code: func (p *Partition) RetainFileSet() (*FileSet, error) {
  	select {
  	case <-p.closing:
  		return nil, errors.New("index is closing")
  	default:
  		p.mu.RLock()
  		defer p.mu.RUnlock()
  		return p.retainFileSet(), nil
  	}
  }
docs: // RetainFileSet returns the current fileset and adds a reference count.