bug#15565: 24.3.50; emacs_backtrace

[Eli Zaretskii <eliz@gnu.org>]

> Date: Tue, 8 Oct 2013 22:53:04 +0200
> From: Dani Moncayo <dmoncayo@gmail.com>
> Cc: 15565@debbugs.gnu.org
> 
> >> I've just bootstrapped the latest trunk, and it crashes very easily.
> >> For example:
> >>   emacs -Q
> >>   C-h r
> >>
> >> This is the backtrace I get:
> >
> > Doesn't happen here, sorry.
> 
> Ok, I've tried to bootstrap again the latest trunk (now at revno
> 114585), and I get the same crash with the same backtrace.

I've succeeded in reproducing this.  To get hit by the bug, you need
(a) to compile with GLYPH_DEBUG=1, and (b) use a new enough compiler
that supports a non-trivial definition of 'assume'.

The problem is that 'eassert' can no longer take expressions that have
side effects, because it evaluates them twice!

I fixed this particular assertion violation in trunk revision 114590,
but I think we had better fixed 'eassert' to not have this nasty
misfeature.

(I guess no one who works on Unix compiles with GLYPH_DEBUG=1, because
the same problem was present on X.  That's too bad, since
GLYPH_DEBUG=1 is a valuable feature for weeding out bugs sooner rather
than later.)