Finality States

[hannahhoward]

Goals

• cleanup go routines for state machines that are not receiving events any longer
• provide a way to identify FSM's that have reached a state of finality -- i.e. their state will no longer change, they should reject all new events and not consume resources
• this is relevant for the ability to restart -- we need need to be able to distinguish between deals that are in some kind of completion vs those that might need to be revisited. While this could be done manually in other code bases, it seems like a good idea to codify it as it also provides tangible benefits in terms of cleaning up go routine usage. Also, a notion of a set of finality states is part of many traditional FSM definitions.

Implementation

• Provide a special error code which returned by a Planner signifies a normal shutdown -- i.e. not an error, but simply that the statemachine is in a state where no further transitions will occur. Also return this error when messages are sent to closed state machine.
• Add a list of FinalityStates to FSM parameters. Check for the FSM being in a finality state inside it's handler, and shut down if neccesary
• Make sure to clear out all FSM events when shutting down to support SendSync
• Add an IsTerminated function of the FSM group which returns whether an FSM is in a final state
  I envision a restart operations looking like this...

var states []SomeFSMState
fsmGroup.List(&states)
for _, state := range states {
   if !fsmGroup.IsTerminated(state) {
      fsmGroup.Send(identifier(state), "restart")
   }
}

• Add FinalityStates to docs

For discussion

In the commit history, you can see I also explored cleaning up the actual statemachine map in statemachine.Group. I ultimately found this untestable and ultimately opted for a more minimal PR that just makes sure we don't leave go routines running longer than they need to. We can however explore this in the future.

[ingar]

Couple of tweaks if you like, otherwise 👍 Nice to not have deals come back alive out of errored states!