bug#55684: 29.0.50; wrong-type-argument in minibuffer-only frames

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sat, 28 May 2022 10:51:25 +0000
> Cc: Stefan Monnier <monnier@iro.umontreal.ca>,
>   martin rudalics <rudalics@gmx.at>, 55684@debbugs.gnu.org, acm@muc.de
> From: Alan Mackenzie <acm@muc.de>
> 
> Hello, Eli.
> 
> On Sat, May 28, 2022 at 08:56:04 +0300, Eli Zaretskii wrote:
> > > Cc: Alan Mackenzie <acm@muc.de>
> > > Date: Fri, 27 May 2022 16:09:13 -0400
> > > From:  Stefan Monnier via "Bug reports for GNU Emacs,
> > >  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> > > Package: Emacs
> > > Version: 29.0.50
> 
> > Doesn't the same problem exist on the emacs-28 branch?  I can
> > reproduce it there.
> 
> > > -    Fset_frame_selected_window (frame, call1 (Qget_mru_window, frame), Qnil);
> > > +    {
> > > +      Lisp_Object w = call1 (Qget_mru_window, frame);
> > > +      if (!NILP (w))            /* Can be nil in minibuffer-only frames.  */
> > > +        Fset_frame_selected_window (frame, w, Qnil);
> 
> > The NILP(w) test should probably be WINDOW_LIVE_P(w).  Martin, do you
> > agree?
> 
> I'm not Martin, but I think WINDOW_LIVE is safe, but might not be
> needed.  I think get-mru-window will only return a live window or nil.

It starts with a list of live windows, that's true, but who will
guarantee that while it's processing some window cannot be deleted?
WINDOW_LIVE_P is not expensive enough to make such assumptions.

In any case, even if we are sure a window returned by get-mru-window
I'd prefer to use WINDOWP instead of NILP here.