Fix callback order of port-dis/connections

Previously the port-engine was a LIFO. Changes were pushed back
and then popped-back. This causes issues when re-connecting
Transport Masters.

The GUI does the following when changing connections:
1. disconnect all
2. connect to new port

which lead to TransportMaster::connection_handler being called
in reverse order: connect, disconnect, and the transport
master was assumed to not be connected.

--

Now connections queue is a FIFO and code was consolidated.

(Note, we cannot use a std::deque because it does not support
memory pre-allocation with ::reserve)

libs/ardour/ardour/port_engine_shared.h

@@ -27,7 +27,6 @@
 #include <string>
 #include <vector>
 
-
 #include "pbd/natsort.h"
 #include "pbd/rcu.h"
 
@@ -203,6 +202,8 @@ protected:
 		pthread_mutex_unlock (&_port_callback_mutex);
 	}
 
+	void process_connection_queue_locked (PortManager& mgr);
+
 	void port_connect_add_remove_callback () {
 		_port_change_flag.store (1);
 	}

libs/ardour/port_engine_shared.cc

@@ -23,6 +23,7 @@
 #include "pbd/error.h"
 
 #include "ardour/port_engine_shared.h"
+#include "ardour/port_manager.h"
 
 #include "pbd/i18n.h"
 
@@ -828,6 +829,16 @@ PortEngineSharedImpl::update_system_port_latencies ()
 	}
 }
 
+void
+PortEngineSharedImpl::process_connection_queue_locked (PortManager& mgr)
+{
+	for (auto& c : _port_connection_queue) {
+		mgr.connect_callback (c->a, c->b, c->c);
+		delete c;
+	}
+	_port_connection_queue.clear ();
+}
+
 #ifndef NDEBUG
 void
 PortEngineSharedImpl::list_ports () const

libs/backends/alsa/alsa_audiobackend.cc

@@ -2092,12 +2092,7 @@ AlsaAudioBackend::main_process_thread ()
 			if (!_port_connection_queue.empty ()) {
 				connections_changed = true;
 			}
-			while (!_port_connection_queue.empty ()) {
-				PortConnectData* c = _port_connection_queue.back ();
-				manager.connect_callback (c->a, c->b, c->c);
-				_port_connection_queue.pop_back ();
-				delete c;
-			}
+			process_connection_queue_locked (manager);
 			pthread_mutex_unlock (&_port_callback_mutex);
 		}
 		if (ports_changed) {

libs/backends/coreaudio/coreaudio_backend.cc

@@ -1282,12 +1282,7 @@ CoreAudioBackend::pre_process ()
 		if (!_port_connection_queue.empty ()) {
 			connections_changed = true;
 		}
-		while (!_port_connection_queue.empty ()) {
-			PortConnectData *c = _port_connection_queue.back ();
-			manager.connect_callback (c->a, c->b, c->c);
-			_port_connection_queue.pop_back ();
-			delete c;
-		}
+		process_connection_queue_locked (manager);
 		pthread_mutex_unlock (&_port_callback_mutex);
 	}
 	if (ports_changed) {

libs/backends/dummy/dummy_audiobackend.cc

@@ -1029,12 +1029,7 @@ DummyAudioBackend::main_process_thread ()
 			if (!_port_connection_queue.empty ()) {
 				connections_changed = true;
 			}
-			while (!_port_connection_queue.empty ()) {
-				PortConnectData *c = _port_connection_queue.back ();
-				manager.connect_callback (c->a, c->b, c->c);
-				_port_connection_queue.pop_back ();
-				delete c;
-			}
+			process_connection_queue_locked (manager);
 			pthread_mutex_unlock (&_port_callback_mutex);
 		}
 		if (ports_changed) {

libs/backends/portaudio/portaudio_backend.cc

@@ -1827,12 +1827,7 @@ PortAudioBackend::process_port_connection_changes ()
 		if (!_port_connection_queue.empty ()) {
 			connections_changed = true;
 		}
-		while (!_port_connection_queue.empty ()) {
-			PortConnectData *c = _port_connection_queue.back ();
-			manager.connect_callback (c->a, c->b, c->c);
-			_port_connection_queue.pop_back ();
-			delete c;
-		}
+		process_connection_queue_locked (manager);
 		pthread_mutex_unlock (&_port_callback_mutex);
 	}
 	if (ports_changed) {

libs/backends/pulseaudio/pulseaudio_backend.cc

@@ -1098,12 +1098,7 @@ PulseAudioBackend::main_process_thread ()
 			if (!_port_connection_queue.empty ()) {
 				connections_changed = true;
 			}
-			while (!_port_connection_queue.empty ()) {
-				PortConnectData* c = _port_connection_queue.back ();
-				manager.connect_callback (c->a, c->b, c->c);
-				_port_connection_queue.pop_back ();
-				delete c;
-			}
+			process_connection_queue_locked (manager);
 			pthread_mutex_unlock (&_port_callback_mutex);
 		}
 		if (ports_changed) {