FIX: when entering topics "tracking" would not be set

There was a timing issue when subscribing to messages for topics.

Old flow:

- We generate JSON for topic
- We subscribe to messages for topic

New flow:

- We keep track of last id in the topic message bus channel
- We generate JSON
- We subscribe to messages for topic starting at saved message id

This ensures that there is complete overlap for message consumption
and that there are no cases where an update may go missing due to timing

app/assets/javascripts/discourse/controllers/topic.js.es6

@@ -946,7 +946,7 @@ export default Ember.Controller.extend(SelectedPostsCount, BufferedContent, {
           this._scrollToPost(data.post_number);
         }
       }
-    });
+    }, this.get('model.message_bus_last_id'));
   },
 
   _scrollToPost: debounce(function(postNumber) {

app/assets/javascripts/discourse/routes/topic-from-params.js.es6

@@ -7,6 +7,11 @@ export default Discourse.Route.extend({
   // Avoid default model hook
   model(params) { return params; },
 
+  deactivate() {
+    this._super();
+    this.controllerFor('topic').unsubscribe();
+  },
+
   setupController(controller, params) {
     params = params || {};
     params.track_visit = true;
@@ -36,6 +41,8 @@ export default Discourse.Route.extend({
         enteredAt: new Date().getTime().toString(),
       });
 
+      topicController.subscribe();
+
       // Highlight our post after the next render
       Ember.run.scheduleOnce('afterRender', function() {
         self.appEvents.trigger('post:highlight', closest);

app/assets/javascripts/discourse/routes/topic.js.es6

@@ -189,7 +189,6 @@ const TopicRoute = Discourse.Route.extend({
     postStream.cancelFilter();
 
     topicController.set('multiSelect', false);
-    topicController.unsubscribe();
     this.controllerFor('composer').set('topic', null);
     this.screenTrack.stop();
 
@@ -216,7 +215,6 @@ const TopicRoute = Discourse.Route.extend({
 
     this.controllerFor('composer').set('topic', model);
     this.topicTrackingState.trackIncoming('all');
-    controller.subscribe();
 
     // We reset screen tracking every time a topic is entered
     this.screenTrack.start(model.get('id'), controller);

app/serializers/topic_view_serializer.rb

@@ -61,7 +61,8 @@ class TopicViewSerializer < ApplicationSerializer
              :tags,
              :featured_link,
              :topic_timer,
-             :unicode_title
+             :unicode_title,
+             :message_bus_last_id
 
   # TODO: Split off into proper object / serializer
   def details
@@ -125,6 +126,10 @@ class TopicViewSerializer < ApplicationSerializer
     result
   end
 
+  def message_bus_last_id
+    object.message_bus_last_id
+  end
+
   def chunk_size
     object.chunk_size
   end

lib/topic_view.rb

@@ -5,7 +5,7 @@ require_dependency 'gaps'
 
 class TopicView
 
-  attr_reader :topic, :posts, :guardian, :filtered_posts, :chunk_size, :print
+  attr_reader :topic, :posts, :guardian, :filtered_posts, :chunk_size, :print, :message_bus_last_id
   attr_accessor :draft, :draft_key, :draft_sequence, :user_custom_fields, :post_custom_fields
 
   def self.slow_chunk_size
@@ -38,6 +38,7 @@ class TopicView
   end
 
   def initialize(topic_id, user=nil, options={})
+    @message_bus_last_id = MessageBus.last_id("/topic/#{topic_id}")
     @user = user
     @guardian = Guardian.new(@user)
     @topic = find_topic(topic_id)