Have SnowflakeClientConn implement io.WriterTo.

By forwarding the method to the inner smux.Stream. This is to prevent
io.Copy in the top-level proxy function from allocating a buffer per
client.

The smux.Stream WriteTo method returns io.EOF on success, contrary to
the contract of io.Copy that says it should return nil. Ignore io.EOF in
the proxy loop to avoid a log message.

/anti-censorship/pluggable-transports/snowflake/-/issues/40177

server/lib/snowflake.go

@@ -232,7 +232,7 @@ func (l *SnowflakeListener) acceptStreams(conn *kcp.UDPSession) error {
 			}
 			return err
 		}
-		l.queueConn(&SnowflakeClientConn{Conn: stream, address: addr})
+		l.queueConn(&SnowflakeClientConn{stream: stream, address: addr})
 	}
 }
 
@@ -280,15 +280,35 @@ func (l *SnowflakeListener) queueConn(conn net.Conn) error {
 	}
 }
 
-// SnowflakeClientConn is a wrapper for the underlying turbotunnel
-// conn. We need to reference our client address map to determine the
-// remote address
+// SnowflakeClientConn is a wrapper for the underlying turbotunnel conn
+// (smux.Stream). It implements the net.Conn and io.WriterTo interfaces. The
+// RemoteAddr method is overridden to refer to a real IP address, looked up from
+// the client address map, rather than an abstract client ID.
 type SnowflakeClientConn struct {
-	net.Conn
+	stream  *smux.Stream
 	address net.Addr
 }
 
-// RemoteAddr returns the mapped client address of the Snowflake connection
+// Forward net.Conn methods, other than RemoteAddr, to the inner stream.
+func (conn *SnowflakeClientConn) Read(b []byte) (int, error)    { return conn.stream.Read(b) }
+func (conn *SnowflakeClientConn) Write(b []byte) (int, error)   { return conn.stream.Write(b) }
+func (conn *SnowflakeClientConn) Close() error                  { return conn.stream.Close() }
+func (conn *SnowflakeClientConn) LocalAddr() net.Addr           { return conn.stream.LocalAddr() }
+func (conn *SnowflakeClientConn) SetDeadline(t time.Time) error { return conn.stream.SetDeadline(t) }
+func (conn *SnowflakeClientConn) SetReadDeadline(t time.Time) error {
+	return conn.stream.SetReadDeadline(t)
+}
+func (conn *SnowflakeClientConn) SetWriteDeadline(t time.Time) error {
+	return conn.stream.SetWriteDeadline(t)
+}
+
+// RemoteAddr returns the mapped client address of the Snowflake connection.
 func (conn *SnowflakeClientConn) RemoteAddr() net.Addr {
 	return conn.address
 }
+
+// WriteTo implements the io.WriterTo interface by passing the call to the
+// underlying smux.Stream.
+func (conn *SnowflakeClientConn) WriteTo(w io.Writer) (int64, error) {
+	return conn.stream.WriteTo(w)
+}

server/server.go

@@ -57,7 +57,7 @@ func proxy(local *net.TCPConn, conn net.Conn) {
 		wg.Done()
 	}()
 	go func() {
-		if _, err := io.Copy(local, conn); err != nil && !errors.Is(err, io.ErrClosedPipe) {
+		if _, err := io.Copy(local, conn); err != nil && !errors.Is(err, io.EOF) && !errors.Is(err, io.ErrClosedPipe) {
 			log.Printf("error copying WebSocket to ORPort %v", err)
 		}
 		local.CloseWrite()