chan []byte, 1)
 	defer close(out)
 	go func() {
 		var shadow int64
@@ -255,13 +255,17 @@ func (iface *tcpInterface) handler(sock net.Conn, incoming bool) {
 				shadow++
 			}
 		}
-		send := func(msg []byte) {
+		send := make(chan []byte)
+		defer close(send)
+		go func() {
+			for msg := range send {
 				msgLen := wire_encode_uint64(uint64(len(msg)))
 				buf := net.Buffers{tcp_msg[:], msgLen, msg}
 				buf.WriteTo(sock)
 				atomic.AddUint64(&p.bytesSent, uint64(len(tcp_msg)+len(msgLen)+len(msg)))
 				util_putBytes(msg)
 			}
+		}()
 		timerInterval := tcp_timeout * 2 / 3
 		timer := time.NewTimer(timerInterval)
 		defer timer.Stop()
@@ -278,9 +282,9 @@ func (iface *tcpInterface) handler(sock net.Conn, incoming bool) {
 			timer.Reset(timerInterval)
 			select {
 			case _ = <-timer.C:
-				send(nil) // TCP keep-alive traffic
+				send <- nil // TCP keep-alive traffic
 			case msg := <-p.linkOut:
-				send(msg)
+				send <- msg
 			case msg, ok := <-out:
 				if !ok {
 					return
@@ -288,31 +292,32 @@ func (iface *tcpInterface) handler(sock net.Conn, incoming bool) {
 				put(msg)
 			}
 			for len(stack) > 0 {
+				// First make sure linkOut gets sent first, if it's non-empty
 				select {
 				case msg := <-p.linkOut:
-					send(msg)
+					send <- msg
+				default:
+				}
+				// Then block until we send or receive something
+				select {
+				case msg := <-p.linkOut:
+					send <- msg
 				case msg, ok := <-out:
 					if !ok {
 						return
 					}
 					put(msg)
-				default:
-					msg := stack[len(stack)-1]
+				case send <- stack[len(stack)-1]:
 					stack = stack[:len(stack)-1]
-					send(msg)
 					p.updateQueueSize(-1)
 				}
 			}
 		}
 	}()
 	p.out = func(msg []byte) {
-		defer func() { recover() }()
-		select {
-		case out <- msg:
 		p.updateQueueSize(1)
-		default:
-			util_putBytes(msg)
-		}
+		defer func() { recover() }()
+		out <- msg
 	}
 	p.close = func() { sock.Close() }
 	setNoDelay(sock, true)