code: func (f *IndexFile) Close() error {
  	// Wait until all references are released.
  	f.wg.Wait()
  
  	f.sfile = nil
  	f.tblks = nil
  	f.mblk = MeasurementBlock{}
  	return mmap.Unmap(f.data)
  }
docs: // Close unmaps the data file.