Re: MPS: face-cache

[Eli Zaretskii <eliz@gnu.org>]

> From: Gerd Möllmann <gerd.moellmann@gmail.com>
> Cc: eller.helmut@gmail.com,  emacs-devel@gnu.org
> Date: Sun, 28 Apr 2024 10:05:03 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > I looked, but couldn't find it.  All I see is that the driver's
> > _type_ (a symbol) is used in font objects.  Otherwise, what I see is
> > that we invoke driver's methods, like 'list', 'match', and 'has_char',
> > but do not use the driver struct itself in dealing with fonts.
> 
> Eglot/clangd shows me it's used in font.c, for example
> 
> Lisp_Object
> font_list_entities (struct frame *f, Lisp_Object spec)
> {
>   struct font_driver_list *driver_list = f->font_driver_list;

That function calls the 'list' method of each of the registered font
drivers to produce fonts that match a given spec:

  for (; driver_list; driver_list = driver_list->next)
    if (driver_list->on
	&& (NILP (ftype) || EQ (driver_list->driver->type, ftype)))
      {
	Lisp_Object cache = font_get_cache (f, driver_list->driver);

	ASET (scratch_font_spec, FONT_TYPE_INDEX, driver_list->driver->type);
	val = assoc_no_quit (scratch_font_spec, XCDR (cache));
	if (CONSP (val))
	  val = XCDR (val);
	else
	  {
	    Lisp_Object copy;

	    val = (driver_list->driver->list) (f, scratch_font_spec);
	    /* We put zero_vector in the font-cache to indicate that
	       no fonts matching SPEC were found on the system.
	       Failure to have this indication in the font cache can
	       cause severe performance degradation in some rare
	       cases, see bug#21028.  */
	    if (NILP (val))
	      val = zero_vector;
	    else
	      val = Fvconcat (1, &val);
	    copy = copy_font_spec (scratch_font_spec);
	    ASET (copy, FONT_TYPE_INDEX, driver_list->driver->type);
	    XSETCDR (cache, Fcons (Fcons (copy, val), XCDR (cache)));
	  }

Then it filters the fonts according to some criteria.

So my reading of this is that the 'list' method can create font
entities, but the list of drivers itself is not used for the font
object, only the driver's 'type' (as symbol) is inserted into the
FONT_TYPE_INDEX slot of the font entities.  IOW, the font objects come
from calls to font driver methods, not from the list of drivers.