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