tentative fix for losing (empty) MIDI files. Incomplete because testing shows issues with some workflows

libs/ardour/ardour/source.h

@@ -47,7 +47,8 @@ class Source : public SessionObject
 		RemovableIfEmpty = 0x10,
 		RemoveAtDestroy = 0x20,
 		NoPeakFile = 0x40,
-		Destructive = 0x80
+		Destructive = 0x80,
+		Empty = 0x100, /* used for MIDI only */
 	};
 
 	Source (Session&, DataType type, const std::string& name, Flag flags=Flag(0));

libs/ardour/enums.cc

@@ -447,6 +447,7 @@ setup_enum_writer ()
 	REGISTER_CLASS_ENUM (Source, RemoveAtDestroy);
 	REGISTER_CLASS_ENUM (Source, NoPeakFile);
 	REGISTER_CLASS_ENUM (Source, Destructive);
+	REGISTER_CLASS_ENUM (Source, Empty);
 	REGISTER_BITS (_Source_Flag);
 
 	REGISTER_ENUM (FadeLinear);

libs/ardour/session.cc

@@ -3603,7 +3603,11 @@ Session::create_midi_source_by_stealing_name (boost::shared_ptr<Track> track)
 		return boost::shared_ptr<MidiSource>();
 	}
 
-	const string path = new_midi_source_path (name);
+	/* MIDI files are small, just put them in the first location of the
+	   session source search path.
+	*/
+
+	const string path = Glib::build_filename (source_search_path (DataType::MIDI).front(), name);
 
 	return boost::dynamic_pointer_cast<SMFSource> (
 		SourceFactory::createWritable (

libs/ardour/smf_source.cc

@@ -71,6 +71,8 @@ SMFSource::SMFSource (Session& s, const string& path, Source::Flag flags)
         assert (!Glib::file_test (_path, Glib::FILE_TEST_EXISTS));
 	existence_check ();
 
+	_flags = Source::Flag (_flags | Empty);
+
 	/* file is not opened until write */
 
 	if (flags & Writable) {
@@ -131,12 +133,23 @@ SMFSource::SMFSource (Session& s, const XMLNode& node, bool must_exist)
 		throw failed_constructor ();
 	}
 
-	if (init (_path, true)) {
+	/* we expect the file to exist, but if no MIDI data was ever added
+	   it will have been removed at last session close. so, we don't
+	   require it to exist if it was marked Empty.
+	*/
+
+	if (init (_path, !(_flags & Source::Empty))) {
 		throw failed_constructor ();
 	}
 
-        assert (Glib::file_test (_path, Glib::FILE_TEST_EXISTS));
-	existence_check ();
+	if (!(_flags & Source::Empty)) {
+		assert (Glib::file_test (_path, Glib::FILE_TEST_EXISTS));
+		existence_check ();
+	} else {
+		assert (_flags & Source::Writable);
+		/* file will be opened on write */
+		return;
+	}
 
 	if (open(_path)) {
 		throw failed_constructor ();
@@ -379,6 +392,7 @@ SMFSource::append_event_unlocked_beats (const Evoral::Event<double>& ev)
 
 	Evoral::SMF::append_event_delta(delta_time_ticks, ev.size(), ev.buffer(), event_id);
 	_last_ev_time_beats = ev.time();
+	_flags = Source::Flag (_flags & ~Empty);
 }
 
 /** Append an event with a timestamp in frames (framepos_t) */
@@ -425,6 +439,7 @@ SMFSource::append_event_unlocked_frames (const Evoral::Event<framepos_t>& ev, fr
 
 	Evoral::SMF::append_event_delta(delta_time_ticks, ev.size(), ev.buffer(), event_id);
 	_last_ev_time_frames = ev.time();
+	_flags = Source::Flag (_flags & ~Empty);
 }
 
 XMLNode&
@@ -667,10 +682,12 @@ SMFSource::destroy_model ()
 void
 SMFSource::flush_midi ()
 {
-	if (!writable() || (writable() && !_open)) {
+	if (!writable() || _length_beats == 0.0) {
 		return;
 	}
 
+	ensure_disk_file ();
+
 	Evoral::SMF::end_write ();
 	/* data in the file means its no longer removable */
 	mark_nonremovable ();
@@ -702,9 +719,6 @@ SMFSource::ensure_disk_file ()
 		if (!_open) {
 			open_for_write ();
 		}
-
-		/* Flush, which will definitely put something on disk */
-		flush_midi ();
 	}
 }