code: def ping(self, status=None):
          
          newping = not self.ping_watchers
          d = defer.Deferred()
          self.ping_watchers.append(d)
          if newping:
              Ping().ping(self.worker.conn).addBoth(self._pong)
  
          return d
docs: Ping the worker to make sure it is still there. Returns a Deferred
          that fires with True if it is.
  
          @param status: if you point this at a BuilderStatus, a 'pinging'
                         event will be pushed.