[fuchsia][shader warmup] fix for fxbug.dev/90387

Warmup surface was being initialized on the platform thread rather than
the raster thread which was causing races in the skia cache, this change
just moves the warmup surface intiialization to the raster thread.

This change also addresses another minor problem where invoking shader
warmup with 0 skps would drop the callback on the floor and that would
cause the dart future to hang forever. Simply invoke the dart future if
no SKP's are provided.

Author: freiling

shell/platform/fuchsia/flutter/engine.cc

@@ -861,24 +861,10 @@ void Engine::WarmupSkps(
     std::shared_ptr<flutter::AssetManager> asset_manager,
     std::optional<const std::vector<std::string>> skp_names,
     std::optional<std::function<void(uint32_t)>> completion_callback) {
-  // We use a raw pointer here because we want to keep this alive until all gpu
-  // work is done and the callbacks skia takes for this are function pointers
-  // so we are unable to use a lambda that captures the smart pointer.
-  SurfaceProducerSurface* skp_warmup_surface =
-      surface_producer->ProduceOffscreenSurface(size).release();
-  if (!skp_warmup_surface) {
-    FML_LOG(ERROR) << "Failed to create offscreen warmup surface";
-    // Tell client that zero shaders were warmed up because warmup failed.
-    if (completion_callback.has_value() && completion_callback.value()) {
-      completion_callback.value()(0);
-    }
-    return;
-  }
-
   // tell concurrent task runner to deserialize all skps available from
   // the asset manager
-  concurrent_task_runner->PostTask([raster_task_runner, skp_warmup_surface,
-                                    surface_producer, asset_manager, skp_names,
+  concurrent_task_runner->PostTask([raster_task_runner, size, surface_producer,
+                                    asset_manager, skp_names,
                                     completion_callback]() {
     TRACE_DURATION("flutter", "DeserializeSkps");
     std::vector<std::unique_ptr<fml::Mapping>> skp_mappings;
@@ -895,6 +881,15 @@ void Engine::WarmupSkps(
       skp_mappings = asset_manager->GetAsMappings(".*\\.skp$", "shaders");
     }
 
+    if (skp_mappings.size() == 0) {
+      FML_LOG(WARNING)
+          << "Engine::WarmupSkps got zero SKP mappings, returning early";
+      if (completion_callback.has_value() && completion_callback.value()) {
+        completion_callback.value()(0);
+      }
+      return;
+    }
+
     size_t total_size = 0;
     for (auto& mapping : skp_mappings) {
       total_size += mapping->GetSize();
@@ -920,10 +915,25 @@ void Engine::WarmupSkps(
 
       // Tell raster task runner to warmup have the compositor
       // context warm up the newly deserialized picture
-      raster_task_runner->PostTask([skp_warmup_surface, picture,
-                                    surface_producer, completion_callback, i,
+      raster_task_runner->PostTask([picture, size, surface_producer,
+                                    completion_callback, i,
                                     count = skp_mappings.size()] {
         TRACE_DURATION("flutter", "WarmupSkp");
+
+        // We use a raw pointer here because we want to keep this alive until
+        // all gpu work is done and the callbacks skia takes for this are
+        // function pointers so we are unable to use a lambda that captures the
+        // smart pointer.
+        static SurfaceProducerSurface* skp_warmup_surface =
+            surface_producer->ProduceOffscreenSurface(size).release();
+        if (!skp_warmup_surface) {
+          FML_LOG(ERROR) << "Failed to create offscreen warmup surface";
+          // Tell client that zero shaders were warmed up because warmup failed.
+          if (completion_callback.has_value() && completion_callback.value()) {
+            completion_callback.value()(0);
+          }
+          return;
+        }
         skp_warmup_surface->GetSkiaSurface()->getCanvas()->drawPicture(picture);
 
         if (i == count - 1) {