code: func newPublisher(reg registry.Registry, ttl time.Duration) publishFunc {
  	return func(name string, us *unit.UnitState) {
  		if us == nil {
  			log.Debugf("Destroying UnitState(%s) in Registry", name)
  			err := reg.RemoveUnitState(name)
  			if err != nil {
  				log.Errorf("Failed to destroy UnitState(%s) in Registry: %v", name, err)
  			}
  		} else {
  			// Sanity check - don't want to publish incomplete UnitStates
  			// TODO(jonboulle): consider teasing apart a separate UnitState-like struct
  			// so we can rely on a UnitState always being fully hydrated?
  			if len(us.UnitHash) == 0 {
  				log.Errorf("Refusing to push UnitState(%s), no UnitHash: %#v", name, us)
  			} else if len(us.MachineID) == 0 {
  				log.Errorf("Refusing to push UnitState(%s), no MachineID: %#v", name, us)
  			} else {
  				log.Debugf("Pushing UnitState(%s) to Registry: %#v", name, us)
  				reg.SaveUnitState(name, us, ttl)
  			}
  		}
  	}
  }
docs: // newPublisher returns a publishFunc that publishes a single UnitState
  // by the given name to the provided Registry, with the given TTL