Executor does not maintain a reference to nodes

Bug report

Required Info:

• Operating System:
  • Ubuntu 18.04.2 LTS (Bionic Beaver)
• Installation type:
  • from source
• Version or commit hash:
  • 005131dba8d7992d9f93b889d0eb265db663b8c5
• DDS implementation:
  • Fast-RTPS
• Client library (if applicable):
  • rclcpp

Steps to reproduce issue

Create a shared pointer to a Node, pass it to SingleThreadedExecutor::add_node. Delete or reset the shared pointer. Spin the executor.

#include "rclcpp/rclcpp.hpp"

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

  rclcpp::executors::SingleThreadedExecutor executor;
  executor.add_node(std::make_shared<rclcpp::Node>("Test"));
  executor.spin();
  return 0;
}

Expected behavior

The node should run.

Actual behavior

ROS crashes with an error like:

[ERROR] [rclcpp]: Couldn't add guard_condition to wait set: guard condition implementation is invalid, at /opt/ros/master/src/ros2/rcl/rcl/src/rcl/guard_condition.c:174, at /opt/ros/master/src/ros2/rcl/rcl/src/rcl/wait.c:455
terminate called after throwing an instance of 'std::runtime_error'
  what():  Couldn't fill wait set

Additional information

Start at rclcpp::executors::SingleThreadedExecutor::add_node and reproduce the failure with the shared-pointer example in the issue. Trace how the executor retains the node while the original pointer is reset; done means the node remains usable during spin without the wait-set error or runtime exception.