Implement receiving cancel messages

client.go

@@ -425,6 +425,11 @@ func (me *Client) connectionLoop(torrent *torrent, conn *connection) error {
 				Begin: msg.Begin,
 				Piece: p,
 			})
+		case peer_protocol.Cancel:
+			req := newRequest(msg.Index, msg.Begin, msg.Length)
+			if !conn.PeerCancel(req) {
+				log.Printf("received unexpected cancel: %v", req)
+			}
 		case peer_protocol.Bitfield:
 			if len(msg.Bitfield) < len(torrent.Pieces) {
 				err = errors.New("received invalid bitfield")

connection.go

@@ -88,6 +88,18 @@ func (c *connection) Request(chunk Request) bool {
 	return true
 }
 
+// Returns true if an unsatisfied request was canceled.
+func (c *connection) PeerCancel(r Request) bool {
+	if c.PeerRequests == nil {
+		return false
+	}
+	if _, ok := c.PeerRequests[r]; !ok {
+		return false
+	}
+	delete(c.PeerRequests, r)
+	return true
+}
+
 func (c *connection) Unchoke() {
 	if !c.Choked {
 		return

misc.go

@@ -65,6 +65,10 @@ type Request struct {
 	chunkSpec
 }
 
+func newRequest(index, begin, length peer_protocol.Integer) Request {
+	return Request{index, chunkSpec{begin, length}}
+}
+
 type pieceByBytesPendingSlice struct {
 	Pending, Indices []peer_protocol.Integer
 }