bug#30874: 27.0.50; Emacs crashes

[Eli Zaretskii <eliz@gnu.org>]

> From: Robert Pluim <rpluim@gmail.com>
> Cc: 30874@debbugs.gnu.org,  jsynacek@redhat.com
> Date: Mon, 26 Mar 2018 18:52:17 +0200
> 
> > But now that I actually see it, I don't think I understand the reason:
> > the call to XftTextExtents8 asks the xft font back-end to produce the
> > extents for an all-ASCII string, so the fact that it may not have
> > glyphs for some exotic non-ASCII characters couldn't be the culprit.
> 
> OK. Is it possible that because we're in synchronous mode that the
> signal has been received just at an inopportune moment?

I doubt that: the backtrace looks very much like describing the actual
call into the X libraries.

> It doesn't crash if I do eg (insert-char ?a), nor (insert-char #x700).

As expected.  But if you put a breakpoint at line 378 of xftfont.c, do
you see the same call to XftTextExtents8 with the same arguments in
the case of 'a'?

> > Can you figure out what's going on here, and why?
> 
> Looks like I'll have to go poking around in the guts of Xft. Pointers
> appreciated.

Sorry, I don't know enough about the xftfont back-end to provide any
pointers.  Maybe someone else here would.