breaker can block when event listeners are added with AddListener

[aryszka]

When adding a listener (buffered) channel with AddListener(), and consuming the events, the breaker can block forever. The test code in the linked gist can reproduce the issue with a reasonable chance:

https://gist.github.com/aryszka/115fea73da80422a6d46ff058c8dcb0b

this is a bug

func (cb *Breaker) sendEvent(event BreakerEvent) {
	for _, receiver := range cb.eventReceivers {
		receiver <- event
	}
	for _, listener := range cb.listeners {
		le := ListenerEvent{CB: cb, Event: event}
		select {
		case listener <- le: //step 1 :this channel have not space to receive
		default: // but between step 1 and 2,another coroutine have receive the event from listener  and listener  is empty ,so  <listener  block !
			//<-listener //step: 2 so you want to release a event and send to it again
			//listener <- le
		}
	}
}

i think if step 1 fail ,step 2 is unnecessary ,ignore this le ,this is not import;
and event channel in your test should better have a bigger capacity,10 may be good,this
guarantee the channel have cache to receive more elemet in highly Concurrent;you can test again use c := make(chan circuit.ListenerEvent, 10);it will not block anymore(In theory, this still has a certain probability happen);
But it would be best to annotation the default select operation ; this can fix the bug,

func (cb *Breaker) Subscribe() <-chan BreakerEvent {
	eventReader := make(chan BreakerEvent)
	output := make(chan BreakerEvent, 100)

	go func() {
		for v := range eventReader {
			select {
			case output <- v:
			default:
				<-output
				output <- v
			}
		}
	}()
	cb.eventReceivers = append(cb.eventReceivers, eventReader)
	return output
}

this also have this bug