MDL-48266 backup: Validate backup_auto_storage config

lib/adminlib.php

@@ -4390,6 +4390,44 @@ class admin_setting_special_backupdays extends admin_setting_configmulticheckbox
     }
 }
 
+/**
+ * Special setting for backup auto destination.
+ *
+ * @package    core
+ * @subpackage admin
+ * @copyright  2014 Frédéric Massart - FMCorz.net
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+class admin_setting_special_backup_auto_destination extends admin_setting_configdirectory {
+
+    /**
+     * Calls parent::__construct with specific arguments.
+     */
+    public function __construct() {
+        parent::__construct('backup/backup_auto_destination', new lang_string('saveto'), new lang_string('backupsavetohelp'), '');
+    }
+
+    /**
+     * Check if the directory must be set, depending on backup/backup_auto_storage.
+     *
+     * Note: backup/backup_auto_storage must be specified BEFORE this setting otherwise
+     * there will be conflicts if this validation happens before the other one.
+     *
+     * @param string $data Form data.
+     * @return string Empty when no errors.
+     */
+    public function write_setting($data) {
+        $storage = (int) get_config('backup', 'backup_auto_storage');
+        if ($storage !== 0) {
+            if (empty($data) || !file_exists($data) || !is_dir($data) || !is_writable($data) ) {
+                // The directory must exist and be writable.
+                return get_string('backuperrorinvaliddestination');
+            }
+        }
+        return parent::write_setting($data);
+    }
+}
+
 
 /**
  * Special debug setting