Allow to change process graph while auditioning

This fixes an issue with adding/removing tracks while auditioning.

Session::remove_routes() calls Graph::clear_other_chain(),
which will block until the graph chains have been swapped.

libs/ardour/ardour/graph.h

@@ -74,6 +74,7 @@ public:
 	void process_one_route (Route* route);
 
 	void clear_other_chain ();
+	void swap_process_chain ();
 
 	bool in_process_thread () const;
 
@@ -85,7 +86,7 @@ private:
 	void drop_threads ();
 	void run_one ();
 	void main_thread ();
-	void prep ();
+	void prep (bool check_pending_chain = true);
 	void dump (int chain) const;
 
 	node_list_t _nodes_rt[2];

libs/ardour/graph.cc

@@ -227,9 +227,37 @@ Graph::clear_other_chain ()
 }
 
 void
-Graph::prep ()
+Graph::swap_process_chain ()
 {
+	/* Intended to be called Session::process_audition.
+	 * Must not be called while the graph is processing.
+	 */
+	bool need_prep = false;
 	if (_swap_mutex.trylock ()) {
+		/* swap mutex acquired */
+		if (_current_chain != _pending_chain) {
+			/* use new chain */
+			_setup_chain   = _current_chain;
+			_current_chain = _pending_chain;
+			_trigger_queue.clear ();
+			/* ensure that all nodes can be queued */
+			_trigger_queue.reserve (_nodes_rt[_current_chain].size ());
+			g_atomic_int_set (&_trigger_queue_size, 0);
+			_cleanup_cond.signal ();
+			need_prep = true;
+		}
+		_swap_mutex.unlock ();
+	}
+
+	if (need_prep) {
+		prep (false);
+	}
+}
+
+void
+Graph::prep (bool check_pending_chain)
+{
+	if (check_pending_chain && _swap_mutex.trylock ()) {
 		/* swap mutex acquired */
 		if (_current_chain != _pending_chain) {
 			/* use new chain */

libs/ardour/session_process.cc

@@ -730,6 +730,10 @@ Session::process_audition (pframes_t nframes)
 		}
 	}
 
+	if (_process_graph) {
+		_process_graph->swap_process_chain ();
+	}
+
 	/* run the auditioner, and if it says we need butler service, ask for it */
 
 	if (auditioner->play_audition (nframes) > 0) {