p2p: fix DiscReason encoding/decoding (#30855)

This fixes an issue where the disconnect message was not wrapped in a list.
The specification requires it to be a list like any other message.

In order to remain compatible with legacy geth versions, we now accept both
encodings when parsing a disconnect message.

---------

Co-authored-by: Felix Lange <fjl@twurst.com>

p2p/peer.go

@@ -345,9 +345,7 @@ func (p *Peer) handle(msg Msg) error {
 	case msg.Code == discMsg:
 		// This is the last message. We don't need to discard or
 		// check errors because, the connection will be closed after it.
-		var m struct{ R DiscReason }
-		rlp.Decode(msg.Payload, &m)
-		return m.R
+		return decodeDisconnectMessage(msg.Payload)
 	case msg.Code < baseProtocolLength:
 		// ignore other base protocol messages
 		return msg.Discard()
@@ -372,6 +370,27 @@ func (p *Peer) handle(msg Msg) error {
 	return nil
 }
 
+// decodeDisconnectMessage decodes the payload of discMsg.
+func decodeDisconnectMessage(r io.Reader) (reason DiscReason) {
+	s := rlp.NewStream(r, 100)
+	k, _, err := s.Kind()
+	if err != nil {
+		return DiscInvalid
+	}
+	if k == rlp.List {
+		s.List()
+		err = s.Decode(&reason)
+	} else {
+		// Legacy path: some implementations, including geth, used to send the disconnect
+		// reason as a byte array by accident.
+		err = s.Decode(&reason)
+	}
+	if err != nil {
+		reason = DiscInvalid
+	}
+	return reason
+}
+
 func countMatchingProtocols(protocols []Protocol, caps []Cap) int {
 	n := 0
 	for _, cap := range caps {

p2p/peer_error.go

@@ -70,6 +70,8 @@ const (
 	DiscSelf
 	DiscReadTimeout
 	DiscSubprotocolError = DiscReason(0x10)
+
+	DiscInvalid = 0xff
 )
 
 var discReasonToString = [...]string{
@@ -86,10 +88,11 @@ var discReasonToString = [...]string{
 	DiscSelf:                "connected to self",
 	DiscReadTimeout:         "read timeout",
 	DiscSubprotocolError:    "subprotocol error",
+	DiscInvalid:             "invalid disconnect reason",
 }
 
 func (d DiscReason) String() string {
-	if len(discReasonToString) <= int(d) {
+	if len(discReasonToString) <= int(d) || discReasonToString[d] == "" {
 		return fmt.Sprintf("unknown disconnect reason %d", d)
 	}
 	return discReasonToString[d]

p2p/transport.go

@@ -113,15 +113,14 @@ func (t *rlpxTransport) close(err error) {
 	// Tell the remote end why we're disconnecting if possible.
 	// We only bother doing this if the underlying connection supports
 	// setting a timeout tough.
-	if t.conn != nil {
-		if r, ok := err.(DiscReason); ok && r != DiscNetworkError {
-			deadline := time.Now().Add(discWriteTimeout)
-			if err := t.conn.SetWriteDeadline(deadline); err == nil {
-				// Connection supports write deadline.
-				t.wbuf.Reset()
-				rlp.Encode(&t.wbuf, []DiscReason{r})
-				t.conn.Write(discMsg, t.wbuf.Bytes())
-			}
+	if reason, ok := err.(DiscReason); ok && reason != DiscNetworkError {
+		// We do not use the WriteMsg func since we want a custom deadline
+		deadline := time.Now().Add(discWriteTimeout)
+		if err := t.conn.SetWriteDeadline(deadline); err == nil {
+			// Connection supports write deadline.
+			t.wbuf.Reset()
+			rlp.Encode(&t.wbuf, []any{reason})
+			t.conn.Write(discMsg, t.wbuf.Bytes())
 		}
 	}
 	t.conn.Close()
@@ -163,11 +162,8 @@ func readProtocolHandshake(rw MsgReader) (*protoHandshake, error) {
 	if msg.Code == discMsg {
 		// Disconnect before protocol handshake is valid according to the
 		// spec and we send it ourself if the post-handshake checks fail.
-		// We can't return the reason directly, though, because it is echoed
-		// back otherwise. Wrap it in a string instead.
-		var reason [1]DiscReason
-		rlp.Decode(msg.Payload, &reason)
-		return nil, reason[0]
+		r := decodeDisconnectMessage(msg.Payload)
+		return nil, r
 	}
 	if msg.Code != handshakeMsg {
 		return nil, fmt.Errorf("expected handshake, got %x", msg.Code)

p2p/transport_test.go

@@ -97,7 +97,7 @@ func TestProtocolHandshake(t *testing.T) {
 			return
 		}
 
-		if err := ExpectMsg(rlpx, discMsg, []DiscReason{DiscQuitting}); err != nil {
+		if err := ExpectMsg(rlpx, discMsg, []any{DiscQuitting}); err != nil {
 			t.Errorf("error receiving disconnect: %v", err)
 		}
 	}()
@@ -112,7 +112,13 @@ func TestProtocolHandshakeErrors(t *testing.T) {
 	}{
 		{
 			code: discMsg,
-			msg:  []DiscReason{DiscQuitting},
+			msg:  []any{DiscQuitting},
+			err:  DiscQuitting,
+		},
+		{
+			// legacy disconnect encoding as byte array
+			code: discMsg,
+			msg:  []byte{byte(DiscQuitting)},
 			err:  DiscQuitting,
 		},
 		{