FontContext: Cache data fetched from the cache thread

Before this change, if we needed to create a Font which we've already
created, but at a new size, then we'd fetch the FontTemplateInfo again.
If the bytes of the font are held in memory, then this could be
expensive as we need to pass those bytes over IPC.

components/gfx/font.rs

@@ -562,7 +562,7 @@ pub struct FontFamilyDescriptor {
 }
 
 impl FontFamilyDescriptor {
-    fn new(name: FontFamilyName, scope: FontSearchScope) -> FontFamilyDescriptor {
+    pub fn new(name: FontFamilyName, scope: FontSearchScope) -> FontFamilyDescriptor {
         FontFamilyDescriptor { name, scope }
     }
 

components/gfx/font_cache_thread.rs

@@ -32,7 +32,7 @@ pub struct FontTemplates {
     templates: Vec<FontTemplate>,
 }
 
-#[derive(Debug, Deserialize, Serialize)]
+#[derive(Clone, Debug, Deserialize, Serialize)]
 pub struct FontTemplateInfo {
     pub font_template: Arc<FontTemplateData>,
     pub font_key: webrender_api::FontKey,

components/gfx/font_context.rs

@@ -50,6 +50,7 @@ pub struct FontContext<S: FontSource> {
     // so they will never be released. Find out a good time to drop them.
     // See bug https://github.com/servo/servo/issues/3300
     font_cache: HashMap<FontCacheKey, Option<FontRef>>,
+    font_template_cache: HashMap<FontTemplateCacheKey, Option<FontTemplateInfo>>,
 
     font_group_cache:
         HashMap<FontGroupCacheKey, Rc<RefCell<FontGroup>>, BuildHasherDefault<FnvHasher>>,
@@ -64,6 +65,7 @@ impl<S: FontSource> FontContext<S> {
             platform_handle: handle,
             font_source,
             font_cache: HashMap::new(),
+            font_template_cache: HashMap::new(),
             font_group_cache: HashMap::with_hasher(Default::default()),
             epoch: 0,
         }
@@ -76,6 +78,7 @@ impl<S: FontSource> FontContext<S> {
         }
 
         self.font_cache.clear();
+        self.font_template_cache.clear();
         self.font_group_cache.clear();
         self.epoch = current_epoch
     }
@@ -120,18 +123,42 @@ impl<S: FontSource> FontContext<S> {
             );
 
             let font =
-                self.font_source.font_template(
+                self.font_template(&font_descriptor.template_descriptor, family_descriptor)
-                    font_descriptor.template_descriptor.clone(),
+                    .and_then(|template_info| self.create_font(template_info, font_descriptor.to_owned()).ok())
-                    family_descriptor.clone(),
+                    .map(|font| Rc::new(RefCell::new(font)));
-                )
-                .and_then(|template_info| self.create_font(template_info, font_descriptor.to_owned()).ok())
-                .map(|font| Rc::new(RefCell::new(font)));
 
             self.font_cache.insert(cache_key, font.clone());
             font
         })
     }
 
+    fn font_template(
+        &mut self,
+        template_descriptor: &FontTemplateDescriptor,
+        family_descriptor: &FontFamilyDescriptor
+    ) -> Option<FontTemplateInfo> {
+        let cache_key = FontTemplateCacheKey {
+            template_descriptor: template_descriptor.clone(),
+            family_descriptor: family_descriptor.clone(),
+        };
+
+        self.font_template_cache.get(&cache_key).map(|v| v.clone()).unwrap_or_else(|| {
+            debug!(
+                "FontContext::font_template cache miss for template_descriptor={:?} family_descriptor={:?}",
+                template_descriptor,
+                family_descriptor
+            );
+
+            let template_info = self.font_source.font_template(
+                template_descriptor.clone(),
+                family_descriptor.clone(),
+            );
+
+            self.font_template_cache.insert(cache_key, template_info.clone());
+            template_info
+        })
+    }
+
     /// Create a `Font` for use in layout calculations, from a `FontTemplateData` returned by the
     /// cache thread and a `FontDescriptor` which contains the styling parameters.
     fn create_font(
@@ -171,6 +198,12 @@ struct FontCacheKey {
     family_descriptor: FontFamilyDescriptor,
 }
 
+#[derive(Debug, Eq, Hash, PartialEq)]
+struct FontTemplateCacheKey {
+    template_descriptor: FontTemplateDescriptor,
+    family_descriptor: FontFamilyDescriptor,
+}
+
 #[derive(Debug)]
 struct FontGroupCacheKey {
     style: Arc<FontStyleStruct>,

components/gfx/tests/font_context.rs

@@ -10,7 +10,7 @@ extern crate style;
 extern crate webrender_api;
 
 use app_units::Au;
-use gfx::font::{fallback_font_families, FontFamilyDescriptor, FontHandleMethods};
+use gfx::font::{fallback_font_families, FontDescriptor, FontFamilyDescriptor, FontFamilyName, FontSearchScope};
 use gfx::font_cache_thread::{FontTemplates, FontTemplateInfo};
 use gfx::font_context::{FontContext, FontContextHandle, FontSource};
 use gfx::font_template::FontTemplateDescriptor;
@@ -195,3 +195,38 @@ fn test_font_fallback() {
         "a fallback font should be used if there is no matching glyph in the group"
     );
 }
+
+#[test]
+fn test_font_template_is_cached() {
+    let source = TestFontSource::new();
+    let count = source.find_font_count.clone();
+    let mut context = FontContext::new(source);
+
+    let mut font_descriptor = FontDescriptor {
+        template_descriptor: FontTemplateDescriptor {
+            weight: FontWeight::normal(),
+            stretch: FontStretch::hundred(),
+            style: FontStyle::Normal,
+        },
+        variant: FontVariantCaps::Normal,
+        pt_size: Au(10),
+    };
+
+    let family_descriptor = FontFamilyDescriptor::new(
+        FontFamilyName::from("CSSTest Basic"),
+        FontSearchScope::Any,
+    );
+
+    let font1 = context.font(&font_descriptor, &family_descriptor).unwrap();
+
+    font_descriptor.pt_size = Au(20);
+    let font2 = context.font(&font_descriptor, &family_descriptor).unwrap();
+
+    assert_ne!(
+        font1.borrow().actual_pt_size,
+        font2.borrow().actual_pt_size,
+        "the same font should not have been returned"
+    );
+
+    assert_eq!(count.get(), 1, "we should only have fetched the template data from the cache thread once");
+}