Server/bus.go

package lucifer3

import (
	"fmt"
	"sync"
)

type ServiceKey struct{}

type Event interface {
	EventDescription() string
}

// A TriggerEvent is a catch-all for events that may trigger a command from the event handler.
type TriggerEvent interface {
	Event

	TriggerKind() string
	TriggerValue(key string) (string, bool)
}

type Command interface {
	CommandDescription() string
}

type EventBus struct {
	mu             sync.Mutex
	listeners      []*serviceListener
	privilegedList []ActiveService
	signal         chan struct{}
}

// JoinCallback joins the event bus for a moment.
func (b *EventBus) JoinCallback(cb func(event Event) bool) {
	b.Join(newCallbackService(cb))
}

func (b *EventBus) Join(service Service) {
	// Take the signal here so that it will receive the events sent by the calling function
	// so that they're processed without having to wait for a new event.
	signal := b.signalCh()

	listener := &serviceListener{
		bus:     b,
		queue:   make([]serviceMessage, 0, 16),
		service: service,
	}
	go listener.run(signal)

	b.mu.Lock()
	b.listeners = append(b.listeners, listener)
	b.mu.Unlock()
}

// JoinPrivileged will add the services to a list that gets first look at
// all events, but they are not allowed to issue any commands. This is for
// things that cannot have eventual consistency.
func (b *EventBus) JoinPrivileged(service ActiveService) {
	b.mu.Lock()
	b.privilegedList = append(b.privilegedList, service)
	b.mu.Unlock()
}

func (b *EventBus) RunCommand(command Command) {
	fmt.Println("[COMMAND]", command.CommandDescription())
	b.send(serviceMessage{command: command})
}

func (b *EventBus) RunEvent(event Event) {
	fmt.Println("[EVENT]", event.EventDescription())
	b.send(serviceMessage{event: event})
}

func (b *EventBus) send(message serviceMessage) {
	b.mu.Lock()
	defer b.mu.Unlock()

	for _, service := range b.privilegedList {
		if message.command != nil {
			service.HandleCommand(nil, message.command)
		}
		if message.event != nil {
			service.HandleEvent(nil, message.event)
		}
	}

	deleteList := make([]int, 0, 0)
	for i, listener := range b.listeners {
		if !listener.service.Active() {
			deleteList = append(deleteList, i-len(deleteList))
			continue
		}

		listener.mu.Lock()
		listener.queue = append(listener.queue, message)
		listener.mu.Unlock()
	}

	for i := range deleteList {
		b.listeners = append(b.listeners[:i], b.listeners[i+1:]...)
	}

	if b.signal != nil {
		close(b.signal)
		b.signal = nil
	}
}

func (b *EventBus) signalCh() <-chan struct{} {
	b.mu.Lock()
	defer b.mu.Unlock()

	if b.signal == nil {
		b.signal = make(chan struct{})
	}

	return b.signal
}

type serviceListener struct {
	mu      sync.Mutex
	bus     *EventBus
	queue   []serviceMessage
	service Service
}

func (l *serviceListener) run(signal <-chan struct{}) {
	// Detect command support
	var activeService ActiveService
	if as, ok := l.service.(ActiveService); ok {
		activeService = as
	}

	queue := make([]serviceMessage, 0, 16)

	for {
		// Listen for the signal, but stop here if the service has marked
		// itself as inactive.
		<-signal
		if !l.service.Active() {
			return
		}

		// Take a new signal before copying the queue to avoid delay
		// in case a message is about to be enqueued right now!
		signal = l.bus.signalCh()

		// Take a copy of the queue.
		l.mu.Lock()
		for _, message := range l.queue {
			queue = append(queue, message)
		}
		l.queue = l.queue[:0]
		l.mu.Unlock()

		// Handle the messages in the queue, but stop mid-queue if the previous message
		// or external circumstances marked the service inactive.
		for _, message := range queue {
			if !l.service.Active() {
				return
			}

			if message.event != nil {
				l.service.HandleEvent(l.bus, message.event)
			}

			if message.command != nil && activeService != nil {
				activeService.HandleCommand(l.bus, message.command)
			}
		}
		queue = queue[:0]
	}
}

type serviceMessage struct {
	event   Event
	command Command
}