fonts: Make `FontContext` thread-safe and share it per-Layout (#32205)

This allows sharing font templates, fonts, and platform fonts across
layout threads. It's the first step toward storing web fonts in the
layout versus the shared `FontCacheThread`. Now fonts and font groups
have some locking (especially on FreeType), which will probably affect
performance. On the other hand, we measured memory usage and this saves
roughly 40 megabytes of memory when loading servo.org based on data from
the memory profiler.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Mukilan Thiyagarajan <mukilan@igalia.com>

1 files changed, 11 insertions, 16 deletions

diff --git a/components/layout/text.rs b/components/layout/text.rs
index b11a25f8dee..2875ccd0d77 100644
--- a/components/layout/text.rs
+++ b/components/layout/text.rs
@@ -70,7 +70,7 @@ impl TextRunScanner {
 
     pub fn scan_for_runs(
         &mut self,
-        font_context: &mut LayoutFontContext,
+        font_context: &LayoutFontContext,
         mut fragments: LinkedList<Fragment>,
     ) -> InlineFragments {
         debug!(
@@ -150,7 +150,7 @@ impl TextRunScanner {
     /// be adjusted.
     fn flush_clump_to_list(
         &mut self,
-        font_context: &mut LayoutFontContext,
+        font_context: &LayoutFontContext,
         out_fragments: &mut Vec<Fragment>,
         paragraph_bytes_processed: &mut usize,
         bidi_levels: Option<&[bidi::Level]>,
@@ -203,10 +203,9 @@ impl TextRunScanner {
                     .map(|l| l.into())
                     .unwrap_or_else(|| {
                         let space_width = font_group
-                            .borrow_mut()
+                            .write()
                             .find_by_codepoint(font_context, ' ')
                             .and_then(|font| {
-                                let font = font.borrow();
                                 font.glyph_index(' ')
                                     .map(|glyph_id| font.glyph_h_advance(glyph_id))
                             })
@@ -248,7 +247,7 @@ impl TextRunScanner {
                 for (byte_index, character) in text.char_indices() {
                     if !character.is_control() {
                         let font = font_group
-                            .borrow_mut()
+                            .write()
                             .find_by_codepoint(font_context, character);
 
                         let bidi_level = match bidi_levels {
@@ -367,7 +366,7 @@ impl TextRunScanner {
                 // If no font is found (including fallbacks), there's no way we can render.
                 let font = match run_info
                     .font
-                    .or_else(|| font_group.borrow_mut().first(font_context))
+                    .or_else(|| font_group.write().first(font_context))
                 {
                     Some(font) => font,
                     None => {
@@ -377,7 +376,7 @@ impl TextRunScanner {
                 };
 
                 let (run, break_at_zero) = TextRun::new(
-                    &mut font.borrow_mut(),
+                    font,
                     run_info.text,
                     &options,
                     run_info.bidi_level,
@@ -535,14 +534,12 @@ fn bounding_box_for_run_metrics(
 /// Panics if no font can be found for the given font style.
 #[inline]
 pub fn font_metrics_for_style(
-    font_context: &mut LayoutFontContext,
+    font_context: &LayoutFontContext,
     style: crate::ServoArc<FontStyleStruct>,
 ) -> FontMetrics {
     let font_group = font_context.font_group(style);
-    let font = font_group.borrow_mut().first(font_context);
-    let font = font.as_ref().unwrap().borrow();
-
-    font.metrics.clone()
+    let font = font_group.write().first(font_context);
+    font.as_ref().unwrap().metrics.clone()
 }
 
 /// Returns the line block-size needed by the given computed style and font size.
@@ -664,10 +661,8 @@ impl RunInfo {
 
     fn has_font(&self, font: &Option<FontRef>) -> bool {
         fn identifier_and_pt_size(font: &Option<FontRef>) -> Option<(FontIdentifier, Au)> {
-            font.as_ref().map(|font| {
-                let font = font.borrow();
-                (font.identifier().clone(), font.descriptor.pt_size)
-            })
+            font.as_ref()
+                .map(|font| (font.identifier().clone(), font.descriptor.pt_size))
         }
 
         identifier_and_pt_size(&self.font) == identifier_and_pt_size(font)