nsqd: fix new topic channel lookup/creation

when an nsqd first sees a topic queries nsqlookupd to identify
any channels that the cluster might know about in order
to proactively create them (in the case where there is more than one
channel this prevents data loss due to timing of consumers subscribing).

in nsqio#176 a regression was introduced that prevent the topic's messagePump
state from being updated in the code path described above.  This meant
that an nsqd that identified channels during creation of a topic would
not begin writing those messages into its channels.

nsqd/nsqd.go

@@ -275,7 +275,6 @@ func (n *NSQd) GetTopic(topicName string) *Topic {
 		// release our global nsqd lock, and switch to a more granular topic lock while we init our
 		// channels from lookupd. This blocks concurrent PutMessages to this topic.
 		t.Lock()
-		defer t.Unlock()
 		n.Unlock()
 		// if using lookupd, make a blocking call to get the topics, and immediately create them.
 		// this makes sure that any message received is buffered to the right channels
@@ -285,6 +284,18 @@ func (n *NSQd) GetTopic(topicName string) *Topic {
 				t.getOrCreateChannel(channelName)
 			}
 		}
+		t.Unlock()
+
+		// NOTE: I would prefer for this to only happen in topic.GetChannel() but we're special
+		// casing the code above so that we can control the locks such that it is impossible
+		// for a message to be written to a (new) topic while we're looking up channels
+		// from lookupd...
+		//
+		// update messagePump state
+		select {
+		case t.channelUpdateChan <- 1:
+		case <-t.exitChan:
+		}
 	}
 	return t
 }

nsqd/topic.go

@@ -60,12 +60,15 @@ func (t *Topic) GetChannel(channelName string) *Channel {
 	t.Lock()
 	channel, isNew := t.getOrCreateChannel(channelName)
 	t.Unlock()
+
 	if isNew {
+		// update messagePump state
 		select {
 		case t.channelUpdateChan <- 1:
 		case <-t.exitChan:
 		}
 	}
+
 	return channel
 }
 
@@ -112,6 +115,7 @@ func (t *Topic) DeleteExistingChannel(channelName string) error {
 	// (so that we dont leave any messages around)
 	channel.Delete()
 
+	// update messagePump state
 	select {
 	case t.channelUpdateChan <- 1:
 	case <-t.exitChan: