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: Tue, 27 Mar 2018 10:57:03 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > So how do we end up loading that problematic font, and why does that
> > happen with the recipe for this bug, but not if set-fonset-font on the
> > command line is omitted?
> 
> Hereʼs what the file loading looks like from Xft's perspective:
> 
> XFT_DEBUG=16 LD_LIBRARY_PATH=/home/rpluim/repos/src/libXft-2.3.2/src/.libs/ ./emacs -Q
> 
> XFT_DEBUG=16
> FontFile /home/rpluim/.local/share/fonts/Inconsolata-Regular.ttf/0 matches new
> Loading file /home/rpluim/.local/share/fonts/Inconsolata-Regular.ttf/0
> FontFile /home/rpluim/.local/share/fonts/Inconsolata-Regular.ttf/0 matches existing (2)
> FontFile /usr/share/fonts/inconsolata/Inconsolata-Bold.ttf/0 matches new
> Loading file /usr/share/fonts/inconsolata/Inconsolata-Bold.ttf/0
> 
> # Inconsolata is my system default monospace font. Now I insert #x274c :
> 
> FontFile /usr/share/fonts/inconsolata/Inconsolata-Regular.ttf/0 matches new
> Loading file /usr/share/fonts/inconsolata/Inconsolata-Regular.ttf/0
> FontFile /usr/share/fonts/vlgothic/VL-Gothic-Regular.ttf/0 matches new
> Loading file /usr/share/fonts/vlgothic/VL-Gothic-Regular.ttf/0

What does "matches new" mean in this log?  And what does "matches
existing" (below) mean?

> # I think this means Inconsolata doesnʼt have a glyph for that
> # codepoint, although I thought the default fontset specified Symbola
> # for that codepoint (and Symbola is installed), so I donʼt understand
> # why VL-Gothic is chosen.

Strange indeed.  Does setting use-default-font-for-symbols to a nil
value change this in any way?

> # Now I change the fontset, and this time it finds the
> # emojione-android font :
>   
> FontFile /usr/share/fonts/dejavu/DejaVuSansMono.ttf/0 matches new
> Loading file /usr/share/fonts/dejavu/DejaVuSansMono.ttf/0
> FontFile /home/rpluim/.local/share/fonts/Inconsolata-Regular.ttf/0 matches existing (2)
> FontFile /usr/share/fonts/eosrei-emojione/emojione-android.ttf/0 matches new
> Loading file /usr/share/fonts/eosrei-emojione/emojione-android.ttf/0

Right.  Does use-default-font-for-symbols change anything in this
case?

> > It looks like this is a problem with all color emoji fonts, so this is
> > indeed a duplicate of bug#30045.  See this bug:
> >
> >   https://bugzilla.redhat.com/show_bug.cgi?id=1498269
> >
> > The question now becomes: how do we avoid loading such fonts, at least
> > when the xftfont back-end is in use?  Is there any alternative except
> > telling users to "move such fonts out of the way"?
> 
> Accoding to that bug, the solution is for the application to 'move
> away from legacy Xft to fontconfig', whatever that means. I can say
> that building '--without-xft' is definitely sub-optimal (the buffer
> text isnʼt scaled, and Emacs doesnʼt find a font to display #x274c).

We already use fontconfig to some extent, and xftfont is AFAIK the
most advanced font backend we have.  Patches for switching to using
more of fontconfig's features (assuming it can replace Xft), or for
switching to a more modern back-end (harfbuzz?) are welcome, but I
don't hold my breath, as I don't think we have expert on board who
know enough about complex script shaping to make progress in those
directions.

As a stopgap, I think we should find a way of ignoring the problematic
fonts.  Is there some way of detecting them?  AFAICT, we could do that
either in ftfont_match or in its subroutine ftfont_spec_pattern.  We
could then pretend that these fonts don't match any font spec, perhaps
subject to some variable (which would provide a 'fire escape"), which
I think would fix the problem.

Failing that, we could have a non-empty list in face-ignored-fonts,
but that would be an inferior solution, and it would take us more time
to come up with the full list of the problematic fonts.

Thanks.