How to handle exceptions in the user-defined-callback in service-server and how to inform service-client about the failed-service-call in ros2 rclcpp ?

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Hi, I just faced the same problem and investigated the rclcpp code a bit, it seems like making the service fail is not currently possible (although the docs recommend to throw exceptions but that doesn't work).

The only workaround in your case is to add a timeout on the client side and make the server not return a response. Here is an example of the migration from ROS1:

// ROS 1 style is in comments, ROS 2 follows, uncommented.
// #include "nav_msgs/GetMap.h"
#include "nav_msgs/srv/get_map.hpp"

// bool service_callback(
//   nav_msgs::GetMap::Request & request,
//   nav_msgs::GetMap::Response & response)
void service_callback(
  const std::shared_ptr<rmw_request_id_t> request_header,
  const std::shared_ptr<nav_msgs::srv::GetMap::Request> request)
{
  auto response = std::make_shared<nav_msgs::srv::GetMap::Response>();
  // ...
  if (!has_map_) {
    // return false;
    return;
  }
  // return true;
  map_server_->send_response(*request_header, *response);
}

On the client side you just add a timeout:

auto result = client->async_send_request(request);
// Wait for the result.
if (sync_response_executor->spin_until_future_complete(result, 800ms) != rclcpp::FutureReturnCode::SUCCESS) {
    RCLCPP_ERROR(get_logger(), "Service failed");
    return;
}