Transport: engaging loop play while rolling with no audio tracks now works

At present only audio data from disk readers is declicked. MIDI tracks with audio output should
likely also be declicked, at which time Session::need_declick_before_locate() will require
amending

libs/ardour/ardour/session.h

@@ -337,6 +337,7 @@ public:
 	uint32_t nstripables (bool with_monitor = false) const;
 	uint32_t nroutes() const { return routes.reader()->size(); }
 	uint32_t ntracks () const;
+	uint32_t naudiotracks () const;
 	uint32_t nbusses () const;
 
 	bool plot_process_graph (std::string const& file_name) const;
@@ -1296,6 +1297,7 @@ protected:
 	double speed() const { return _transport_speed; }
 	samplepos_t position() const { return _transport_sample; }
 	void set_transport_speed (double speed, bool abort, bool clear_state, bool as_default);
+	bool need_declick_before_locate () const;
 
 private:
 	int  create (const std::string& mix_template, BusProfile const *, bool unnamed);

libs/ardour/ardour/transport_api.h

@@ -43,6 +43,7 @@ class LIBARDOUR_API TransportAPI
 	virtual double speed() const = 0;
 	virtual void set_transport_speed (double speed, bool abort_capture, bool clear_state, bool as_default) = 0;
 	virtual samplepos_t position() const = 0;
+   	virtual bool need_declick_before_locate () const = 0;
 };
 
 } /* end namespace ARDOUR */

libs/ardour/session.cc

@@ -6002,6 +6002,8 @@ Session::get_mix_buffers (ChanCount count)
 uint32_t
 Session::ntracks () const
 {
+	/* XXX Could be optimized by caching */
+
 	uint32_t n = 0;
 	boost::shared_ptr<RouteList> r = routes.reader ();
 
@@ -6014,6 +6016,23 @@ Session::ntracks () const
 	return n;
 }
 
+uint32_t
+Session::naudiotracks () const
+{
+	/* XXX Could be optimized by caching */
+
+	uint32_t n = 0;
+	boost::shared_ptr<RouteList> r = routes.reader ();
+
+	for (RouteList::const_iterator i = r->begin(); i != r->end(); ++i) {
+		if (boost::dynamic_pointer_cast<AudioTrack> (*i)) {
+			++n;
+		}
+	}
+
+	return n;
+}
+
 uint32_t
 Session::nbusses () const
 {

libs/ardour/session_transport.cc

@@ -647,6 +647,15 @@ Session::start_transport ()
 	TransportStateChange (); /* EMIT SIGNAL */
 }
 
+bool
+Session::need_declick_before_locate () const
+{
+	/* At this time (July 2020) only audio playback from disk readers is
+	   de-clicked. MIDI tracks with audio output really need it too.
+	*/
+	return naudiotracks() > 0;
+}
+
 bool
 Session::should_roll_after_locate () const
 {

libs/ardour/transport_fsm.cc

@@ -292,6 +292,7 @@ TransportFSM::process_event (Event& ev, bool already_deferred, bool& deferred)
 			break;
 		case Rolling:
 			if (ev.for_loop_end) {
+
 				/* we will finish the locate synchronously, so
 				 * that after returning from
 				 * ::locate_for_loop() we will already have
@@ -308,6 +309,7 @@ TransportFSM::process_event (Event& ev, bool already_deferred, bool& deferred)
 				 */
 				transition (WaitingForLocate);
 				locate_for_loop (ev);
+
 			} else if (DiskReader::no_disk_output()) {
 
 				/* separate clause to allow a comment that is
@@ -326,8 +328,14 @@ TransportFSM::process_event (Event& ev, bool already_deferred, bool& deferred)
 				transition (WaitingForLocate);
 				locate_for_loop (ev);
 			} else {
-				transition (DeclickToLocate);
-				start_declick_for_locate (ev);
+
+				if (api->need_declick_before_locate()) {
+					transition (DeclickToLocate);
+					start_declick_for_locate (ev);
+				} else {
+					transition (WaitingForLocate);
+					locate_for_loop (ev);
+				}
 			}
 			break;
 		case WaitingForLocate:
@@ -675,4 +683,3 @@ TransportFSM::will_roll_fowards () const
 	}
 	return (_direction_state == Forwards);
 }
-