func (c *Cache) PathToKey(key string) string {
	return strings.TrimPrefix(key, c.diskRoot+string(os.PathSeparator))
}
func (c *Cache) PathToKey(key string) string {
	return strings.TrimPrefix(key, c.diskRoot+string(os.PathSeparator))
}
// PathToKey converts a path on disk to a key, assuming the path is actually
// under DiskRoot() ...