code: func (handler *UnroutedHandler) sizeOfUploads(ids []string) (size int64, err error) {
  	for _, id := range ids {
  		info, err := handler.composer.Core.GetInfo(id)
  		if err != nil {
  			return size, err
  		}
  
  		if info.SizeIsDeferred || info.Offset != info.Size {
  			err = ErrUploadNotFinished
  			return size, err
  		}
  
  		size += info.Size
  	}
  
  	return
  }
docs: // The get sum of all sizes for a list of upload ids while checking whether
  // all of these uploads are finished yet. This is used to calculate the size
  // of a final resource.