Fix latency compensation race-condition

Remove need for explicit `initialize_latencies` call that used
to be called from GUI-thread post_engine_init(), as well as
Session::engine_running().

Further reduce calls, `graph_reordered` implies a latency-update
and fix ordering issue. update_latency_compensation() must be called
*after* resort_routes().

libs/ardour/ardour/session.h

@@ -1314,7 +1314,6 @@ private:
 	void add_monitor_section ();
 	void remove_monitor_section ();
 
-	void initialize_latencies ();
 	void update_latency (bool playback);
 	bool update_route_latency (bool reverse, bool apply_to_delayline);
 

libs/ardour/session.cc

@@ -3016,9 +3016,6 @@ Session::add_routes (RouteList& new_routes, bool input_auto_connect, bool output
 
 	graph_reordered ();
 
-	update_latency (false);
-	update_latency (true);
-
 	set_dirty();
 
 	if (save) {
@@ -5222,7 +5219,7 @@ Session::graph_reordered ()
 	/* force all diskstreams to update their capture offset values to
 	 * reflect any changes in latencies within the graph.
 	 */
-	update_route_latency (false, true);
+	update_latency_compensation (true);
 }
 
 /** @return Number of samples that there is disk space available to write,
@@ -6387,7 +6384,11 @@ restart:
 void
 Session::update_latency (bool playback)
 {
-	DEBUG_TRACE (DEBUG::Latency, string_compose ("JACK latency callback: %1\n", (playback ? "PLAYBACK" : "CAPTURE")));
+	/* called only from AudioEngine::latency_callback.
+	 * but may indirectly be triggered from
+	 * Session::update_latency_compensation -> _engine.update_latencies
+	 */
+	DEBUG_TRACE (DEBUG::LatencyCompensation, string_compose ("Engine latency callback: %1\n", (playback ? "PLAYBACK" : "CAPTURE")));
 
 	if (inital_connect_or_deletion_in_progress () || _adding_routes_in_progress || _route_deletion_in_progress) {
 		return;
@@ -6412,32 +6413,24 @@ Session::update_latency (bool playback)
 	}
 
 	if (playback) {
+		Glib::Threads::Mutex::Lock lx (_update_latency_lock);
 		set_worst_output_latency ();
 		update_route_latency (true, true);
 	} else {
+		Glib::Threads::Mutex::Lock lx (_update_latency_lock);
 		set_worst_input_latency ();
 		update_route_latency (false, false);
 	}
 
-	DEBUG_TRACE (DEBUG::Latency, "JACK latency callback: DONE\n");
+	DEBUG_TRACE (DEBUG::LatencyCompensation, "Engine latency callback: DONE\n");
 	LatencyUpdated (); /* EMIT SIGNAL */
 }
 
-void
-Session::initialize_latencies ()
-{
-	{
-		Glib::Threads::Mutex::Lock lm (_engine.process_lock());
-		update_latency (false);
-		update_latency (true);
-	}
-
-	set_worst_io_latencies ();
-}
-
 void
 Session::set_worst_io_latencies ()
 {
+	DEBUG_TRACE (DEBUG::LatencyCompensation, "Session::set_worst_io_latencies\n");
+	Glib::Threads::Mutex::Lock lx (_update_latency_lock);
 	set_worst_output_latency ();
 	set_worst_input_latency ();
 }
@@ -6463,7 +6456,7 @@ Session::set_worst_output_latency ()
 
 	_worst_output_latency = max (_worst_output_latency, _click_io->latency());
 
-	DEBUG_TRACE (DEBUG::Latency, string_compose ("Worst output latency: %1\n", _worst_output_latency));
+	DEBUG_TRACE (DEBUG::LatencyCompensation, string_compose ("Worst output latency: %1\n", _worst_output_latency));
 }
 
 void
@@ -6485,7 +6478,7 @@ Session::set_worst_input_latency ()
 		_worst_input_latency = max (_worst_input_latency, (*i)->input()->latency());
 	}
 
-	DEBUG_TRACE (DEBUG::Latency, string_compose ("Worst input latency: %1\n", _worst_input_latency));
+	DEBUG_TRACE (DEBUG::LatencyCompensation, string_compose ("Worst input latency: %1\n", _worst_input_latency));
 }
 
 void
@@ -6506,20 +6499,25 @@ Session::update_latency_compensation (bool force_whole_graph)
 		return;
 	}
 
+	DEBUG_TRACE (DEBUG::LatencyCompensation, string_compose ("update_latency_compensation %1\n", (force_whole_graph ? "of whole graph" : "")));
+
 	bool some_track_latency_changed = update_route_latency (false, false);
 
 	if (some_track_latency_changed || force_whole_graph)  {
+		DEBUG_TRACE (DEBUG::LatencyCompensation, "update_latency_compensation: delegate to engine\n");
 		_engine.update_latencies ();
 		/* above call will ask the backend up update its latencies, which
 		 * eventually will trigger  AudioEngine::latency_callback () and
 		 * call Session::update_latency ()
 		 */
 	} else {
+		DEBUG_TRACE (DEBUG::LatencyCompensation, "update_latency_compensation: directly apply to routes\n");
 		boost::shared_ptr<RouteList> r = routes.reader ();
 		for (RouteList::iterator i = r->begin(); i != r->end(); ++i) {
 			(*i)->apply_latency_compensation ();
 		}
 	}
+	DEBUG_TRACE (DEBUG::LatencyCompensation, "update_latency_compensation: DONE\n");
 }
 
 char

libs/ardour/session_state.cc

@@ -365,10 +365,6 @@ Session::post_engine_init ()
 
 		_state_of_the_state = StateOfTheState (_state_of_the_state & ~(CannotSave | Dirty));
 
-		/* update latencies */
-
-		initialize_latencies ();
-
 		_locations->added.connect_same_thread (*this, boost::bind (&Session::location_added, this, _1));
 		_locations->removed.connect_same_thread (*this, boost::bind (&Session::location_removed, this, _1));
 		_locations->changed.connect_same_thread (*this, boost::bind (&Session::locations_changed, this));

libs/ardour/session_transport.cc

@@ -1869,7 +1869,6 @@ Session::engine_halted ()
 void
 Session::engine_running ()
 {
-	initialize_latencies ();
 	_transport_fsm->start ();
 }
 
@@ -1907,8 +1906,8 @@ Session::route_processors_changed (RouteProcessorChange c)
 		return;
 	}
 
-	update_latency_compensation ();
 	resort_routes ();
+	update_latency_compensation ();
 
 	set_dirty ();
 }