From dd9244c01d67ffef0050e6a6b7cef216f944f926 Mon Sep 17 00:00:00 2001
From: Matt Joiner <anacrolix@gmail.com>
Date: Tue, 13 Feb 2018 00:21:28 +1100
Subject: [PATCH] bencode: Unmarshal now returns an error on unused trailing
 bytes

---
 bencode/api.go | 19 ++++++++++++++++---
 client.go      |  5 +++--
 2 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/bencode/api.go b/bencode/api.go
index 1b77f588..12511997 100644
--- a/bencode/api.go
+++ b/bencode/api.go
@@ -123,9 +123,22 @@ func Marshal(v interface{}) ([]byte, error) {
 
 // Unmarshal the bencode value in the 'data' to a value pointed by the 'v'
 // pointer, return a non-nil error if any.
-func Unmarshal(data []byte, v interface{}) error {
-	e := Decoder{r: bytes.NewBuffer(data)}
-	return e.Decode(v)
+func Unmarshal(data []byte, v interface{}) (err error) {
+	buf := bytes.NewBuffer(data)
+	e := Decoder{r: buf}
+	err = e.Decode(v)
+	if err == nil && buf.Len() != 0 {
+		err = ErrUnusedTrailingBytes{buf.Len()}
+	}
+	return
+}
+
+type ErrUnusedTrailingBytes struct {
+	NumUnusedBytes int
+}
+
+func (me ErrUnusedTrailingBytes) Error() string {
+	return fmt.Sprintf("%d unused trailing bytes", me.NumUnusedBytes)
 }
 
 func NewDecoder(r io.Reader) *Decoder {
diff --git a/client.go b/client.go
index 60bd43b4..deb09a3f 100644
--- a/client.go
+++ b/client.go
@@ -912,8 +912,9 @@ func (cl *Client) sendInitialMessages(conn *connection, torrent *Torrent) {
 func (cl *Client) gotMetadataExtensionMsg(payload []byte, t *Torrent, c *connection) error {
 	var d map[string]int
 	err := bencode.Unmarshal(payload, &d)
-	if err != nil {
-		return fmt.Errorf("error unmarshalling payload: %s: %q", err, payload)
+	if _, ok := err.(bencode.ErrUnusedTrailingBytes); ok {
+	} else if err != nil {
+		return fmt.Errorf("error unmarshalling bencode: %s", err)
 	}
 	msgType, ok := d["msg_type"]
 	if !ok {