bug#61521: "default" is now the first item returned from (font-faces), breaking various code.

[Eli Zaretskii <eliz@gnu.org>]

> Date: Wed, 15 Feb 2023 11:19:54 -0500
> Cc: 61521@debbugs.gnu.org
> From: Brennan Vincent <brennan@umanwizard.com>
> 
> > Good question.  AFAICT, the sorting was added when we switched from
> > storing faces in alists to storing them in hash tables.  It probably
> > sorted faces to be more compatible with what face-list returned before
> > the switch to hash table.  So I suspect the order we have now is
> > simply a bug, and we do need to change the order of sorting to get
> > back the original order.
> 
> I tend to agree. Sorry for not explaining this reasoning more fully in my
> original message.
> 
> Here's what I suspect happened (not 100% sure, it's just a theory):
> 
> (1) Initially set of faces was stored as a list, so it was naturally maintained
> in the inverse order that things were added to it (thus default would be at the
> end).
> 
> (2) Now faces are stored in a hash table whose key is the face and whose value
> contains various pieces of data, including the face ID.
> 
> (3) This face ID is allocated in increasing order (see e.g. this code in xfaces.c:
>       Lisp_Object face_id = make_fixnum (next_lface_id);
>       lface_id_to_name[next_lface_id] = face;
>       Fput (face, Qface, face_id);
>       ++next_lface_id;
> 
> (4) Thus, `face-list` and `frame-face-alist` sorted the faces by face ID in
> order to maintain the old ordering behavior. However, the author accidentally
> inverted the comparison when doing so.
> 
> 
> > Gregory, any counter-arguments?

No further comments, so I've now installed the proposed changes on the
emacs-29 branch, and I'm boldly closing this bug as done.