func (e *dataUsageEntry) addChild(hash dataUsageHash) {
	if _, ok := e.Children[hash.Key()]; ok {
		return
	}
	if e.Children == nil {
		e.Children = make(dataUsageHashMap, 1)
	}
	e.Children[hash.Key()] = struct{}{}
}

// Create a clone of the entry.
func (e dataUsageEntry) clone() dataUsageEntry {
	// We operate on a copy from the receiver.
	if e.Children != nil {
		ch := make(dataUsageHashMap, len(e.Children))

		var compact bool
		if flat.Objects < dataScannerCompactLeastObject {
			compact = true
		} else {
			// Compact if we only have objects as children...
			compact = true
			for k := range into.Children {
				if v, ok := f.newCache.Cache[k]; ok {
					if len(v.Children) > 0 || v.Objects > 1 {
						compact = false
						break
					}
				}
			}

		}
		if compact {

		field, err = dc.ReadMapKeyPtr()
		if err != nil {
			err = msgp.WrapError(err)
			return
		}
		switch msgp.UnsafeString(field) {
		case "ch":
			err = z.Children.DecodeMsg(dc)
			if err != nil {
				err = msgp.WrapError(err, "Children")
				return
			}
		case "sz":
			z.Size, err = dc.ReadInt64()
			if err != nil {
				err = msgp.WrapError(err, "Size")
				return
			}
		case "os":

			// here.
			return errFileAccessDenied
		}
	}
	return err
}

// Reliably retries os.RemoveAll if for some reason os.RemoveAll returns
// syscall.ENOTEMPTY (children has files).
func reliableRemoveAll(dirPath string) (err error) {
	i := 0
	for {
		// Removes all the directories and files.
		if err = RemoveAll(dirPath); err != nil {
			// Retry only for the first retryable error.