[coverage] Add event for coverage reporting

This CL adds a new event that enables the back-end to send coverage
updates on its own initiative. This event can be triggered via the C++
method `triggerPreciseCoverageDeltaUpdate` on the agent in a way that
causes coverage data to be immediatelly collected.

This is useful in the back-end to collect coverage at a certain point
in time, i.e. when a lifecycle event such as first contentful paint
occurs.

The previous interface could not support this, because it could not
reasonably be triggered from C++, and if triggered through the protocol,
dispatching messages added delay that invalidated the data (i.e. data
might have been taken too late to be accurate).

TBR=yangguo@chromium.org

Change-Id: I0f7201412a8d64866e6e314e5bc850354c13a9da
Bug: chromium:1022031
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1992437
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65864}

include/js_protocol.pdl

@@ -888,6 +888,19 @@ domain Profiler
       # Profile title passed as an argument to console.profile().
       optional string title
 
+  # Reports coverage delta since the last poll (either from an event like this, or from
+  # `takePreciseCoverage` for the current isolate. May only be sent if precise code
+  # coverage has been started. This event can be trigged by the embedder to, for example,
+  # trigger collection of coverage data immediatelly at a certain point in time.
+  experimental event preciseCoverageDeltaUpdate
+    parameters
+      # Monotonically increasing time (in seconds) when the coverage update was taken in the backend.
+      number timestamp
+      # Identifier for distinguishing coverage events.
+      string occassion
+      # Coverage data for the current isolate.
+      array of ScriptCoverage result
+
 # Runtime domain exposes JavaScript runtime by means of remote evaluation and mirror objects.
 # Evaluation results are returned as mirror object that expose object type, string representation
 # and unique identifier that can be used for further object reference. Original objects are

include/v8-inspector.h

@@ -161,6 +161,8 @@ class V8_EXPORT V8InspectorSession {
                             v8::Local<v8::Context>*,
                             std::unique_ptr<StringBuffer>* objectGroup) = 0;
   virtual void releaseObjectGroup(const StringView&) = 0;
+  virtual void triggerPreciseCoverageDeltaUpdate(
+      const StringView& occassion) = 0;
 };
 
 class V8_EXPORT V8InspectorClient {

src/inspector/v8-inspector-session-impl.cc

@@ -464,4 +464,9 @@ V8InspectorSessionImpl::searchInTextByLines(const StringView& text,
   return result;
 }
 
+void V8InspectorSessionImpl::triggerPreciseCoverageDeltaUpdate(
+    const StringView& occassion) {
+  m_profilerAgent->triggerPreciseCoverageDeltaUpdate(toString16(occassion));
+}
+
 }  // namespace v8_inspector

src/inspector/v8-inspector-session-impl.h

@@ -92,6 +92,8 @@ class V8InspectorSessionImpl : public V8InspectorSession,
   V8InspectorSession::Inspectable* inspectedObject(unsigned num);
   static const unsigned kInspectedObjectBufferSize = 5;
 
+  void triggerPreciseCoverageDeltaUpdate(const StringView& occassion) override;
+
  private:
   V8InspectorSessionImpl(V8InspectorImpl*, int contextGroupId, int sessionId,
                          V8Inspector::Channel*, const StringView& state);

src/inspector/v8-profiler-agent-impl.cc

@@ -414,6 +414,22 @@ Response V8ProfilerAgentImpl::takePreciseCoverage(
   return coverageToProtocol(m_session->inspector(), coverage, out_result);
 }
 
+void V8ProfilerAgentImpl::triggerPreciseCoverageDeltaUpdate(
+    const String16& occassion) {
+  if (!m_state->booleanProperty(ProfilerAgentState::preciseCoverageStarted,
+                                false)) {
+    return;
+  }
+  v8::HandleScope handle_scope(m_isolate);
+  v8::debug::Coverage coverage = v8::debug::Coverage::CollectPrecise(m_isolate);
+  std::unique_ptr<protocol::Array<protocol::Profiler::ScriptCoverage>>
+      out_result;
+  coverageToProtocol(m_session->inspector(), coverage, &out_result);
+  double now =
+      v8::base::TimeTicks::HighResolutionNow().since_origin().InSecondsF();
+  m_frontend.preciseCoverageDeltaUpdate(now, occassion, std::move(out_result));
+}
+
 Response V8ProfilerAgentImpl::getBestEffortCoverage(
     std::unique_ptr<protocol::Array<protocol::Profiler::ScriptCoverage>>*
         out_result) {

src/inspector/v8-profiler-agent-impl.h

@@ -65,6 +65,8 @@ class V8ProfilerAgentImpl : public protocol::Profiler::Backend {
   void consoleProfile(const String16& title);
   void consoleProfileEnd(const String16& title);
 
+  void triggerPreciseCoverageDeltaUpdate(const String16& occassion);
+
  private:
   String16 nextProfileId();