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

[Pip Cet]

On Fri, May 29, 2020 at 1:53 PM Eli Zaretskii <eliz@gnu.org> wrote:
> > From: Pip Cet <pipcet@gmail.com>
> > Date: Fri, 29 May 2020 11:47:46 +0000
> > Cc: eggert@cs.ucla.edu, Stefan Monnier <monnier@iro.umontreal.ca>, 
> > 41321@debbugs.gnu.org
> >
> > > There's no bug:
> >
> > I believe there is.
> >
> > > the $size variable was not updated inside pvectype
> > > because the 'set' command tried to access invalid memory.
> >
> > Why would pvectype be called at all? xtype should have said
> > "Lisp_Symbol", not "Lisp_Vectorlike", and never gotten to pvectype at
> > all.
>
> Look at what xtype does, and you will see.

So you think it's a bug in xtype?

The relevant definitions are:

define xtype
  xgettype $
  output $type
  echo \n
  if $type == Lisp_Vectorlike
    xvectype
  end
end

define xgettype
  if (CHECK_LISP_OBJECT_TYPE)
    set $bugfix = $arg0.i
  else
    set $bugfix = $arg0
  end
  set $type = (enum Lisp_Type) (USE_LSB_TAG ? (EMACS_INT) $bugfix & (1
<< GCTYPEBITS) - 1 : (EMACS_UINT) $bugfix >> VALBITS)
end

Both look fine to me: xtype calls xgettype (not xvectype), which sets
$type to the type bits, then outputs them. But the bug must have
happened by then, because what's output is "Lisp_Vectorlike" even
though $ is a Lisp_Symbol. I fail to see how xvectype and pvectype are
relevant at all...