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

[Alexander Shukaev]

Here is also a cleaner stack traceback, where I didn't issue 'USR2' 
multiple times, but only one time:

Thread 1 "emacs" received signal SIGUSR2, User defined signal 2.
0x00007ffff6107823 in ?? () from /usr/lib/libfontconfig.so.1
(gdb) bt
#0  0x00007ffff6107823 in  () at /usr/lib/libfontconfig.so.1
#1  0x00007ffff61079b5 in  () at /usr/lib/libfontconfig.so.1
#2  0x00007ffff6107cc2 in  () at /usr/lib/libfontconfig.so.1
#3  0x00007ffff6107dee in  () at /usr/lib/libfontconfig.so.1
#4  0x00007ffff6108b82 in FcFontMatch () at /usr/lib/libfontconfig.so.1
#5  0x00005555558e523c in ftcrfont_open (f=0x5555564cfb40, entity=..., 
pixel_size=18) at ftcrfont.c:137
#6  0x00005555558466c8 in font_open_entity (f=0x5555564cfb40, 
entity=..., pixel_size=13) at font.c:2913
#7  0x0000555555848372 in font_open_for_lface (f=0x5555564cfb40, 
entity=..., attrs=0x555556114b80, spec=...) at font.c:3350
#8  0x00005555558ead90 in fontset_find_font (fontset=..., c=30693, 
face=0x555556114b80, charset_id=-1, fallback=false) at fontset.c:668
#9  0x00005555558eb440 in fontset_font (fontset=..., c=30693, 
face=0x555556114b80, id=-1) at fontset.c:782
#10 0x00005555558ec00b in face_for_char (f=0x5555564cfb40, 
face=0x555556114b80, c=30693, pos=257426, object=...) at fontset.c:996
#11 0x00005555555bd121 in FACE_FOR_CHAR (f=0x5555564cfb40, 
face=0x555556114b80, character=30693, pos=257426, object=...) at 
dispextern.h:1882
#12 0x00005555555db943 in get_next_display_element (it=0x7fffffff80c0) 
at xdisp.c:7753
#13 0x0000555555610bda in display_line (it=0x7fffffff80c0, 
cursor_vpos=1) at xdisp.c:23350
#14 0x0000555555601099 in try_window (window=..., pos=..., flags=0) at 
xdisp.c:19297
#15 0x00005555555fcc94 in redisplay_window (window=..., 
just_this_one_p=false) at xdisp.c:18448
#16 0x00005555555f4f38 in redisplay_window_0 (window=...) at xdisp.c:16429
#17 0x000055555581ccff in internal_condition_case_1 (bfun=0x5555555f4ef6 
<redisplay_window_0>, arg=..., handlers=..., hfun=0x5555555f4ebe 
<redisplay_window_error>) at eval.c:1380
#18 0x00005555555f4e92 in redisplay_windows (window=...) at xdisp.c:16409
#19 0x00005555555f4e44 in redisplay_windows (window=...) at xdisp.c:16403
#20 0x00005555555f37c4 in redisplay_internal () at xdisp.c:15877
#21 0x00005555555f1158 in redisplay () at xdisp.c:15099
#22 0x000055555573f13d in read_char (commandflag=1, map=..., 
prev_event=..., used_mouse_menu=0x7fffffffd47d, end_time=0x0) at 
keyboard.c:2493
#23 0x0000555555752a81 in read_key_sequence (keybuf=0x7fffffffd660, 
prompt=..., dont_downcase_last=false, can_return_switch_frame=true, 
fix_current_buffer=true, prevent_redisplay=false) at keyboard.c:9547
#24 0x000055555573b3d6 in command_loop_1 () at keyboard.c:1350
#25 0x000055555581cc20 in internal_condition_case (bfun=0x55555573af39 
<command_loop_1>, handlers=..., hfun=0x55555573a516 <cmd_error>) at 
eval.c:1356
#26 0x000055555573ab17 in command_loop_2 (ignore=...) at keyboard.c:1091
#27 0x000055555581c045 in internal_catch (tag=..., func=0x55555573aaea 
<command_loop_2>, arg=...) at eval.c:1117
#28 0x000055555573aab6 in command_loop () at keyboard.c:1070
#29 0x0000555555739ff9 in recursive_edit_1 () at keyboard.c:714
#30 0x000055555573a1f5 in Frecursive_edit () at keyboard.c:786
#31 0x0000555555735f3a in main (argc=1, argv=0x7fffffffdae8) at emacs.c:2047