Synchronize commits at end of JS task

third_party/WebKit/Source/core/offscreencanvas/OffscreenCanvas.cpp

Index: third_party/WebKit/Source/core/offscreencanvas/OffscreenCanvas.cpp
diff --git a/third_party/WebKit/Source/core/offscreencanvas/OffscreenCanvas.cpp b/third_party/WebKit/Source/core/offscreencanvas/OffscreenCanvas.cpp
index a41f0c69a83ff962d9490753f956682b135ca320..7db0de08caa1b4bbaedaf9c5015aa6e37886c702 100644
--- a/third_party/WebKit/Source/core/offscreencanvas/OffscreenCanvas.cpp
+++ b/third_party/WebKit/Source/core/offscreencanvas/OffscreenCanvas.cpp
@@ -4,6 +4,7 @@
 
 #include "core/offscreencanvas/OffscreenCanvas.h"
 
+#include <memory>
 #include "core/dom/ExceptionCode.h"
 #include "core/fileapi/Blob.h"
 #include "core/frame/ImageBitmap.h"
@@ -17,8 +18,8 @@
 #include "platform/graphics/OffscreenCanvasFrameDispatcherImpl.h"
 #include "platform/graphics/StaticBitmapImage.h"
 #include "platform/image-encoders/ImageEncoderUtils.h"
+#include "public/platform/Platform.h"
 #include "wtf/MathExtras.h"
-#include <memory>
 
 namespace blink {
 
@@ -237,20 +238,40 @@ ScriptPromise OffscreenCanvas::commit(RefPtr<StaticBitmapImage> image,
                                       bool isWebGLSoftwareRendering,
                                       ScriptState* scriptState) {
   getOrCreateFrameDispatcher()->setNeedsBeginFrame(true);
-  if (m_commitPromiseResolver) {
+
+  if (!m_commitPromiseResolver) {
+    m_commitPromiseResolver = ScriptPromiseResolver::create(scriptState);
+    m_commitPromiseResolver->keepAliveWhilePending();
+
+    m_overdrawFrame = nullptr;

[Justin Novosad, 2017/03/24 17:41:53]

We don't to track m_overdrawFrame anymore.

     if (image) {
+      // We defer the submission of commit frames at the end of JS task
+      m_currentFrame = std::move(image);
+      m_currentFrameIsWebGLSoftwareRendering = isWebGLSoftwareRendering;
+      Platform::current()->currentThread()->addTaskObserver(m_context);
+    }
+  } else if (image) {
+    if (m_currentFrame) {
+      // An override of m_currentFrame can happen when there are multiple
+      // frames committed before JS task finishes.
+      m_currentFrame = std::move(image);
+      m_currentFrameIsWebGLSoftwareRendering = isWebGLSoftwareRendering;
+    } else {
+      // The current frame has been dispatched but the promise is not resolved
+      // yet.
       m_overdrawFrame = std::move(image);
       m_overdrawFrameIsWebGLSoftwareRendering = isWebGLSoftwareRendering;
     }
-  } else {
-    m_overdrawFrame = nullptr;
-    m_commitPromiseResolver = ScriptPromiseResolver::create(scriptState);
-    m_commitPromiseResolver->keepAliveWhilePending();
-    doCommit(std::move(image), isWebGLSoftwareRendering);
   }
+
   return m_commitPromiseResolver->promise();
 }
 
+void OffscreenCanvas::doCommitAtEndOfTask() {

[Justin Novosad, 2017/03/24 17:41:53]

What happens if we reach this point and m_commitPromiseResolver is still
pending?  IMHO, we need to treat m_currentFrame as an overdraw frame in that
case, and postpone the commit until beginFrame().

[xlai (Olivia), 2017/03/24 18:18:52]

Actually, this situation was already handled in the original code. But I think I
used more than enough variables to hold the frames; m_currentFrame can be
reused in the overdraw situation. So I remove the
m_overdrawFrame now.

+  doCommit(std::move(m_currentFrame), m_currentFrameIsWebGLSoftwareRendering);
+  Platform::current()->currentThread()->removeTaskObserver(m_context);
+}
+
 void OffscreenCanvas::doCommit(RefPtr<StaticBitmapImage> image,
                                bool isWebGLSoftwareRendering) {
   double commitStartTime = WTF::monotonicallyIncreasingTime();
@@ -264,7 +285,7 @@ void OffscreenCanvas::beginFrame() {
     // first and save the promise resolution for later.
     doCommit(std::move(m_overdrawFrame),
              m_overdrawFrameIsWebGLSoftwareRendering);
-  } else if (m_commitPromiseResolver) {
+  } else if (!m_currentFrame && m_commitPromiseResolver) {
     m_commitPromiseResolver->resolve();
     m_commitPromiseResolver.clear();
     // We need to tell parent frame to stop sending signals on begin frame to