code: func (v *view) availableShards() *roaring.Bitmap {
  	v.mu.RLock()
  	defer v.mu.RUnlock()
  
  	b := roaring.NewBitmap()
  	for shard := range v.fragments {
  		_, _ = b.Add(shard) // ignore error, no writer attached
  	}
  	return b
  }
docs: // availableShards returns a bitmap of shards which contain data.