code: def shutdown(self):
          
          with self._lock:
              for cid in list(self.connections.keys()):
                  if self.connections[cid].executing:
                      raise ConnectionBusyError(cid)
                  if self.connections[cid].locked:
                      self.connections[cid].free()
                  self.connections[cid].close()
                  del self.connections[cid]
docs: Forcefully shutdown the entire pool, closing all non-executing
          connections.
  
          :raises: ConnectionBusyError