Flush audio-buffers for internal connections

Forward data for ports that are both internally and
externally connected.

e.g. click-io may be connected to a track as well as physical
outputs. Since the port is externally connected, data is written
to an internal buffer _data[], and only at cycle-end resampled
to engine-buffers.

However a track's recording the metronome is not externally
connected. Hence data is directly read from the engine-buffers.
Summing also happens at engine level, so data has to
be written back for downstream ports to read them in the same cycle.

libs/ardour/ardour/audio_port.h

@@ -42,6 +42,8 @@ public:
 	void cycle_end (pframes_t);
 	void cycle_split ();
 
+	void flush_buffers (pframes_t nframes);
+
 	/* reset SRC, clear out any state */
 	void reinit (bool with_ratio);
 

libs/ardour/audio_port.cc

@@ -127,6 +127,22 @@ AudioPort::cycle_split ()
 {
 }
 
+void
+AudioPort::flush_buffers (pframes_t nframes)
+{
+	if (!sends_output() || !_port_handle || !in_cycle ()) {
+		return;
+	}
+	if (!externally_connected () || !internally_connected ()) {
+		return;
+	}
+	/* port is both internally and externnally connected, make
+	 * data available to internal connections which directly use
+	 * port_engine.get_buffer () without resampling.
+	 */
+	copy_vector ((float*)port_engine.get_buffer (_port_handle, nframes), &_data[_global_port_buffer_offset], nframes);
+}
+
 void
 AudioPort::reinit (bool with_ratio)
 {

libs/ardour/io.cc

@@ -1700,6 +1700,12 @@ IO::copy_to_outputs (BufferSet& bufs, DataType type, pframes_t nframes, samplecn
 		port_buffer.read_from (*prev, nframes, offset);
 		++o;
 	}
+
+	/* when port is both externally and internally connected,
+	 * make data directly available to downstream internal ports */
+	for (auto const& p : _ports) {
+		p->flush_buffers (nframes);
+	}
 }
 
 boost::shared_ptr<Port>