sensors/vehicle_gps_position: only register callbacks once topic published

- this avoids creating unnecessary uORB device nodes for GPS instances that might never exist

src/modules/sensors/vehicle_gps_position/VehicleGPSPosition.cpp

@@ -59,16 +59,9 @@ bool VehicleGPSPosition::Start()
 	// force initial updates
 	ParametersUpdate(true);
 
-	// needed to change the active sensor if the primary stops updating
-	bool anyGpsRegistered = false;
-	bool registered[GPS_MAX_RECEIVERS];
+	ScheduleNow();
 
-	for (uint8_t i = 0; i < GPS_MAX_RECEIVERS; i++) {
-		registered[i] = _sensor_gps_sub[i].registerCallback();
-		anyGpsRegistered |= registered[i];
-	}
-
-	return anyGpsRegistered;
+	return true;
 }
 
 void VehicleGPSPosition::Stop()
@@ -98,21 +91,26 @@ void VehicleGPSPosition::Run()
 	perf_begin(_cycle_perf);
 	ParametersUpdate();
 
+	// backup schedule
+	ScheduleDelayed(500_ms);
+
 	// Check all GPS instance
 	bool any_gps_updated = false;
 	bool gps_updated[GPS_MAX_RECEIVERS];
 
 	for (uint8_t i = 0; i < GPS_MAX_RECEIVERS; i++) {
-
 		gps_updated[i] = _sensor_gps_sub[i].updated();
 
 		if (gps_updated[i]) {
 			any_gps_updated = true;
 			_sensor_gps_sub[i].copy(&_gps_state[i]);
+
+			if (!_sensor_gps_sub[i].registered()) {
+				_sensor_gps_sub[i].registerCallback();
+			}
 		}
 	}
 
-
 	if ((_param_sens_gps_mask.get() == 0) && gps_updated[0]) {
 		// When GPS blending is disabled we always use the first receiver instance
 		Publish(_gps_state[0]);

src/modules/uORB/SubscriptionCallback.hpp

@@ -68,22 +68,24 @@ public:
 
 	bool registerCallback()
 	{
-		if (_subscription.get_node() && _subscription.get_node()->register_callback(this)) {
-			// registered
-			_registered = true;
+		if (!_registered) {
+			if (_subscription.get_node() && _subscription.get_node()->register_callback(this)) {
+				// registered
+				_registered = true;
 
-		} else {
-			// force topic creation by subscribing with old API
-			int fd = orb_subscribe_multi(_subscription.get_topic(), _subscription.get_instance());
+			} else {
+				// force topic creation by subscribing with old API
+				int fd = orb_subscribe_multi(_subscription.get_topic(), _subscription.get_instance());
 
-			// try to register callback again
-			if (_subscription.subscribe()) {
-				if (_subscription.get_node() && _subscription.get_node()->register_callback(this)) {
-					_registered = true;
+				// try to register callback again
+				if (_subscription.subscribe()) {
+					if (_subscription.get_node() && _subscription.get_node()->register_callback(this)) {
+						_registered = true;
+					}
 				}
-			}
 
-			orb_unsubscribe(fd);
+				orb_unsubscribe(fd);
+			}
 		}
 
 		return _registered;
@@ -131,6 +133,8 @@ public:
 
 	virtual void call() = 0;
 
+	bool registered() const { return _registered; }
+
 protected:
 
 	bool _registered{false};