bug#23259: 25.1.50; Xerror: "BadLength" on extreme zoom causing emacs to segfault

[Eli Zaretskii <eliz@gnu.org>]

> From: Matthew Leach <matthew@mattleach.net>
> Date: Sun, 10 Apr 2016 11:49:12 +0100
> 
> emacs -Q
> open a file, emacs/src/process.c, say.
> C-x C-=
> and keep pressing
> C-=
> 
> Emacs will grind to a halt a little but will eventually segfault when
> the zoom is extreme enough.
> 
> Attached is the backtrace.  It looks as though X has thrown an error,
> maybe we need to limit the amount of zoom that Emacs allows?

Probably.  However, the interesting question is: what values to use as
limits?

Also, this backtrace is not useful, since the X errors are by default
asynchronous.  Could you please run Emacs in synchronous X mode (see
etc/DEBUG for how), and present the backtrace then?  Also, please
tell:

  . what is the maximum value of the scale factor you see on the mode
    line before Emacs crashes

  . what is the value returned by display-pixel-width

  . what is the values returned by frame-char-width and
    frame-char-height

I'll try to prepare a patch using the above data.

Thanks.