[Rasterizer] Make resubmit information temporary (flutter/engine#42001)

This PR refactors `Rasterizer` so that the resubmit information is
returned as a temporary struct instead of stored permanently as class
member. This limits the lifetime of these values and reduces class
members.

Additionally, this benefits the multi-view project: if the rasterizer is
going to store multiple surfaces, what variables should be stored in a
map from view IDs? Should all surfaces have a to-be-submitted record?
The answer is "no", because these information is only temporary.

This PR should not need unit tests since it's only a refactor.

## Pre-launch Checklist

- [ ] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [ ] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [ ] I read and followed the [Flutter Style Guide] and the [C++,
Objective-C, Java style guides].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [ ] I added new tests to check the change I am making or feature I am
adding, or Hixie said the PR is test-exempt. See [testing the engine]
for instructions on writing and running engine tests.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [ ] I signed the [CLA].
- [ ] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

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

@@ -193,18 +193,19 @@ RasterStatus Rasterizer::Draw(
                  .GetRasterTaskRunner()
                  ->RunsTasksOnCurrentThread());
 
-  RasterStatus raster_status = RasterStatus::kFailed;
+  DoDrawResult draw_result;
   LayerTreePipeline::Consumer consumer =
-      [&](std::unique_ptr<LayerTreeItem> item) {
+      [this, &draw_result,
+       &discard_callback](std::unique_ptr<LayerTreeItem> item) {
         std::unique_ptr<LayerTree> layer_tree = std::move(item->layer_tree);
         std::unique_ptr<FrameTimingsRecorder> frame_timings_recorder =
             std::move(item->frame_timings_recorder);
         float device_pixel_ratio = item->device_pixel_ratio;
         if (discard_callback(*layer_tree.get())) {
-          raster_status = RasterStatus::kDiscarded;
+          draw_result.raster_status = RasterStatus::kDiscarded;
         } else {
-          raster_status = DoDraw(std::move(frame_timings_recorder),
-                                 std::move(layer_tree), device_pixel_ratio);
+          draw_result = DoDraw(std::move(frame_timings_recorder),
+                               std::move(layer_tree), device_pixel_ratio);
         }
       };
 
@@ -215,18 +216,15 @@ RasterStatus Rasterizer::Draw(
   // if the raster status is to resubmit the frame, we push the frame to the
   // front of the queue and also change the consume status to more available.
 
-  bool should_resubmit_frame = ShouldResubmitFrame(raster_status);
+  bool should_resubmit_frame = ShouldResubmitFrame(draw_result.raster_status);
   if (should_resubmit_frame) {
-    auto resubmitted_layer_tree_item = std::make_unique<LayerTreeItem>(
-        std::move(resubmitted_layer_tree_), std::move(resubmitted_recorder_),
-        resubmitted_pixel_ratio_);
     auto front_continuation = pipeline->ProduceIfEmpty();
-    PipelineProduceResult result =
-        front_continuation.Complete(std::move(resubmitted_layer_tree_item));
-    if (result.success) {
+    PipelineProduceResult pipeline_result = front_continuation.Complete(
+        std::move(draw_result.resubmitted_layer_tree_item));
+    if (pipeline_result.success) {
       consume_result = PipelineConsumeResult::MoreAvailable;
     }
-  } else if (raster_status == RasterStatus::kEnqueuePipeline) {
+  } else if (draw_result.raster_status == RasterStatus::kEnqueuePipeline) {
     consume_result = PipelineConsumeResult::MoreAvailable;
   }
 
@@ -255,7 +253,7 @@ RasterStatus Rasterizer::Draw(
       break;
   }
 
-  return raster_status;
+  return draw_result.raster_status;
 }
 
 bool Rasterizer::ShouldResubmitFrame(const RasterStatus& raster_status) {
@@ -376,7 +374,7 @@ fml::Milliseconds Rasterizer::GetFrameBudget() const {
   return delegate_.GetFrameBudget();
 };
 
-RasterStatus Rasterizer::DoDraw(
+Rasterizer::DoDrawResult Rasterizer::DoDraw(
     std::unique_ptr<FrameTimingsRecorder> frame_timings_recorder,
     std::unique_ptr<flutter::LayerTree> layer_tree,
     float device_pixel_ratio) {
@@ -387,7 +385,9 @@ RasterStatus Rasterizer::DoDraw(
                  ->RunsTasksOnCurrentThread());
 
   if (!layer_tree || !surface_) {
-    return RasterStatus::kFailed;
+    return DoDrawResult{
+        .raster_status = RasterStatus::kFailed,
+    };
   }
 
   PersistentCache* persistent_cache = PersistentCache::GetCacheForProcess();
@@ -399,13 +399,18 @@ RasterStatus Rasterizer::DoDraw(
     last_layer_tree_ = std::move(layer_tree);
     last_device_pixel_ratio_ = device_pixel_ratio;
   } else if (ShouldResubmitFrame(raster_status)) {
-    resubmitted_pixel_ratio_ = device_pixel_ratio;
-    resubmitted_layer_tree_ = std::move(layer_tree);
-    resubmitted_recorder_ = frame_timings_recorder->CloneUntil(
-        FrameTimingsRecorder::State::kBuildEnd);
-    return raster_status;
+    return DoDrawResult{
+        .raster_status = raster_status,
+        .resubmitted_layer_tree_item = std::make_unique<LayerTreeItem>(
+            std::move(layer_tree),
+            frame_timings_recorder->CloneUntil(
+                FrameTimingsRecorder::State::kBuildEnd),
+            device_pixel_ratio),
+    };
   } else if (raster_status == RasterStatus::kDiscarded) {
-    return raster_status;
+    return DoDrawResult{
+        .raster_status = raster_status,
+    };
   }
 
   if (persistent_cache->IsDumpingSkp() &&
@@ -473,11 +478,15 @@ RasterStatus Rasterizer::DoDraw(
   if (raster_thread_merger_) {
     if (raster_thread_merger_->DecrementLease() ==
         fml::RasterThreadStatus::kUnmergedNow) {
-      return RasterStatus::kEnqueuePipeline;
+      return DoDrawResult{
+          .raster_status = RasterStatus::kEnqueuePipeline,
+      };
     }
   }
 
-  return raster_status;
+  return DoDrawResult{
+      .raster_status = raster_status,
+  };
 }
 
 RasterStatus Rasterizer::DrawToSurface(

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

@@ -500,6 +500,19 @@ class Rasterizer final : public SnapshotDelegate,
   void DisableThreadMergerIfNeeded();
 
  private:
+  // The result of `DoDraw`.
+  //
+  // Normally `DoDraw` returns simply a raster status. However, sometimes we
+  // need to attempt to rasterize the layer tree again. This happens when
+  // layer_tree has not successfully rasterized due to changes in the thread
+  // configuration, in which case the resubmitted task will be inserted to the
+  // front of the pipeline.
+  struct DoDrawResult {
+    RasterStatus raster_status = RasterStatus::kFailed;
+
+    std::unique_ptr<LayerTreeItem> resubmitted_layer_tree_item;
+  };
+
   // |SnapshotDelegate|
   std::unique_ptr<GpuImageResult> MakeSkiaGpuImage(
       sk_sp<DisplayList> display_list,
@@ -554,7 +567,7 @@ class Rasterizer final : public SnapshotDelegate,
       GrDirectContext* surface_context,
       bool compressed);
 
-  RasterStatus DoDraw(
+  DoDrawResult DoDraw(
       std::unique_ptr<FrameTimingsRecorder> frame_timings_recorder,
       std::unique_ptr<flutter::LayerTree> layer_tree,
       float device_pixel_ratio);
@@ -581,12 +594,6 @@ class Rasterizer final : public SnapshotDelegate,
   // This is the last successfully rasterized layer tree.
   std::unique_ptr<flutter::LayerTree> last_layer_tree_;
   float last_device_pixel_ratio_;
-  // Set when we need attempt to rasterize the layer tree again. This layer_tree
-  // has not successfully rasterized. This can happen due to the change in the
-  // thread configuration. This will be inserted to the front of the pipeline.
-  std::unique_ptr<flutter::LayerTree> resubmitted_layer_tree_;
-  std::unique_ptr<FrameTimingsRecorder> resubmitted_recorder_;
-  float resubmitted_pixel_ratio_;
   fml::closure next_frame_callback_;
   bool user_override_resource_cache_bytes_;
   std::optional<size_t> max_cache_bytes_;