[ros2/rclcpp] Threadsafety of Node::create_publisher<>

edit

Creating and destroying (allowing them to go out of scope) publishers and timers is thread-safe right now. For Subscriptions, Service Servers, and Service Clients, creation is currently thread-safe, and destroying them was intended to be, but there are some on going issues about that here:

https://github.com/ros2/rclcpp/issues...

and

https://github.com/ros2/rclcpp/pull/431

Long term hopefully all of the actions will be thread-safe (that's the goal). Also we hope to improve the documentation soon too, but anyone could help with that I think.

On a related note, adding and removing nodes to/from executors is also thread-safe. So you could do something like this:

#include <thread>

#include <rclcpp/rclcpp.hpp>
#include <std_msgs/msg/string.hpp>

using namespace std::chrono_literals;

int main(int argc, char ** argv) {
  rclcpp::init(argc, argv);

  rclcpp::executors::SingleThreadedExecutor executor;

  auto node = std::make_shared<rclcpp::Node>("test");
  executor.add_node(node);

  std::thread t([&executor]() {
    executor.spin();
  });

  {
    std::this_thread::sleep_for(1s);
    auto pub = node->create_publisher<std_msgs::msg::String>("chatter");
    std::this_thread::sleep_for(1s);
    pub.reset();
  }

  {
    std::this_thread::sleep_for(1s);
    auto sub = node->create_subscription<std_msgs::msg::String>(
      "chatter",
      [](const std_msgs::msg::String::SharedPtr msg) {(void)msg;});
    std::this_thread::sleep_for(1s);
    sub.reset();
  }

  {
    std::this_thread::sleep_for(1s);
    executor.remove_node(node);
    std::this_thread::sleep_for(1s);
    executor.add_node(node);
    std::this_thread::sleep_for(1s);
    node.reset();
    std::this_thread::sleep_for(1s);
  }


  executor.cancel();
  t.join();
}