bug#25945: Emacs aborts while calling FT_Load_Glyph

[Eli Zaretskii <eliz@gnu.org>]

Ping!

> Date: Fri, 03 Mar 2017 10:08:42 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 25945@debbugs.gnu.org
> 
> > Date: Fri, 03 Mar 2017 08:04:29 +0100 (CET)
> > From: Werner LEMBERG <wl@gnu.org>
> > 
> > For me, Emacs aborts at ftfont.c:1550 while `mew' tries to display an
> > e-mail.
> > 
> >   1549  if (FT_Load_Glyph (ft_face, g->g.code, FT_LOAD_DEFAULT) != 0)
> >   1550    emacs_abort ();
> > 
> > Examining `ft_face' and `g->g.code' I see that the font in question is
> > `Padauk Book Bold' (PadaukBook-Bold.ttf), glyph 376.  Examining this
> > font further with `ftview' I see that bytecode of this font is broken,
> > and that the font can only be displayed successfully without bytecode.
> > [This is version 3.002 of the font, taken from the current TeXLive
> > repository.]
> > 
> > I think there is no reason that Emacs aborts for such broken fonts.
> > Instead I suggest that (a) Emacs tries to load the glyph again without
> > hinting, and (b) if that fails, it should display a missing glyph,
> > using the standard rectangle with hex digits in it.
> > 
> > New code for (a) is quite simple:
> > 
> >   if (FT_Load_Glyph (ft_face, g->g.code, FT_LOAD_DEFAULT) != 0)
> >     if (FT_Load_Glyph (ft_face, g->g.code, FT_LOAD_NO_HINTING) != 0)
> >       ...
> 
> This should probably be accompanied by a suitable FONT_ADD_LOG call,
> to mention that this fallback was taken.
> 
> > [The code might be further improved by implementing a per-font glyph
> >  load mode, to be initialized with `FT_LOAD_DEFAULT'.  If, say, 100
> >  calls using `FT_LOAD_DEFAULT' for a given font fails, and
> >  `FT_LOAD_NO_HINTING' is successful, the default loading behaviour for
> >  this font could be switched to `FT_LOAD_NO_HINTING'.  Given that such
> >  broken fonts are rare my suggestion is probably overkill, however.]
> > 
> > My knowledge of Emacs internals is too small to provide an
> > implementation for (b).
> 
> I think it's too late for (b) when we discover this problem in
> ftfont_get_metrics.  To do (b) we should have discovered this in
> ftfont_has_char, or thereabouts.
> 
> I'm CC'ing Handa-san in the hope that he will have insights and
> comments on this.
> 
> Thanks.