code: func CreateHandler(typ string, loader Loader, config jsonconfig.Obj) (http.Handler, error) {
  	mapLock.Lock()
  	ctor, ok := handlerConstructors[typ]
  	mapLock.Unlock()
  	if !ok {
  		return nil, fmt.Errorf("blobserver: Handler type %q not known or loaded", typ)
  	}
  	return ctor(loader, config)
  }
docs: // CreateHandler instantiates an http Handler of type 'typ' from the
  // provided JSON configuration, and finding peer handlers and
  // configuration from the environment in 'loader'.
  //
  // The handler 'typ' must have been previously registered with
  // RegisterHandlerConstructor.