Executor quick destruction after spin hangs

Bug report

Required Info:

• Operating System: Ubuntu 20.04
• Installation type: Binaries
• Version or commit hash: 5.0.0-1focal.20201007.211227
• DDS implementation: Fast DDS
• Client library (if applicable): rclcpp

Steps to reproduce issue

TEST(Foo, bar) {
  auto node = std::make_shared<rclcpp::Node>("test");
  auto executor = std::make_shared<rclcpp::executors::SingleThreadedExecutor>();
  executor->add_node(node);
  auto executor_spin_future = std::async(
    std::launch::async, [&executor]() -> void {
      executor->spin();
    });
  //  std::this_thread::sleep_for(50ms);
  executor->cancel();
}

Expected behavior

That the test ends.

Actual behavior

Very frequently, the test hangs on the future destruction, waiting for thread join();
Because the thread is stuck on executor->spin() (wait_for_work exactly)

Additional information

Uncommenting the sleep_for fixes the issue on my machine, so there seems to be a race condition.
In our use case, we have test than end faster than that time and trigger this issue, we can add the sleep as a workaround but it's ugly: https://github.com/ros-controls/ros2_control/pull/234/files/833b1661209cc12a9d6f9ef45c87f4072e641aa7#diff-28c9aa0318c48cffb66c294bbf17394b6f9295bb8b9fb83d2cac539699f7e354R130

Start with the provided test reproducer and trace SingleThreadedExecutor::spin() into wait_for_work(), where the thread remains blocked after cancel(). Reproduce the hang on Ubuntu 20.04 and compare it with the sleep_for workaround; done means the test completes reliably without the delay.