statistics: delete chart items when root node is deleted

When reparenting the statistics widget, QtQuick deletes
the rootNode and all the child nodes. It is unclear whether
this is a bug or intended behavior. In any case, it means
that the pointers to QSG nodes in the chart items become
stale.

To avoid this, delete all chart items in the root node's
destructor, before QtQuick can do anything. It is unclear
from which context this is called (render or UI) and whether
this is even valid. In some tests, it seemed to work.

The difficulty is that all the stale pointers to chart items
have to be deleted as well. All in all, the QSG memory
management is a big nuisance and very brittle.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

1 files changed, 7 insertions, 1 deletions

diff --git a/stats/statsview.h b/stats/statsview.h
index 3623c01eb..cc497792b 100644
--- a/stats/statsview.h
+++ b/stats/statsview.h
@@ -43,13 +43,14 @@ public:
 	~StatsView();
 
 	void plot(const StatsState &state);
-	void updateFeatures(const StatsState &state); // Updates the visibility of chart features, such as legend, regression, etc.
+	void updateFeatures(const StatsState &state);	// Updates the visibility of chart features, such as legend, regression, etc.
 	QQuickWindow *w() const;			// Make window available to items
 	QSizeF size() const;
 	QRectF plotArea() const;
 	void addQSGNode(QSGNode *node, ChartZValue z);	// Must only be called in render thread!
 	void registerChartItem(ChartItem &item);
 	void registerDirtyChartItem(ChartItem &item);
+	void emergencyShutdown();			// Called when QQuick decides to delete out root node.
 
 	// Create a chart item and add it to the scene.
 	// The item must not be deleted by the caller, but can be
@@ -151,6 +152,10 @@ private:
 
 	// There are three double linked lists of chart items:
 	// clean items, dirty items and items to be deleted.
+	// Note that only the render thread must delete chart items,
+	// and therefore these lists are the only owning pointers
+	// to chart items. All other pointers are non-owning and
+	// can therefore become stale.
 	struct ChartItemList {
 		ChartItemList();
 		ChartItem *first, *last;
@@ -161,6 +166,7 @@ private:
 	};
 	ChartItemList cleanItems, dirtyItems, deletedItems;
 	void deleteChartItemInternal(ChartItem &item);
+	void freeDeletedChartItems();
 };
 
 // This implementation detail must be known to users of the class.