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

[Pip Cet <pipcet@gmail.com>]

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On Sun, Aug 30, 2015 at 3:01 PM, Eli Zaretskii <eliz@gnu.org> wrote:

> > Date: Sun, 30 Aug 2015 12:51:26 +0000
> > From: Pip Cet <pipcet@gmail.com>
> > Somehow, the argument to Fcopy_sequence was changed while concat was
> > underway.
>
> How do you see that?
>

I originally concluded it was the only way to trigger the bug, but I just
managed to trigger it again and have it open in a GDB session:

#1  0x00000000005efdb3 in concat (nargs=1, args=0x7fffffff76e8,
target_type=Lisp_Cons, last_special=false) at fns.c:747
747                     XSETCAR (tail, elt);
(gdb) p result_len
$22 = 4
(gdb) p debug_print(Flength(args[0]))
5
$23 = void
(gdb)


> > 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?

Thanks,
Pip

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