bug#40097: 28.0.50; Preferred font ignored for specific charset

[Eli Zaretskii <eliz@gnu.org>]

> From: Robert Pluim <rpluim@gmail.com>
> Cc: Sergey Organov <sorganov@gmail.com>,  Kenichi Handa <handa@gnu.org>,
>   40097@debbugs.gnu.org
> Date: Wed, 18 Mar 2020 12:07:54 +0100
> 
> Ah, now I see where this is coming from: I was looking down in font.c,
> but this is a fontset.c feature.

Yes.

> Iʼm not sure how useful it is, I donʼt think fontconfig has any notion
> of 'charset' beyond 'does this font support this Unicode character'.

Indeed; and you will see in ftfont.c that ftfont_list (or, rather, one
of the subroutines it calls) concocts a "charset" by using the few
representative characters from fc_charset_table, and then asks
Fontconfig to list fonts which support those characters (at least
that's my reading of the code).  Which is why I'm puzzled how come
DejaVu Sans Mono is not in the list and we proceed to the xfont
backend, because I'm quite sure DejaVu Sans does support the Cyrillic
characters that represent windows-1251.  If you can tell what I'm
missing here, maybe we could make some progress even without changing
the design.