[Threaded Compositor] Flickering and rendering artifacts when resizin…

…g the web view

https://bugs.webkit.org/show_bug.cgi?id=154070

Reviewed by Žan Doberšek.

Resizing the web view is expected to be a sync operation, the UI process creates a new backing store state ID,
sends UpdateBackingStoreState message with the flag RespondImmediately to the web process and waits up to 500ms
for the reply (DidUpdateBackingStoreState message). When using the threaded compositor, we schedule a task in
the compositing thread to update the viewport size, and return immediately, so that we reply to the UI process
before the compositing thread has actually updated its size. There's a moment in which sizes are out of sync
causing the flickering and rendering artifacts, the UI process continues rendering at the new size, while the
web process is still rendering at the previous size. We can prevent this from happening just by making the
resize task synchronous in the threaded compositor.

* Shared/CoordinatedGraphics/threadedcompositor/CompositingRunLoop.cpp:
(WebKit::CompositingRunLoop::performTaskSync): Add sync version of performTask().
* Shared/CoordinatedGraphics/threadedcompositor/CompositingRunLoop.h:
* Shared/CoordinatedGraphics/threadedcompositor/ThreadedCompositor.cpp:
(WebKit::ThreadedCompositor::didChangeViewportSize): Use performTaskSync().

Canonical link: https://commits.webkit.org/176816@main
git-svn-id: https://svn.webkit.org/repository/webkit/trunk@202037 268f45cc-cd09-0410-ab3c-d52691b4dbfc

Source/WebKit2/ChangeLog

@@ -1,3 +1,25 @@
+2016-06-14  Carlos Garcia Campos  <cgarcia@igalia.com>
+
+        [Threaded Compositor] Flickering and rendering artifacts when resizing the web view
+        https://bugs.webkit.org/show_bug.cgi?id=154070
+
+        Reviewed by Žan Doberšek.
+
+        Resizing the web view is expected to be a sync operation, the UI process creates a new backing store state ID,
+        sends UpdateBackingStoreState message with the flag RespondImmediately to the web process and waits up to 500ms
+        for the reply (DidUpdateBackingStoreState message). When using the threaded compositor, we schedule a task in
+        the compositing thread to update the viewport size, and return immediately, so that we reply to the UI process
+        before the compositing thread has actually updated its size. There's a moment in which sizes are out of sync
+        causing the flickering and rendering artifacts, the UI process continues rendering at the new size, while the
+        web process is still rendering at the previous size. We can prevent this from happening just by making the
+        resize task synchronous in the threaded compositor.
+
+        * Shared/CoordinatedGraphics/threadedcompositor/CompositingRunLoop.cpp:
+        (WebKit::CompositingRunLoop::performTaskSync): Add sync version of performTask().
+        * Shared/CoordinatedGraphics/threadedcompositor/CompositingRunLoop.h:
+        * Shared/CoordinatedGraphics/threadedcompositor/ThreadedCompositor.cpp:
+        (WebKit::ThreadedCompositor::didChangeViewportSize): Use performTaskSync().
+
 2016-06-14  Carlos Garcia Campos  <cgarcia@igalia.com>
 
         Unreviewed. Fix Soup downloads after r201943.

Source/WebKit2/Shared/CoordinatedGraphics/threadedcompositor/CompositingRunLoop.cpp

@@ -37,7 +37,6 @@ CompositingRunLoop::CompositingRunLoop(std::function<void ()>&& updateFunction)
     : m_runLoop(RunLoop::current())
     , m_updateTimer(m_runLoop, this, &CompositingRunLoop::updateTimerFired)
     , m_updateFunction(WTFMove(updateFunction))
-    , m_lastUpdateTime(0)
 {
 }
 
@@ -47,6 +46,18 @@ void CompositingRunLoop::performTask(std::function<void ()>&& function)
     m_runLoop.dispatch(WTFMove(function));
 }
 
+void CompositingRunLoop::performTaskSync(std::function<void ()>&& function)
+{
+    ASSERT(isMainThread());
+    LockHolder locker(m_dispatchSyncConditionMutex);
+    m_runLoop.dispatch([this, function = WTFMove(function)] {
+        LockHolder locker(m_dispatchSyncConditionMutex);
+        function();
+        m_dispatchSyncCondition.notifyOne();
+    });
+    m_dispatchSyncCondition.wait(m_dispatchSyncConditionMutex);
+}
+
 void CompositingRunLoop::setUpdateTimer(UpdateTiming timing)
 {
     if (m_updateTimer.isActive())

Source/WebKit2/Shared/CoordinatedGraphics/threadedcompositor/CompositingRunLoop.h

@@ -47,6 +47,7 @@ class CompositingRunLoop {
     CompositingRunLoop(std::function<void ()>&&);
 
     void performTask(std::function<void ()>&&);
+    void performTaskSync(std::function<void ()>&&);
 
     void setUpdateTimer(UpdateTiming timing = Immediate);
     void stopUpdateTimer();
@@ -59,8 +60,10 @@ class CompositingRunLoop {
     RunLoop& m_runLoop;
     RunLoop::Timer<CompositingRunLoop> m_updateTimer;
     std::function<void ()> m_updateFunction;
+    Lock m_dispatchSyncConditionMutex;
+    Condition m_dispatchSyncCondition;
 
-    double m_lastUpdateTime;
+    double m_lastUpdateTime { 0 };
 };
 
 } // namespace WebKit

Source/WebKit2/Shared/CoordinatedGraphics/threadedcompositor/ThreadedCompositor.cpp

@@ -82,7 +82,7 @@ void ThreadedCompositor::setDeviceScaleFactor(float scale)
 void ThreadedCompositor::didChangeViewportSize(const IntSize& size)
 {
     RefPtr<ThreadedCompositor> protector(this);
-    m_compositingRunLoop->performTask([protector, size] {
+    m_compositingRunLoop->performTaskSync([protector, size] {
         protector->viewportController()->didChangeViewportSize(size);
     });
 }