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

[Robert Pluim]

>>>>> On Thu, 3 Sep 2020 09:18:53 +0200, Alexander Shukaev 
>>>>> <emacs@Alexander.Shukaev.name> said:

    Alexander> Hi,
    Alexander> With latest Emacs, I have a sporadic hang issue when either 
opening a
    Alexander> file or scrolling a buffer.  The best part is that it's 
impossible to
    Alexander> get break back to evaluation loop of Emacs even with 'USR2'
    Alexander> signal. That is when 'USR2' is sent, it does show "Entering
    Alexander> debugger...", though it remains frozen afterwards.  Resending 
'USR2'
    Alexander> leads to the same outcome.  One thing is for certain, every time
    Alexander> 'USR2' is sent into such a hang, the stack traceback leads to
    Alexander> `FcCharSetSubtractCount' from '/usr/lib/libfontconfig.so.1':

If you've still got this in gdb, can you go to frame #6 and do

pp entity

(this is assuming you've sourced the .gdbinit in src/)

That will tell us which font weʼre trying to open. Perhaps itʼs a bad
font or fontconfig configuration.

Robert