[AdTagging] Experiment with only looking at the top of the stack

What: Add a feature to cause AdTagging to only tag a v8 stack as an ad
stack if the top of the stack is an ad script. E.g., ignore the
bottom.

Why: Because sometimes an event on an ad script is used to create
content (e.g., the ad finishes showing so now reveal the page
content by calling content script).

Bug: 1022802
Change-Id: I54ef3f0930f886df01320a714e72e8e4554a0357
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1906467Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Josh Karlin <jkarlin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714675}

third_party/blink/renderer/core/frame/ad_tracker.cc

@@ -38,6 +38,11 @@ namespace features {
 // the currently running stack is ad related.
 const base::Feature kAsyncStackAdTagging{"AsyncStackAdTagging",
                                          base::FEATURE_DISABLED_BY_DEFAULT};
+
+// Controls whether the AdTracker analyzes the whole pseudo-stack or just the
+// top of the stack when detecting ads.
+const base::Feature kTopOfStackAdTagging{"TopOfStackAdTagging",
+                                         base::FEATURE_DISABLED_BY_DEFAULT};
 }  // namespace features
 
 // static
@@ -57,7 +62,9 @@ AdTracker* AdTracker::FromExecutionContext(
 AdTracker::AdTracker(LocalFrame* local_root)
     : local_root_(local_root),
       async_stack_enabled_(
-          base::FeatureList::IsEnabled(features::kAsyncStackAdTagging)) {
+          base::FeatureList::IsEnabled(features::kAsyncStackAdTagging)),
+      top_of_stack_only_(
+          base::FeatureList::IsEnabled(features::kTopOfStackAdTagging)) {
   local_root_->GetProbeSink()->AddAdTracker(this);
 }
 
@@ -88,6 +95,9 @@ ExecutionContext* AdTracker::GetCurrentExecutionContext() {
 
 void AdTracker::WillExecuteScript(ExecutionContext* execution_context,
                                   const String& script_url) {
+  if (top_of_stack_only_)
+    return;
+
   bool is_ad = script_url.IsEmpty()
                    ? false
                    : IsKnownAdScript(execution_context, script_url);
@@ -97,6 +107,9 @@ void AdTracker::WillExecuteScript(ExecutionContext* execution_context,
 }
 
 void AdTracker::DidExecuteScript() {
+  if (top_of_stack_only_)
+    return;
+
   if (stack_frame_is_ad_.back()) {
     DCHECK_LT(0u, num_ads_in_stack_);
     num_ads_in_stack_ -= 1;

third_party/blink/renderer/core/frame/ad_tracker.h

@@ -28,6 +28,7 @@ class ExecuteScript;
 
 namespace features {
 CORE_EXPORT extern const base::Feature kAsyncStackAdTagging;
+CORE_EXPORT extern const base::Feature kTopOfStackAdTagging;
 }
 
 // Tracker for tagging resources as ads based on the call stack scripts.
@@ -107,6 +108,7 @@ class CORE_EXPORT AdTracker : public GarbageCollected<AdTracker> {
   uint32_t running_ad_async_tasks_ = 0;
 
   const bool async_stack_enabled_;
+  const bool top_of_stack_only_;
 
   DISALLOW_COPY_AND_ASSIGN(AdTracker);
 };

third_party/blink/renderer/core/frame/ad_tracker_test.cc

@@ -136,6 +136,36 @@ TEST_F(AdTrackerTest, AnyExecutingScriptsTaggedAsAdResource) {
   EXPECT_TRUE(AnyExecutingScriptsTaggedAsAdResource());
 }
 
+TEST_F(AdTrackerTest, TopOfStackOnly_NoAdsOnTop) {
+  base::test::ScopedFeatureList feature_list;
+  feature_list.InitAndEnableFeature(features::kTopOfStackAdTagging);
+  CreateAdTracker();
+
+  String ad_script_url("https://example.com/bar.js");
+  AppendToKnownAdScripts(ad_script_url);
+
+  WillExecuteScript(ad_script_url);
+  WillExecuteScript("https://example.com/foo.js");
+  ad_tracker_->SetScriptAtTopOfStack("https://www.example.com/baz.js");
+
+  EXPECT_FALSE(AnyExecutingScriptsTaggedAsAdResource());
+}
+
+TEST_F(AdTrackerTest, TopOfStackOnly_AdsOnTop) {
+  base::test::ScopedFeatureList feature_list;
+  feature_list.InitAndEnableFeature(features::kTopOfStackAdTagging);
+  CreateAdTracker();
+
+  String ad_script_url("https://example.com/bar.js");
+  AppendToKnownAdScripts(ad_script_url);
+
+  WillExecuteScript(ad_script_url);
+  WillExecuteScript("https://example.com/foo.js");
+  ad_tracker_->SetScriptAtTopOfStack(ad_script_url);
+
+  EXPECT_TRUE(AnyExecutingScriptsTaggedAsAdResource());
+}
+
 // Tests that if neither script in the stack is an ad,
 // AnyExecutingScriptsTaggedAsAdResource should return false.
 TEST_F(AdTrackerTest, AnyExecutingScriptsTaggedAsAdResource_False) {