disallow invalid port-removal

do not allow port-removal if the port would be re-added immediately
after that again because the main-delivery actually needs it.

As a side effect this prevents this crash:

 * create a stereo-track, then remove one output
  -> unhandled exception "AudioEngine::PortRegistrationFailure&"

 The problem:
 - the port is removed from the RCU ports list,
   but Port::drop() (which calls jack_port_unregister) is only called
   from the Port's destructor at some later time.
   (because a reference to the port still exists elsewhere)
 - the jack-port is not yet removed.
 - meanwhile Delivery::configure_io comes along and notices that
   there are more audio-buffers than ports and tries to re-register the port.
 - but the port still exists in jack, so it fails and throws an exception
   ...which is not handled.

gtk2_ardour/port_matrix.cc

@@ -733,7 +733,7 @@ PortMatrix::remove_channel (ARDOUR::BundleChannel b)
 			int const r = io->remove_port (p, this);
 			if (r == -1) {
 				ArdourDialog d (_("Port removal not allowed"));
-				Label l (_("This port cannot be removed, as the first plugin in the track or buss cannot accept the new number of inputs."));
+				Label l (_("This port cannot be removed.\nEither the first plugin in the track or buss cannot accept\nthe new number of inputs or the last plugin has more outputs."));
 				d.get_vbox()->pack_start (l);
 				d.add_button (Stock::OK, RESPONSE_ACCEPT);
 				d.set_modal (true);

libs/ardour/ardour/route.h

@@ -530,6 +530,7 @@ class Route : public SessionObject, public Automatable, public RouteGroupMember,
 	void silence_unlocked (framecnt_t);
 
 	ChanCount processor_max_streams;
+	ChanCount processor_out_streams;
 
 	uint32_t pans_required() const;
 	ChanCount n_process_buffers ();
@@ -553,6 +554,7 @@ class Route : public SessionObject, public Automatable, public RouteGroupMember,
 	void output_change_handler (IOChange, void *src);
 
 	bool input_port_count_changing (ChanCount);
+	bool output_port_count_changing (ChanCount);
 
 	bool _in_configure_processors;
 

libs/ardour/route.cc

@@ -136,6 +136,7 @@ Route::init ()
 	_input->PortCountChanging.connect_same_thread (*this, boost::bind (&Route::input_port_count_changing, this, _1));
 
 	_output->changed.connect_same_thread (*this, boost::bind (&Route::output_change_handler, this, _1, _2));
+	_output->PortCountChanging.connect_same_thread (*this, boost::bind (&Route::output_port_count_changing, this, _1));
 
 	/* add amp processor  */
 
@@ -1704,6 +1705,8 @@ Route::configure_processors_unlocked (ProcessorStreams* err)
 	}
 
 	ChanCount out;
+	bool seen_mains_out = false;
+	processor_out_streams = _input->n_ports();
 
 	list< pair<ChanCount,ChanCount> >::iterator c = configuration.begin();
 	for (ProcessorList::iterator p = _processors.begin(); p != _processors.end(); ++p, ++c) {
@@ -1716,8 +1719,21 @@ Route::configure_processors_unlocked (ProcessorStreams* err)
 		processor_max_streams = ChanCount::max(processor_max_streams, c->first);
 		processor_max_streams = ChanCount::max(processor_max_streams, c->second);
 		out = c->second;
+
+		if (boost::dynamic_pointer_cast<Delivery> (*p)
+				&& boost::dynamic_pointer_cast<Delivery> (*p)->role() == Delivery::Main) {
+			/* main delivery will increase port count to match input.
+			 * the Delivery::Main is usually the last processor - followed only by
+			 * 'MeterOutput'.
+			 */
+			seen_mains_out = true;
+		}
+		if (!seen_mains_out) {
+			processor_out_streams = out;
+		}
 	}
 
+
 	if (_meter) {
 		_meter->reset_max_channels (processor_max_streams);
 	}
@@ -3754,6 +3770,19 @@ Route::input_port_count_changing (ChanCount to)
 	return false;
 }
 
+/** Called when there is a proposed change to the output port count */
+bool
+Route::output_port_count_changing (ChanCount to)
+{
+	for (DataType::iterator t = DataType::begin(); t != DataType::end(); ++t) {
+		if (processor_out_streams.get(*t) > to.get(*t)) {
+			return true;
+		}
+	}
+	/* The change is ok */
+	return false;
+}
+
 list<string>
 Route::unknown_processors () const
 {