bug#41321: 27.0.91; Emacs aborts due to invalid pseudovector objects

[Eli Zaretskii]

> Cc: pipcet@gmail.com, 41321@debbugs.gnu.org, monnier@iro.umontreal.ca
> From: Paul Eggert <eggert@cs.ucla.edu>
> Date: Fri, 29 May 2020 13:24:55 -0700
> 
> There are really two bugs here.
> 
> 1. The idea of taking the address modulo LISP_ALIGNMENT is wrong, as a pointer
> can point into the middle of (say) a pseudovector and not be
> LISP_ALIGNMENT-aligned. Replacing LISP_ALIGNMENT with GCALIGNMENT does not fix
> this bug in general, because such a pointer might not be GCALIGNMENT-aligned
> either. This bug can cause crashes because it causes GC to think an object is
> garbage when it's not garbage.
> 
> 2. LISP_ALIGNMENT is too large on MinGW and some other platforms.
> 
> The patch I sent earlier attempted to be the simplest patch that would fix the
> bug you observed on MinGW, which is a special case of (1). It does not attempt
> to fix all plausible cases of (1), nor does it address (2).
> 
> We can fix these two bugs separately, by installing the attached patches into
> emacs-27. The first patch fixes (1) and thus fixes the crash along with other
> plausible crashes. The second one fixes (2), and this fixes the MinGW crash 
> in a
> different way but does not fix the crash on other plausible platforms. (1)
> probably has better performance than (2), though I doubt whether users will 
> notice.

Since (1) is for now purely theoretical (and rare even in that
theoretical case), I'd like to see (2) applied to emacs-27.  Let's do
that soon, as I'd like to have another pretest in the near future.

Thanks.