Use flip buffering for connection writing

client.go

@@ -2,6 +2,7 @@ package torrent
 
 import (
 	"bufio"
+	"bytes"
 	"context"
 	"crypto/rand"
 	"errors"
@@ -1233,10 +1234,10 @@ func (cl *Client) banPeerIP(ip net.IP) {
 func (cl *Client) newConnection(nc net.Conn) (c *connection) {
 	c = &connection{
 		conn:            nc,
-
 		Choked:          true,
 		PeerChoked:      true,
 		PeerMaxRequests: 250,
+		writeBuffer:     new(bytes.Buffer),
 	}
 	c.writerCond.L = &cl.mu
 	c.setRW(connStatsReadWriter{nc, &cl.mu, c})

connection.go

@@ -93,7 +93,7 @@ type connection struct {
 	pieceInclination  []int
 	pieceRequestOrder prioritybitmap.PriorityBitmap
 
-	writeBuffer bytes.Buffer
+	writeBuffer *bytes.Buffer
 	uploadTimer *time.Timer
 	writerCond  sync.Cond
 }
@@ -404,10 +404,9 @@ func (cn *connection) fillWriteBuffer(msg func(pp.Message) bool) {
 // connection is writable.
 func (cn *connection) writer(keepAliveTimeout time.Duration) {
 	var (
-		// buf       bytes.Buffer
 		lastWrite      time.Time = time.Now()
+		keepAliveTimer *time.Timer
 	)
-	var keepAliveTimer *time.Timer
 	keepAliveTimer = time.AfterFunc(keepAliveTimeout, func() {
 		cn.mu().Lock()
 		defer cn.mu().Unlock()
@@ -420,6 +419,7 @@ func (cn *connection) writer(keepAliveTimeout time.Duration) {
 	defer cn.mu().Unlock()
 	defer cn.Close()
 	defer keepAliveTimer.Stop()
+	frontBuf := new(bytes.Buffer)
 	for {
 		if cn.closed.IsSet() {
 			return
@@ -440,12 +440,10 @@ func (cn *connection) writer(keepAliveTimeout time.Duration) {
 			cn.writerCond.Wait()
 			continue
 		}
-		var buf bytes.Buffer
-		buf.Write(cn.writeBuffer.Bytes())
-		cn.writeBuffer.Reset()
+		// Flip the buffers.
+		frontBuf, cn.writeBuffer = cn.writeBuffer, frontBuf
 		cn.mu().Unlock()
-		// log.Printf("writing %d bytes", buf.Len())
-		n, err := cn.w.Write(buf.Bytes())
+		n, err := cn.w.Write(frontBuf.Bytes())
 		cn.mu().Lock()
 		if n != 0 {
 			lastWrite = time.Now()
@@ -454,10 +452,10 @@ func (cn *connection) writer(keepAliveTimeout time.Duration) {
 		if err != nil {
 			return
 		}
-		if n != buf.Len() {
+		if n != frontBuf.Len() {
 			panic("short write")
 		}
-		buf.Reset()
+		frontBuf.Reset()
 	}
 }
 

connection_test.go

@@ -20,14 +20,12 @@ import (
 // Have that would potentially alter it.
 func TestSendBitfieldThenHave(t *testing.T) {
 	r, w := io.Pipe()
-	c := &connection{
-		t: &Torrent{
-			cl: &Client{},
-		},
-		r: r,
-		w: w,
-	}
-	c.writerCond.L = &c.t.cl.mu
+	var cl Client
+	cl.initLogger()
+	c := cl.newConnection(nil)
+	c.setTorrent(cl.newTorrent(metainfo.Hash{}, nil))
+	c.r = r
+	c.w = w
 	go c.writer(time.Minute)
 	c.mu().Lock()
 	c.Bitfield([]bool{false, true, false})