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, 12 insertions, 31 deletions

diff --git a/components/canvas/canvas_data.rs b/components/canvas/canvas_data.rs
index 5bb4640dfd1..0175242521e 100644
--- a/components/canvas/canvas_data.rs
+++ b/components/canvas/canvas_data.rs
@@ -2,9 +2,8 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
-use std::cell::RefCell;
 use std::mem;
-use std::sync::{Arc, Mutex};
+use std::sync::Arc;
 
 use canvas_traits::canvas::*;
 use euclid::default::{Point2D, Rect, Size2D, Transform2D, Vector2D};
@@ -359,21 +358,6 @@ pub enum Filter {
     Nearest,
 }
 
-pub(crate) type CanvasFontContext = FontContext<FontCacheThread>;
-
-thread_local!(static FONT_CONTEXT: RefCell<Option<CanvasFontContext>> = RefCell::new(None));
-
-pub(crate) fn with_thread_local_font_context<F, R>(canvas_data: &CanvasData, f: F) -> R
-where
-    F: FnOnce(&mut CanvasFontContext) -> R,
-{
-    FONT_CONTEXT.with(|font_context| {
-        f(font_context.borrow_mut().get_or_insert_with(|| {
-            FontContext::new(canvas_data.font_cache_thread.lock().unwrap().clone())
-        }))
-    })
-}
-
 pub struct CanvasData<'a> {
     backend: Box<dyn Backend>,
     drawtarget: Box<dyn GenericDrawTarget>,
@@ -386,7 +370,7 @@ pub struct CanvasData<'a> {
     old_image_key: Option<ImageKey>,
     /// An old webrender image key that can be deleted when the current epoch ends.
     very_old_image_key: Option<ImageKey>,
-    font_cache_thread: Mutex<FontCacheThread>,
+    font_context: Arc<FontContext<FontCacheThread>>,
 }
 
 fn create_backend() -> Box<dyn Backend> {
@@ -398,7 +382,7 @@ impl<'a> CanvasData<'a> {
         size: Size2D<u64>,
         webrender_api: Box<dyn WebrenderApi>,
         antialias: AntialiasMode,
-        font_cache_thread: FontCacheThread,
+        font_context: Arc<FontContext<FontCacheThread>>,
     ) -> CanvasData<'a> {
         let backend = create_backend();
         let draw_target = backend.create_drawtarget(size);
@@ -412,7 +396,7 @@ impl<'a> CanvasData<'a> {
             image_key: None,
             old_image_key: None,
             very_old_image_key: None,
-            font_cache_thread: Mutex::new(font_cache_thread),
+            font_context,
         }
     }
 
@@ -494,17 +478,14 @@ impl<'a> CanvasData<'a> {
         let font = font_style.map_or_else(
             || load_system_font_from_style(None),
             |style| {
-                with_thread_local_font_context(self, |font_context| {
-                    let font_group = font_context.font_group(ServoArc::new(style.clone()));
-                    let font = font_group
-                        .borrow_mut()
-                        .first(font_context)
-                        .expect("couldn't find font");
-                    let font = font.borrow_mut();
-                    Font::from_bytes(font.template.data(), 0)
-                        .ok()
-                        .or_else(|| load_system_font_from_style(Some(style)))
-                })
+                let font_group = self.font_context.font_group(ServoArc::new(style.clone()));
+                let font = font_group
+                    .write()
+                    .first(&self.font_context)
+                    .expect("couldn't find font");
+                Font::from_bytes(font.template.data(), 0)
+                    .ok()
+                    .or_else(|| load_system_font_from_style(Some(style)))
             },
         );
         let font = match font {