Deferrable + Cancelable lifecycle change_state transition functions

[tgroechel]

Feature request

Allow for user transition callbacks within lifecycle nodes (e.g., on_configure) to defer work and be cancelable.

Backward compatible PR: #2214
Demo Repo

Feature description

Lifecycle transition callbacks currently must be synchronous. With the limitation of calling service within a service #773 and the advent of async services #1709, we would like to propose adding the ability to defer work within a user transition callback (e.g., on_configure) and allowing these transitions to be cancelled.

Deferral + Cancelable transitions would allow for:

1. nodes/entities with external transition dependencies to defer work until those dependencies are met (e.g., requests to external service(s) is/are ready/complete)
2. attempting recovery mid-transition instead of needing to fully shutdown + fully bring the node back to a desired state

The change_state process would ideally be deferrable + cancelable. This would allow for both long running functions (e.g., a long load in on_configure) as well as free up the executor to process new events (e.g., nodes waiting on external dependencies).

Example code reference

void on_activate_async(
     const rclcpp_lifecycle::State &state,
     std::shared_ptr<rclcpp_lifecycle::ChangeStateHandler> change_state_hdl) {
   std::thread t(&LifecycleTalker::defer_on_activate_work, this, change_state_hdl);
   t.detach();
} // Off Executor

void defer_on_activate_work(
   std::shared_ptr<rclcpp_lifecycle::ChangeStateHandler> change_state_hdl) {
   /* Do work & */
   /* Monitor for cancellation via change_state_hdl->is_cancelling() */
   LifecycleNode::on_activate(state); // activate ManagedEntities
   change_state_hdl->send_callback_resp(CallbackReturn::SUCCESS);
}

[ros-discourse]

This issue has been mentioned on ROS Discourse. There might be relevant details there:

https://discourse.ros.org/t/deferrable-canceleable-lifecycle-transitions/32318/1