net: Don't load the placeholder image for background images, only for

image fragments.

This also changes the way the placeholder is handled in the image cache
task to decode it up front instead of each time an image fails to load,
both because it was more convenient to implement that way and because
it saves CPU cycles to do so.

This matches the behavior of Gecko and WebKit. It improves the look of
our cached copy of Wikipedia.

components/layout/context.rs

@@ -15,7 +15,8 @@ use gfx::font_context::FontContext;
 use msg::compositor_msg::LayerId;
 use msg::constellation_msg::ConstellationChan;
 use net_traits::image::base::Image;
-use net_traits::image_cache_task::{ImageCacheChan, ImageCacheTask, ImageState};
+use net_traits::image_cache_task::{ImageCacheChan, ImageCacheTask, ImageResponse, ImageState};
+use net_traits::image_cache_task::{UsePlaceholder};
 use script::layout_interface::{Animation, LayoutChan, ReflowGoal};
 use std::boxed;
 use std::cell::Cell;
@@ -154,9 +155,11 @@ impl<'a> LayoutContext<'a> {
         }
     }
 
-    pub fn get_or_request_image(&self, url: Url) -> Option<Arc<Image>> {
+    pub fn get_or_request_image(&self, url: Url, use_placeholder: UsePlaceholder)
+                                -> Option<Arc<Image>> {
         // See if the image is already available
-        let result = self.shared.image_cache_task.get_image_if_available(url.clone());
+        let result = self.shared.image_cache_task.get_image_if_available(url.clone(),
+                                                                         use_placeholder);
 
         match result {
             Ok(image) => Some(image),
@@ -174,7 +177,11 @@ impl<'a> LayoutContext<'a> {
                         self.shared.image_cache_task.request_image(url,
                                                                    ImageCacheChan(sync_tx),
                                                                    None);
-                        sync_rx.recv().unwrap().image
+                        match sync_rx.recv().unwrap().image_response {
+                            ImageResponse::Loaded(image) |
+                            ImageResponse::PlaceholderLoaded(image) => Some(image),
+                            ImageResponse::None => None,
+                        }
                     }
                     // Not yet requested, async mode - request image from the cache
                     (ImageState::NotRequested, false) => {

components/layout/display_list_builder.rs

@@ -35,6 +35,7 @@ use gfx::paint_task::{PaintLayer, THREAD_TINT_COLORS};
 use msg::compositor_msg::ScrollPolicy;
 use msg::constellation_msg::ConstellationChan;
 use msg::constellation_msg::Msg as ConstellationMsg;
+use net_traits::image_cache_task::UsePlaceholder;
 use png::{self, PixelsByColorType};
 use std::cmp;
 use std::default::Default;
@@ -432,7 +433,7 @@ impl FragmentDisplayListBuilding for Fragment {
                                                clip: &ClippingRegion,
                                                image_url: &Url) {
         let background = style.get_background();
-        let image = layout_context.get_or_request_image(image_url.clone());
+        let image = layout_context.get_or_request_image(image_url.clone(), UsePlaceholder::No);
         if let Some(image) = image {
             debug!("(building display list) building background image");
 

components/layout/fragment.rs

@@ -27,6 +27,7 @@ use gfx::text::glyph::CharIndex;
 use gfx::text::text_run::{TextRun, TextRunSlice};
 use msg::constellation_msg::{ConstellationChan, Msg, PipelineId, SubpageId};
 use net_traits::image::base::Image;
+use net_traits::image_cache_task::UsePlaceholder;
 use rustc_serialize::{Encodable, Encoder};
 use script_traits::UntrustedNodeAddress;
 use std::borrow::ToOwned;
@@ -337,7 +338,9 @@ impl ImageFragmentInfo {
                    .map(Au::from_px)
         }
 
-        let image = url.and_then(|url| layout_context.get_or_request_image(url));
+        let image = url.and_then(|url| {
+            layout_context.get_or_request_image(url, UsePlaceholder::Yes)
+        });
 
         ImageFragmentInfo {
             replaced_image_fragment_info: ReplacedImageFragmentInfo::new(node,