PERF: Send down gaps as the relevant posts load instead of front loading.

app/assets/javascripts/discourse/models/post-stream.js.es6

@@ -850,6 +850,10 @@ export default RestModel.extend({
       if (posts) {
         posts.forEach(p => this.storePost(store.createRecord("post", p)));
       }
+
+      if (result.post_stream.gaps) {
+        this.set("gaps", _.merge(this.get("gaps"), result.post_stream.gaps));
+      }
     });
   },
 

lib/topic_view.rb

@@ -5,7 +5,15 @@ require_dependency 'gaps'
 
 class TopicView
 
-  attr_reader :topic, :posts, :guardian, :filtered_posts, :chunk_size, :print, :message_bus_last_id
+  attr_reader :topic,
+              :posts,
+              :guardian,
+              :filtered_posts,
+              :chunk_size,
+              :print,
+              :message_bus_last_id,
+              :contains_gaps
+
   attr_accessor :draft, :draft_key, :draft_sequence, :user_custom_fields, :post_custom_fields, :post_number
 
   def self.slow_chunk_size
@@ -98,13 +106,28 @@ class TopicView
     path
   end
 
-  def contains_gaps?
-    @contains_gaps
-  end
-
   def gaps
     return unless @contains_gaps
-    @gaps ||= Gaps.new(filtered_post_ids, unfiltered_posts.order(:sort_order).pluck(:id))
+
+    @gaps ||= begin
+      sort_order_max_range =
+        if @posts.offset(@limit).exists? && unfiltered_posts.order(:sort_order)
+            .where("sort_order > ?", @posts.last.sort_order)
+            .offset(@limit)
+            .exists?
+
+          @posts.last.sort_order
+        else
+          @topic.highest_post_number
+        end
+
+      unfiltered_ids = unfiltered_posts.order(:sort_order).where("posts.sort_order BETWEEN ? AND ?",
+        @posts.first.sort_order,
+        sort_order_max_range
+      ).pluck(:id)
+
+      Gaps.new(@posts.pluck(:id), unfiltered_ids)
+    end
   end
 
   def last_post
@@ -504,6 +527,7 @@ class TopicView
     # This should be last - don't want to tell the admin about deleted posts that clicking the button won't show
     # copy the filter for has_deleted? method
     @predelete_filtered_posts = @filtered_posts.spawn
+
     if @guardian.can_see_deleted_posts? && !@show_deleted && has_deleted?
       @filtered_posts = @filtered_posts.where("deleted_at IS NULL OR post_number = 1")
       @contains_gaps = true

spec/components/topic_view_spec.rb

@@ -344,23 +344,67 @@ describe TopicView do
 
       # Update them to the sort order we're checking for
       [p1, p2, p3, p4, p5, p6, p7].each_with_index do |p, idx|
-        p.sort_order = idx + 1
-        p.save
+        p.update!(sort_order: idx + 1)
       end
-      p6.user_id = nil # user got nuked
-      p6.save!
+
+      p6.update!(user_id: nil) # user got nuked
     end
 
-    describe "contains_gaps?" do
-      it "works" do
-        # does not contain contains_gaps with default filtering
-        expect(topic_view.contains_gaps?).to eq(false)
-        # contains contains_gaps when filtered by username" do
-        expect(TopicView.new(topic.id, evil_trout, username_filters: ['eviltrout']).contains_gaps?).to eq(true)
-        # contains contains_gaps when filtered by summary
-        expect(TopicView.new(topic.id, evil_trout, filter: 'summary').contains_gaps?).to eq(true)
-        # contains contains_gaps when filtered by best
-        expect(TopicView.new(topic.id, evil_trout, best: 5).contains_gaps?).to eq(true)
+    describe "#gaps" do
+      describe 'no filter' do
+        it 'should not contain gaps' do
+          expect(topic_view.contains_gaps).to eq(false)
+          expect(topic_view.gaps).to eq(nil)
+        end
+
+        describe 'as an admin' do
+          it 'should contain gaps' do
+            p2.update!(deleted_at: Time.zone.now)
+            topic_view = TopicView.new(topic.id, Fabricate(:admin))
+
+            expect(topic_view.contains_gaps).to eq(true)
+            expect(topic_view.posts).to eq([p1, p3, p5])
+
+            expect(topic_view.gaps.before).to eq(
+              p5.id => [p4.id],
+              p3.id => [p2.id]
+            )
+
+            expect(topic_view.gaps.after).to eq(p5.id => [p6.id, p7.id])
+          end
+        end
+      end
+
+      describe 'filtered by username' do
+        it 'should contain gaps' do
+          topic_view = TopicView.new(topic.id, evil_trout,
+            username_filters: ['eviltrout']
+          )
+
+          expect(topic_view.contains_gaps).to eq(true)
+          expect(topic_view.gaps.before).to eq(p5.id => [p3.id])
+          expect(topic_view.gaps.after).to eq({})
+        end
+      end
+
+      describe 'filtered by summary' do
+        it 'should contain gaps' do
+          topic_view = TopicView.new(topic.id, evil_trout, filter: 'summary')
+
+          expect(topic_view.contains_gaps).to eq(true)
+          expect(topic_view.gaps.before).to eq({})
+          expect(topic_view.gaps.after).to eq({})
+        end
+      end
+
+      describe 'filtered by best' do
+        it 'should contain gaps' do
+          topic_view = TopicView.new(topic.id, evil_trout, best: 5)
+
+          expect(topic_view.contains_gaps).to eq(true)
+          expect(topic_view.gaps.before).to eq({})
+          expect(topic_view.gaps.after).to eq({})
+        end
       end
     end
 
@@ -403,21 +447,21 @@ describe TopicView do
         near_view = topic_view_near(p1)
         expect(near_view.desired_post).to eq(p1)
         expect(near_view.posts).to eq([p1, p2, p3])
-        expect(near_view.contains_gaps?).to eq(false)
+        expect(near_view.contains_gaps).to eq(false)
       end
 
       it "snaps to the upper boundary" do
         near_view = topic_view_near(p5)
         expect(near_view.desired_post).to eq(p5)
         expect(near_view.posts).to eq([p2, p3, p5])
-        expect(near_view.contains_gaps?).to eq(false)
+        expect(near_view.contains_gaps).to eq(false)
       end
 
       it "returns the posts in the middle" do
         near_view = topic_view_near(p2)
         expect(near_view.desired_post).to eq(p2)
         expect(near_view.posts).to eq([p1, p2, p3])
-        expect(near_view.contains_gaps?).to eq(false)
+        expect(near_view.contains_gaps).to eq(false)
       end
 
       it "gaps deleted posts to an admin" do
@@ -434,7 +478,7 @@ describe TopicView do
         near_view = topic_view_near(p3, true)
         expect(near_view.desired_post).to eq(p3)
         expect(near_view.posts).to eq([p2, p3, p4])
-        expect(near_view.contains_gaps?).to eq(false)
+        expect(near_view.contains_gaps).to eq(false)
       end
 
       it "gaps deleted posts by nuked users to an admin" do
@@ -452,7 +496,7 @@ describe TopicView do
         near_view = topic_view_near(p5, true)
         expect(near_view.desired_post).to eq(p5)
         expect(near_view.posts).to eq([p4, p5, p6])
-        expect(near_view.contains_gaps?).to eq(false)
+        expect(near_view.contains_gaps).to eq(false)
       end
 
       context "when 'posts per page' exceeds the number of posts" do
@@ -461,7 +505,7 @@ describe TopicView do
         it 'returns all the posts' do
           near_view = topic_view_near(p5)
           expect(near_view.posts).to eq([p1, p2, p3, p5])
-          expect(near_view.contains_gaps?).to eq(false)
+          expect(near_view.contains_gaps).to eq(false)
         end
 
         it 'gaps deleted posts to admins' do
@@ -476,7 +520,7 @@ describe TopicView do
           evil_trout.admin = true
           near_view = topic_view_near(p5, true)
           expect(near_view.posts).to eq([p1, p2, p3, p4, p5, p6, p7])
-          expect(near_view.contains_gaps?).to eq(false)
+          expect(near_view.contains_gaps).to eq(false)
         end
       end
     end