In case of DB access error, Mediawiki 1.28 shows an html page defined in "includes/exception/MWExceptionRenderer.php".
In method "output", line 45 :

if ( $e instanceof DBConnectionError ) {
​ self::reportOutageHTML( $e );
}

Actual Results: it seems the page is served with HTTP code 200 by default (checked with Firefox developer tool, Network tab). Our frontend (Varnish) also sees the page as a normal 200 response and starts caching the error pages.

Expected Results: serve the page with a 503 error code, maybe by adding a "self::statusHeader( 500 );" just before "self::reportOutageHTML( $e );", so frontends can detect the page is not the content they expect.