Fix spawning actors with duplicate id bug

[troygilman0]

This PR addresses this issue #163.

• Registry.add() now returns an error if the process exists.
• Instead of running through the actor startup sequence for the duplicate process, we will just return the PID as it should be a valid PID for the original process.
• Added test case to ensure only one process is started.

[troygilman0]

renamed this existing test case to be more accurate

[tprifti]

If you check Registry, it's not possible to add 2 actors with same ID. An event is broadcasted and you can subscribe into that in case you want to have a specific logic.
For reference: https://github.com/anthdm/hollywood/blob/master/actor/registry.go#L60-L70

[troygilman0]

If you check Registry, it's not possible to add 2 actors with same ID. An event is broadcasted and you can subscribe into that in case you want to have a specific logic. For reference: https://github.com/anthdm/hollywood/blob/master/actor/registry.go#L60-L70

I understand that there is a DuplicateIdEvent that is published in this scenario, but the issue is that the startup sequence (actor.Initialized and actor.Started events) is still triggered for the duplicate actor. This can be seen in the issue I raised #163. In my opinion, this is unexpected behavior that can cause bugs.

[tprifti]

If you check Registry, it's not possible to add 2 actors with same ID. An event is broadcasted and you can subscribe into that in case you want to have a specific logic. For reference: https://github.com/anthdm/hollywood/blob/master/actor/registry.go#L60-L70

I understand that there is a DuplicateIdEvent that is published in this scenario, but the issue is that the startup sequence (actor.Initialized and actor.Started events) is still triggered for the duplicate actor. This can be seen in the issue I raised #163. In my opinion, this is unexpected behavior that can cause bugs.

I will investigate it further and see what is causing the issue. Normally, the registry should not do anything when the process exists, thus no duplicate events should be present

[anthdm]

@troygilman0 @tprifti After some investigation I think we need to prevent engine.SpawnProc from calling Start() and move this to the bottom of the registry.Add function.

func (e *Engine) SpawnProc(p Processer) *PID {
	e.Registry.add(p)
	// p.Start() remove this!
	return p.PID()
}

func (r *Registry) add(proc Processer) {
	r.mu.Lock()
	id := proc.PID().ID
	if _, ok := r.lookup[id]; ok {
		r.mu.Unlock()
		r.engine.BroadcastEvent(ActorDuplicateIdEvent{PID: proc.PID()})
		return
	}
	r.lookup[id] = proc
	r.mu.Unlock()

	proc.Start() // Add this
}

[anthdm]

Looks good to go. Thanks for all the effort.