bug#70989: 29.3; Calling isearch-forward when using Unifont throws error "Invalid font name"

[Po Lu via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Po Lu <luangruo@yahoo.com>
>> Cc: me@rodrigomorales.site,  70989@debbugs.gnu.org
>> Date: Sun, 19 May 2024 15:37:09 +0800
>> 
>> Eli Zaretskii <eliz@gnu.org> writes:
>> 
>> > What I see there is the call to fond_intern_prop, and if the result is
>> > accepted by font_style_to_value as a valid value of FONT_WIDTH_INDEX,
>> > it is returned.  But "Sans-Serif" is not supposed to be a valid width
>> > of a font, so I'm confused.
>> 
>> Where does FONT_WIDTH_INDEX come into play?
>
> Inside get_adstyle_property:
>
>   adstyle = font_intern_prop (str, end - str, 1);
>   if (font_style_to_value (FONT_WIDTH_INDEX, adstyle, 0) >= 0)
>     return Qnil;
>   return adstyle;

This tests whether the adstyle duplicates information that the pattern
might provide as the font width, since if so, the adstyle is entirely
redundant.  You'll observe that font_style_to_value returning >= 0
indicates that the adstyle is a valid width, and therefore the function
rejects, and does not validate, width values.

>> The attribute responsible is FONT_ADSTYLE_INDEX, which is set to
>> "Sans-Serif" by:
>> 
>> 	 font. */
>>       FT_Face ft_face;
>> 
>>       ASET (entity, FONT_ADSTYLE_INDEX, get_adstyle_property (p));
>>       if ((ft_library || FT_Init_FreeType (&ft_library) == 0)
>> 
>> in ftfont_pattern_entity.  The XLFD generated from a font with such an
>> adstyle is not parsable, the character `-' being the field separator in
>> XLFDs, and gives rise to parser errors such as Rodrigo encountered when
>> I-search, by proxy, attempted to deconstruct the XLFD back into a font
>> spec.
>
> Yes, I understand all that.  That was not my question.
>
> So what is the patch you'd propose.

diff --git a/src/ftfont.c b/src/ftfont.c
index 2e37b62ea35..efb6bf668bb 100644
--- a/src/ftfont.c
+++ b/src/ftfont.c
@@ -149,7 +149,8 @@ matching_prefix (char const *str, ptrdiff_t len, char const *pat)
 get_adstyle_property (FcPattern *p)
 {
   FcChar8 *fcstr;
-  char *str, *end;
+  char *str, *end, *tmp;
+  size_t i;
   Lisp_Object adstyle;
 
 #ifdef FC_FONTFORMAT
@@ -168,7 +169,18 @@ get_adstyle_property (FcPattern *p)
       || matching_prefix (str, end - str, "Oblique")
       || matching_prefix (str, end - str, "Italic"))
     return Qnil;
-  adstyle = font_intern_prop (str, end - str, 1);
+  /* The characters `-', `?', `*', and `"' are not representable in XLFDs
+     and therefore must be replaced by substitutes.  (bug#70989) */
+  USE_SAFE_ALLOCA;
+  tmp = SAFE_ALLOCA (end - str);
+  for (i = 0; i < end - str; ++i)
+    tmp[i] = (str[i] == '-'
+	      ? '_' : ((str[i] != '?'
+			&& str[i] != '*'
+			&& str[i] != '"')
+		       ? str[i] : ' '));
+  adstyle = font_intern_prop (tmp, end - str, 1);
+  SAFE_FREE ();
   if (font_style_to_value (FONT_WIDTH_INDEX, adstyle, 0) >= 0)
     return Qnil;
   return adstyle;