Add PubSub topics.

A TopicFunc is an interface to let the pubisher decide whether it needs
to send a message to a subscriber or not. It returns true if the
publisher must send the message and false otherwise.

Users of the pubsub package can create a subscriber with a topic
function by calling `pubsub.SubscribeTopic`.

Message delivery has also been modified to use concurrent channels per
subscriber. That way, topic verification and message delivery is not
o(N+M) anymore, based on the number of subscribers and topic verification
complexity.

Using pubsub topics, the API stops controlling the message delivery,
delegating that function to a topic generated with the filtering
provided by the user. The publisher sends every message to the
subscriber if there is no filter, but the api doesn't have to select
messages to return anymore.

Signed-off-by: David Calavera <david.calavera@gmail.com>

pubsub/publisher.go

@@ -13,11 +13,12 @@ func NewPublisher(publishTimeout time.Duration, buffer int) *Publisher {
 	return &Publisher{
 		buffer:      buffer,
 		timeout:     publishTimeout,
-		subscribers: make(map[subscriber]struct{}),
+		subscribers: make(map[subscriber]topicFunc),
 	}
 }
 
 type subscriber chan interface{}
+type topicFunc func(v interface{}) bool
 
 // Publisher is basic pub/sub structure. Allows to send events and subscribe
 // to them. Can be safely used from multiple goroutines.
@@ -25,7 +26,7 @@ type Publisher struct {
 	m           sync.RWMutex
 	buffer      int
 	timeout     time.Duration
-	subscribers map[subscriber]struct{}
+	subscribers map[subscriber]topicFunc
 }
 
 // Len returns the number of subscribers for the publisher
@@ -38,9 +39,14 @@ func (p *Publisher) Len() int {
 
 // Subscribe adds a new subscriber to the publisher returning the channel.
 func (p *Publisher) Subscribe() chan interface{} {
+	return p.SubscribeTopic(nil)
+}
+
+// SubscribeTopic adds a new subscriber that filters messages sent by a topic.
+func (p *Publisher) SubscribeTopic(topic topicFunc) chan interface{} {
 	ch := make(chan interface{}, p.buffer)
 	p.m.Lock()
-	p.subscribers[ch] = struct{}{}
+	p.subscribers[ch] = topic
 	p.m.Unlock()
 	return ch
 }
@@ -56,20 +62,13 @@ func (p *Publisher) Evict(sub chan interface{}) {
 // Publish sends the data in v to all subscribers currently registered with the publisher.
 func (p *Publisher) Publish(v interface{}) {
 	p.m.RLock()
-	for sub := range p.subscribers {
-		// send under a select as to not block if the receiver is unavailable
-		if p.timeout > 0 {
-			select {
-			case sub <- v:
-			case <-time.After(p.timeout):
-			}
-			continue
-		}
-		select {
-		case sub <- v:
-		default:
-		}
+	wg := new(sync.WaitGroup)
+	for sub, topic := range p.subscribers {
+		wg.Add(1)
+
+		go p.sendTopic(sub, topic, v, wg)
 	}
+	wg.Wait()
 	p.m.RUnlock()
 }
 
@@ -82,3 +81,24 @@ func (p *Publisher) Close() {
 	}
 	p.m.Unlock()
 }
+
+func (p *Publisher) sendTopic(sub subscriber, topic topicFunc, v interface{}, wg *sync.WaitGroup) {
+	defer wg.Done()
+	if topic != nil && !topic(v) {
+		return
+	}
+
+	// send under a select as to not block if the receiver is unavailable
+	if p.timeout > 0 {
+		select {
+		case sub <- v:
+		case <-time.After(p.timeout):
+		}
+		return
+	}
+
+	select {
+	case sub <- v:
+	default:
+	}
+}