Fix playback alignment when adding/removing channels

The disk-reader assumes that all playback ringbuffers are in sync
and have the same fill_level.

libs/ardour/ardour/disk_io.h

@@ -113,6 +113,7 @@ public:
 protected:
 	friend class Auditioner;
 	virtual int  seek (samplepos_t which_sample, bool complete_refill = false) = 0;
+	virtual void configuration_changed () = 0;
 
 protected:
 	Flag         _flags;

libs/ardour/ardour/disk_reader.h

@@ -223,6 +223,8 @@ private:
 	void get_midi_playback (MidiBuffer& dst, samplepos_t start_sample, samplepos_t end_sample, MonitorState, BufferSet&, double speed, samplecnt_t distance);
 	void maybe_xfade_loop (Sample*, samplepos_t read_start, samplepos_t read_end, ReaderChannelInfo*);
 
+	void configuration_changed ();
+
 	bool overwrite_existing_audio ();
 	bool overwrite_existing_midi ();
 };

libs/ardour/ardour/disk_writer.h

@@ -145,6 +145,8 @@ protected:
 
 	int do_flush (RunContext context, bool force = false);
 
+	void configuration_changed ();
+
 private:
 	static samplecnt_t _chunk_samples;
 

libs/ardour/disk_io.cc

@@ -193,7 +193,7 @@ DiskIOProcessor::configure_io (ChanCount in, ChanCount out)
 	}
 
 	if (changed) {
-		seek (_session.transport_sample());
+		configuration_changed ();
 	}
 
 	return Processor::configure_io (in, out);

libs/ardour/disk_reader.cc

@@ -507,6 +507,12 @@ DiskReader::declick_in_progress () const
 	return (_declick_amp.gain () != 0); // declick-out
 }
 
+void
+DiskReader::configuration_changed ()
+{
+	_session.request_overwrite_buffer (_track, LoopDisabled);
+}
+
 bool
 DiskReader::pending_overwrite () const
 {
@@ -521,6 +527,24 @@ DiskReader::set_pending_overwrite (OverwriteReason why)
 	/* called from audio thread, so we can use the read ptr and playback sample as we wish */
 
 	if (!c->empty ()) {
+
+		if (c->size () > 1) {
+			/* Align newly added buffers.
+			 *
+			 * overwrite_sample and file_sample[] are are maintained
+			 * per DiskReader, not per channel.
+			 * ::refill_audio() and ::overwrite_existing_audio() expect
+			 * that read-pointers and fill_level of all buffers are in sync.
+			 */
+			ChannelList::iterator chan = c->begin ();
+			for (++chan; chan != c->end (); ++chan) {
+				ReaderChannelInfo* chaninfo = dynamic_cast<ReaderChannelInfo*> (*chan);
+				if (!chaninfo->initialized) {
+					(*chan)->rbuf->align_to (*(c->front ()->rbuf));
+				}
+			}
+		}
+
 		const samplecnt_t reserved_size = c->front ()->rbuf->reserved_size ();
 		const samplecnt_t bufsize       = c->front ()->rbuf->bufsize ();
 

libs/ardour/disk_writer.cc

@@ -798,8 +798,14 @@ DiskWriter::set_note_mode (NoteMode m)
 		_midi_write_source->model()->set_note_mode(m);
 }
 
+void
+DiskWriter::configuration_changed ()
+{
+	seek (_session.transport_sample(), false);
+}
+
 int
-DiskWriter::seek (samplepos_t sample, bool complete_refill)
+DiskWriter::seek (samplepos_t sample, bool /*complete_refill*/)
 {
 	uint32_t n;
 	ChannelList::iterator chan;

libs/pbd/pbd/playback_buffer.h

@@ -65,10 +65,20 @@ public:
 		/* writer, when seeking, may block */
 		Glib::Threads::Mutex::Lock lm (_reset_lock);
 		SpinLock sl (_reservation_lock);
-		g_atomic_int_set (&write_idx, g_atomic_int_get (&read_idx));
+		g_atomic_int_set (&read_idx, 0);
+		g_atomic_int_set (&write_idx, 0);
 		g_atomic_int_set (&reserved, 0);
 	}
 
+	/* called from rt (reader) thread for new buffers */
+	void align_to (PlaybackBuffer const& other) {
+		Glib::Threads::Mutex::Lock lm (_reset_lock);
+		write_idx = other.write_idx;
+		read_idx  = other.read_idx;
+		reserved  = other.reserved;
+		memset (buf, 0, size * sizeof (T));
+	}
+
 	/* write-thread */
 	guint write_space () const {
 		guint w, r;