Service and Message definitions for actions

[sloretz]

Feature request

This is a ticket to create message and service definitions for actions in the rcl_interfaces package.

Feature description

In the current proposal there are 3 services and 2 topics.

1. Send Goal Service
   Service generated for each action, nothing to add to rcl_interfaces
2. Cancel Goal Service
   Same for all actions, could live in rcl_interfaces
   Request is the goal id and a time stamp
   Response is a list of goal ids
3. Get Result Service
   Service generated for each action, generated service response will include enum goal status
4. Goal Status Topic
   Same for all actions, could live in rcl_interfaces
   List of (goal id, the time the goal was accepted, and an enum for goal status)
5. Feedback Topic
   Message generated for each action, nothing to add to rcl_interfaces.

Implementation considerations

The state/status of a goal is used in two places: the status topic and the response to GetResult. It would be convenient if goal status constants were in a message that could be used by both.

# GoalStatus.msg
uint8 UNKNOWN=0
uint8 ACCEPTED=1
...
uint8 status

relates to ros2/design#193
connects to ros2/ros2#583

[jacobperron]

We might consider defining a GoalDescription.msg (analogous to actionlib_msgs/GoalID.msg) for reuse in all of the services and topics related to actions (generated or defined in rcl_interfaces). Suggesting the word "Description" instead of "ID" because it also contains a timestamp.

From the design doc, the plan is to use a UUID for the goal ID. There is the uuid_msgs package from ROS 1. We can fork this as a core ROS 2 package along with helper libraries (e.g. unique_id) for working with UUIDs.

[jacobperron]

Ready for review.

I've opted for the shorter name GoalInfo.msg (instead of GoalDescription.msg), if anyone has a strong opinion about it I can change it easily.