Don't send a load event when a loaded image is actually the placeholder

The image cache returns an `ImageCacheResult::ImageAvailable `the second
time you try getting the placeholder. This means that in some cases, the
loading of an image would fail, then the same image would get fetched
from the cache, the placeholder would be loaded from that but would be
seen as a normal image, firing a load event.

This made the tests in
`fetch/cross-origin-resource-policy/image-loads.html` fail depending on
their order.

components/layout/context.rs

@@ -163,7 +163,7 @@ impl<'a> LayoutContext<'a> {
         }
 
         match self.get_or_request_image_or_meta(node, url.clone(), use_placeholder) {
-            Some(ImageOrMetadataAvailable::ImageAvailable(image, _)) => {
+            Some(ImageOrMetadataAvailable::ImageAvailable { image, .. }) => {
                 let image_info = WebRenderImageInfo::from_image(&*image);
                 if image_info.key.is_none() {
                     Some(image_info)

components/layout/fragment.rs

@@ -431,8 +431,8 @@ impl ImageFragmentInfo {
                     layout_context
                         .get_or_request_image_or_meta(node.opaque(), url, UsePlaceholder::Yes)
                         .map(|result| match result {
-                            ImageOrMetadataAvailable::ImageAvailable(i, _) => {
-                                ImageOrMetadata::Image(i)
+                            ImageOrMetadataAvailable::ImageAvailable { image, .. } => {
+                                ImageOrMetadata::Image(image)
                             },
                             ImageOrMetadataAvailable::MetadataAvailable(m) => {
                                 ImageOrMetadata::Metadata(m)

components/net/image_cache.rs

@@ -466,9 +466,12 @@ impl ImageCache for ImageCacheImpl {
             match result {
                 Ok((image, image_url)) => {
                     debug!("{} is available", url);
-                    return ImageCacheResult::Available(ImageOrMetadataAvailable::ImageAvailable(
-                        image, image_url,
-                    ));
+                    let is_placeholder = image_url == store.placeholder_url;
+                    return ImageCacheResult::Available(ImageOrMetadataAvailable::ImageAvailable {
+                        image,
+                        url: image_url,
+                        is_placeholder,
+                    });
                 },
                 Err(()) => {
                     debug!("{} is not available", url);
@@ -515,9 +518,14 @@ impl ImageCache for ImageCacheImpl {
         // TODO: make this behaviour configurable according to the caller's needs.
         store.handle_decoder(decoded);
         match store.get_completed_image_if_available(url, origin, cors_setting, use_placeholder) {
-            Some(Ok((image, image_url))) => ImageCacheResult::Available(
-                ImageOrMetadataAvailable::ImageAvailable(image, image_url),
-            ),
+            Some(Ok((image, image_url))) => {
+                let is_placeholder = image_url == store.placeholder_url;
+                ImageCacheResult::Available(ImageOrMetadataAvailable::ImageAvailable {
+                    image,
+                    url: image_url,
+                    is_placeholder,
+                })
+            },
             _ => ImageCacheResult::LoadError,
         }
     }

components/net_traits/image_cache.rs

@@ -17,7 +17,12 @@ use std::sync::Arc;
 /// Indicating either entire image or just metadata availability
 #[derive(Clone, Debug, Deserialize, MallocSizeOf, Serialize)]
 pub enum ImageOrMetadataAvailable {
-    ImageAvailable(#[ignore_malloc_size_of = "Arc"] Arc<Image>, ServoUrl),
+    ImageAvailable {
+        #[ignore_malloc_size_of = "Arc"]
+        image: Arc<Image>,
+        url: ServoUrl,
+        is_placeholder: bool,
+    },
     MetadataAvailable(ImageMetadata),
 }
 

components/script/dom/htmlimageelement.rs

@@ -329,8 +329,16 @@ impl HTMLImageElement {
         );
 
         match cache_result {
-            ImageCacheResult::Available(ImageOrMetadataAvailable::ImageAvailable(image, url)) => {
-                self.process_image_response(ImageResponse::Loaded(image, url))
+            ImageCacheResult::Available(ImageOrMetadataAvailable::ImageAvailable {
+                image,
+                url,
+                is_placeholder,
+            }) => {
+                if is_placeholder {
+                    self.process_image_response(ImageResponse::PlaceholderLoaded(image, url))
+                } else {
+                    self.process_image_response(ImageResponse::Loaded(image, url))
+                }
             },
             ImageCacheResult::Available(ImageOrMetadataAvailable::MetadataAvailable(m)) => {
                 self.process_image_response(ImageResponse::MetadataLoaded(m))
@@ -1109,7 +1117,7 @@ impl HTMLImageElement {
         );
 
         match cache_result {
-            ImageCacheResult::Available(ImageOrMetadataAvailable::ImageAvailable(_image, _url)) => {
+            ImageCacheResult::Available(ImageOrMetadataAvailable::ImageAvailable { .. }) => {
                 // Step 15
                 self.finish_reacting_to_environment_change(
                     selected_source,

components/script/dom/htmlvideoelement.rs

@@ -166,8 +166,12 @@ impl HTMLVideoElement {
         );
 
         match cache_result {
-            ImageCacheResult::Available(ImageOrMetadataAvailable::ImageAvailable(img, url)) => {
-                self.process_image_response(ImageResponse::Loaded(img, url));
+            ImageCacheResult::Available(ImageOrMetadataAvailable::ImageAvailable {
+                image,
+                url,
+                ..
+            }) => {
+                self.process_image_response(ImageResponse::Loaded(image, url));
             },
             ImageCacheResult::ReadyForRequest(id) => {
                 self.do_fetch_poster_frame(poster_url, id, cancel_receiver)