Add write buffer pooling

Add WriteBufferPool to Dialer and Upgrader. This field specifies a pool
to use for write operations on a connection.  Use of the pool can reduce
memory use when there is a modest write volume over a large number of
connections.

Use larger of hijacked buffer and buffer allocated for connection (if
any) as buffer for building handshake response. This decreases possible
allocations when building the handshake response.

Modify bufio reuse test to call Upgrade instead of the internal
newConnBRW. Move the test from conn_test.go to server_test.go because
it's a serer test.

Update newConn and newConnBRW:

- Move the bufio "hacks" from newConnBRW to separate functions and call
these functions directly from Upgrade.
- Rename newConn to newTestConn and move to conn_test.go. Shorten
argument list to common use case.
- Rename newConnBRW to newConn.
- Add pool code to newConn.

client.go

@@ -69,6 +69,17 @@ type Dialer struct {
 	// do not limit the size of the messages that can be sent or received.
 	ReadBufferSize, WriteBufferSize int
 
+	// WriteBufferPool is a pool of buffers for write operations. If the value
+	// is not set, then write buffers are allocated to the connection for the
+	// lifetime of the connection.
+	//
+	// A pool is most useful when the application has a modest volume of writes
+	// across a large number of connections.
+	//
+	// Applications should use a single pool for each unique value of
+	// WriteBufferSize.
+	WriteBufferPool BufferPool
+
 	// Subprotocols specifies the client's requested subprotocols.
 	Subprotocols []string
 
@@ -277,7 +288,7 @@ func (d *Dialer) Dial(urlStr string, requestHeader http.Header) (*Conn, *http.Re
 		}
 	}
 
-	conn := newConn(netConn, false, d.ReadBufferSize, d.WriteBufferSize)
+	conn := newConn(netConn, false, d.ReadBufferSize, d.WriteBufferSize, d.WriteBufferPool, nil, nil)
 
 	if err := req.Write(netConn); err != nil {
 		return nil, nil, err

compression_test.go

@@ -43,7 +43,7 @@ func textMessages(num int) [][]byte {
 
 func BenchmarkWriteNoCompression(b *testing.B) {
 	w := ioutil.Discard
-	c := newConn(fakeNetConn{Reader: nil, Writer: w}, false, 1024, 1024)
+	c := newTestConn(nil, w, false)
 	messages := textMessages(100)
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
@@ -54,7 +54,7 @@ func BenchmarkWriteNoCompression(b *testing.B) {
 
 func BenchmarkWriteWithCompression(b *testing.B) {
 	w := ioutil.Discard
-	c := newConn(fakeNetConn{Reader: nil, Writer: w}, false, 1024, 1024)
+	c := newTestConn(nil, w, false)
 	messages := textMessages(100)
 	c.enableWriteCompression = true
 	c.newCompressionWriter = compressNoContextTakeover
@@ -66,7 +66,7 @@ func BenchmarkWriteWithCompression(b *testing.B) {
 }
 
 func TestValidCompressionLevel(t *testing.T) {
-	c := newConn(fakeNetConn{}, false, 1024, 1024)
+	c := newTestConn(nil, nil, false)
 	for _, level := range []int{minCompressionLevel - 1, maxCompressionLevel + 1} {
 		if err := c.SetCompressionLevel(level); err == nil {
 			t.Errorf("no error for level %d", level)

conn.go

@@ -223,6 +223,20 @@ func isValidReceivedCloseCode(code int) bool {
 	return validReceivedCloseCodes[code] || (code >= 3000 && code <= 4999)
 }
 
+// BufferPool represents a pool of buffers. The *sync.Pool type satisfies this
+// interface.  The type of the value stored in a pool is not specified.
+type BufferPool interface {
+	// Get gets a value from the pool or returns nil if the pool is empty.
+	Get() interface{}
+	// Put adds a value to the pool.
+	Put(interface{})
+}
+
+// writePoolData is the type added to the write buffer pool. This wrapper is
+// used to prevent applications from peeking at and depending on the values
+// added to the pool.
+type writePoolData struct{ buf []byte }
+
 // The Conn type represents a WebSocket connection.
 type Conn struct {
 	conn        net.Conn
@@ -232,6 +246,8 @@ type Conn struct {
 	// Write fields
 	mu            chan bool // used as mutex to protect write to conn
 	writeBuf      []byte    // frame is constructed in this buffer.
+	writePool     BufferPool
+	writeBufSize  int
 	writeDeadline time.Time
 	writer        io.WriteCloser // the current writer returned to the application
 	isWriting     bool           // for best-effort concurrent write detection
@@ -263,71 +279,38 @@ type Conn struct {
 	newDecompressionReader func(io.Reader) io.ReadCloser
 }
 
-func newConn(conn net.Conn, isServer bool, readBufferSize, writeBufferSize int) *Conn {
-	return newConnBRW(conn, isServer, readBufferSize, writeBufferSize, nil)
-}
-
-type writeHook struct {
-	p []byte
-}
-
-func (wh *writeHook) Write(p []byte) (int, error) {
-	wh.p = p
-	return len(p), nil
-}
-
-func newConnBRW(conn net.Conn, isServer bool, readBufferSize, writeBufferSize int, brw *bufio.ReadWriter) *Conn {
-	mu := make(chan bool, 1)
-	mu <- true
+func newConn(conn net.Conn, isServer bool, readBufferSize, writeBufferSize int, writeBufferPool BufferPool, br *bufio.Reader, writeBuf []byte) *Conn {
 
-	var br *bufio.Reader
-	if readBufferSize == 0 && brw != nil && brw.Reader != nil {
-		// Reuse the supplied bufio.Reader if the buffer has a useful size.
-		// This code assumes that peek on a reader returns
-		// bufio.Reader.buf[:0].
-		brw.Reader.Reset(conn)
-		if p, err := brw.Reader.Peek(0); err == nil && cap(p) >= 256 {
-			br = brw.Reader
-		}
-	}
 	if br == nil {
 		if readBufferSize == 0 {
 			readBufferSize = defaultReadBufferSize
-		}
-		if readBufferSize < maxControlFramePayloadSize {
+		} else if readBufferSize < maxControlFramePayloadSize {
+			// must be large enough for control frame
 			readBufferSize = maxControlFramePayloadSize
 		}
 		br = bufio.NewReaderSize(conn, readBufferSize)
 	}
 
-	var writeBuf []byte
-	if writeBufferSize == 0 && brw != nil && brw.Writer != nil {
-		// Use the bufio.Writer's buffer if the buffer has a useful size. This
-		// code assumes that bufio.Writer.buf[:1] is passed to the
-		// bufio.Writer's underlying writer.
-		var wh writeHook
-		brw.Writer.Reset(&wh)
-		brw.Writer.WriteByte(0)
-		brw.Flush()
-		if cap(wh.p) >= maxFrameHeaderSize+256 {
-			writeBuf = wh.p[:cap(wh.p)]
-		}
+	if writeBufferSize <= 0 {
+		writeBufferSize = defaultWriteBufferSize
 	}
+	writeBufferSize += maxFrameHeaderSize
 
-	if writeBuf == nil {
-		if writeBufferSize == 0 {
-			writeBufferSize = defaultWriteBufferSize
-		}
-		writeBuf = make([]byte, writeBufferSize+maxFrameHeaderSize)
+	if writeBuf == nil && writeBufferPool == nil {
+		writeBuf = make([]byte, writeBufferSize)
 	}
 
+	mu := make(chan bool, 1)
+	mu <- true
 	c := &Conn{
 		isServer:               isServer,
 		br:                     br,
 		conn:                   conn,
 		mu:                     mu,
 		readFinal:              true,
 		writeBuf:               writeBuf,
+		writePool:              writeBufferPool,
+		writeBufSize:           writeBufferSize,
 		enableWriteCompression: true,
 		compressionLevel:       defaultCompressionLevel,
 	}
@@ -484,7 +467,19 @@ func (c *Conn) prepWrite(messageType int) error {
 	c.writeErrMu.Lock()
 	err := c.writeErr
 	c.writeErrMu.Unlock()
-	return err
+	if err != nil {
+		return err
+	}
+
+	if c.writeBuf == nil {
+		wpd, ok := c.writePool.Get().(writePoolData)
+		if ok {
+			c.writeBuf = wpd.buf
+		} else {
+			c.writeBuf = make([]byte, c.writeBufSize)
+		}
+	}
+	return nil
 }
 
 // NextWriter returns a writer for the next message to send. The writer's Close
@@ -610,6 +605,10 @@ func (w *messageWriter) flushFrame(final bool, extra []byte) error {
 
 	if final {
 		c.writer = nil
+		if c.writePool != nil {
+			c.writePool.Put(writePoolData{buf: c.writeBuf})
+			c.writeBuf = nil
+		}
 		return nil
 	}
 

conn_broadcast_test.go

@@ -70,7 +70,7 @@ func (b *broadcastBench) makeConns(numConns int) {
 	conns := make([]*broadcastConn, numConns)
 
 	for i := 0; i < numConns; i++ {
-		c := newConn(fakeNetConn{Reader: nil, Writer: b.w}, true, 1024, 1024)
+		c := newTestConn(nil, b.w, true)
 		if b.compression {
 			c.enableWriteCompression = true
 			c.newCompressionWriter = compressNoContextTakeover