Do chunk pooling at Torrent instead of connection level

client.go

@@ -1169,10 +1169,9 @@ func (cl *Client) badPeerIPPort(ip net.IP, port int) bool {
 // Return a Torrent ready for insertion into a Client.
 func (cl *Client) newTorrent(ih metainfo.Hash) (t *Torrent) {
 	t = &Torrent{
-		cl:        cl,
-		infoHash:  ih,
-		chunkSize: defaultChunkSize,
-		peers:     make(map[peersKey]Peer),
+		cl:       cl,
+		infoHash: ih,
+		peers:    make(map[peersKey]Peer),
 
 		halfOpen:          make(map[string]struct{}),
 		pieceStateChanges: pubsub.NewPubSub(),
@@ -1180,6 +1179,7 @@ func (cl *Client) newTorrent(ih metainfo.Hash) (t *Torrent) {
 		storageOpener:       cl.defaultStorage,
 		maxEstablishedConns: defaultEstablishedConnsPerTorrent,
 	}
+	t.setChunkSize(defaultChunkSize)
 	return
 }
 
@@ -1228,7 +1228,7 @@ func (cl *Client) AddTorrentSpec(spec *TorrentSpec) (t *Torrent, new bool, err e
 	cl.mu.Lock()
 	defer cl.mu.Unlock()
 	if spec.ChunkSize != 0 {
-		t.chunkSize = pp.Integer(spec.ChunkSize)
+		t.setChunkSize(pp.Integer(spec.ChunkSize))
 	}
 	t.addTrackers(spec.Trackers)
 	t.maybeNewConns()

connection.go

@@ -698,16 +698,11 @@ func (c *connection) lastHelpful() (ret time.Time) {
 func (c *connection) mainReadLoop() error {
 	t := c.t
 	cl := t.cl
-	pool := &sync.Pool{
-		New: func() interface{} {
-			return make([]byte, t.chunkSize)
-		},
-	}
 
 	decoder := pp.Decoder{
 		R:         bufio.NewReader(c.rw),
 		MaxLength: 256 * 1024,
-		Pool:      pool,
+		Pool:      t.chunkPool,
 	}
 	for {
 		cl.mu.Unlock()
@@ -782,7 +777,7 @@ func (c *connection) mainReadLoop() error {
 		case pp.Piece:
 			cl.downloadedChunk(t, c, &msg)
 			if len(msg.Piece) == int(t.chunkSize) {
-				pool.Put(msg.Piece)
+				t.chunkPool.Put(msg.Piece)
 			}
 		case pp.Extended:
 			switch msg.ExtendedID {

connection_test.go

@@ -144,10 +144,10 @@ func BenchmarkConnectionMainReadLoop(b *testing.B) {
 			Length:      1 << 20,
 			PieceLength: 1 << 20,
 		},
-		chunkSize:         defaultChunkSize,
 		storage:           &storage.Torrent{ts},
 		pieceStateChanges: pubsub.NewPubSub(),
 	}
+	t.setChunkSize(defaultChunkSize)
 	t.makePieces()
 	t.pendingPieces.Add(0)
 	r, w := io.Pipe()

torrent.go

@@ -52,6 +52,7 @@ type Torrent struct {
 	// The size of chunks to request from peers over the wire. This is
 	// normally 16KiB by convention these days.
 	chunkSize pp.Integer
+	chunkPool *sync.Pool
 	// Total length of the torrent in bytes. Stored because it's not O(1) to
 	// get this from the info dict.
 	length int64
@@ -115,6 +116,15 @@ type Torrent struct {
 	stats TorrentStats
 }
 
+func (t *Torrent) setChunkSize(size pp.Integer) {
+	t.chunkSize = size
+	t.chunkPool = &sync.Pool{
+		New: func() interface{} {
+			return make([]byte, size)
+		},
+	}
+}
+
 func (t *Torrent) setDisplayName(dn string) {
 	if t.haveInfo() {
 		return