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: Thu, 29 Mar 2018 18:14:11 +0200
> 
> > 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 I understand it, we should use fontconfig for finding fonts,
> harfbuzz for doing the layout, and cairo to actually render the
> result

Does harfbuzz require Cairo?  If it does, that's unfortunate, because
the Cairo rendering option currently has a few known and annoying
redisplay bugs, which no one seems to be willing/capable of fixing.

> plus itʼs Emacs redisplay, which Iʼm told is scary

Don't believe the rumors too much.  Besides, adding a font backend
doesn't require to mess with the display code in any way, all you need
is implement the interfaces you see documented in 'struct font_driver'
defined in font.h (reusing the methods of existing backends where
appropriate, as xftfont does with ftfont methods); all the rest is
already taken care of in the infrastructure.  The only interface
between font back-end methods and the display engine is via 'struct
glyph_string', which is a relatvely simple data structure.

> I do note we have an Xft font backend, a freetype one, and a
> freetype+cairo one already, which seems excessive.

You forgot xfont and freetype-without-XFT.  It could be that we could
remove some of them, but I don't know which ones are used and how
much.  And freetype+cairo is not used much because of the Cairo
problems.

> > As a stopgap, I think we should find a way of ignoring the problematic
> > fonts.  Is there some way of detecting them?
> 
> You mean short of running ftfont_get_bitmap over every available
> codepoint in the font and skipping it if the resulting width * height
> is > 4096 [1]?  That would probably detect a problematic font pretty
> quickly, but is not very elegant. Also Iʼm not sure we get to
> intervene before Xft decides that it needs to fall back to that font.

AFAIK, there's no "fallback" per se.  Whenever the already-loaded
fonts don't support a character, Emacs looks for the fonts that do
using the "match" method.  If we always fail these fonts in that
method, they will never be used.

> > 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.
> 
> Iʼm hoping that matching on something like !'family emoji' would work,
> although Iʼve not figured out how to express that in fontconfig-speak.

I thought there could be a way of detecting those "color bitmap fonts"
by examining their attributes in ftfont_match and/or
ftfont_spec_pattern.  Then we could return a failure indication for
them.