Add an explicit flush after copyTextureCHROMIUM() to make the WebGL layer contents updated.

Source/platform/graphics/gpu/DrawingBuffer.cpp

Index: Source/platform/graphics/gpu/DrawingBuffer.cpp
diff --git a/Source/platform/graphics/gpu/DrawingBuffer.cpp b/Source/platform/graphics/gpu/DrawingBuffer.cpp
index 8b3bd008303b195f7fdd492ce4be180db5346f18..6a191d70bd37dd27aa76acff3cc53811dc191ef6 100644
--- a/Source/platform/graphics/gpu/DrawingBuffer.cpp
+++ b/Source/platform/graphics/gpu/DrawingBuffer.cpp
@@ -389,6 +389,7 @@ void DrawingBuffer::paintCompositedResultsToCanvas(ImageBuffer* imageBuffer)
     if (tex) {
         m_context->copyTextureCHROMIUM(GL_TEXTURE_2D, m_frontColorBuffer,
             tex, 0, GL_RGBA, GL_UNSIGNED_BYTE);
+        m_context->flush();

[Ken Russell (switch to Gerrit), 2014/03/17 19:17:32]

Uh oh. If this is really the needed fix then there are a bunch of other places
throughout the code that also need to be fixed.

[no sievers, 2014/03/17 19:48:09]

We do need to glFlush() whenever we need the changes to be visible in another
context. We seem to do this here in DrawingBuffer::prepareMailbox() where we
pass the texture to the compositor in a mailbox
(Canvas2DLayerBridge::prepareMailbox() also does this, see webContext->flush()).

I don't know what the return path in here implies (also vs. the path below) and
who will be consuming this texture.

[Ken Russell (switch to Gerrit), 2014/03/17 21:34:16]

My point is that there are other callers of copyTextureCHROMIUM which do not
issue a flush immediately afterward. At least there's one in
RendererGpuVideoAcceleratorFactories::ReadPixels
(src/content/renderer/media/renderer_gpu_video_accelerator_factories.cc).

I strongly feel that we should add a Telemetry test for this right away so that
it doesn't regress again.

[vmiura, 2014/03/17 21:35:08]

Checked if other sites have calls to flush().

ImageBuffer::copyToPlatformTexture                       YES
DrawingBuffer::prepareMailbox                            YES
DrawingBuffer::copyToPlatformTexture                     YES
WebMediaPlayerAndroid::copyVideoTextureToPlatformTexture NO
RendererGpuVideoAcceleratorFactories::ReadPixels         NO
WebMediaPlayerImpl::copyVideoTextureToPlatformTexture    YES
DrawingBuffer::paintCompositedResultsToCanvas            NO  <- this CL

Flushing automatically in copyTextureCHROMIUM may not be sufficient as
copyTextureCHROMIUM may not be the last command.

example:
  context->copyTextureCHROMIUM(GL_TEXTURE_2D, getBackingTexture(), texture,
level, internalFormat, destType);
  context->pixelStorei(GC3D_UNPACK_FLIP_Y_CHROMIUM, false);
  context->pixelStorei(GC3D_UNPACK_UNPREMULTIPLY_ALPHA_CHROMIUM, false);
  context->flush();

[piman, 2014/03/18 22:43:40]

Right, what matters is not the copyTextureCHROMIUM per se, it's about when we
need ordering between contexts.
The convention is that flush() is needed to ensure ordering. The reason is that
mac needs a (deep) flush.

         return;
     }