layout/context: Wrap image_cache_thread in a Mutex<> as well

`SharedLayoutContext.image_cache_thread` didn't get the `Mutex<>`
treatment along with all the other channels in there, because
`ipc-channel::Sender` is presently inherently `Sync`. I believe this to
be an implementation accident though, that should be rectified in the
future -- not something users should actually rely on...

Note that sharing senders (be it `mpsc` or `ipc-channel`) in is probably
not a good idea anyway: just cloning them -- and letting them handle the
sharing internally -- should be both simpler and cheaper. But right now
that's how things are handled here; so let's go with the flow...

components/layout/context.rs

@@ -76,7 +76,7 @@ pub struct SharedLayoutContext {
     pub style_context: SharedStyleContext,
 
     /// The shared image cache thread.
-    pub image_cache_thread: ImageCacheThread,
+    pub image_cache_thread: Mutex<ImageCacheThread>,
 
     /// A channel for the image cache to send responses to.
     pub image_cache_sender: Mutex<ImageCacheChan>,
@@ -132,7 +132,8 @@ impl SharedLayoutContext {
         debug_assert!(opts::get().output_file.is_some() || opts::get().exit_after_load);
 
         // See if the image is already available
-        let result = self.image_cache_thread.find_image(url.clone(), use_placeholder);
+        let result = self.image_cache_thread.lock().unwrap()
+                                            .find_image(url.clone(), use_placeholder);
 
         match result {
             Ok(image) => return Some(image),
@@ -146,7 +147,7 @@ impl SharedLayoutContext {
         // If we are emitting an output file, then we need to block on
         // image load or we risk emitting an output file missing the image.
         let (sync_tx, sync_rx) = ipc::channel().unwrap();
-        self.image_cache_thread.request_image(url, ImageCacheChan(sync_tx), None);
+        self.image_cache_thread.lock().unwrap().request_image(url, ImageCacheChan(sync_tx), None);
         loop {
             match sync_rx.recv() {
                 Err(_) => return None,
@@ -170,7 +171,8 @@ impl SharedLayoutContext {
                        .map(|img| ImageOrMetadataAvailable::ImageAvailable(img));
         }
         // See if the image is already available
-        let result = self.image_cache_thread.find_image_or_metadata(url.clone(),
+        let result = self.image_cache_thread.lock().unwrap()
+                                            .find_image_or_metadata(url.clone(),
                                                                     use_placeholder);
         match result {
             Ok(image_or_metadata) => Some(image_or_metadata),
@@ -179,7 +181,8 @@ impl SharedLayoutContext {
             // Not yet requested, async mode - request image or metadata from the cache
             Err(ImageState::NotRequested) => {
                 let sender = self.image_cache_sender.lock().unwrap().clone();
-                self.image_cache_thread.request_image_and_metadata(url, sender, None);
+                self.image_cache_thread.lock().unwrap()
+                                       .request_image_and_metadata(url, sender, None);
                 None
             }
             // Image has been requested, is still pending. Return no image for this paint loop.

components/layout_thread/lib.rs

@@ -513,7 +513,7 @@ impl LayoutThread {
                 local_context_creation_data: Mutex::new(local_style_context_creation_data),
                 timer: self.timer.clone(),
             },
-            image_cache_thread: self.image_cache_thread.clone(),
+            image_cache_thread: Mutex::new(self.image_cache_thread.clone()),
             image_cache_sender: Mutex::new(self.image_cache_sender.clone()),
             font_cache_thread: Mutex::new(self.font_cache_thread.clone()),
             webrender_image_cache: self.webrender_image_cache.clone(),