bug#43177: Bug: Emacs 27.1 hangs forever in `FcCharSetSubtractCount' from '/usr/lib/libfontconfig.so.1'

[Robert Pluim]

>>>>> On Thu, 03 Sep 2020 16:19:56 +0300, Eli Zaretskii <eliz@gnu.org> said:

    >> From: Robert Pluim <rpluim@gmail.com>
    >> Date: Thu, 03 Sep 2020 11:58:50 +0200
    >> Cc: 43177@debbugs.gnu.org
    >> 
    >> So this is the 'x' font backend, rather than the 'ftcr' font
    >> backend, which I would not expect emacs to be falling back to.

    Eli> Why do you think this is evidence of using the fallback backend?
    Eli> AFAIR, when Emacs needs to find a font for a character, it loops over
    Eli> all the fonts with all the available backends, and only later decides
    Eli> which of the fonts to use.  Don't you see that if you type some
    Eli> character unlikely to have been seen in an "emacs -Q" session?

Youʼre right, Iʼd run through this in gdb, and set my breakpoint on
the wrong function.

'C-u C-x =' would tell us which backend ended up being used.

Robert