There is an unused argument of _dbus_auth_return_buffer(), its definition is:

/**
 * Returns a buffer with new data read into it.
 *
 * @param auth the auth conversation
 * @param buffer the buffer being returned
 * @param bytes_read number of new bytes added
 */
void
_dbus_auth_return_buffer (DBusAuth               *auth,
                          DBusString             *buffer,
                          int                     bytes_read)
{
  _dbus_assert (buffer == &auth->incoming);
  _dbus_assert (auth->buffer_outstanding);

  auth->buffer_outstanding = FALSE;
}


The bytes_read in fact doesn't used at all, a bug like bug#70218