优化File缓存自动生成空目录的问题

library/think/cache/driver/File.php

@@ -63,10 +63,11 @@ class File extends Driver
     /**
      * 取得变量的存储文件名
      * @access protected
-     * @param string $name 缓存变量名
+     * @param  string $name 缓存变量名
+     * @param  bool   $auto 是否自动创建目录
      * @return string
      */
-    protected function getCacheKey($name)
+    protected function getCacheKey($name, $auto = false)
     {
         $name = md5($name);
         if ($this->options['cache_subdir']) {
@@ -78,7 +79,8 @@ class File extends Driver
         }
         $filename = $this->options['path'] . $name . '.php';
         $dir      = dirname($filename);
-        if (!is_dir($dir)) {
+
+        if ($auto && !is_dir($dir)) {
             mkdir($dir, 0755, true);
         }
         return $filename;
@@ -144,7 +146,7 @@ class File extends Driver
         if ($expire instanceof \DateTime) {
             $expire = $expire->getTimestamp() - time();
         }
-        $filename = $this->getCacheKey($name);
+        $filename = $this->getCacheKey($name, true);
         if ($this->tag && !is_file($filename)) {
             $first = true;
         }