block thread merging with shared engines

shell/common/rasterizer.cc

@@ -466,6 +466,17 @@ RasterStatus Rasterizer::DrawToSurface(flutter::LayerTree& layer_tree) {
         raster_status == RasterStatus::kSkipAndRetry) {
       return raster_status;
     }
+    if (shared_engine_block_thread_merging_ && raster_thread_merger_ &&
+        raster_thread_merger_->IsMerged()) {
+      // TODO(73620): Remove when platform views are accounted for.
+      FML_LOG(ERROR)
+          << "Error: Thread merging not implemented for engines with shared "
+             "components.\n\n"
+             "This is likely a result of using platform views with enigne "
+             "groups.  See "
+             "https://github.com/flutter/flutter/issues/73620.";
+      fml::KillProcess();
+    }
     if (external_view_embedder_ &&
         (!raster_thread_merger_ || raster_thread_merger_->IsMerged())) {
       FML_DCHECK(!frame->IsSubmitted());

shell/common/rasterizer.h

@@ -434,6 +434,15 @@ class Rasterizer final : public SnapshotDelegate {
   ///
   void DisableThreadMergerIfNeeded();
 
+  /// @brief   Mechanism to stop thread merging when using shared engine
+  ///          components.
+  /// @details This is a temporary workaround until thread merging can be
+  ///          supported correctly.  This should be called on the raster
+  ///          thread.
+  /// @see     https://github.com/flutter/flutter/issues/73620
+  ///
+  void BlockThreadMerging() { shared_engine_block_thread_merging_ = true; }
+
  private:
   Delegate& delegate_;
   std::unique_ptr<Surface> surface_;
@@ -450,6 +459,7 @@ class Rasterizer final : public SnapshotDelegate {
   fml::RefPtr<fml::RasterThreadMerger> raster_thread_merger_;
   fml::TaskRunnerAffineWeakPtrFactory<Rasterizer> weak_factory_;
   std::shared_ptr<ExternalViewEmbedder> external_view_embedder_;
+  bool shared_engine_block_thread_merging_ = false;
 
   // |SnapshotDelegate|
   sk_sp<SkImage> MakeRasterSnapshot(sk_sp<SkPicture> picture,

shell/common/shell.cc

@@ -500,6 +500,18 @@ std::unique_ptr<Shell> Shell::Spawn(
       }));
   result->shared_resource_context_ = io_manager_->GetSharedResourceContext();
   result->RunEngine(std::move(run_configuration));
+
+  task_runners_.GetRasterTaskRunner()->PostTask(
+      [rasterizer = rasterizer_->GetWeakPtr(),
+       spawn_rasterizer = result->rasterizer_->GetWeakPtr()]() {
+        if (rasterizer) {
+          rasterizer->BlockThreadMerging();
+        }
+        if (spawn_rasterizer) {
+          spawn_rasterizer->BlockThreadMerging();
+        }
+      });
+
   return result;
 }