fix(simulation): don't publish gimbal when not available (#27908)

We should not publish gimbal things if we don't actually have a gimbal
model in Gz. Otherwise this confuses any other gimbal setups that one
might try.

src/modules/simulation/gz_bridge/GZGimbal.cpp

@@ -84,8 +84,10 @@ void GZGimbal::Run()
 		publishJointCommand(_gimbal_yaw_cmd_publisher, _yaw_stp, _yaw_rate_stp, _last_yaw_stp, _yaw_min, _yaw_max, dt);
 	}
 
-	if (_mnt_mode_out == 2) {
-		// We have a Mavlink gimbal capable of sending messages
+	if (_mnt_mode_out == 2 && _gimbal_present) {
+		// We have a Mavlink gimbal capable of sending messages. Only present it if the
+		// model actually has a gimbal (_gimbal_present), otherwise PX4 would advertise a
+		// phantom gimbal device and could not manage an external MAVLink gimbal.
 		publishDeviceInfo();
 		publishDeviceAttitude();
 	}
@@ -102,6 +104,9 @@ void GZGimbal::gimbalIMUCallback(const gz::msgs::IMU &IMU_data)
 {
 	pthread_mutex_lock(&_node_mutex);
 
+	// Receiving gimbal IMU data means the model actually has a gimbal.
+	_gimbal_present = true;
+
 	static const matrix::Quatf q_FLU_to_FRD = matrix::Quatf(0.0f, 1.0f, 0.0f, 0.0f);
 	static const matrix::Quatf q_ENU_to_NED = matrix::Quatf(0.0f, cosf(M_PI_4_F), cosf(M_PI_4_F), 0.0f);
 

src/modules/simulation/gz_bridge/GZGimbal.hpp

@@ -80,6 +80,11 @@ private:
 	matrix::Quatf _q_gimbal = matrix::Quatf(1.0f, 0.0f, 0.0f, 0.0f);
 	float _gimbal_rate[3] = {NAN};
 
+	// Set once we receive gimbal IMU data from Gazebo, i.e. the model actually has a
+	// gimbal. Until then we don't present a MAVLink gimbal device, so that a model
+	// without a gimbal lets PX4 act as a manager for an external MAVLink gimbal instead.
+	bool _gimbal_present = false;
+
 	// Device information attributes
 	const char _vendor_name[32] = "PX4";
 	const char _model_name[32] = "Gazebo Gimbal";