Ignore drops-onto-items from invalid places items

If the QMimeData object created by PlacesItemModel doesn't have any url
set (e.g. when dragging unmounted devices), it is detected by the
resulting DropJob as "drop raw data" because the mimeData has one format set
(the internalMimeType() used for dragging between places items).

This results in a crash because the DropJob schedules a PasteJob, but in
the meantime the QDrag from Dolphin ends and deletes the mimeData object
that was passed to the paste job.

The fix is to prevent the DropJob in the first place. We can introduce a
new internal mimetype that we use to blacklist drops-onto-items (while
still allowing drops-between-items). This way PlacesItemModel can set
the blacklist flag if the mimeData is being created without urls.

BUG: 373005
FIXED-IN: 17.04.3

Test Plan:
Drag and drop an unmounted device to another place item or the DolphinView, doesn't crash anymore.
Dropping the unmounted device between two places item still works.

Differential Revision: https://phabricator.kde.org/D5535

src/kitemviews/kitemlistcontroller.cpp

@@ -928,7 +928,7 @@ bool KItemListController::dropEvent(QGraphicsSceneDragDropEvent* event, const QT
         // Something has been dropped between two items.
         m_view->hideDropIndicator();
         emit aboveItemDropEvent(dropAboveIndex, event);
-    } else {
+    } else if (!event->mimeData()->hasFormat(m_model->blacklistItemDropEventMimeType())) {
         // Something has been dropped on an item or on an empty part of the view.
         emit itemDropEvent(m_view->itemAt(pos), event);
     }

src/kitemviews/kitemmodelbase.cpp

@@ -142,6 +142,11 @@ bool KItemModelBase::supportsDropping(int index) const
     return false;
 }
 
+QString KItemModelBase::blacklistItemDropEventMimeType() const
+{
+    return QStringLiteral("application/x-dolphin-blacklist-drop");
+}
+
 void KItemModelBase::onGroupedSortingChanged(bool current)
 {
     Q_UNUSED(current);

src/kitemviews/kitemmodelbase.h

@@ -172,6 +172,16 @@ public:
     // decision whether it accepts the drop?
     virtual bool supportsDropping(int index) const;
 
+    /**
+     * @return An internal mimetype to signal that an itemDropEvent() should be rejected by
+     *         the receiving model.
+     *
+     *         This mimeType can be used in createMimeData() to notify that the
+     *         drop-onto-items events should be ignored, while the drop-between-items
+     *         ones should be still accepted.
+     */
+    QString blacklistItemDropEventMimeType() const;
+
 signals:
     /**
      * Is emitted if one or more items have been inserted. Each item-range consists

src/panels/places/placesitemmodel.cpp

@@ -376,6 +376,9 @@ QMimeData* PlacesItemModel::createMimeData(const KItemSet& indexes) const
     QMimeData* mimeData = new QMimeData();
     if (!urls.isEmpty()) {
         mimeData->setUrls(urls);
+    } else {
+        // #378954: prevent itemDropEvent() drops if there isn't a source url.
+        mimeData->setData(blacklistItemDropEventMimeType(), QByteArrayLiteral("true"));
     }
     mimeData->setData(internalMimeType(), itemData);