docs: def backup_db(self):
          """
          " Generate a xxxxx.backup.json.
          """
          with self.db_mutex:
              if os.path.exists(self.json_db_path):
                  try:
                      shutil.copy2(self.json_db_path, self.backup_json_db_path)
                  except (IOError, OSError):
                      _logger.debug("*** No file to copy.")