docs: def save_db(self):
          """
          " Save json db to file system.
          """
          with self.db_mutex:
              if not isinstance(self.db, dict) and not isinstance(self.db, list):
                  return False
              try:
                  with open(self.json_db_path, "w") as fp:
                      json.dump(self.db, fp, indent=4)
              except Exception as e:
                  # disk full or something.
                  _logger.debug("*** Write JSON DB to file error.")
                  raise e
  
              else:
                  self.sync()
                  return True