code: func (b *Backend) lockGroupMeta(groupUUID string) error {
  	query := bson.M{
  		"_id":  groupUUID,
  		"lock": false,
  	}
  	change := bson.M{
  		"$set": bson.M{
  			"lock": true,
  		},
  	}
  
  	_, err := b.groupMetasCollection.UpdateOne(context.Background(), query, change, options.Update().SetUpsert(true))
  
  	return err
  }
docs: // lockGroupMeta acquires lock on groupUUID document