Fix race condition on mWaitingForUpdateDelivery

Change-Id: I0e91bda73722468b9339fc434fe04420b5e7d3da
Reviewed-by: David Edmundson <davidedmundson@kde.org>

src/plugins/platforms/wayland/qwaylandwindow.cpp

@@ -716,18 +716,17 @@ void QWaylandWindow::handleFrameCallback()
     mFrameCallbackElapsedTimer.invalidate();
 
     // The rest can wait until we can run it on the correct thread
-    if (!mWaitingForUpdateDelivery) {
+    if (mWaitingForUpdateDelivery.testAndSetAcquire(false, true)) {
         // Queued connection, to make sure we don't call handleUpdate() from inside waitForFrameSync()
         // in the single-threaded case.
-        mWaitingForUpdateDelivery = true;
         QMetaObject::invokeMethod(this, &QWaylandWindow::doHandleFrameCallback, Qt::QueuedConnection);
     }
-
     mFrameSyncWait.notify_all();
 }
 
 void QWaylandWindow::doHandleFrameCallback()
 {
+    mWaitingForUpdateDelivery.storeRelease(false);
     bool wasExposed = isExposed();
     mFrameCallbackTimedOut = false;
     if (!wasExposed && isExposed()) // Did setting mFrameCallbackTimedOut make the window exposed?
@@ -735,7 +734,6 @@ void QWaylandWindow::doHandleFrameCallback()
     if (wasExposed && hasPendingUpdateRequest())
         deliverUpdateRequest();
 
-    mWaitingForUpdateDelivery = false;
 }
 
 bool QWaylandWindow::waitForFrameSync(int timeout)

src/plugins/platforms/wayland/qwaylandwindow_p.h

@@ -283,7 +283,7 @@ protected:
     WId mWindowId;
     bool mWaitingForFrameCallback = false;
     bool mFrameCallbackTimedOut = false; // Whether the frame callback has timed out
-    bool mWaitingForUpdateDelivery = false;
+    QAtomicInt mWaitingForUpdateDelivery = false;
     int mFrameCallbackCheckIntervalTimerId = -1;
     QElapsedTimer mFrameCallbackElapsedTimer;
     struct ::wl_callback *mFrameCallback = nullptr;