How to make a service request inside a callback function

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The problem is, that you can't use the spin_node_until_future_complete() command inside the outer callback. The solution is to use an MultiThreadedExecutor and different callback groups. Then you can call the inner service by giving it a callback function (instead of waiting for the return code with spin_node_until_future_complete).

If you want to wait for the result from the inner service call before the outer service callback finishes you need to block the thread with a while(return_from_inner_service == false) loop. This is important if your outer callback is from a service, because then you want to receive the inner service result and use it to compute the result of the outer service. But this is bad design, because you block the spin() call from the outer service. Luckily we can still handle other callbacks with the MultiThreadedExecutor as long as we assign them to a different callback group. If the outer callback is from a topic you don't want to block the callback, just handle the inner service call inside the assigned callback function. It will execute asynchronous.

Here is some code example in C++ (assuming all necessary headers included and ROS services created)

Create the MultiThreadedExecuter and an instance of the Node in main:

int main(int argc, char *argv[])
{
    rclcpp::init(argc, argv);
    rclcpp::executors::MultiThreadedExecutor executor;
    auto node = std::make_shared<Communication>();
    executor.add_node(node);
    executor.spin();
    rclcpp::shutdown();
    return 0;
}

Create seperate Callback group and assign it to the inner service client (inside the Node/class constructor):

callback_group_input_ = this->create_callback_group(rclcpp::callback_group::CallbackGroupType::MutuallyExclusive);
get_input_client_ = this->create_client<petra_core::srv::GetInput>("GetInput", rmw_qos_profile_services_default, callback_group_input_);

Inside the outer callback from a topic or service, send the inner_request and define a callback for it:

int choice = -1;
    auto inner_client_callback = [&,this](rclcpp::Client<petra_core::srv::GetInput>::SharedFuture inner_future)
        { 
            auto result = inner_future.get();
            choice = stoi(result->input);
            RCLCPP_INFO(this->get_logger(), "[inner service] callback executed");
        };
    auto inner_future_result = get_input_client_->async_send_request(inner_request, inner_client_callback);

If it is necessary to wait inside the outer callback for the result block the thread:

// Wait for the result.
    while (choice < 0 && rclcpp::ok())
    {
        std::this_thread::sleep_for(std::chrono::milliseconds(10));
    }

I hope it helps you, or whoever has the same problem. I was stuck with this for weeks...