code: def loop_once(self):
          
          while 1:
              if not self._active_nodes:
                  # If everything has died stop looping
                  self.triggershutdown()
                  raise RuntimeError("Unexpectedly no active workers available")
              try:
                  eventcall = self.queue.get(timeout=2.0)
                  break
              except Empty:
                  continue
          callname, kwargs = eventcall
          assert callname, kwargs
          method = "worker_" + callname
          call = getattr(self, method)
          self.log("calling method", method, kwargs)
          call(**kwargs)
          if self.sched.tests_finished:
              self.triggershutdown()
docs: Process one callback from one of the workers.