[ROS2]: image_transport does not resolve base topic in correct namespace

[devrite]

Hi everyone,

I think I came across a bug yesterday (using foxy, but I gues it is in rolling too) as I just tried the way image_transport resolves the base topic manually to verify this (see below). So this should affect any ROS2 version and not only foxy.

If you inspect the following excerpt of the camerapublisher:

image_common/image_transport/src/camera_publisher.cpp

Lines 82 to 96 in 4c5053e

	CameraPublisher::CameraPublisher(
	rclcpp::Node * node,
	const std::string & base_topic,
	rmw_qos_profile_t custom_qos)
	: impl_(std::make_shared<Impl>(node))
	{
	// Explicitly resolve name here so we compute the correct CameraInfo topic when the
	// image topic is remapped (#4539).
	std::string image_topic = rclcpp::expand_topic_or_service_name(base_topic,
	node->get_name(), node->get_namespace());
	std::string info_topic = getCameraInfoTopic(image_topic);
	auto qos = rclcpp::QoS(rclcpp::QoSInitialization::from_rmw(custom_qos), custom_qos);
	impl_->image_pub_ = image_transport::create_publisher(node, image_topic, custom_qos);
	impl_->info_pub_ = node->create_publisher<sensor_msgs::msg::CameraInfo>(info_topic, qos);

It uses the expand mechanism but only using the node namespace. ROS2 Nodes feature the ability to create sub-nodes with an associated sub-namespace. If you look into the official templates of create a subscriber or publisher they will use the function resolve_topic_name'), which does all this sub-namespace handling correctly.

I suggest we switch to this or as similar function or call node->get_effective_namespace()instead.

I will try to provide a merge request for the latter today.

BR, Markus