Seperate freetype face and named instance indices (#39213)

In servo, each `LocalFontIdentifier` has an `fn index() -> u32`, which
returns the index of the font within the font file in case it is a font
collection (`.ttc` instead of `.ttf`). The way this index is obtained is
platform-dependent.

On systems using `fontconfig`, we get the index by querying `FC_INDEX`:


d2c78db981/components/fonts/platform/freetype/font_list.rs (L109-L112)

There is a sneaky bug here: In addition to the aforementioned face
index, the value for `FC_INDEX` contains the index of the named instance
of the face in the upper 16 bits. This behaviour is completely
undocumented, but FreeType uses the same format:
https://freetype.org/freetype2/docs/reference/ft2-face_creation.html#ft_open_face.

This wasn't a problem for the longest time, because the only consumer of
the `index` value coming from fontconfig was FreeType. However, sometime
after
29e618dcf7,
we started also passing it to`read-fonts` . `read-fonts` expects only
the face index, causing it to return errors as seen in
https://github.com/servo/servo/issues/39209.

The fix is to seperate the two indices when storing them in a
`LocalFontDescriptor` and pass only the lower 16 bits to `read-fonts`.
I'm unsure whether we should continue passing the named instance index
to FreeType since we don't actually support variable fonts, but I've
opted to keep the current behaviour for now.

Fixes: https://github.com/servo/servo/issues/39209

Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>

components/fonts/platform/freetype/android/font_list.rs

@@ -432,7 +432,8 @@ where
     let mut produce_font = |font: &Font| {
         let local_font_identifier = LocalFontIdentifier {
             path: Atom::from(FontList::font_absolute_path(&font.filename)),
-            variation_index: 0,
+            face_index: 0,
+            named_instance_index: 0,
         };
         let stretch = StyleFontStretch::NORMAL;
         let weight = font

components/fonts/platform/freetype/font.rs

@@ -96,7 +96,11 @@ impl PlatformFontMethods for PlatformFont {
         let library = FreeTypeLibraryHandle::get().lock();
         let filename = CString::new(&*font_identifier.path).expect("filename contains NUL byte!");
 
-        let face = FreeTypeFace::new_from_file(&library, &filename, font_identifier.index())?;
+        let face = FreeTypeFace::new_from_file(
+            &library,
+            &filename,
+            font_identifier.face_index_for_freetype(),
+        )?;
 
         let normalized_variations = face.set_variations_for_font(variations, &library)?;
 

components/fonts/platform/freetype/font_list.rs

@@ -123,7 +123,8 @@ where
 
             let local_font_identifier = LocalFontIdentifier {
                 path: Atom::from(c_str_to_string(path as *const c_char)),
-                variation_index: index as i32,
+                face_index: (index & 0xFFFF) as u16,
+                named_instance_index: (index >> 16) as u16,
             };
             let descriptor = FontTemplateDescriptor::new(weight, stretch, style);
 

components/fonts/platform/freetype/ohos/font_list.rs

@@ -444,7 +444,8 @@ where
     let mut produce_font = |font: &Font| {
         let local_font_identifier = LocalFontIdentifier {
             path: Atom::from(font.filepath.clone()),
-            variation_index: 0,
+            face_index: 0,
+            named_instance_index: 0,
         };
         let stretch = font.width.into();
         let weight = font

components/fonts/tests/font_context.rs

@@ -186,7 +186,8 @@ mod font_context {
 
             let local_font_identifier = LocalFontIdentifier {
                 path: path.to_str().expect("Could not load test font").into(),
-                variation_index: 0,
+                face_index: 0,
+                named_instance_index: 0,
             };
             let handle = PlatformFont::new_from_local_font_identifier(
                 local_font_identifier.clone(),

components/shared/fonts/font_identifier.rs

@@ -41,18 +41,26 @@ mod platform {
         /// The path to the font.
         pub path: Atom,
         /// The variation index within the font.
-        pub variation_index: i32,
+        pub face_index: u16,
+        /// The index of the named instance within the font.
+        ///
+        /// For non-variable fonts, this is ignored.
+        pub named_instance_index: u16,
     }
 
     impl LocalFontIdentifier {
         pub fn index(&self) -> u32 {
-            self.variation_index.try_into().unwrap()
+            self.face_index as u32
+        }
+
+        pub fn named_instance_index(&self) -> u32 {
+            self.named_instance_index as u32
         }
 
         pub fn native_font_handle(&self) -> NativeFontHandle {
             NativeFontHandle {
                 path: PathBuf::from(&*self.path),
-                index: self.variation_index as u32,
+                index: self.face_index as u32,
             }
         }
 
@@ -64,9 +72,20 @@ mod platform {
 
             Some(FontDataAndIndex {
                 data,
-                index: self.variation_index as u32,
+                index: self.face_index as u32,
             })
         }
+
+        /// Fontconfig and FreeType use a packed format to represent face and
+        /// named instance indexes in a single integer. The first 16 bits make
+        /// up the named instance index and the second 16 bits make up the
+        /// face index.
+        ///
+        /// See <https://freetype.org/freetype2/docs/reference/ft2-face_creation.html#ft_open_face>
+        /// for more information.
+        pub fn face_index_for_freetype(&self) -> u32 {
+            ((self.named_instance_index()) << 16) | self.index()
+        }
     }
 }