Be more specific about RouteProcessorChange signals

Session::route_processors_changed accumulates signals emitted
in realtime and processing is delegated to a dedicated rt-safe
thread. Previously this resulted in any changed to be converted
to a `GeneralChange`, which unconditionally triggered a route-reorder.

Record-arming a track causes a MeterPointChange (meters change to "in"),
and this caused routes to be resorted and a latency-update.

While the former is reasonable (Ardour prefers to process
rec-armed routes first), the latter certainly is not.

libs/ardour/ardour/session.h

@@ -686,23 +686,9 @@ public:
 
 	class ProcessorChangeBlocker {
 	public:
-		ProcessorChangeBlocker (Session* s, bool rc = true)
-			: _session (s)
-			, _reconfigure_on_delete (rc)
-		{
-			g_atomic_int_inc (&s->_ignore_route_processor_changes);
-		}
+		ProcessorChangeBlocker (Session* s, bool rc = true);
+		~ProcessorChangeBlocker ();
 
-		~ProcessorChangeBlocker ()
-		{
-			if (g_atomic_int_dec_and_test (&_session->_ignore_route_processor_changes)) {
-				if (g_atomic_int_compare_and_exchange (&_session->_ignored_a_processor_change, 1, 0)) {
-					if (_reconfigure_on_delete) {
-						_session->route_processors_changed (RouteProcessorChange ());
-					}
-				}
-			}
-		}
 	private:
 		Session* _session;
 		bool _reconfigure_on_delete;

libs/ardour/ardour/types.h

@@ -674,12 +674,16 @@ struct CleanupReport {
 /** A struct used to describe changes to processors in a route.
  *  This is useful because objects that respond to a change in processors
  *  can optimise what work they do based on details of what has changed.
+ *
+ *  While the signal themselves are distinct values, the Session
+ *  can accumulate then via ProcessorChangeBlocker and batch process
+ *  them.
  */
 struct RouteProcessorChange {
 	enum Type {
-		GeneralChange = 0x0,
 		MeterPointChange = 0x1,
-		RealTimeChange = 0x2
+		RealTimeChange   = 0x2,
+		GeneralChange    = 0x4
 	};
 
 	RouteProcessorChange () : type (GeneralChange), meter_visibly_changed (true)

libs/ardour/session.cc

@@ -7539,3 +7539,29 @@ Session::mixer_scenes () const
 	Glib::Threads::RWLock::ReaderLock lm (_mixer_scenes_lock);
 	return _mixer_scenes;
 }
+
+Session::ProcessorChangeBlocker::ProcessorChangeBlocker (Session* s, bool rc)
+	: _session (s)
+	, _reconfigure_on_delete (rc)
+{
+	g_atomic_int_inc (&s->_ignore_route_processor_changes);
+}
+
+Session::ProcessorChangeBlocker::~ProcessorChangeBlocker ()
+{
+	if (g_atomic_int_dec_and_test (&_session->_ignore_route_processor_changes)) {
+		gint type = g_atomic_int_and (&_session->_ignored_a_processor_change, 0);
+		if (_reconfigure_on_delete) {
+			if (type & RouteProcessorChange::GeneralChange) {
+				_session->route_processors_changed (RouteProcessorChange ());
+			} else {
+				if (type & RouteProcessorChange::MeterPointChange) {
+					_session->route_processors_changed (RouteProcessorChange (RouteProcessorChange::MeterPointChange));
+				}
+				if (type & RouteProcessorChange::RealTimeChange) {
+					_session->route_processors_changed (RouteProcessorChange (RouteProcessorChange::RealTimeChange));
+				}
+			}
+		}
+	}
+}

libs/ardour/session_transport.cc

@@ -1882,11 +1882,13 @@ void
 Session::route_processors_changed (RouteProcessorChange c)
 {
 	if (g_atomic_int_get (&_ignore_route_processor_changes) > 0) {
-		g_atomic_int_set (&_ignored_a_processor_change, 1);
+		g_atomic_int_or (&_ignored_a_processor_change, (int)c.type);
 		return;
 	}
 
 	if (c.type == RouteProcessorChange::MeterPointChange) {
+		/* sort rec-armed routes first */
+		resort_routes ();
 		set_dirty ();
 		return;
 	}