Add test for FontContext/FontGroup functionality

Unfortunately, this required quite a bit of changes to the non-test
code. That's because FontContext depends on a FontCacheThread, which in
turn depends on a CoreResourceThread and therefore lots of other data
structures.

It seemed like it would be very difficult to instantiate a FontContext
as it was, and even if we could it seems like overkill to have all these
data structures present for a relatively focused test.

Therefore, I created a FontSource trait which represents the interface
which FontContext uses to talk to FontCacheThread. FontCacheThread then
implements FontSource. Then, in the test, we can create a dummy
implementation of FontSource rather than using FontCacheThread.

This actually has the advantage that we can make our dummy
implementation behave in certain specific way which are useful for
testing, for example it can count the number of times
find_font_template() is called, which helps us verify that
caching/lazy-loading is working as intended.

components/gfx/font_context.rs

@@ -5,7 +5,8 @@
 use app_units::Au;
 use fnv::FnvHasher;
 use font::{Font, FontDescriptor, FontGroup, FontHandleMethods, FontRef};
-use font_cache_thread::{FontCacheThread, FontTemplateInfo};
+use font_cache_thread::FontTemplateInfo;
+use font_template::FontTemplateDescriptor;
 use malloc_size_of::{MallocSizeOf, MallocSizeOfOps};
 use platform::font::FontHandle;
 pub use platform::font_context::FontContextHandle;
@@ -20,6 +21,7 @@ use std::sync::atomic::{AtomicUsize, Ordering, ATOMIC_USIZE_INIT};
 use style::computed_values::font_variant_caps::T as FontVariantCaps;
 use style::properties::style_structs::Font as FontStyleStruct;
 use style::values::computed::font::SingleFontFamily;
+use webrender_api;
 
 static SMALL_CAPS_SCALE_FACTOR: f32 = 0.8;      // Matches FireFox (see gfxFont.h)
 
@@ -58,14 +60,26 @@ impl FallbackFontCacheEntry {
 /// this one.
 static FONT_CACHE_EPOCH: AtomicUsize = ATOMIC_USIZE_INIT;
 
+pub trait FontSource {
+    fn get_font_instance(&mut self, key: webrender_api::FontKey, size: Au) -> webrender_api::FontInstanceKey;
+
+    fn find_font_template(
+        &mut self,
+        family: SingleFontFamily,
+        desc: FontTemplateDescriptor
+    ) -> Option<FontTemplateInfo>;
+
+    fn last_resort_font_template(&mut self, desc: FontTemplateDescriptor) -> FontTemplateInfo;
+}
+
 /// The FontContext represents the per-thread/thread state necessary for
 /// working with fonts. It is the public API used by the layout and
 /// paint code. It talks directly to the font cache thread where
 /// required.
 #[derive(Debug)]
-pub struct FontContext {
+pub struct FontContext<S: FontSource> {
     platform_handle: FontContextHandle,
-    font_cache_thread: FontCacheThread,
+    font_source: S,
 
     // TODO: The font context holds a strong ref to the cached fonts
     // so they will never be released. Find out a good time to drop them.
@@ -81,12 +95,12 @@ pub struct FontContext {
     epoch: usize,
 }
 
-impl FontContext {
-    pub fn new(font_cache_thread: FontCacheThread) -> FontContext {
+impl<S: FontSource> FontContext<S> {
+    pub fn new(font_source: S) -> FontContext<S> {
         let handle = FontContextHandle::new();
         FontContext {
             platform_handle: handle,
-            font_cache_thread: font_cache_thread,
+            font_source,
             font_cache: vec!(),
             fallback_font_cache: vec!(),
             font_group_cache: HashMap::with_hasher(Default::default()),
@@ -97,7 +111,7 @@ impl FontContext {
     /// Create a `Font` for use in layout calculations, from a `FontTemplateInfo` returned by the
     /// cache thread (which contains the underlying font data) and a `FontDescriptor` which
     /// contains the styling parameters.
-    fn create_font(&self, info: FontTemplateInfo, descriptor: FontDescriptor) -> Result<Font, ()> {
+    fn create_font(&mut self, info: FontTemplateInfo, descriptor: FontDescriptor) -> Result<Font, ()> {
         // TODO: (Bug #3463): Currently we only support fake small-caps
         // painting. We should also support true small-caps (where the
         // font supports it) in the future.
@@ -110,8 +124,7 @@ impl FontContext {
                                                         info.font_template,
                                                         Some(actual_pt_size))?;
 
-        let font_instance_key = self.font_cache_thread
-                                    .get_font_instance(info.font_key, actual_pt_size);
+        let font_instance_key = self.font_source.get_font_instance(info.font_key, actual_pt_size);
         Ok(Font::new(handle, descriptor.to_owned(), actual_pt_size, font_instance_key))
     }
 
@@ -155,9 +168,9 @@ impl FontContext {
     }
 
     /// Creates a new font cache entry matching `descriptor` and `family`.
-    fn create_font_cache_entry(&self, descriptor: &FontDescriptor, family: &SingleFontFamily) -> FontCacheEntry {
+    fn create_font_cache_entry(&mut self, descriptor: &FontDescriptor, family: &SingleFontFamily) -> FontCacheEntry {
         let font =
-            self.font_cache_thread.find_font_template(family.clone(), descriptor.template_descriptor.clone())
+            self.font_source.find_font_template(family.clone(), descriptor.template_descriptor.clone())
                 .and_then(|template_info|
                     self.create_font(template_info, descriptor.to_owned()).ok()
                 )
@@ -187,8 +200,8 @@ impl FontContext {
     }
 
     /// Creates a new fallback font cache entry matching `descriptor`.
-    fn create_fallback_font_cache_entry(&self, descriptor: &FontDescriptor) -> Option<FallbackFontCacheEntry> {
-        let template_info = self.font_cache_thread.last_resort_font_template(descriptor.template_descriptor.clone());
+    fn create_fallback_font_cache_entry(&mut self, descriptor: &FontDescriptor) -> Option<FallbackFontCacheEntry> {
+        let template_info = self.font_source.last_resort_font_template(descriptor.template_descriptor.clone());
 
         match self.create_font(template_info, descriptor.to_owned()) {
             Ok(font) =>
@@ -220,7 +233,7 @@ impl FontContext {
     }
 }
 
-impl MallocSizeOf for FontContext {
+impl<S: FontSource> MallocSizeOf for FontContext<S> {
     fn size_of(&self, ops: &mut MallocSizeOfOps) -> usize {
         // FIXME(njn): Measure other fields eventually.
         self.platform_handle.size_of(ops)