docs: def sigusr1_handler(self, unused_signum, unused_frame):
          """
          Handle SIGUSR1 signal. Call function which is defined in the
          **settings.SIGUSR1_HANDLER**. If main process, forward the
          signal to all child processes.
          """
          for process in self.processes:
              if process.pid and os.getpid() == self.main_pid:
                  try:
                      os.kill(process.pid, signal.SIGUSR1)
                  except ProcessLookupError:
                      pass
          if self._sigusr1_handler_func is not None:
              self._sigusr1_handler_func(self.context)