Bug 1457466 - Make the GenerateFrame transaction bypass the scene builder thread. r=nical

This allows frames to be generated by the render backend thread even
while the scene builder thread is busy with a long scene build. The
GenerateFrame transaction also contains APZ and OMTA information, so
this allows the user to scroll and view OMTAnimations during long scene
builds.

MozReview-Commit-ID: KG5YC2KwIaH

--HG--
extra : rebase_source : 3ba559aa22a3a036a3b3a034ea20caacdc8c864a

gfx/layers/wr/WebRenderBridgeParent.cpp

@@ -1296,14 +1296,17 @@ WebRenderBridgeParent::CompositeToTarget(gfx::DrawTarget* aTarget, const gfx::In
 
   mAsyncImageManager->SetCompositionTime(TimeStamp::Now());
 
-  // TODO: We can improve upon this by using two transactions: one for everything that
-  // doesn't change the display list (in other words does not cause the scene to be
-  // re-built), and one for the rest. This way, if an async pipeline needs to re-build
-  // its display list, other async pipelines can still be rendered while the scene is
-  // building.
-  wr::TransactionBuilder txn;
-  mAsyncImageManager->ApplyAsyncImages(txn);
-  mApi->SendTransaction(txn);
+  {
+    // TODO: We can improve upon this by using two transactions: one for everything that
+    // doesn't change the display list (in other words does not cause the scene to be
+    // re-built), and one for the rest. This way, if an async pipeline needs to re-build
+    // its display list, other async pipelines can still be rendered while the scene is
+    // building. Those other async pipelines can go in the other transaction that
+    // we create below.
+    wr::TransactionBuilder txn;
+    mAsyncImageManager->ApplyAsyncImages(txn);
+    mApi->SendTransaction(txn);
+  }
 
   if (!mAsyncImageManager->GetCompositeUntilTime().IsNull()) {
     // Trigger another CompositeToTarget() call because there might be another
@@ -1319,6 +1322,11 @@ WebRenderBridgeParent::CompositeToTarget(gfx::DrawTarget* aTarget, const gfx::In
     return;
   }
 
+  // Ensure this GenerateFrame is handled on the render backend thread rather
+  // than going through the scene builder thread. That way we continue generating
+  // frames with the old scene even during slow scene builds.
+  wr::TransactionBuilder txn(/* aUseSceneBuilderThread */ false);
+
   nsTArray<wr::WrOpacityProperty> opacityArray;
   nsTArray<wr::WrTransformProperty> transformArray;
 

gfx/webrender_bindings/WebRenderAPI.cpp

@@ -133,8 +133,8 @@ private:
 };
 
 
-TransactionBuilder::TransactionBuilder()
-  : mUseSceneBuilderThread(gfxPrefs::WebRenderAsyncSceneBuild())
+TransactionBuilder::TransactionBuilder(bool aUseSceneBuilderThread)
+  : mUseSceneBuilderThread(gfxPrefs::WebRenderAsyncSceneBuild() && aUseSceneBuilderThread)
 {
   mTxn = wr_transaction_new(mUseSceneBuilderThread);
   mResourceUpdates = wr_resource_updates_new();

gfx/webrender_bindings/WebRenderAPI.h

@@ -51,7 +51,7 @@ struct Line {
 
 class TransactionBuilder {
 public:
-  TransactionBuilder();
+  explicit TransactionBuilder(bool aUseSceneBuilderThread = true);
 
   ~TransactionBuilder();