uORB::Subscription minor API cleanup

* the forceInit() method was combined with the existing subscribe()
 * delete unused last_update()

src/modules/uORB/Subscription.cpp

@@ -42,36 +42,59 @@
 namespace uORB
 {
 
-bool Subscription::subscribe()
+bool
+Subscription::subscribe()
 {
-	DeviceMaster *device_master = uORB::Manager::get_instance()->get_device_master();
-	_node = device_master->getDeviceNode(_meta, _instance);
+	// valid ORB_ID required
+	if (_meta == nullptr) {
+		return false;
+	}
 
+	// check if already subscribed
 	if (_node != nullptr) {
-		_node->add_internal_subscriber();
-
-		// If there were any previous publications, allow the subscriber to read them
-		const unsigned curr_gen = _node->published_message_count();
-		const unsigned q_size = _node->get_queue_size();
-
-		_last_generation = curr_gen - (q_size < curr_gen ? q_size : curr_gen);
-
 		return true;
 	}
 
+	DeviceMaster *device_master = uORB::Manager::get_instance()->get_device_master();
+
+	if (device_master != nullptr) {
+		uORB::DeviceNode *node = device_master->getDeviceNode(_meta, _instance);
+
+		if (node != nullptr) {
+			_node = node;
+			_node->add_internal_subscriber();
+
+			// If there were any previous publications, allow the subscriber to read them
+			const unsigned curr_gen = _node->published_message_count();
+			const uint8_t q_size = _node->get_queue_size();
+
+			if (q_size < curr_gen) {
+				_last_generation = curr_gen - q_size;
+
+			} else {
+				_last_generation = 0;
+			}
+
+			return true;
+		}
+	}
+
 	return false;
 }
 
-void Subscription::unsubscribe()
+void
+Subscription::unsubscribe()
 {
 	if (_node != nullptr) {
 		_node->remove_internal_subscriber();
 	}
 
+	_node = nullptr;
 	_last_generation = 0;
 }
 
-bool Subscription::init()
+bool
+Subscription::init()
 {
 	if (_meta != nullptr) {
 		// this throttles the relatively expensive calls to getDeviceNode()
@@ -90,18 +113,8 @@ bool Subscription::init()
 	return false;
 }
 
-bool Subscription::forceInit()
-{
-	if (_node == nullptr) {
-		// reset generation to force subscription attempt
-		_last_generation = 0;
-		return subscribe();
-	}
-
-	return false;
-}
-
-bool Subscription::update(uint64_t *time, void *dst)
+bool
+Subscription::update(uint64_t *time, void *dst)
 {
 	if ((time != nullptr) && (dst != nullptr) && published()) {
 		// always copy data to dst regardless of update