avoid recursion on the root of the file system

fixes #9791

framework/CHANGELOG.md

@@ -20,6 +20,7 @@ Yii Framework 2 Change Log
 - Bug #9681: `Json::encode()` was erroring under CYGWIN (samdark)
 - Bug #9714: Fixed `yii\rbac\PhpManager::updateItem()` unable to save users assignments (rezident1307)
 - Bug #9754: Fixed `yii\web\Request` error when path info is empty (dynasource)
+- Bug #9791: Fixed endless loop on file creation for non-existing device letters on windows (lukos, cebe)
 - Enh #7581: Added ability to specify range using anonymous function in `RangeValidator` (RomeroMsk)
 - Enh #8613: `yii\widgets\FragmentCache` will not store empty content anymore which fixes some problems related to `yii\filters\PageCache` (kidol)
 - Enh #9476: Added DI injection via controller action method signature (mdmunir)

framework/helpers/BaseFileHelper.php

@@ -455,7 +455,8 @@ class BaseFileHelper
             return true;
         }
         $parentDir = dirname($path);
-        if ($recursive && !is_dir($parentDir)) {
+        // recurse if parent dir does not exist and we are not at the root of the file system.
+        if ($recursive && !is_dir($parentDir) && $parentDir !== $path) {
             static::createDirectory($parentDir, $mode, true);
         }
         try {