cont’d work on a16dd7c, fixes #6170

libs/ardour/ardour/midi_ring_buffer.h

@@ -49,6 +49,7 @@ public:
 	inline bool read_contents(uint32_t size, uint8_t* buf);
 
 	size_t read(MidiBuffer& dst, framepos_t start, framepos_t end, framecnt_t offset=0, bool stop_on_overflow_in_destination=false);
+	size_t skip_to(framepos_t start);
 
 	void dump(std::ostream& dst);
 	void flush (framepos_t start, framepos_t end);

libs/ardour/midi_diskstream.cc

@@ -621,7 +621,7 @@ MidiDiskstream::commit (framecnt_t playback_distance)
 	 *
 	 * In those cases the butler needs to be summed to refill the buffer (done now)
 	 * AND we need to skip (frames_read - frames_written). ie remove old events
-	 * before playback_sample from the rinbuffer. (not yet done)
+	 * before playback_sample from the rinbuffer.
 	 *
 	 * [1] one way to do so is described at #6170.
 	 * For me just popping up the context-menu on a MIDI-track header
@@ -1440,6 +1440,8 @@ MidiDiskstream::get_playback (MidiBuffer& dst, framecnt_t nframes)
 
 	size_t events_read = 0;	
 
+	_playback_buf->skip_to (playback_sample);
+
 	if (loc) {
 		framepos_t effective_start;
 

libs/ardour/midi_ring_buffer.cc

@@ -60,6 +60,10 @@ MidiRingBuffer<T>::read(MidiBuffer& dst, framepos_t start, framepos_t end, frame
 		ev_time = *(reinterpret_cast<T*>((uintptr_t)peekbuf));
 		ev_size = *(reinterpret_cast<uint32_t*>((uintptr_t)(peekbuf + sizeof(T) + sizeof (Evoral::EventType))));
 
+		if (this->read_space() < ev_size) {
+			break;;
+		}
+
 		if (ev_time >= end) {
 			DEBUG_TRACE (DEBUG::MidiDiskstreamIO, string_compose ("MRB event @ %1 past end @ %2\n", ev_time, end));
 			break;
@@ -124,6 +128,59 @@ MidiRingBuffer<T>::read(MidiBuffer& dst, framepos_t start, framepos_t end, frame
 	return count;
 }
 
+template<typename T>
+size_t
+MidiRingBuffer<T>::skip_to(framepos_t start)
+{
+	if (this->read_space() == 0) {
+		return 0;
+	}
+
+	T                 ev_time;
+	uint32_t          ev_size;
+	size_t            count = 0;
+	const size_t      prefix_size = sizeof(T) + sizeof(Evoral::EventType) + sizeof(uint32_t);
+
+	while (this->read_space() >= prefix_size) {
+
+		uint8_t peekbuf[prefix_size];
+		this->peek (peekbuf, prefix_size);
+
+		ev_time = *(reinterpret_cast<T*>((uintptr_t)peekbuf));
+		ev_size = *(reinterpret_cast<uint32_t*>((uintptr_t)(peekbuf + sizeof(T) + sizeof (Evoral::EventType))));
+
+		if (ev_time >= start) {
+			return count;
+		}
+
+		if (this->read_space() < ev_size) {
+			continue;
+		}
+
+		this->increment_read_ptr (prefix_size);
+
+		uint8_t status;
+		bool r = this->peek (&status, sizeof(uint8_t));
+		assert (r); // If this failed, buffer is corrupt, all hope is lost
+
+		++count;
+
+		if (ev_size < 8) {
+			this->increment_read_ptr (ev_size);
+		} else {
+			// we only track note on/off, 8 bytes are plenty.
+			uint8_t write_loc[8];
+			bool success = read_contents (ev_size, write_loc);
+			if (success) {
+				_tracker.track(write_loc);
+			}
+		}
+	}
+	return count;
+}
+
+
+
 template<typename T>
 void
 MidiRingBuffer<T>::flush (framepos_t /*start*/, framepos_t end)