Remove common wsutil.Reader allocations (#189)

![image](https://github.com/gobwas/ws/assets/5663952/b7f5a544-f6f0-44cf-b38c-e89fd3de0bd4)

* `ws.ReadHeader` allocates a 12 byte temporary buffer to read the header into. Since `io.ReadFull` is used, it escapes to the heap.

* `wsutil.NewCipherReader` is used for many calls.

Since the `NextFrame` shouldn't have concurrent calls, both of these should be possible to have internally.

I am forced to copy "readHeader" from `ws`, since there is not way to eliminate the alloc otherwise as far as I can tell.

wsutil/reader.go

@@ -1,6 +1,7 @@
 package wsutil
 
 import (
+	"encoding/binary"
 	"errors"
 	"io"
 	"io/ioutil"
@@ -56,10 +57,12 @@ type Reader struct {
 	OnContinuation FrameHandlerFunc
 	OnIntermediate FrameHandlerFunc
 
-	opCode ws.OpCode        // Used to store message op code on fragmentation.
-	frame  io.Reader        // Used to as frame reader.
-	raw    io.LimitedReader // Used to discard frames without cipher.
-	utf8   UTF8Reader       // Used to check UTF8 sequences if CheckUTF8 is true.
+	opCode ws.OpCode                  // Used to store message op code on fragmentation.
+	frame  io.Reader                  // Used to as frame reader.
+	raw    io.LimitedReader           // Used to discard frames without cipher.
+	utf8   UTF8Reader                 // Used to check UTF8 sequences if CheckUTF8 is true.
+	tmp    [ws.MaxHeaderSize - 2]byte // Used for reading headers.
+	cr     *CipherReader              // Used by NextFrame() to unmask frame payload.
 }
 
 // NewReader creates new frame reader that reads from r keeping given state to
@@ -165,7 +168,7 @@ func (r *Reader) Discard() (err error) {
 // Note that next NextFrame() call must be done after receiving or discarding
 // all current message bytes.
 func (r *Reader) NextFrame() (hdr ws.Header, err error) {
-	hdr, err = ws.ReadHeader(r.Source)
+	hdr, err = r.readHeader(r.Source)
 	if err == io.EOF && r.fragmented() {
 		// If we are in fragmented state EOF means that is was totally
 		// unexpected.
@@ -196,7 +199,12 @@ func (r *Reader) NextFrame() (hdr ws.Header, err error) {
 
 	frame := io.Reader(&r.raw)
 	if hdr.Masked {
-		frame = NewCipherReader(frame, hdr.Mask)
+		if r.cr == nil {
+			r.cr = NewCipherReader(frame, hdr.Mask)
+		} else {
+			r.cr.Reset(frame, hdr.Mask)
+		}
+		frame = r.cr
 	}
 
 	for _, x := range r.Extensions {
@@ -261,6 +269,82 @@ func (r *Reader) reset() {
 	r.opCode = 0
 }
 
+// readHeader reads a frame header from in.
+func (r *Reader) readHeader(in io.Reader) (h ws.Header, err error) {
+	// Make slice of bytes with capacity 12 that could hold any header.
+	//
+	// The maximum header size is 14, but due to the 2 hop reads,
+	// after first hop that reads first 2 constant bytes, we could reuse 2 bytes.
+	// So 14 - 2 = 12.
+	bts := r.tmp[:2]
+
+	// Prepare to hold first 2 bytes to choose size of next read.
+	_, err = io.ReadFull(in, bts)
+	if err != nil {
+		return h, err
+	}
+	const bit0 = 0x80
+
+	h.Fin = bts[0]&bit0 != 0
+	h.Rsv = (bts[0] & 0x70) >> 4
+	h.OpCode = ws.OpCode(bts[0] & 0x0f)
+
+	var extra int
+
+	if bts[1]&bit0 != 0 {
+		h.Masked = true
+		extra += 4
+	}
+
+	length := bts[1] & 0x7f
+	switch {
+	case length < 126:
+		h.Length = int64(length)
+
+	case length == 126:
+		extra += 2
+
+	case length == 127:
+		extra += 8
+
+	default:
+		err = ws.ErrHeaderLengthUnexpected
+		return h, err
+	}
+
+	if extra == 0 {
+		return h, err
+	}
+
+	// Increase len of bts to extra bytes need to read.
+	// Overwrite first 2 bytes that was read before.
+	bts = bts[:extra]
+	_, err = io.ReadFull(in, bts)
+	if err != nil {
+		return h, err
+	}
+
+	switch {
+	case length == 126:
+		h.Length = int64(binary.BigEndian.Uint16(bts[:2]))
+		bts = bts[2:]
+
+	case length == 127:
+		if bts[0]&0x80 != 0 {
+			err = ws.ErrHeaderLengthMSB
+			return h, err
+		}
+		h.Length = int64(binary.BigEndian.Uint64(bts[:8]))
+		bts = bts[8:]
+	}
+
+	if h.Masked {
+		copy(h.Mask[:], bts)
+	}
+
+	return h, nil
+}
+
 // NextReader prepares next message read from r. It returns header that
 // describes the message and io.Reader to read message's payload. It returns
 // non-nil error when it is not possible to read message's initial frame.