bug#40661: Crash in regex search during redisplay

[Richard Copley <rcopley@gmail.com>]

On Thu, 16 Apr 2020 at 17:42, Daniel Colascione <dancol@dancol.org> wrote:
>
> On April 16, 2020 9:33:16 AM PDT, Eli Zaretskii <eliz@gnu.org> wrote:
> >> Date: Thu, 16 Apr 2020 18:36:36 +0300
> >> From: Eli Zaretskii <eliz@gnu.org>
> >> Cc: 40661@debbugs.gnu.org
> >>
> >> Looks like GC sometimes kicks in while we are inside re_search_2
> >
> >Or not.  I cannot get a breakpoint inside GC to fire while we are in
> >search_buffer_re, so maybe my hypothesis was wrong.  Although the
> >symptoms are all there: when the segfault hits, the pointers passed to
> >re_search_2 are invalid, but BEGV_ADDR and GAP_END_ADDR, from which
> >they are supposed to be computed, are valid (and different).  And the
> >patch does seem to avoid the segfaults.  But maybe it's just a
> >coincidence or a side effect...
>
> Try using rr and see where those pointers came from

It seems clear from "str1=str1@entry=0xc607fd", etc., that they come
from the caller, search_buffer_re. The question is, why are they no
longer valid after updating syntax?