Comment 4 for bug 1177995

firefox is calling FcFreeTypeQueryFace for webfonts like this:

        pattern =
            (*sQueryFacePtr)(mFace, gfxFontconfigUtils::ToFcChar8(""), 0, NULL);

This fails now because we require the valid filename to generate hash value and if not, simply fails because the matcher behaves wrong if some objects are available or unavailable in the cache.

One idea to support this case is to define constant value and add an invalid hash to the cache?

FWIW FcPatternDel should warn if one is going to delete objects which is in the matcher perhaps.