bug#47094: 27.1; emacs dies with XBM display

[Eli Zaretskii]

> Date: Fri, 12 Mar 2021 22:08:10 +0000
> From: Alan Third <alan@idiocy.org>
> 
> > > (let* ((w 256)
> > >        (h 256)
> > >        (s (make-string (* (/ w 8) h) #x55)))
> > >   (insert-image (create-image s 'xbm t :width w :height h)))
> > 
> > I can reproduce in Emacs 27, but not on the current master branch.  So
> > I guess this has been fixed already, and I'm closing the bug.
> 
> I can reproduce on the master branch. It looks like a stack overflow
> in xbm_read_bitmap_data.

I did indeed get an infinite recursion on the emacs-27 branch, but not
on master.

> I tried it in a debugger, but the backtrace looks truncated.

Before or after SIGSEGV?  If it's after, then it isn't surprising you
get a truncated backtrace.

I think if you see the infinite recursion we should understand why it
happens in the first place, and try to prevent it.