code: func (d *fsDest) numOpenFiles() (n int) {
  	d.lock.RLock()
  	n = len(d.openFiles)
  	d.lock.RUnlock()
  	return
  }
docs: // numOpenFiles returns a number of currently open files.
  //
  // Used by txnFSDest to make sure all files are closed before finalizing the
  // transaction.