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

[Pip Cet]

On Fri, May 22, 2020 at 12:48 PM Eli Zaretskii <eliz@gnu.org> wrote:
> > From: Pip Cet <pipcet@gmail.com>
> > Date: Fri, 22 May 2020 12:39:27 +0000
> > Cc: Stefan Monnier <monnier@iro.umontreal.ca>, 41321@debbugs.gnu.org
> >
> > > Sorry, I don't follow.  "xtype" shows the type of the last result,
> > > AFAIK, in this case the type of $29.  If this changed somehow, either
> > > we have a bug in .gdbinit or I have been using GDB incorrectly for I
> > > don't know how many years.
> >
> > I think it's most likely to be a GDB bug, and I can't reproduce it here.
> >
> > But it's definitely trying to access memory at address 0x18ac04f8,
> > which corresponds to start_marker.
>
> My interpretation of that equality was that both start_marker and the
> buffer's overlay chain git invalidated because some code relocated
> objects and unmapped the previously referenced memory, perhaps due to
> GC.  I don't yet have an explanation for how this could happen, so
> maybe this hypothesis is wrong.

I think it has to be, because the error message would then read
"Cannot access memory at address 0x170cb040", which is the only
address xvectype is supposed to look at.

> But I very much doubt there's such a blatant bug in GDB: this is the
> latest GDB 9.1, and I'm using these commands from .gdbinit all the
> time.  I tend to think this is somehow part of the bug that caused the
> crash.

I'm not sure how it could be. I don't think posting the disassembled
code for `signal_before_change' can hurt, since there's no easy way
for anyone else to reproduce it.