bug#63204: 29.0.90; vc-annotate does not properly test whether a face exits

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 63204@debbugs.gnu.org
> From: Philip Kaludercic <philipk@posteo.net>
> Date: Mon, 01 May 2023 18:32:49 +0000
> 
> Tobias Bading <tbading@web.de> writes:
> 
> > 1. emacs -Q
> >
> > 2. Imagine that in a previous Emacs session you have copied a string with
> >    text properties from a buffer in vc-annotate-mode, maybe also searched
> >    for it somewhere. IOW, your ~/.emacs.d/.emacs.desktop file may contain
> >    something like this:
> >
> >    (setq some-search-ring-or-whatever
> >          '(#("heisenbug" 0 9 (face vc-annotate-face-CCCCFF))))
> >
> >    Evaluate this expression to simulate a desktop-read of such a previous
> >    session.
> >
> > 3. Open lisp/vc/vc-annotate.el from the Git branch emacs-29.
> >
> > 4. C-x v g (vc-annotate)
> >
> > 5. There are lines with white background caused by the use of undefined face
> >    vc-annotate-face-CCCCFF. C-h e should contain:
> >
> >    Invalid face reference: vc-annotate-face-CCCCFF
> >
> >    If for whatever reason you don’t see these bad lines, use C-u C-x = to
> >    see what face a line uses and repeat these steps with this face instead
> >    of vc-annotate-face-CCCCFF.
> >
> >
> > This can be fixed with:
> >
> > diff --git a/lisp/vc/vc-annotate.el b/lisp/vc/vc-annotate.el
> > index 70057a6aac..ae8db021ad 100644
> > --- a/lisp/vc/vc-annotate.el
> > +++ b/lisp/vc/vc-annotate.el
> > @@ -724,7 +724,9 @@ vc-annotate-lines
> >                                        (substring (cdr color) 1)
> >                                      (cdr color))))
> >                 ;; Make the face if not done.
> > -               (face (or (intern-soft face-name)
> > +               (face (or (let ((sym (intern-soft face-name)))
> > +                           (if (facep sym)
> > +                               sym))
> 
> I'd write this as (and (facep sym) sym).
> 
> >                           (let ((tmp-face (make-face (intern face-name))))
> >                             (set-face-extend tmp-face t)
> >                             (cond
> >
> > My Emacs Lisp isn’t that great so please feel free to improve this code
> > if needed. The important part is that (intern-soft face-name) is not
> > sufficient to check whether a face exists.
> 
> I think a better solution would be to revise general approach
> vc-annotate takes, and instead of hard-coding faces/colours to use (that
> break when switching between light and dark themes), to have a list of
> faces that could be used independently of vc-annotate.

We should probably do that on master, but for emacs-29 this is too
much of a churn, I think.  So for emacs-29, I think we should install
something like the proposed patch, just a bit cleaned-up.