bug#21380: 25.0.50; GTK-induced segfault when scheduling timer from window-configuration-change-hook

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sun, 30 Aug 2015 15:24:26 +0000
> From: Pip Cet <pipcet@gmail.com>
> Cc: 21380@debbugs.gnu.org
> 
>     > Further investigation indicates that
>     > window-configuration-change-hook was called in the middle of concat:
>     
>     Did you understand how this fact is related to the segfault?
>     
> 
> I _think_ I do.
> 
> 1. concat called with args[0] == Vtimer_list
> 2. concat stores result_len (=4)
> 3. concat calls make_list (4)
> 4. make_list interrupted by QUIT
> 5. see stack trace
> 6. window-configuration-change-hook modifies Vtimer_list, which now has length
> 5
> 7. control returns to concat
> 8. concat tries to write 5 elements into a 4-element list, which causes the
> segfault because `tail' is unexpectedly NULL.
> 
> Does that make sense to you?

It does, but there's one additional factor that was supposed to
prevent such problems: the first thing timer_check does is copy
Vtimer_list to a local variable; then it works on that copy.  So
whatever happens in the meantime to Vtimer_list should not have
affected concat, because concat is called on a copy.

Which part of this doesn't work, and why?