bug#29066: 26.0.90; crash in gc involving buffer local symbols

[Eli Zaretskii <eliz@gnu.org>]

> From: Valentin Gatien-Baron <vgatien-baron@janestreet.com>
> Date: Mon, 30 Oct 2017 18:04:14 -0400
> Cc: 29066@debbugs.gnu.org,
> 	Mark Shinwell <mshinwell@janestreet.com>
> 
> Yes, it fixes the problem.

Thanks.

> I also checked the following works, and seems better to me (stop having dangling pointers, instead of being
> careful with them):
> 
> diff --git a/src/alloc.c b/src/alloc.c
> index da0c3ad4b3..44dfa95cf5 100644
> --- a/src/alloc.c
> +++ b/src/alloc.c
> @@ -7030,8 +7030,10 @@ sweep_symbols (void)
>          {
>            if (!sym->s.gcmarkbit)
>              {
> -              if (sym->s.redirect == SYMBOL_LOCALIZED)
> +              if (sym->s.redirect == SYMBOL_LOCALIZED) {
>                  xfree (SYMBOL_BLV (&sym->s));
> +                sym->s.val.blv = NULL;
> +              }

That was my first attempt, but various macros like SYMBOL_BLV and
SET_SYMBOL_BLV insist on val.blv being non-NULL.  I guess you've built
Emacs without --enable-checking, so you don't see the effect of that,
but if you do, you will have assertion violations with your patch.