peer_protocol.Decoder.Decode: Avoid allocating another intermediate reader

On my system, BenchmarkConnectionMainReadLoop goes from 596 to 1311 MB/s.

peer_protocol/protocol.go

@@ -128,6 +128,20 @@ type Decoder struct {
 	MaxLength Integer // TODO: Should this include the length header or not?
 }
 
+func readByte(r io.Reader) (b byte, err error) {
+	var arr [1]byte
+	n, err := r.Read(arr[:])
+	b = arr[0]
+	if n == 1 {
+		err = nil
+		return
+	}
+	if err == nil {
+		panic(err)
+	}
+	return
+}
+
 // io.EOF is returned if the source terminates cleanly on a message boundary.
 func (d *Decoder) Decode(msg *Message) (err error) {
 	var length Integer
@@ -146,29 +160,18 @@ func (d *Decoder) Decode(msg *Message) (err error) {
 		return
 	}
 	msg.Keepalive = false
-	b := make([]byte, length)
+	r := &io.LimitedReader{d.R, int64(length)}
-	_, err = io.ReadFull(d.R, b)
-	if err != nil {
-		if err == io.EOF {
-			err = io.ErrUnexpectedEOF
-		}
-		if err != io.ErrUnexpectedEOF {
-			err = fmt.Errorf("error reading message: %s", err)
-		}
-		return
-	}
-	r := bytes.NewReader(b)
 	// Check that all of r was utilized.
 	defer func() {
 		if err != nil {
 			return
 		}
-		if r.Len() != 0 {
+		if r.N != 0 {
-			err = fmt.Errorf("%d bytes unused in message type %d", r.Len(), msg.Type)
+			err = fmt.Errorf("%d bytes unused in message type %d", r.N, msg.Type)
 		}
 	}()
 	msg.Keepalive = false
-	c, err := r.ReadByte()
+	c, err := readByte(r)
 	if err != nil {
 		return
 	}
@@ -210,7 +213,7 @@ func (d *Decoder) Decode(msg *Message) (err error) {
 		}
 		msg.Piece = b
 	case Extended:
-		msg.ExtendedID, err = r.ReadByte()
+		msg.ExtendedID, err = readByte(r)
 		if err != nil {
 			break
 		}