private function purgeByCacheTag($tag)
    {
        $cacheTagsCacheDir = $this->getTagPath($tag);
        if (!file_exists($cacheTagsCacheDir) || !is_dir($cacheTagsCacheDir)) {
            return;
        }

        $files = (new Finder())->files()->in($cacheTagsCacheDir);
        foreach ($files as $file) {
            // @todo Change to be able to reuse parent::invalidate() or parent::purge() ?
            if ($digest = file_get_contents($file->getRealPath())) {
                @unlink($this->getPath($digest));
            }
            @unlink($file);
        }
        // We let folder stay in case another process have just written new cache tags.
    }