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

[Eli Zaretskii]

> 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.

>   (gdb) p rvoe_arg.location
>   $35 = (Lisp_Object *) 0x15c9298 <globals+120>
>   (gdb) xtype
>   Lisp_Vectorlike
>   Cannot access memory at address 0x18ac04f8
>   (gdb) p rvoe_arg.errorp
>   $36 = false
> 
> Surely rvoe_arg.location isn't a vectorlike, so that also points to
> GDB not dealing with things correctly.

rvoe_arg.location should be a pointer to the value of
before-change-functions, so yes, it isn't supposed to be vectorlike.
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.