diff -upr drupal-7.9/includes/file.inc drupal-7.9-patched-is-writable/includes/file.inc
--- drupal-7.9/includes/file.inc	2011-11-07 21:34:27.282166217 +0400
+++ drupal-7.9-patched-is-writable/includes/file.inc	2011-11-07 22:05:45.247478554 +0400
@@ -450,7 +450,7 @@ function file_prepare_directory(&$direct
     return FALSE;
   }
   // The directory exists, so check to see if it is writable.
-  $writable = is_writable($directory);
+  $writable = drupal_is_writable($directory);
   if (!$writable && ($options & FILE_MODIFY_PERMISSIONS)) {
     return drupal_chmod($directory);
   }
@@ -2404,7 +2404,7 @@ function file_directory_temp() {
     }
 
     foreach ($directories as $directory) {
-      if (is_dir($directory) && is_writable($directory)) {
+      if (is_dir($directory) && drupal_is_writable($directory)) {
         $temporary_directory = $directory;
         break;
       }
@@ -2459,3 +2459,24 @@ function file_get_content_headers($file)
 /**
  * @} End of "defgroup file".
  */
+
+function drupal_is_writable($path) {
+  //NOTE: use a trailing slash for folders!!!
+  if ($path{strlen($path)-1} == '/') { // recursively return a temporary file path
+    return drupal_is_writable($path.uniqid(mt_rand()).'.tmp');
+  } elseif (is_dir($path)) {
+    return drupal_is_writable($path.'/'.uniqid(mt_rand()).'.tmp');
+  }
+
+  // check tmp file for read/write capabilities
+  $rm = file_exists($path);
+  $f = @fopen($path, 'a');
+  if ($f === FALSE) {
+    return FALSE;
+  }
+  fclose($f);
+  if (!$rm) {
+    unlink($path);
+  }
+  return TRUE;
+}
diff -upr drupal-7.9/includes/install.inc drupal-7.9-patched-is-writable/includes/install.inc
--- drupal-7.9/includes/install.inc	2011-11-07 21:34:27.274166178 +0400
+++ drupal-7.9-patched-is-writable/includes/install.inc	2011-11-07 21:43:43.400923861 +0400
@@ -847,7 +847,7 @@ function drupal_verify_install_file($fil
             }
             break;
           case FILE_WRITABLE:
-            if (!is_writable($file) && !drupal_install_fix_file($file, $mask)) {
+            if (!drupal_is_writable($file) && !drupal_install_fix_file($file, $mask)) {
               $return = FALSE;
             }
             break;
@@ -862,7 +862,7 @@ function drupal_verify_install_file($fil
             }
             break;
           case FILE_NOT_WRITABLE:
-            if (is_writable($file) && !drupal_install_fix_file($file, $mask)) {
+            if (drupal_is_writable($file) && !drupal_install_fix_file($file, $mask)) {
               $return = FALSE;
             }
             break;
@@ -960,7 +960,7 @@ function drupal_install_fix_file($file,
           }
           break;
         case FILE_WRITABLE:
-          if (!is_writable($file)) {
+          if (!drupal_is_writable($file)) {
             $mod |= 0222;
           }
           break;
@@ -975,7 +975,7 @@ function drupal_install_fix_file($file,
           }
           break;
         case FILE_NOT_WRITABLE:
-          if (is_writable($file)) {
+          if (drupal_is_writable($file)) {
             $mod &= ~0222;
           }
           break;
diff -upr drupal-7.9/includes/updater.inc drupal-7.9-patched-is-writable/includes/updater.inc
--- drupal-7.9/includes/updater.inc	2011-11-07 21:34:27.274166178 +0400
+++ drupal-7.9-patched-is-writable/includes/updater.inc	2011-11-07 21:43:56.900990802 +0400
@@ -309,7 +309,7 @@ class Updater {
     // Make the parent dir writable if need be and create the dir.
     if (!is_dir($directory)) {
       $parent_dir = dirname($directory);
-      if (!is_writable($parent_dir)) {
+      if (!drupal_is_writable($parent_dir)) {
         @chmod($parent_dir, 0755);
         // It is expected that this will fail if the directory is owned by the
         // FTP user. If the FTP user == web server, it will succeed.
diff -upr drupal-7.9/modules/system/system.admin.inc drupal-7.9-patched-is-writable/modules/system/system.admin.inc
--- drupal-7.9/modules/system/system.admin.inc	2011-11-07 21:34:27.698168270 +0400
+++ drupal-7.9-patched-is-writable/modules/system/system.admin.inc	2011-11-07 21:44:33.785173700 +0400
@@ -1674,7 +1674,7 @@ function system_performance_settings() {
   );
 
   $directory = 'public://';
-  $is_writable = is_dir($directory) && is_writable($directory);
+  $is_writable = is_dir($directory) && drupal_is_writable($directory);
   $disabled = !$is_writable;
   $disabled_message = '';
   if (!$is_writable) {
diff -upr drupal-7.9/modules/system/system.module drupal-7.9-patched-is-writable/modules/system/system.module
--- drupal-7.9/modules/system/system.module	2011-11-07 21:34:27.694168262 +0400
+++ drupal-7.9-patched-is-writable/modules/system/system.module	2011-11-07 21:44:44.913228881 +0400
@@ -2156,7 +2156,7 @@ function system_check_directory($form_el
     watchdog('file system', 'The directory %directory does not exist and could not be created.', array('%directory' => $directory), WATCHDOG_ERROR);
   }
 
-  if (is_dir($directory) && !is_writable($directory) && !drupal_chmod($directory)) {
+  if (is_dir($directory) && !drupal_is_writable($directory) && !drupal_chmod($directory)) {
     // If the directory is not writable and cannot be made so.
     form_set_error($form_element['#parents'][0], t('The directory %directory exists but is not writable and could not be made writable.', array('%directory' => $directory)));
     watchdog('file system', 'The directory %directory exists but is not writable and could not be made writable.', array('%directory' => $directory), WATCHDOG_ERROR);