Aborting of in-progress push payments

[mattsaxon]

With respect to Roy's demo of push payments using the 'fb' PMI, I think it would be useful to more specifically define the behaviour of when Abort() is called when a payment in 'processing'.

In particular, I wonder if a specific state of "Processing" would be useful to define in the state machine in addition to "Created", "Interactive" and "Closed".

This would allow the definition of a specific exception to be thrown in the abort() algorithm to disambiguate the various reasons for failure, e.g. "AlreadyProcessing"

[mattsaxon]

With regard to the proposal for a "processing" status, it is worth highlighting that section 16.10 explicitly calls out an addition state of pending, "the payment request user interface remains in a pending state". It is worth clarifiying what this pending state means and if it is part of the state model in section 3.10 (internal slot: state)

[mattsaxon]

this issue is related to one in the payment handler, issue 117.

[zkoch]

Marking as postponed since we don't have this in any current implementation, and we'd have to mark as "at-risk" for now anyway because of that. We'd prefer to postpone for now, though we think it's a good and useful idea.