UseCounter: Introduce UseCounter::Observer for layout tests

third_party/WebKit/Source/core/frame/UseCounter.cpp

Index: third_party/WebKit/Source/core/frame/UseCounter.cpp
diff --git a/third_party/WebKit/Source/core/frame/UseCounter.cpp b/third_party/WebKit/Source/core/frame/UseCounter.cpp
index f424c1430b58935699917abd1a4a054bb88f957c..eebae4bd6a329a0cb9ca5bd79c7326b5e674fbbe 100644
--- a/third_party/WebKit/Source/core/frame/UseCounter.cpp
+++ b/third_party/WebKit/Source/core/frame/UseCounter.cpp
@@ -1143,6 +1143,10 @@ bool UseCounter::hasRecordedMeasurement(Feature feature) const {
   return m_featuresRecorded.quickGet(feature);
 }
 
+DEFINE_TRACE(UseCounter) {
+  visitor->trace(m_observers);
+}
+
 void UseCounter::didCommitLoad(KURL url) {
   m_legacyCounter.updateMeasurements();
 
@@ -1173,6 +1177,12 @@ void UseCounter::count(const Frame* frame, Feature feature) {
     return;
 
   host->useCounter().count(feature);
+  HeapHashSet<Member<Observer>> toBeRemoved;

[Rick Byers, 2017/02/11 15:01:51]

IIRC, this code path isn't triggered on the deprecation case.  If that's
correct, why is the test passing?  Is it just relying on the race and could
still be flaky?  I think you want to move this logic down into
recordMeasurement.

In fact, do you really want the semantics to be that the observer is invoked for
EVERY call to 'count'?  That can be a LOT of calls .  Perhaps the semantics
should be that it's invoked when the histogram is updated - i.e. the first time
a feature is used on a page.  That's much rarer and so less performance
sensitive.  So maybe you want to put this logic in a helper function that's
called at the same time as we generate the blink.feature_usage TRACE?

Regardless, this choice of observation semantics should be documented in a
comment in UseCounter.h.

[nhiroki, 2017/02/13 04:35:07]

Good point. This is because both countFeature() and countDeprecation() on
SharedWorkerGlobalScope get to countFeature() on Document.
Yes, it's preferable to call it only one time per feature. Moved the call into
recordMeasurement.
Added.

+  for (auto observer : host->useCounter().m_observers) {
+    if (observer->onCountFeature(feature))
+      toBeRemoved.insert(observer);
+  }
+  host->useCounter().m_observers.removeAll(toBeRemoved);
 }
 
 void UseCounter::count(const Document& document, Feature feature) {
@@ -1193,6 +1203,16 @@ bool UseCounter::isCounted(CSSPropertyID unresolvedProperty) {
   return m_CSSRecorded.quickGet(unresolvedProperty);
 }
 
+void UseCounter::addObserver(Document& document, Observer* observer) {
+  Frame* frame = document.frame();
+  if (!frame)
+    return;
+  FrameHost* host = frame->host();
+  if (!host)
+    return;
+  host->useCounter().m_observers.insert(observer);
+}
+
 bool UseCounter::isCounted(Document& document, const String& string) {
   Frame* frame = document.frame();
   if (!frame)