Unique frame number for each frame (flutter/engine#26021)

This gets created during vsync and stays for the
lifecycle of the frame.

engine/src/flutter/flow/frame_timings.cc

@@ -11,7 +11,10 @@
 
 namespace flutter {
 
-FrameTimingsRecorder::FrameTimingsRecorder() = default;
+std::atomic_int FrameTimingsRecorder::frame_number_gen_ = {1};
+
+FrameTimingsRecorder::FrameTimingsRecorder()
+    : frame_number_(frame_number_gen_++) {}
 
 FrameTimingsRecorder::~FrameTimingsRecorder() = default;
 
@@ -132,4 +135,8 @@ std::unique_ptr<FrameTimingsRecorder> FrameTimingsRecorder::CloneUntil(
   return recorder;
 }
 
+uint64_t FrameTimingsRecorder::GetFrameNumber() const {
+  return frame_number_;
+}
+
 }  // namespace flutter

engine/src/flutter/flow/frame_timings.h

@@ -79,10 +79,19 @@ class FrameTimingsRecorder {
   /// the events. This summary is sent to the framework.
   FrameTiming RecordRasterEnd(fml::TimePoint raster_end);
 
+  /// Returns the frame number. Frame number is unique per frame and a frame
+  /// built earlier will have a frame number less than a frame that has been
+  /// built at a later point of time.
+  uint64_t GetFrameNumber() const;
+
  private:
+  static std::atomic_int frame_number_gen_;
+
   mutable std::mutex state_mutex_;
   State state_ = State::kUninitialized;
 
+  const uint64_t frame_number_;
+
   fml::TimePoint vsync_start_;
   fml::TimePoint vsync_target_;
   fml::TimePoint build_start_;

engine/src/flutter/flow/frame_timings_recorder_unittests.cc

@@ -87,5 +87,22 @@ TEST(FrameTimingsRecorderTest, ThrowWhenRecordRasterBeforeBuildEnd) {
 
 #endif
 
+TEST(FrameTimingsRecorderTest, RecordersHaveUniqueFrameNumbers) {
+  auto recorder1 = std::make_unique<FrameTimingsRecorder>();
+  auto recorder2 = std::make_unique<FrameTimingsRecorder>();
+
+  ASSERT_TRUE(recorder2->GetFrameNumber() > recorder1->GetFrameNumber());
+}
+
+TEST(FrameTimingsRecorderTest, ClonedHasUniqueFrameNumber) {
+  auto recorder = std::make_unique<FrameTimingsRecorder>();
+
+  const auto now = fml::TimePoint::Now();
+  recorder->RecordVsync(now, now);
+
+  auto cloned = recorder->CloneUntil(FrameTimingsRecorder::State::kVsync);
+  ASSERT_NE(recorder->GetFrameNumber(), cloned->GetFrameNumber());
+}
+
 }  // namespace testing
 }  // namespace flutter

engine/src/flutter/shell/common/animator.cc

@@ -41,7 +41,6 @@ Animator::Animator(Delegate& delegate,
               : 2)),
 #endif  // SHELL_ENABLE_METAL
       pending_frame_semaphore_(1),
-      frame_number_(1),
       paused_(false),
       regenerate_layer_tree_(false),
       frame_scheduled_(false),
@@ -86,7 +85,11 @@ void Animator::EnqueueTraceFlowId(uint64_t trace_flow_id) {
 // This Parity is used by the timeline component to correctly align
 // GPU Workloads events with their respective Framework Workload.
 const char* Animator::FrameParity() {
-  return (frame_number_ % 2) ? "even" : "odd";
+  if (!frame_timings_recorder_) {
+    return "even";
+  }
+  uint64_t frame_number = frame_timings_recorder_->GetFrameNumber();
+  return (frame_number % 2) ? "even" : "odd";
 }
 
 static int64_t FxlToDartOrEarlier(fml::TimePoint time) {
@@ -97,7 +100,8 @@ static int64_t FxlToDartOrEarlier(fml::TimePoint time) {
 
 void Animator::BeginFrame(
     std::unique_ptr<FrameTimingsRecorder> frame_timings_recorder) {
-  TRACE_EVENT_ASYNC_END0("flutter", "Frame Request Pending", frame_number_++);
+  TRACE_EVENT_ASYNC_END0("flutter", "Frame Request Pending",
+                         frame_timings_recorder->GetFrameNumber());
 
   frame_timings_recorder_ = std::move(frame_timings_recorder);
   frame_timings_recorder_->RecordBuildStart(fml::TimePoint::Now());
@@ -239,8 +243,13 @@ void Animator::RequestFrame(bool regenerate_layer_tree) {
   // started an expensive operation right after posting this message however.
   // To support that, we need edge triggered wakes on VSync.
 
+  uint64_t frame_number = 0;
+  if (frame_timings_recorder_) {
+    frame_number = frame_timings_recorder_->GetFrameNumber();
+  }
+
   task_runners_.GetUITaskRunner()->PostTask([self = weak_factory_.GetWeakPtr(),
-                                             frame_number = frame_number_]() {
+                                             frame_number = frame_number]() {
     if (!self) {
       return;
     }

engine/src/flutter/shell/common/animator.h

@@ -108,7 +108,6 @@ class Animator final {
   fml::RefPtr<LayerTreePipeline> layer_tree_pipeline_;
   fml::Semaphore pending_frame_semaphore_;
   LayerTreePipeline::ProducerContinuation producer_continuation_;
-  int64_t frame_number_;
   bool paused_;
   bool regenerate_layer_tree_;
   bool frame_scheduled_;