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

[Robert Pluim]

>>>>> On Thu, 3 Sep 2020 11:40:07 +0200, Alexander Shukaev 
>>>>> <emacs@Alexander.Shukaev.name> said:
    Alexander> Right, so briefly testing, I think the scrolling hang is gone 
thanks to

    Alexander> (add-to-list 'face-ignored-fonts "Adobe.*Blank")

    Alexander> However, hang from opening certain files is still there and is 
also
    Alexander> font-related, though with a different symptom:

    Alexander> Thread 1 "emacs" received signal SIGABRT, Aborted.
    Alexander> 0x00007ffff5c0646f in poll () from /usr/lib/libc.so.6
    Alexander> (gdb) bt
    Alexander> #0  0x00007ffff5c0646f in poll () at /usr/lib/libc.so.6
    Alexander> #1  0x00007ffff6e2a63b in  () at /usr/lib/libxcb.so.1
    Alexander> #2  0x00007ffff6e2c08f in  () at /usr/lib/libxcb.so.1
    Alexander> #3  0x00007ffff6e2c203 in xcb_wait_for_reply64 () at 
/usr/lib/libxcb.so.1
    Alexander> #4  0x00007ffff6e904b9 in _XReply () at /usr/lib/libX11.so.6
    Alexander> #5  0x00007ffff6e71e71 in  () at /usr/lib/libX11.so.6
    Alexander> #6  0x00007ffff6e7254e in XLoadQueryFont () at 
/usr/lib/libX11.so.6
    Alexander> #7  0x0000555555a8d875 in xfont_supported_scripts
    Alexander>     (display=0x555556594940, fontname=0x55556a79e428 
"-misc-trirong
    Alexander>     medium-medium-i-normal--0-0-0-0-p-0-iso10646-1",
    Alexander>     props=XIL(0x7ffff27926e5), encoding=0x7ffff23ab8d0)
    Alexander> at xfont.c:266

So this is the 'x' font backend, rather than the 'ftcr' font
backend, which I would not expect emacs to be falling back to. Do you
have any face/fontset customisations? Could you show them to us? (Iʼm
hoping youʼre not messing with the font-backend frame parameter anywhere)

Robert