Wikimedia Bugzilla is closed!

It appears the bug only applies to searches with the parameter srsearch=title which is the default for this parameter.

Working: http://en.wikipedia.org/w/api.php?action=query&list=search&srwhat=text&srsearch=meaning
Not working: http://en.wikipedia.org/w/api.php?action=query&list=search&srwhat=title&srsearch=meaning

Bumped severity to major.

That clarifies a lot. See this code in the MWSearch extension:

        // this stuff's a little broken atm
        global $wgLuceneDisableTitleMatches;
        if( $wgLuceneDisableTitleMatches ) {
            return null;
        } else {
            return LuceneSearchSet::newFromQuery( 'titlematch',
                $term, $this->namespaces, $this->limit, $this->offset );
        }

Apparently a search can return null sometimes. The question is what to do when null is returned. Should we raise an error or silently fail and return an empty list?

Fixed in r32349. Will raise a search-text-disabled or search-title-disabled when either search type is disabled.

Note that the split between title and text searching will probably go away entirely soon.