Fix unreliable MediaStream capture from WebGL canvases

third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp

Index: third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp
diff --git a/third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp b/third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp
index 413841dd2ab3079a3dab294718cf741890dad911..355af9166312627ab05f91701b37b7ad17fb0d44 100644
--- a/third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp
+++ b/third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp
@@ -329,7 +329,17 @@ void HTMLCanvasElement::didDraw(const FloatRect& rect) {
 void HTMLCanvasElement::finalizeFrame() {
   if (hasImageBuffer())
     m_imageBuffer->finalizeFrame();
-  notifyListenersCanvasChanged();
+
+  // If the canvas is visible, notifying listeners is taken
+  // care of in the in doDeferredPaintInvalidation, which allows
+  // the frame to be grabbed prior to compositing, which is
+  // critically important because compositing may clear the canvas's
+  // image. (e.g. WebGL context with preserveDrawingBuffer=false).
+  // If the canvas is not visible, doDeferredPaintInvalidation
+  // will not get called, so we need to take care of business here.
+  if (!m_didNotifyListenersForCurrentFrame)
+    notifyListenersCanvasChanged();
+  m_didNotifyListenersForCurrentFrame = false;

[xlai (Olivia), 2017/03/22 17:06:21]

I'm thinking of a scenario when the canvas is visible and finalizeFrame() and
doPaintInvalidation() occur interleaving each other...Then the
m_didNotifyListenersForCurrentFrame is set to true and false back and forth...Is
that what you intend to do?

 }
 
 void HTMLCanvasElement::didDisableAcceleration() {
@@ -371,6 +381,9 @@ void HTMLCanvasElement::doDeferredPaintInvalidation() {
   if (m_dirtyRect.isEmpty())
     return;
 
+  notifyListenersCanvasChanged();
+  m_didNotifyListenersForCurrentFrame = true;
+
   // Propagate the m_dirtyRect accumulated so far to the compositor
   // before restarting with a blank dirty rect.
   FloatRect srcRect(0, 0, size().width(), size().height());