copy header and body of message into the same buffer before sending

That way, we don't send a bunch of tiny packets. The extra copy is nothing
compared to that.

multiplex.go

@@ -55,7 +55,6 @@ type Multiplex struct {
 	wrLock sync.Mutex
 
 	nstreams chan *Stream
-	hdrBuf   []byte
 
 	channels map[streamID]*Stream
 	chLock   sync.Mutex
@@ -70,7 +69,6 @@ func NewMultiplex(con net.Conn, initiator bool) *Multiplex {
 		closed:    make(chan struct{}),
 		shutdown:  make(chan struct{}),
 		nstreams:  make(chan *Stream, 16),
-		hdrBuf:    make([]byte, 20),
 	}
 
 	go mp.handleIncoming()
@@ -137,19 +135,19 @@ func (mp *Multiplex) sendMsg(header uint64, data []byte, dl time.Time) error {
 			return err
 		}
 	}
-	n := binary.PutUvarint(mp.hdrBuf, header)
-	n += binary.PutUvarint(mp.hdrBuf[n:], uint64(len(data)))
-	_, err := mp.con.Write(mp.hdrBuf[:n])
+	buf := mpool.ByteSlicePool.Get(uint32(len(data)) + 20).([]byte)
+	defer mpool.ByteSlicePool.Put(uint32(cap(buf)), buf)
+
+	n := 0
+	n += binary.PutUvarint(buf[n:], header)
+	n += binary.PutUvarint(buf[n:], uint64(len(data)))
+	n += copy(buf[n:], data)
+
+	_, err := mp.con.Write(buf[:n])
 	if err != nil {
 		return err
 	}
 
-	if len(data) != 0 {
-		_, err = mp.con.Write(data)
-		if err != nil {
-			return err
-		}
-	}
 	if !dl.IsZero() {
 		if err := mp.con.SetWriteDeadline(time.Time{}); err != nil {
 			return err