Flush GraphNode RCU when removing Routes

Since 44610c787 RCU keeps references until another write happens.
even before then, some shared_ptr references may have been kept.

When using a process graph, a route's activision-set can
hold references to other graph-nodes (routes). This lead
to Routes not being deleted until a second graph-reorder
flushed the RCU.

libs/ardour/ardour/graphnode.h

@@ -60,6 +60,7 @@ public:
 
 	node_set_t const& activation_set (GraphChain const* const g) const;
 	int               init_refcount (GraphChain const* const g) const;
+	void              flush_graph_activision_rcu ();
 
 protected:
 	friend struct GraphChain;

libs/ardour/graphnode.cc

@@ -46,6 +46,12 @@ GraphActivision::init_refcount (GraphChain const* const g) const
 	return m->at (g);
 }
 
+void
+GraphActivision::flush_graph_activision_rcu ()
+{
+	_activation_set.flush ();
+}
+
 /* ****************************************************************************/
 
 GraphNode::GraphNode (std::shared_ptr<Graph> graph)

libs/ardour/session.cc

@@ -3849,12 +3849,10 @@ Session::remove_routes (std::shared_ptr<RouteList> routes_to_remove)
 	 * going away, then flush old references out of the graph.
 	 */
 
-	routes.flush (); // maybe unsafe, see below.
 	resort_routes ();
 
 	/* get rid of it from the dead wood collection in the route list manager */
 	/* XXX i think this is unsafe as it currently stands, but i am not sure. (pd, october 2nd, 2006) */
-
 	routes.flush ();
 
 	/* remove these routes from the selection if appropriate, and signal
@@ -3882,6 +3880,15 @@ Session::remove_routes (std::shared_ptr<RouteList> routes_to_remove)
 		return;
 	}
 
+	/* really drop reference to the Surround Master to
+	 * unload the vapor plugin. While the RCU keeps a refecent the
+	 * SurroundMaster, a new SurroundMaster cannot be added.
+	 */
+	std::shared_ptr<RouteList const> r = routes.reader ();
+	for (auto const& rt : *r) {
+		rt->flush_graph_activision_rcu ();
+	}
+
 	PropertyChange pc;
 	pc.add (Properties::order);
 	PresentationInfo::Change (pc);