API for action servers and clients

[sloretz]

This PR proposes an API for action clients and action services. It is incomplete, but what is here is ready for review.

connects to ros2/design#193

[Jmeyer1292]

I'm concerned about the use of a synchronous ``execute()` function here. It has been my experience in ROS1 that many of the actions that control the high level behavior of our robots themselves dispatch to one or more asynchronous tasks (be they library calls, another set of actions, etc). I find it more elegant to handle these via the callbacks of these sub-functions rather than spinning in a given thread. Furthermore, using an execute callback means that you require a 1 to 1 mapping of goals to threads. This likely won't scale well when you hit many goals active at once.

So, in my humble opinion, I would prefer to see a base API that mimics the original (not simple) action server.

[wjwwood]

My understanding is that the signature of the execute callback can optionally return void, which essentially means you can dispatch any asynchronous tasks and return from it immediately, electing to asynchronously "finish" the execution of the action in the future by using set_canceled, set_aborted, or set_success asynchronously.

In that case the execute function is just your notification that the goal is ready to be executed.

Also, if you don't want the asynchronous execute notice at all, one possible value for the "execution policy" is "do nothing". Then it's up to the user to poll the action server object to "drain" ready goals and execute them to one of the terminal states.

So in short, I think what you're asking for should be possible in the current design.

[Jmeyer1292]

👍 Sounds good.

Please forgive a dumb question: Is the contents of the rclcpp_action/rclcpp_action.hpp file available somewhere for review so that I might intuit that answer myself?

I also have one more sort-of-pedantic question: The given execution callback does not appear to return a value if the loop aborts early because of rclcpp::ok() returning false. In this case, what would the appropriate value be aborted?

[wjwwood]

Please forgive a dumb question: Is the contents of the rclcpp_action/rclcpp_action.hpp file available somewhere for review so that I might intuit that answer myself?

No unfortunately, we're sort in the process of developing these API's. This pr/examples is meant to be a motivating example for how the API should work. So basically it's a made up API at the moment 😄. I also expect it will evolve more. So for now, asking us questions about the examples is the most reasonable thing.

I also have one more sort-of-pedantic question: The given execution callback does not appear to return a value if the loop aborts early because of rclcpp::ok() returning false. In this case, what would the appropriate value be aborted?

The user defines the "execute" callback so if in the process they check for and notice the rclcpp::ok is returning false, then they can choose to call set_abort themselves.

If it occurs inbetween calls to the user (e.g. after handle_goal has completed but before execute is called), I'm not 100% sure right now. I'd assume the action server would cancel all outstanding goals and then wait for any on going user callbacks to finish executing. It's something to maybe keep an eye and ask about again when we have a more complete prototype.

[sloretz]

Replaced by #216