Prototype Priority Hints Infrastructure

Initial infrastructure changes to support Priority Hints. This includes
adding a FetchImportanceMode enum to fetch_api_request.mojom, some
supporting getters/setters to FetchParameters as well as ResourceRequest,
and priority-modifying logic to ResourceFetcher.

R=kinuko@chromium.org, yhirano@chromium.org, yoav@yoav.ws

Bug: 821464
Change-Id: I5ab7f70f44e80d01b7d47ad4c00ae3f4817815ee
Reviewed-on: https://chromium-review.googlesource.com/1053221
Commit-Queue: Dominic Farolino <domfarolino@gmail.com>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Yoav Weiss <yoav@yoav.ws>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558150}

third_party/blink/public/platform/modules/fetch/fetch_api_request.mojom

@@ -86,6 +86,15 @@ enum FetchCacheMode {
   kUnspecifiedForceCacheMiss,
 };
 
+// Corresponds to Fetch request's "importance mode"
+// Currently discussed at: https://github.com/WICG/priority-hints
+// TODO(domfarolino): add a spec link to this once specified.
+enum FetchImportanceMode {
+  kImportanceLow,
+  kImportanceAuto,
+  kImportanceHigh
+};
+
 struct FetchAPIRequest {
   network.mojom.FetchRequestMode mode;
   bool is_main_resource_load;

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

@@ -93,6 +93,10 @@ class PLATFORM_EXPORT FetchParameters {
     resource_request_.SetRequestContext(context);
   }
 
+  void SetFetchImportanceMode(mojom::FetchImportanceMode importance_mode) {
+    resource_request_.SetFetchImportanceMode(importance_mode);
+  }
+
   const TextResourceDecoderOptions& DecoderOptions() const {
     return decoder_options_;
   }

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

@@ -168,6 +168,56 @@ static ResourceFetcher::ResourceFetcherSet& MainThreadFetchersSet() {
   return fetchers;
 }
 
+ResourceLoadPriority AdjustPriorityWithPriorityHint(
+    ResourceLoadPriority priority_so_far,
+    Resource::Type type,
+    const ResourceRequest& resource_request,
+    FetchParameters::DeferOption defer_option,
+    bool is_link_preload) {
+  mojom::FetchImportanceMode importance_mode =
+      resource_request.GetFetchImportanceMode();
+
+  ResourceLoadPriority new_priority = priority_so_far;
+
+  switch (importance_mode) {
+    case mojom::FetchImportanceMode::kImportanceAuto:
+      break;
+    case mojom::FetchImportanceMode::kImportanceHigh:
+      // Boost priority of
+      // - Late and async scripts
+      // - Images
+      // - Prefetch
+      if ((type == Resource::kScript &&
+           (FetchParameters::kLazyLoad == defer_option)) ||
+          type == Resource::kImage || type == Resource::kLinkPrefetch) {
+        new_priority = ResourceLoadPriority::kHigh;
+      }
+
+      DCHECK_LE(priority_so_far, new_priority);
+      break;
+    case mojom::FetchImportanceMode::kImportanceLow:
+      // Demote priority of:
+      // - Images
+      //     Note: this will only have a real effect on in-viewport images since
+      //     out-of-viewport images already have priority set to kLow
+      // - Link preloads
+      //     For this initial implementation we do a blanket demotion regardless
+      //     of `as` value/type. TODO(domfarolino): maybe discuss a more
+      //     granular approach with loading team
+      if (type == Resource::kImage ||
+          resource_request.GetRequestContext() ==
+              WebURLRequest::RequestContext::kRequestContextFetch ||
+          is_link_preload) {
+        new_priority = ResourceLoadPriority::kLow;
+      }
+
+      DCHECK_LE(new_priority, priority_so_far);
+      break;
+  }
+
+  return new_priority;
+}
+
 }  // namespace
 
 ResourceLoadPriority ResourceFetcher::ComputeLoadPriority(
@@ -224,6 +274,11 @@ ResourceLoadPriority ResourceFetcher::ComputeLoadPriority(
     priority = ResourceLoadPriority::kVeryLow;
   }
 
+  if (RuntimeEnabledFeatures::PriorityHintsEnabled()) {
+    priority = AdjustPriorityWithPriorityHint(priority, type, resource_request,
+                                              defer_option, is_link_preload);
+  }
+
   // A manually set priority acts as a floor. This is used to ensure that
   // synchronous requests are always given the highest possible priority, as
   // well as to ensure that there isn't priority churn if images move in and out

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

@@ -68,6 +68,7 @@ ResourceRequest::ResourceRequest(const KURL& url)
       request_context_(WebURLRequest::kRequestContextUnspecified),
       frame_type_(network::mojom::RequestContextFrameType::kNone),
       fetch_request_mode_(network::mojom::FetchRequestMode::kNoCORS),
+      fetch_importance_mode_(mojom::FetchImportanceMode::kImportanceAuto),
       fetch_credentials_mode_(network::mojom::FetchCredentialsMode::kInclude),
       fetch_redirect_mode_(network::mojom::FetchRedirectMode::kFollow),
       referrer_policy_(kReferrerPolicyDefault),
@@ -111,6 +112,7 @@ ResourceRequest::ResourceRequest(CrossThreadResourceRequestData* data)
   SetRequestContext(data->request_context_);
   SetFrameType(data->frame_type_);
   SetFetchRequestMode(data->fetch_request_mode_);
+  SetFetchImportanceMode(data->fetch_importance_mode_);
   SetFetchCredentialsMode(data->fetch_credentials_mode_);
   SetFetchRedirectMode(data->fetch_redirect_mode_);
   SetFetchIntegrity(data->fetch_integrity_.IsolatedCopy());
@@ -204,6 +206,7 @@ std::unique_ptr<CrossThreadResourceRequestData> ResourceRequest::CopyData()
   data->request_context_ = request_context_;
   data->frame_type_ = frame_type_;
   data->fetch_request_mode_ = fetch_request_mode_;
+  data->fetch_importance_mode_ = fetch_importance_mode_;
   data->fetch_credentials_mode_ = fetch_credentials_mode_;
   data->fetch_redirect_mode_ = fetch_redirect_mode_;
   data->fetch_integrity_ = fetch_integrity_.IsolatedCopy();

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

@@ -286,6 +286,21 @@ class PLATFORM_EXPORT ResourceRequest final {
     fetch_request_mode_ = mode;
   }
 
+  // A resource request's fetch_importance_mode_ is a developer-set priority
+  // hint that differs from priority_. It is used in
+  // ResourceFetcher::ComputeLoadPriority to possibly influence the resolved
+  // priority of a resource request.
+  // This member exists both here and in FetchParameters, as opposed just in
+  // the latter because the fetch() API creates a ResourceRequest object long
+  // before its associaed FetchParameters, so this makes it easier to
+  // communicate an importance value down to the lower-level fetching code.
+  mojom::FetchImportanceMode GetFetchImportanceMode() const {
+    return fetch_importance_mode_;
+  }
+  void SetFetchImportanceMode(mojom::FetchImportanceMode mode) {
+    fetch_importance_mode_ = mode;
+  }
+
   network::mojom::FetchCredentialsMode GetFetchCredentialsMode() const {
     return fetch_credentials_mode_;
   }
@@ -415,6 +430,7 @@ class PLATFORM_EXPORT ResourceRequest final {
   WebURLRequest::RequestContext request_context_;
   network::mojom::RequestContextFrameType frame_type_;
   network::mojom::FetchRequestMode fetch_request_mode_;
+  mojom::FetchImportanceMode fetch_importance_mode_;
   network::mojom::FetchCredentialsMode fetch_credentials_mode_;
   network::mojom::FetchRedirectMode fetch_redirect_mode_;
   String fetch_integrity_;
@@ -480,6 +496,7 @@ struct CrossThreadResourceRequestData {
   WebURLRequest::RequestContext request_context_;
   network::mojom::RequestContextFrameType frame_type_;
   network::mojom::FetchRequestMode fetch_request_mode_;
+  mojom::FetchImportanceMode fetch_importance_mode_;
   network::mojom::FetchCredentialsMode fetch_credentials_mode_;
   network::mojom::FetchRedirectMode fetch_redirect_mode_;
   String fetch_integrity_;

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -1001,6 +1001,10 @@
     {
       name: "PrintBrowser",
     },
+    {
+      name: "PriorityHints",
+      status: "experimental",
+    },
     {
       name: "PushMessaging",
       status: "stable",