func visit(tx *bolt.Tx, path string, update updateFn) error {
	for path != "" {
		parent, child := split(path)
		pnode, err := get(tx, parent)
		if err != nil && Code(err) != PathNotFound {
			return err
		}
		if pnode != nil && pnode.nodetype() != directory {
			return errorf(PathConflict, "attempted to visit \"%s\", but it's not a "+
				"directory", path)
		}
		if pnode == nil {
			pnode = &NodeProto{}
		}
		if err := update(pnode, parent, child); err != nil {
			return err
		}
		if err := put(tx, parent, pnode); err != nil {
			return err
		}
		path = parent
	}
	return nil
}
func visit(tx *bolt.Tx, path string, update updateFn) error {
	for path != "" {
		parent, child := split(path)
		pnode, err := get(tx, parent)
		if err != nil && Code(err) != PathNotFound {
			return err
		}
		if pnode != nil && pnode.nodetype() != directory {
			return errorf(PathConflict, "attempted to visit \"%s\", but it's not a "+
				"directory", path)
		}
		if pnode == nil {
			pnode = &NodeProto{}
		}
		if err := update(pnode, parent, child); err != nil {
			return err
		}
		if err := put(tx, parent, pnode); err != nil {
			return err
		}
		path = parent
	}
	return nil
}
// visit visits every ancestor of 'path' (excluding 'path' itself), leaf to
// root (i.e.  end of 'path' to beginning), and calls 'update' on each node
// along the way. For example, if 'visit' is called with 'path'="/path/to/file",
// then updateFn is called as follows:
//
// 1. update(node at "/path/to" or nil, "/path/to", "file")
// 2. update(node at "/path"    or nil, "/path",    "to")
// 3. update(node at "/"        or nil, "",         "path")
//
// This is useful for propagating changes to size upwards.