[server] Better split flushing events and dispatch events

Running real world applications (Xwayland) showed that it doesn't like
at all that we do a dispatch when we are going to flush. This caused
in a very reliable manner a "Connection closed" error in XWayland, taking
down the client and in return the (xwayland-enabled) server.

src/wayland/autotests/client/test_wayland_surface.cpp

@@ -358,7 +358,7 @@ void TestWaylandSurface::testAttachBuffer()
     QVERIFY(!buffer2->isReferenced());
     delete buffer2;
     // TODO: we should have a signal on when the Buffer gets released
-    QTest::qWait(100);
+    QCoreApplication::processEvents(QEventLoop::WaitForMoreEvents);
     QVERIFY(redBuffer.data()->isReleased());
 
     KWayland::Server::BufferInterface *buffer3 = serverSurface->buffer();

src/wayland/display.cpp

@@ -46,6 +46,7 @@ class Display::Private
 public:
     Private(Display *q);
     void flush();
+    void dispatch();
     void setRunning(bool running);
     void installSocketNotifier();
 
@@ -77,7 +78,7 @@ void Display::Private::installSocketNotifier()
         return;
     }
     QSocketNotifier *m_notifier = new QSocketNotifier(fd, QSocketNotifier::Read, q);
-    QObject::connect(m_notifier, &QSocketNotifier::activated, q, [this] { flush(); } );
+    QObject::connect(m_notifier, &QSocketNotifier::activated, q, [this] { dispatch(); } );
     QObject::connect(QThread::currentThread()->eventDispatcher(), &QAbstractEventDispatcher::aboutToBlock, q, [this] { flush(); });
     setRunning(true);
 }
@@ -94,6 +95,14 @@ Display::~Display()
 }
 
 void Display::Private::flush()
+{
+    if (!display || !loop) {
+        return;
+    }
+    wl_display_flush_clients(display);
+}
+
+void Display::Private::dispatch()
 {
     if (!display || !loop) {
         return;
@@ -101,7 +110,6 @@ void Display::Private::flush()
     if (wl_event_loop_dispatch(loop, 0) != 0) {
         qCWarning(KWAYLAND_SERVER) << "Error on dispatching Wayland event loop";
     }
-    wl_display_flush_clients(display);
 }
 
 void Display::setSocketName(const QString &name)

src/wayland/keyboard_interface.cpp

@@ -143,6 +143,7 @@ void KeyboardInterface::setFocusedSurface(SurfaceInterface *surface, quint32 ser
     wl_array_release(&keys);
 
     d->sendModifiers();
+    d->client->flush();
 }
 
 void KeyboardInterface::keyPressed(quint32 key, quint32 serial)

src/wayland/pointer_interface.cpp

@@ -135,6 +135,7 @@ void PointerInterface::setFocusedSurface(SurfaceInterface *surface, quint32 seri
     wl_pointer_send_enter(d->resource, serial,
                           d->focusedSurface->resource(),
                           wl_fixed_from_double(pos.x()), wl_fixed_from_double(pos.y()));
+    d->client->flush();
 }
 
 void PointerInterface::buttonPressed(quint32 button, quint32 serial)

src/wayland/server/buffer_interface.cpp

@@ -181,6 +181,7 @@ void BufferInterface::unref()
     if (d->refCount == 0) {
         if (d->buffer) {
             wl_buffer_send_release(d->buffer);
+            wl_client_flush(wl_resource_get_client(d->buffer));
         }
         deleteLater();
     }