predictors: Mark before_first_contentful_paint for resources fetched before fcp.

chrome/browser/predictors/resource_prefetch_predictor.cc

Index: chrome/browser/predictors/resource_prefetch_predictor.cc
diff --git a/chrome/browser/predictors/resource_prefetch_predictor.cc b/chrome/browser/predictors/resource_prefetch_predictor.cc
index 58d01b5df36a23eabb6ebf86be8c50d4a700047b..a101b6e38098d37c532bb0d76fc85054563e5375 100644
--- a/chrome/browser/predictors/resource_prefetch_predictor.cc
+++ b/chrome/browser/predictors/resource_prefetch_predictor.cc
@@ -469,6 +469,8 @@ ResourcePrefetchPredictor::OriginRequestSummary::~OriginRequestSummary() {}
 ResourcePrefetchPredictor::URLRequestSummary::URLRequestSummary()
     : resource_type(content::RESOURCE_TYPE_LAST_TYPE),
       priority(net::IDLE),
+      response_time(base::TimeTicks()),
+      before_first_contentful_paint(false),
       was_cached(false),
       has_validators(false),
       always_revalidate(false),
@@ -490,6 +492,7 @@ bool ResourcePrefetchPredictor::URLRequestSummary::SummarizeResponse(
   if (!request_info)
     return false;
 
+  summary->response_time = base::TimeTicks::Now();
   summary->resource_url = request.original_url();
   summary->request_url = request.url();
   content::ResourceType resource_type_from_request =
@@ -517,7 +520,9 @@ bool ResourcePrefetchPredictor::URLRequestSummary::SummarizeResponse(
 
 ResourcePrefetchPredictor::PageRequestSummary::PageRequestSummary(
     const GURL& i_main_frame_url)
-    : main_frame_url(i_main_frame_url), initial_url(i_main_frame_url) {}
+    : main_frame_url(i_main_frame_url),
+      initial_url(i_main_frame_url),
+      first_contentful_paint(base::TimeTicks::Max()) {}
 
 ResourcePrefetchPredictor::PageRequestSummary::PageRequestSummary(
     const PageRequestSummary& other) = default;
@@ -645,6 +650,18 @@ void ResourcePrefetchPredictor::RecordMainFrameLoadComplete(
   }
 }
 
+void ResourcePrefetchPredictor::RecordFirstContentfulPaint(
+    const NavigationID& navigation_id,
+    const base::TimeTicks& first_contentful_paint) {
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+  if (initialization_state_ != INITIALIZED)
+    return;
+
+  NavigationMap::iterator nav_it = inflight_navigations_.find(navigation_id);
+  if (nav_it != inflight_navigations_.end())
+    nav_it->second->first_contentful_paint = first_contentful_paint;
+}
+
 void ResourcePrefetchPredictor::StartPrefetching(const GURL& url,
                                                  PrefetchOrigin origin) {
   TRACE_EVENT1("browser", "ResourcePrefetchPredictor::StartPrefetching", "url",
@@ -844,6 +861,12 @@ void ResourcePrefetchPredictor::OnNavigationComplete(
   std::unique_ptr<PageRequestSummary> summary = std::move(nav_it->second);
   inflight_navigations_.erase(nav_it);
 
+  // Set before_first_contentful paint for each resource.

[Benoit L, 2017/04/25 15:30:36]

If I understand the code correctly, this will slightly overestimate the
resources we get before first contentful paint, because we take the time at
which we start reading the response as the reference, not the time at which we
are done with a resource.

This is fine, and doing otherwise would complicate the code.

If my understanding is correct, can you add a comment making that explicit?

[trevordixon, 2017/04/25 20:23:55]

OK, documented above where response_time is recorded.

+  for (auto& request_summary : summary->subresource_requests) {
+    request_summary.before_first_contentful_paint =
+        request_summary.response_time < summary->first_contentful_paint;
+  }
+
   const GURL& initial_url = summary->initial_url;
   ResourcePrefetchPredictor::Prediction prediction;
   bool has_data = GetPrefetchData(initial_url, &prediction);
@@ -1343,6 +1366,8 @@ void ResourcePrefetchPredictor::LearnNavigation(
       resource_to_add->set_average_position(i + 1);
       resource_to_add->set_priority(
           static_cast<ResourceData::Priority>(summary.priority));
+      resource_to_add->set_before_first_contentful_paint(
+          summary.before_first_contentful_paint);
       resource_to_add->set_has_validators(summary.has_validators);
       resource_to_add->set_always_revalidate(summary.always_revalidate);
 
@@ -1392,6 +1417,8 @@ void ResourcePrefetchPredictor::LearnNavigation(
 
         old_resource->set_priority(
             static_cast<ResourceData::Priority>(new_summary.priority));
+        old_resource->set_before_first_contentful_paint(
+            new_summary.before_first_contentful_paint);
 
         int position = new_index[resource_url] + 1;
         int total =
@@ -1419,6 +1446,8 @@ void ResourcePrefetchPredictor::LearnNavigation(
       resource_to_add->set_average_position(i + 1);
       resource_to_add->set_priority(
           static_cast<ResourceData::Priority>(summary.priority));
+      resource_to_add->set_before_first_contentful_paint(
+          summary.before_first_contentful_paint);
       resource_to_add->set_has_validators(new_resources[i].has_validators);
       resource_to_add->set_always_revalidate(
           new_resources[i].always_revalidate);