Add a dedicated export method to MidiRegion

To export a MIDI region to a file, the code used MidiRegion::clone()
since it takes care of creating a new file-backed source with the wanted
contents. Nevertheless, it had several side-effects:
- it created and registered a new region which is confusing to users
- it only exported notes that were in the region range, but didn't
  remove the region start offset from MIDI events, essentially producing
  a spurious silence at the beginning of the exported file (this is not
  a problem for region cloning because the newly created region is made
  aware of the offset and caters for it).

Add a dedicated code path for export, that uses the new offsetting
capabilities of MidiModel::write_section_to().

libs/ardour/ardour/midi_region.h

@@ -61,6 +61,8 @@ class LIBARDOUR_API MidiRegion : public Region
 
 	~MidiRegion();
 
+	bool do_export (std::string path) const;
+
 	boost::shared_ptr<MidiRegion> clone (std::string path = std::string()) const;
 	boost::shared_ptr<MidiRegion> clone (boost::shared_ptr<MidiSource>) const;
 

libs/ardour/ardour/midi_source.h

@@ -63,6 +63,19 @@ class LIBARDOUR_API MidiSource : virtual public Source, public boost::enable_sha
 	              Evoral::Beats                 begin = Evoral::MinBeats,
 	              Evoral::Beats                 end   = Evoral::MaxBeats);
 
+	/** Export the midi data in the given time range to another MidiSource
+	 * \param newsrc MidiSource to which data will be written. Should be a
+	 *        new, empty source. If it already has contents, the results are
+	 *        undefined. Source must be writable.
+	 * \param begin time of earliest event that can be written.
+	 * \param end time of latest event that can be written.
+	 * \return zero on success, non-zero if the write failed for any reason.
+	 */
+	int export_write_to (const Lock&                   lock,
+	                     boost::shared_ptr<MidiSource> newsrc,
+	                     Evoral::Beats                 begin,
+	                     Evoral::Beats                 end);
+
 	/** Read the data in a given time range from the MIDI source.
 	 * All time stamps in parameters are in audio frames (even if the source has tempo time).
 	 * \param dst Ring buffer where read events are written.

libs/ardour/midi_region.cc

@@ -120,6 +120,37 @@ MidiRegion::~MidiRegion ()
 {
 }
 
+/** Export the MIDI data of the MidiRegion to a new MIDI file (SMF).
+ */
+bool
+MidiRegion::do_export (string path) const
+{
+	boost::shared_ptr<MidiSource> newsrc;
+
+	/* caller must check for pre-existing file */
+	assert (!path.empty());
+	assert (!Glib::file_test (path, Glib::FILE_TEST_EXISTS));
+	newsrc = boost::dynamic_pointer_cast<MidiSource>(
+		SourceFactory::createWritable(DataType::MIDI, _session,
+		                              path, false, _session.frame_rate()));
+
+	BeatsFramesConverter bfc (_session.tempo_map(), _position);
+	Evoral::Beats const bbegin = bfc.from (_start);
+	Evoral::Beats const bend = bfc.from (_start + _length);
+
+	{
+		/* Lock our source since we'll be reading from it.  write_to() will
+		   take a lock on newsrc. */
+		Source::Lock lm (midi_source(0)->mutex());
+		if (midi_source(0)->export_write_to (lm, newsrc, bbegin, bend)) {
+			return false;
+		}
+	}
+
+	return true;
+}
+
+
 /** Create a new MidiRegion that has its own version of some/all of the Source used by another.
  */
 boost::shared_ptr<MidiRegion>

libs/ardour/midi_source.cc

@@ -384,6 +384,23 @@ MidiSource::mark_streaming_write_completed (const Lock& lock)
 	mark_midi_streaming_write_completed (lock, Evoral::Sequence<Evoral::Beats>::DeleteStuckNotes);
 }
 
+int
+MidiSource::export_write_to (const Lock& lock, boost::shared_ptr<MidiSource> newsrc, Evoral::Beats begin, Evoral::Beats end)
+{
+	Lock newsrc_lock (newsrc->mutex ());
+
+	if (!_model) {
+		error << string_compose (_("programming error: %1"), X_("no model for MidiSource during export"));
+		return -1;
+	}
+
+	_model->write_section_to (newsrc, newsrc_lock, begin, end, true);
+
+	newsrc->flush_midi(newsrc_lock);
+
+	return 0;
+}
+
 int
 MidiSource::write_to (const Lock& lock, boost::shared_ptr<MidiSource> newsrc, Evoral::Beats begin, Evoral::Beats end)
 {