bug#18573: 24.3.93; set-face-attribute crashes Emacs when started with -nw

[Eli Zaretskii <eliz@gnu.org>]

> From: Jan Djärv <jan.h.d@swipnet.se>
> Date: Sun, 28 Sep 2014 10:44:15 +0200
> Cc: 18573@debbugs.gnu.org
> 
> This seems to be a generic error in xfaces.c.  It tries to load a font without checking the type
> of frame.  The type is tty, but it tries to load a font anyway, and eventually ends up in (font.c) font_pixel_size, which does:
> 
> #define FRAME_RES_Y(f)						\
>   (eassert (FRAME_WINDOW_P (f)), FRAME_DISPLAY_INFO (f)->resy)
> 
> Now, FRAME_DISPLAY_INFO for a NS compiled Emacs is
> 
> #define FRAME_DISPLAY_INFO(f) ((f)->output_data.ns->display_info)
> 
> but the frame is not an NS frame, it is a tty frame, so bad things happen.
> It is the same for X, but there it just happens to return a nonsense value, so the code continues without crashing, and eventually discovers that there are no font dirvers and the load font fails.
> 
> The code is in xfaces.c, Finternal_set_lisp_face_attribute, around line 3120 where it calls
> font_load_for_lface.
> 
> The code in question is not called if compiled for a tty (#ifdef:ed out), but it is called when the frame is a tty frame on a non-tty compiled Emacs.
> 
> I think these cases should be the same, i.e. font_load_for_lface not called for tty frames.

I believe this happens when internal-set-lisp-face-attribute is
called with its FRAME argument t, meaning change the default for new
(i.e. future) frames.  Since the code needs a frame, it just uses the
selected frame, which in this case happens to be a TTY frame.

Is that description correct?

If so, the question is how to fix this.  If we simply do nothing when
the selected frame is a TTY frame, and then create a GUI frame at some
future point, will the new default take effect?  If it will, then I
agree that the code under this condition

	      if (! FONT_OBJECT_P (value))

should not be executed when the selected frame is a TTY frame.

But if this doesn't work, then what are our alternatives?  We could
loop over all the frames looking for a GUI frame, and use that.  But
what if there's no such frame?  Signal an error?