public static function obtainReadLock(Zend_Search_Lucene_Storage_Directory $lockDirectory)
    {
        $lock = $lockDirectory->createFile(self::READ_LOCK_FILE);
        if (!$lock->lock(LOCK_SH)) {
            include_once 'Zend/Search/Lucene/Exception.php';
            throw new Zend_Search_Lucene_Exception('Can\'t obtain shared reading index lock');
        }
        return $lock;
    }