Lower priority for JavaScript requests when defer intervention is enabled

Change-Id: I3ecf89469653e98afed23f4d6cf8185bde032be8
Bug: 978490
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1687483
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Doug Arnett <dougarnett@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676141}

chrome/test/BUILD.gn

@@ -1015,6 +1015,7 @@ if (!is_android) {
       "../browser/prerender/prerender_test_utils.cc",
       "../browser/prerender/prerender_test_utils.h",
       "../browser/previews/defer_all_script_browsertest.cc",
+      "../browser/previews/defer_all_script_priority_browsertest.cc",
       "../browser/previews/hints_fetcher_browsertest.cc",
       "../browser/previews/lazyload_browsertest.cc",
       "../browser/previews/previews_browsertest.cc",

third_party/blink/renderer/core/loader/document_loader.h

@@ -282,6 +282,10 @@ class CORE_EXPORT DocumentLoader
 
   void SetLoadingJavaScriptUrl() { loading_url_as_javascript_ = true; }
 
+  WebURLRequest::PreviewsState previews_state() const {
+    return previews_state_;
+  }
+
  protected:
   bool had_transient_activation() const { return had_transient_activation_; }
 

third_party/blink/renderer/core/loader/frame_fetch_context.cc

@@ -323,6 +323,10 @@ FrameFetchContext::GetPreviewsResourceLoadingHints() const {
   return document_loader->GetPreviewsResourceLoadingHints();
 }
 
+WebURLRequest::PreviewsState FrameFetchContext::previews_state() const {
+  return GetLocalFrameClient()->GetPreviewsStateForFrame();
+}
+
 LocalFrame* FrameFetchContext::GetFrame() const {
   return &frame_or_imported_document_->GetFrame();
 }

third_party/blink/renderer/core/loader/frame_fetch_context.h

@@ -130,6 +130,7 @@ class CORE_EXPORT FrameFetchContext final : public BaseFetchContext {
   SubresourceFilter* GetSubresourceFilter() const override;
   PreviewsResourceLoadingHints* GetPreviewsResourceLoadingHints()
       const override;
+  WebURLRequest::PreviewsState previews_state() const override;
   bool AllowScriptFromSource(const KURL&) const override;
   bool ShouldBlockRequestByInspector(const KURL&) const override;
   void DispatchDidBlockRequest(const ResourceRequest&,

third_party/blink/renderer/platform/loader/fetch/fetch_context.h

@@ -142,6 +142,10 @@ class PLATFORM_EXPORT FetchContext
     return false;
   }
 
+  virtual WebURLRequest::PreviewsState previews_state() const {
+    return WebURLRequest::kPreviewsUnspecified;
+  }
+
  private:
   DISALLOW_COPY_AND_ASSIGN(FetchContext);
 };

third_party/blink/renderer/platform/loader/fetch/resource_fetcher.cc

@@ -243,6 +243,33 @@ ResourceLoadPriority AdjustPriorityWithPriorityHint(
   return new_priority;
 }
 
+ResourceLoadPriority AdjustPriorityWithDeferScriptIntervention(
+    const FetchContext& fetch_context,
+    ResourceLoadPriority priority_so_far,
+    ResourceType type,
+    const ResourceRequest& resource_request,
+    FetchParameters::DeferOption defer_option,
+    bool is_link_preload) {
+  WebURLRequest::PreviewsState context_previews_state =
+      fetch_context.previews_state();
+
+  if (type != ResourceType::kScript)
+    return priority_so_far;
+
+  // If none of the JavaScript resources are render blocking (due to the
+  // DeferAllScript intervention), then lower their priority so they do not
+  // contend for network resources with higher priority resources that may be
+  // render blocking (e.g., html, css). ResourceLoadPriority::kMedium
+  // corresponds to a network priority of
+  // network::mojom::blink::RequestPriority::kLow which is considered delayable
+  // by the resource scheduler on the browser side.
+  if (RuntimeEnabledFeatures::ForceDeferScriptInterventionEnabled() ||
+      (context_previews_state & WebURLRequest::kDeferAllScriptOn)) {
+    return std::min(priority_so_far, ResourceLoadPriority::kMedium);
+  }
+  return priority_so_far;
+}
+
 std::unique_ptr<TracedValue> BeginResourceLoadData(
     const blink::ResourceRequest& request) {
   auto value = std::make_unique<TracedValue>();
@@ -508,6 +535,10 @@ ResourceLoadPriority ResourceFetcher::ComputeLoadPriority(
   priority = AdjustPriorityWithPriorityHint(priority, type, resource_request,
                                             defer_option, is_link_preload);
 
+  priority = AdjustPriorityWithDeferScriptIntervention(
+      Context(), priority, type, resource_request, defer_option,
+      is_link_preload);
+
   if (properties_->IsSubframeDeprioritizationEnabled()) {
     if (properties_->IsMainFrame()) {
       DEFINE_STATIC_LOCAL(