Fix possible race condition leading to a dead lock

It is a bit dangerous to call wl_display_dispatch() in the event thread,
since it may race with the dispatch called e.g. in QWaylandDisplay's
blockingReadEvents() and lead to a dead lock. Instead, use wl_display_prepare_read()
and wl_display_read_events() in the event thread, which doesn't block, and only
dispatch in QWaylandDisplay.
As a result we don't need the additional wayland queue anymore, so remove it.

Change-Id: I9fbbe5d2f38d06773beb7847df1a0212cca92c37
Reviewed-by: Robin Burchell <robin.burchell@viroteck.net>

5 files changed, 6 insertions, 21 deletions

diff --git a/examples/wayland/server-buffer/client/main.cpp b/examples/wayland/server-buffer/client/main.cpp
index 3f7a94d03..1a2c447a7 100644
--- a/examples/wayland/server-buffer/client/main.cpp
+++ b/examples/wayland/server-buffer/client/main.cpp
@@ -95,7 +95,6 @@ public:
 
         QtWaylandClient::QWaylandDisplay *wayland_display = wayland_integration->display();
         struct ::wl_registry *registry = wl_display_get_registry(wayland_display->wl_display());
-        wl_proxy_set_queue(reinterpret_cast<struct wl_proxy *>(registry), wayland_display->wl_event_queue());
         QtWayland::wl_registry::init(registry);
     }
 
diff --git a/src/client/qwaylanddisplay.cpp b/src/client/qwaylanddisplay.cpp
index eb431cf0d..f197b3788 100644
--- a/src/client/qwaylanddisplay.cpp
+++ b/src/client/qwaylanddisplay.cpp
@@ -147,12 +147,7 @@ QWaylandDisplay::QWaylandDisplay(QWaylandIntegration *waylandIntegration)
     mEventThreadObject->displayConnect();
     mDisplay = mEventThreadObject->display(); //blocks until display is available
 
-    //Create a new even queue for the QtGui thread
-    mEventQueue = wl_display_create_queue(mDisplay);
-
     struct ::wl_registry *registry = wl_display_get_registry(mDisplay);
-    wl_proxy_set_queue((struct wl_proxy *)registry, mEventQueue);
-
     init(registry);
 
     connect(mEventThreadObject, SIGNAL(newEventsRead()), this, SLOT(flushRequests()));
@@ -180,7 +175,7 @@ QWaylandDisplay::~QWaylandDisplay(void)
 
 void QWaylandDisplay::flushRequests()
 {
-    if (wl_display_dispatch_queue_pending(mDisplay, mEventQueue) < 0) {
+    if (wl_display_dispatch_pending(mDisplay) < 0) {
         mEventThreadObject->checkError();
         exitWithError();
     }
@@ -191,7 +186,7 @@ void QWaylandDisplay::flushRequests()
 
 void QWaylandDisplay::blockingReadEvents()
 {
-    if (wl_display_dispatch_queue(mDisplay, mEventQueue) < 0) {
+    if (wl_display_dispatch(mDisplay) < 0) {
         mEventThreadObject->checkError();
         exitWithError();
     }
@@ -345,17 +340,16 @@ void QWaylandDisplay::forceRoundTrip()
     int ret = 0;
     bool done = false;
     wl_callback *callback = wl_display_sync(mDisplay);
-    wl_proxy_set_queue((struct wl_proxy *)callback, mEventQueue);
     wl_callback_add_listener(callback, &sync_listener, &done);
     flushRequests();
     if (QThread::currentThread()->eventDispatcher()) {
         while (!done && ret >= 0) {
             QThread::currentThread()->eventDispatcher()->processEvents(QEventLoop::WaitForMoreEvents);
-            ret = wl_display_dispatch_queue_pending(mDisplay, mEventQueue);
+            ret = wl_display_dispatch_pending(mDisplay);
         }
     } else {
         while (!done && ret >= 0)
-            ret = wl_display_dispatch_queue(mDisplay, mEventQueue);
+            ret = wl_display_dispatch(mDisplay);
     }
 
     if (ret == -1 && !done)
diff --git a/src/client/qwaylanddisplay_p.h b/src/client/qwaylanddisplay_p.h
index 7ba431d7c..ee129a50e 100644
--- a/src/client/qwaylanddisplay_p.h
+++ b/src/client/qwaylanddisplay_p.h
@@ -108,7 +108,6 @@ public:
     void setCursor(struct wl_buffer *buffer, struct wl_cursor_image *image);
 
     struct wl_display *wl_display() const { return mDisplay; }
-    struct wl_event_queue *wl_event_queue() const { return mEventQueue; }
     struct ::wl_registry *wl_registry() { return object(); }
 
     const struct wl_compositor *wl_compositor() const { return mCompositor.object(); }
@@ -175,7 +174,6 @@ private:
     };
 
     struct wl_display *mDisplay;
-    struct wl_event_queue *mEventQueue;
     QtWayland::wl_compositor mCompositor;
     struct wl_shm *mShm;
     QThread *mEventThread;
diff --git a/src/client/qwaylandeventthread.cpp b/src/client/qwaylandeventthread.cpp
index e2e21c9e7..e0a3edcb8 100644
--- a/src/client/qwaylandeventthread.cpp
+++ b/src/client/qwaylandeventthread.cpp
@@ -80,13 +80,9 @@ void QWaylandEventThread::checkError() const
 
 void QWaylandEventThread::readWaylandEvents()
 {
-    if (wl_display_dispatch(m_display) < 0) {
-        checkError();
-        m_readNotifier->setEnabled(false);
-        emit fatalError();
-        return;
+    if (wl_display_prepare_read(m_display) == 0) {
+        wl_display_read_events(m_display);
     }
-
     emit newEventsRead();
 }
 
diff --git a/src/client/qwaylandnativeinterface.cpp b/src/client/qwaylandnativeinterface.cpp
index 8170c39f1..0d20075f0 100644
--- a/src/client/qwaylandnativeinterface.cpp
+++ b/src/client/qwaylandnativeinterface.cpp
@@ -58,8 +58,6 @@ void *QWaylandNativeInterface::nativeResourceForIntegration(const QByteArray &re
 
     if (lowerCaseResource == "display" || lowerCaseResource == "wl_display" || lowerCaseResource == "nativedisplay")
         return m_integration->display()->wl_display();
-    if (lowerCaseResource == "wl_event_queue")
-        return m_integration->display()->wl_event_queue();
     if (lowerCaseResource == "compositor")
         return const_cast<wl_compositor *>(m_integration->display()->wl_compositor());
     if (lowerCaseResource == "server_buffer_integration")