Extending ActionServer with Pausing capabilities

[jr-bor]

I have a doubt that hopefully you can help me with 😄

Use case: I have an action_1 running (e.g. DoDishes) and, while performing, I want to perform another action_2 (e.g. pick up the phone).

As far as I can think about it, a way of implementing this with the SimpleActionServer it would be to create two of these. When the second option wants to be executed, I cancel action_1, execute action_2 and, when done, I start action_1 again. This would do the trick for most of the cases.

However, there are some cases where the action_1 might has critical information that we don't want to loose (e.g. number of plates cleaned) and here is might problem. I could store this critical information in the client but it would be sort of a patch to the root problem. So far, the state machine imposed by action_server.h does not contemplate a "Paused" state. Thus it does not provide client-side methods like setPaused, setActive.

If this makes sense, I was wondering what would be the necessary steps to make this happen and I foresee this as:

• Extend common_msgs/actionlib_msgs to have a Paused state
• Extend action_sever.h to have a two new subscribers as "pause" and "resume" to handle the new interactions with the Active state.
• Extend both python and cpp client implementation to add the setPaused and setActive methods that easy the job.

In some sense, it tackles a design problem of actionlib which is a bi-directional interaction of the process, but for a very simple (and generic) application which is pausing and resuming the action. I think this might be a useful feature for some applications like contextual navigation (imagine a cleaning robot that while cleaning comes across with a dirty spot, cleans it, and resumes the previous job), but maybe this can be achieved in a simpler way that I cannot see 😛

Thanks for the help!