MultiThreadedExecutor virtual run

Bug report

Required Info:

• Operating System:
  • Ubuntu 20.04
• Installation type:
  • From sources
• Version or commit hash:
  • Galactic and Humble
• DDS implementation:
  • rmw_cyclonedds_cpp
• Client library (if applicable):
  • rclcpp

Steps to reproduce issue

The multi-threaded executor declares the void run(size_t this_thread_number); method as protected but does not declares it as a virtual method. This prevents a child class from overriding the run method and prevents improving that method in a case by case basis.
For example, the current implementation is not exception safe which means that if an exception is thrown in the lower layer of rcl implementation (possibly at the rmw layer), the multithreaded executor thread might abnormally terminate since the exception is not handled.

Expected behavior

The run method should be override'able by a child class.

Actual behavior

The run method is not override'able by a child class.

Additional information

Feature request

Feature description

Implementation considerations

Start in rclcpp/include/rclcpp/executors/multi_threaded_executor.hpp at the protected run(size_t this_thread_number) declaration, then inspect its implementation in the corresponding executor source. Confirm how a child class currently interacts with run and define done as allowing that method to be overridden without changing unrelated executor behavior; the payload names no test file.