code: func unixListener() (net.Listener, error) {
  	s := os.Getenv("LISTEN_FDS")
  	if s != "" {
  		// socket activated
  		lfds, err := strconv.ParseInt(s, 10, 16)
  		if err != nil {
  			return nil, errwrap.Wrap(errors.New("error parsing LISTEN_FDS env var"), err)
  		}
  		if lfds < 1 {
  			return nil, fmt.Errorf("LISTEN_FDS < 1")
  		}
  
  		return net.FileListener(os.NewFile(uintptr(listenFdsStart), "listen"))
  	} else {
  		dir := filepath.Dir(common.MetadataServiceRegSock)
  		err := os.MkdirAll(dir, 0755)
  		if err != nil {
  			return nil, errwrap.Wrap(fmt.Errorf("failed to create %v", dir), err)
  		}
  
  		return net.ListenUnix("unix", &net.UnixAddr{
  			Net:  "unix",
  			Name: common.MetadataServiceRegSock,
  		})
  	}
  }
docs: // unixListener returns the listener used for registrations (over unix sock)