Client: Don't freeze in QDrag::exec if there was no drag focus

002fade6 fixed a crash when starting a drag without a valid focus, but
there is still a problem, because QDrag::exec will never return because
it's waiting indefinitely in an event loop.

- QWaylandDataDevice::startDrag can now fail by returning false.
- When starting a drag fails, we cancel the drag through
QWaylandDrag::cancelDrag wrapped in invokeMethod.
- Also, don't unnecessarily create a data_source if we cannot start a
drag.

[ChangeLog][QPA plugin] Fixed a freeze that happened when starting a
drag-and-drop operation without a valid source surface.

Change-Id: Iea19b0c92c196a44d1274a966bee4ff519632d34
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>

src/plugins/platforms/wayland/qwaylanddatadevice.cpp

@@ -102,19 +102,22 @@ QWaylandDataOffer *QWaylandDataDevice::dragOffer() const
     return m_dragOffer.data();
 }
 
-void QWaylandDataDevice::startDrag(QMimeData *mimeData, QWaylandWindow *icon)
+bool QWaylandDataDevice::startDrag(QMimeData *mimeData, QWaylandWindow *icon)
 {
-    m_dragSource.reset(new QWaylandDataSource(m_display->dndSelectionHandler(), mimeData));
-    connect(m_dragSource.data(), &QWaylandDataSource::cancelled, this, &QWaylandDataDevice::dragSourceCancelled);
-
     QWaylandWindow *origin = m_display->currentInputDevice()->pointerFocus();
     if (!origin)
         origin = m_display->currentInputDevice()->touchFocus();
 
-    if (origin)
+    if (!origin) {
-        start_drag(m_dragSource->object(), origin->object(), icon->object(), m_display->currentInputDevice()->serial());
-    else
         qCDebug(lcQpaWayland) << "Couldn't start a drag because the origin window could not be found.";
+        return false;
+    }
+
+    m_dragSource.reset(new QWaylandDataSource(m_display->dndSelectionHandler(), mimeData));
+    connect(m_dragSource.data(), &QWaylandDataSource::cancelled, this, &QWaylandDataDevice::dragSourceCancelled);
+
+    start_drag(m_dragSource->object(), origin->object(), icon->object(), m_display->currentInputDevice()->serial());
+    return true;
 }
 
 void QWaylandDataDevice::cancelDrag()

src/plugins/platforms/wayland/qwaylanddatadevice_p.h

@@ -89,7 +89,7 @@ public:
 
 #if QT_CONFIG(draganddrop)
     QWaylandDataOffer *dragOffer() const;
-    void startDrag(QMimeData *mimeData, QWaylandWindow *icon);
+    bool startDrag(QMimeData *mimeData, QWaylandWindow *icon);
     void cancelDrag();
 #endif
 

src/plugins/platforms/wayland/qwaylanddnd.cpp

@@ -66,8 +66,13 @@ void QWaylandDrag::startDrag()
 {
     QBasicDrag::startDrag();
     QWaylandWindow *icon = static_cast<QWaylandWindow *>(shapedPixmapWindow()->handle());
-    m_display->currentInputDevice()->dataDevice()->startDrag(drag()->mimeData(), icon);
+    if (m_display->currentInputDevice()->dataDevice()->startDrag(drag()->mimeData(), icon)) {
         icon->addAttachOffset(-drag()->hotSpot());
+    } else {
+        // Cancelling immediately does not work, since the event loop for QDrag::exec is started
+        // after this function returns.
+        QMetaObject::invokeMethod(this, [this](){ cancelDrag(); }, Qt::QueuedConnection);
+    }
 }
 
 void QWaylandDrag::cancel()