bug#23386: Segfault when messing with font-backend

[Stefan Monnier]

>> % emacs -Q --eval "(push '(font-backend x) default-frame-alist)"
>> I get a segfault.  This is with the latest emacs-25.
> Not reproducible here, but then I cannot run an X version.  (I did try
> an equivalent for Windows; it didn't crash.)

Not surprised: it seems specific to the X11 code.

>> Program received signal SIGSEGV, Segmentation fault.
>> 0xf70b3b36 in XftCharIndex () from /usr/lib/i386-linux-gnu/libXft.so.2
>> (gdb) xbacktrace
>> (gdb) bt
>> #0  0xf70b3b36 in XftCharIndex () from /usr/lib/i386-linux-gnu/libXft.so.2
>> #1  0x082d4d69 in xftfont_encode_char (font=0x8779b00, c=92) at xftfont.c:537
> Can you tell which arguments to XftCharIndex were invalid in this
> case?

I guess it's the second arg (xftfont_info->xftfont), which is NULL.

    Program received signal SIGSEGV, Segmentation fault.
    0xf70b3b36 in XftCharIndex () from /usr/lib/i386-linux-gnu/libXft.so.2
    (gdb) up
    #1  0x082d4d69 in xftfont_encode_char (font=0x8779af0, c=92) at 
xftfont.c:537
    (gdb) p xftfont_info->display
    $1 = (Display *) 0x86f2368
    (gdb) p xftfont_info->xftfont
    $2 = (XftFont *) 0x0
    (gdb) p c
    $3 = 92
    (gdb)

> Also, is 'x' a valid font-backend symbol?

AFAIK yes.

> It's strange that Emacs uses xftfont when it should have been using
> xfont instead (AFAIU).

Indeed, that seems to be a big part of the problem.


        Stefan