code: func (s *fsCacheStore) DiskUsage(ctx context.Context) (int64, error) {
  	s.mu.Lock()
  	defer s.mu.Unlock()
  	var size int64
  
  	for _, snap := range s.sources {
  		if len(snap.refs) == 0 {
  			ss, err := snap.getSize(ctx)
  			if err != nil {
  				return 0, err
  			}
  			size += ss
  		}
  	}
  	return size, nil
  }
docs: // DiskUsage reports how much data is allocated by the cache