bug#49797: 28.0.50; Setting face to custom fontset doesn't work

[Yuan Fu]

> On Oct 6, 2021, at 11:33 AM, Eli Zaretskii <eliz@gnu.org> wrote:
> 
>> From: Yuan Fu <casouri@gmail.com>
>> Date: Wed, 6 Oct 2021 11:11:42 -0700
>> Cc: Kenichi Handa <handa@gnu.org>,
>> 49797@debbugs.gnu.org
>> 
>> Later in that function, we take the fontset of that face
>> 
>>        fontset = FONTSET_FROM_ID (face->fontset);
>> 
>> Then find an appropriate font from it
>> 
>>        rfont_def = fontset_font (fontset, c, face, id);
> 
> Yes, AFAIU that's the part where we check if the current face's
> fontset's font, made for the ASCII characters, happens to have a glyph
> for the non-ASCII character we need to display.

That is not, the part you mentioned is before:

      if (face->ascii_face->font)
        {
          XSETFONT (font_object, face->ascii_face->font);
          if (font_has_char (f, font_object, c))
            return face->ascii_face->id;
        }

And what about the comment I mentioned? Did you read it?

In any case, 

    rfont_def = fontset_font (fontset, c, face, id); 

finds a font in the fontset that can display character c. That’s hard to ignore.

> 
>> So Emacs definitely is designed to support using fontsets to assign 
>> different fonts for different characters to faces. In fact, you can try this 
>> right now (without my patch):
>> 
>>        (set-face-attribute 'variable-pitch :fontset "fontset-serif”)
>> 
>> (Notice I used the :fontset attribute, not :font attribute.) And the 
>> variable-pitch face will work as intended, use CJK font for CJK characters 
>> and Latin font for Latin characters, specified in “fontset-serif”. The only 
>> problem is 1) :fontset attribute is not documented and 2) this doesn’t work 
>> with ‘default face.
> 
> As mentioned earlier, user code is not supposed to set the :fontset
> attribute directly, which is why it isn't documented.

Finternal_set_lisp_face_attribute has code that let me set :fontset. Why is it 
there if Lisp is not supposed to set :fontset? Plus, Emacs doesn’t set :fontset 
by itself, what’s the point for this attribute, then, if Emacs doesn’t set it 
itself and don’t want the user to set it?

> 
>>> Maybe we can extend the design to support "face-specific" fontsets,
>>> but I'm quite sure that will need changes in font.c and fontset.c as
>>> well, because the current design is implicitly assumed there.
>> 
>> Emacs already has an elaborate implementation to support fontsets
> 
> Not fontsets specific to faces, according to my reading of the code,
> as I tried to explain.
> 
>> it is just hindered by the manual and bugs in the Lisp interface.
> 
> Or maybe by design.
> 
>>> That's one way of interpreting what the manual says, but it is not the
>>> only one.  If you look at what the code does, you will arrive at
>>> another interpretation: Emacs allows you to specify a fontset as the
>>> value for the :font attribute, but what it does in that case is take
>>> from the fontset the font for ASCII characters, and then use it as if
>>> you specified that font, not a fontset.  IOW, the fontset in that case
>>> is just used as a method of specifying the ASCII font.
>> 
>> I agree, another way is to document the :fontset attribute, document that 
>> passing a fontset to :font attribute only sets the ASCII font, and fix the 
>> bug where setting :fontset attribute for ‘default face doesn’t work.
> 
> I'm saying that fixing this will probably need more extensive changes
> than in your proposed patch.

I don’t think so. Anyway, you don’t seem convinced albeit clear evidences in my 
eye. Let’s wait for Handa-san to enlighten us.

Yuan