It seems updateSearchIndex.php is broken. The "same" bug was reported in T42302 but on a different table. When adding page_restrictions to the lockSearchindex function in Maintenance.php it seems to work.
/**
* Lock the search index
* @param &$db DatabaseBase object
*/
private function lockSearchindex( &$db ) {
$write = array( 'searchindex' );
$read = array( 'page', 'revision', 'text', 'interwiki', 'l10n_cache', 'user', 'page_restrictions' );
$db->lockTables( $read, $write, __CLASS__ . '::' . __METHOD__ );
}But then the script ends with the below message which seems to mean that something else is not working.
--- Unlocking -- Done *** Couldn't write to the searchUpdate.newwikidb.pos!
This was tested on a clean MW-1.23.7 & MW-1.24.1 install but I found out on our production wiki running MW-1.22.5.