Reduce Reader allocation by 8 bytes

Both Reader and Writer now will only ever allocate 16 bytes
for their entire usage :)

This is the minimum possible while still preventing misuse
of a EOFed Reader or a closed Writer.

Author: nhooyr

websocket.go

@@ -36,9 +36,6 @@ type Conn struct {
 	closer   io.Closer
 	client   bool
 
-	// read limit for a message in bytes.
-	msgReadLimit int64
-
 	closeOnce sync.Once
 	closeErr  error
 	closed    chan struct{}
@@ -50,10 +47,13 @@ type Conn struct {
 	writeFrameLock chan struct{}
 	writeHeaderBuf []byte
 	writeHeader    *header
+	// read limit for a message in bytes.
+	msgReadLimit int64
 
 	// messageWriter state.
 	writeMsgOpcode opcode
 	writeMsgCtx    context.Context
+	readMsgLeft    int64
 
 	// Used to ensure the previous reader is read till EOF before allowing
 	// a new one.
@@ -371,20 +371,19 @@ func (c *Conn) reader(ctx context.Context) (MessageType, io.Reader, error) {
 	c.readMsgHeader = h
 	c.readFrameEOF = false
 	c.readMaskPos = 0
+	c.readMsgLeft = c.msgReadLimit
 
 	r := &messageReader{
-		c:    c,
-		left: c.msgReadLimit,
+		c: c,
 	}
 	c.previousReader = r
 	return MessageType(h.opcode), r, nil
 }
 
 // messageReader enables reading a data frame from the WebSocket connection.
 type messageReader struct {
-	c    *Conn
-	left int64
-	eof  bool
+	c   *Conn
+	eof bool
 }
 
 // Read reads as many bytes as possible into p.
@@ -406,14 +405,14 @@ func (r *messageReader) read(p []byte) (int, error) {
 		return 0, xerrors.Errorf("cannot use EOFed reader")
 	}
 
-	if r.left <= 0 {
+	if r.c.readMsgLeft <= 0 {
 		err := xerrors.Errorf("read limited at %v bytes", r.c.msgReadLimit)
 		r.c.Close(StatusMessageTooBig, err.Error())
 		return 0, err
 	}
 
-	if int64(len(p)) > r.left {
-		p = p[:r.left]
+	if int64(len(p)) > r.c.readMsgLeft {
+		p = p[:r.c.readMsgLeft]
 	}
 
 	if r.c.readFrameEOF {
@@ -441,7 +440,7 @@ func (r *messageReader) read(p []byte) (int, error) {
 	n, err := r.c.readFramePayload(r.c.readMsgCtx, p)
 
 	h.payloadLength -= int64(n)
-	r.left -= int64(n)
+	r.c.readMsgLeft -= int64(n)
 	if h.masked {
 		r.c.readMaskPos = fastXOR(h.maskKey, r.c.readMaskPos, p)
 	}