layout: Add a FontMetricsProvider for resolving font-relative units (#31966)

The only font relative unit that Servo knows how to resolve currently is
`rem` (relative to the root font size). This is because Stylo cannot do
any font queries. This adds a mechanism to allow this, exposing the
ability to properly render `ex` units in Servo.

This change only allows resolving some font size relative units thoug,
as Servo doesn't collect all the FontMetrics it needs to resolve them
all. This capability will be added in followup changes.

Some new tests fail:
 - ex-unit-001.html: This test fails because Servo does not yet have
   support for setting the weight using @font-face rules on web fonts.
 - ex-unit-004.html: This test fails because Servo does not yet have
   support for setting the Unicode range of a web font using @font-face
   rules.
 - first-available-font-001.html: This test fails because the above
   two feature are missing.

components/layout_thread_2020/lib.rs

@@ -42,7 +42,7 @@ use malloc_size_of::{MallocSizeOf, MallocSizeOfOps};
 use metrics::{PaintTimeMetrics, ProfilerMetadataFactory};
 use msg::constellation_msg::{BrowsingContextId, PipelineId};
 use net_traits::image_cache::{ImageCache, UsePlaceholder};
-use parking_lot::RwLock;
+use parking_lot::{ReentrantMutex, RwLock};
 use profile_traits::mem::{Report, ReportKind, ReportsChan};
 use profile_traits::path;
 use profile_traits::time::{
@@ -69,14 +69,15 @@ use style::context::{
     QuirksMode, RegisteredSpeculativePainter, RegisteredSpeculativePainters, SharedStyleContext,
 };
 use style::dom::{OpaqueNode, TElement, TNode};
-use style::driver;
 use style::error_reporting::RustLogReporter;
+use style::font_metrics::FontMetrics;
 use style::global_style_data::{GLOBAL_STYLE_DATA, STYLE_THREAD_POOL};
 use style::invalidation::element::restyle_hints::RestyleHint;
 use style::media_queries::{Device, MediaList, MediaType};
 use style::properties::style_structs::Font;
 use style::properties::PropertyId;
 use style::selector_parser::{PseudoElement, SnapshotMap};
+use style::servo::media_queries::FontMetricsProvider;
 use style::shared_lock::{SharedRwLock, SharedRwLockReadGuard, StylesheetGuards};
 use style::stylesheets::{
     DocumentStyleSheet, Origin, Stylesheet, StylesheetInDocument, UrlExtraData,
@@ -85,6 +86,8 @@ use style::stylesheets::{
 use style::stylist::Stylist;
 use style::traversal::DomTraversal;
 use style::traversal_flags::TraversalFlags;
+use style::values::computed::CSSPixelLength;
+use style::{driver, Zero};
 use style_traits::{CSSPixel, DevicePixel, SpeculativePainter};
 use url::Url;
 use webrender_api::units::LayoutPixel;
@@ -119,8 +122,9 @@ pub struct LayoutThread {
     /// Reference to the script thread image cache.
     image_cache: Arc<dyn ImageCache>,
 
-    /// Public interface to the font cache thread.
-    font_cache_thread: FontCacheThread,
+    /// Public interface to the font cache thread. This needs to be behind a [`ReentrantMutex`],
+    /// because some font cache operations can trigger others.
+    font_cache_thread: Arc<ReentrantMutex<FontCacheThread>>,
 
     /// Is this the first reflow in this LayoutThread?
     first_reflow: Cell<bool>,
@@ -236,6 +240,10 @@ impl Layout for LayoutThread {
         self.handle_request(Request::FromFontCache);
     }
 
+    fn device<'a>(&'a self) -> &'a Device {
+        self.stylist.device()
+    }
+
     fn waiting_for_web_fonts_to_load(&self) -> bool {
         self.outstanding_web_fonts.load(Ordering::SeqCst) != 0
     }
@@ -448,11 +456,13 @@ impl LayoutThread {
 
         // The device pixel ratio is incorrect (it does not have the hidpi value),
         // but it will be set correctly when the initial reflow takes place.
+        let font_cache_thread = Arc::new(ReentrantMutex::new(font_cache_thread));
         let device = Device::new(
             MediaType::screen(),
             QuirksMode::NoQuirks,
             window_size.initial_viewport,
             window_size.device_pixel_ratio,
+            Box::new(LayoutFontMetricsProvider(font_cache_thread.clone())),
         );
 
         // Ask the router to proxy IPC messages from the font cache thread to layout.
@@ -546,7 +556,7 @@ impl LayoutThread {
                 traversal_flags,
             ),
             image_cache: self.image_cache.clone(),
-            font_cache_thread: Mutex::new(self.font_cache_thread.clone()),
+            font_cache_thread: self.font_cache_thread.clone(),
             webrender_image_cache: self.webrender_image_cache.clone(),
             pending_images: Mutex::new(vec![]),
             use_rayon,
@@ -629,8 +639,10 @@ impl LayoutThread {
         // Find all font-face rules and notify the font cache of them.
         // GWTODO: Need to handle unloading web fonts.
         if stylesheet.is_effective_for_device(self.stylist.device(), &guard) {
-            let newly_loading_font_count =
-                self.font_cache_thread.add_all_web_fonts_from_stylesheet(
+            let newly_loading_font_count = self
+                .font_cache_thread
+                .lock()
+                .add_all_web_fonts_from_stylesheet(
                     &*stylesheet,
                     &guard,
                     self.stylist.device(),
@@ -1092,6 +1104,7 @@ impl LayoutThread {
             self.stylist.quirks_mode(),
             window_size_data.initial_viewport,
             window_size_data.device_pixel_ratio,
+            Box::new(LayoutFontMetricsProvider(self.font_cache_thread.clone())),
         );
 
         // Preserve any previously computed root font size.
@@ -1248,3 +1261,44 @@ impl RegisteredSpeculativePainters for RegisteredPaintersImpl {
             .map(|painter| painter as &dyn RegisteredSpeculativePainter)
     }
 }
+
+#[derive(Debug)]
+struct LayoutFontMetricsProvider(Arc<ReentrantMutex<FontCacheThread>>);
+
+impl FontMetricsProvider for LayoutFontMetricsProvider {
+    fn query_font_metrics(
+        &self,
+        _vertical: bool,
+        font: &Font,
+        base_size: CSSPixelLength,
+        _in_media_query: bool,
+        _retrieve_math_scales: bool,
+    ) -> FontMetrics {
+        layout::context::with_thread_local_font_context(&self.0, move |font_context| {
+            let Some(servo_metrics) = font_context
+                .font_group_with_size(ServoArc::new(font.clone()), base_size.into())
+                .borrow_mut()
+                .first(font_context)
+                .map(|font| font.borrow().metrics.clone())
+            else {
+                return Default::default();
+            };
+
+            // Only use the x-height of this font if it is non-zero. Some fonts return
+            // inaccurate metrics, which shouldn't be used.
+            let x_height = Some(servo_metrics.x_height)
+                .filter(|x_height| !x_height.is_zero())
+                .map(CSSPixelLength::from);
+
+            FontMetrics {
+                x_height,
+                zero_advance_measure: None,
+                cap_height: None,
+                ic_width: None,
+                ascent: servo_metrics.ascent.into(),
+                script_percent_scale_down: None,
+                script_script_percent_scale_down: None,
+            }
+        })
+    }
+}