Fix thread depth computation in statuses_controller (#9426)

* Add test that should currently fail

* Fix depth computation (will still fail if statuses have been filtered out)

* Fix handling of broken threads

app/controllers/statuses_controller.rb

@@ -65,12 +65,13 @@ class StatusesController < ApplicationController
 
   private
 
-  def create_descendant_thread(depth, statuses)
+  def create_descendant_thread(starting_depth, statuses)
+    depth = starting_depth + statuses.size
     if depth < DESCENDANTS_DEPTH_LIMIT
-      { statuses: statuses }
+      { statuses: statuses, starting_depth: starting_depth }
     else
       next_status = statuses.pop
-      { statuses: statuses, next_status: next_status }
+      { statuses: statuses, starting_depth: starting_depth, next_status: next_status }
     end
   end
 
@@ -102,15 +103,18 @@ class StatusesController < ApplicationController
 
     if descendants.present?
       statuses       = [descendants.first]
-      depth    = 1
+      starting_depth = 0
 
       descendants.drop(1).each_with_index do |descendant, index|
         if descendants[index].id == descendant.in_reply_to_id
-          depth += 1
           statuses << descendant
         else
-          @descendant_threads << create_descendant_thread(depth, statuses)
+          @descendant_threads << create_descendant_thread(starting_depth, statuses)
 
+          # The thread is broken, assume it's a reply to the root status
+          starting_depth = 0
+
+          # ... unless we can find its ancestor in one of the already-processed threads
           @descendant_threads.reverse_each do |descendant_thread|
             statuses = descendant_thread[:statuses]
 
@@ -119,18 +123,16 @@ class StatusesController < ApplicationController
             end
 
             if index.present?
-              depth += index - statuses.size
+              starting_depth = descendant_thread[:starting_depth] + index + 1
               break
             end
-
-            depth -= statuses.size
           end
 
           statuses = [descendant]
         end
       end
 
-      @descendant_threads << create_descendant_thread(depth, statuses)
+      @descendant_threads << create_descendant_thread(starting_depth, statuses)
     end
 
     @max_descendant_thread_id = @descendant_threads.pop[:statuses].first.id if descendants.size >= DESCENDANTS_LIMIT

spec/controllers/statuses_controller_spec.rb

@@ -115,14 +115,18 @@ describe StatusesController do
       end
 
       it 'assigns @descendant_threads for threads with :next_status key if they are hitting the depth limit' do
-        stub_const 'StatusesController::DESCENDANTS_DEPTH_LIMIT', 1
+        stub_const 'StatusesController::DESCENDANTS_DEPTH_LIMIT', 2
         status = Fabricate(:status)
-        child = Fabricate(:status, in_reply_to_id: status.id)
+        child0 = Fabricate(:status, in_reply_to_id: status.id)
+        child1 = Fabricate(:status, in_reply_to_id: child0.id)
+        child2 = Fabricate(:status, in_reply_to_id: child0.id)
 
         get :show, params: { account_username: status.account.username, id: status.id }
 
-        expect(assigns(:descendant_threads)[0][:statuses].pluck(:id)).not_to include child.id
+        expect(assigns(:descendant_threads)[0][:statuses].pluck(:id)).not_to include child1.id
-        expect(assigns(:descendant_threads)[0][:next_status].id).to eq child.id
+        expect(assigns(:descendant_threads)[1][:statuses].pluck(:id)).not_to include child2.id
+        expect(assigns(:descendant_threads)[0][:next_status].id).to eq child1.id
+        expect(assigns(:descendant_threads)[1][:next_status].id).to eq child2.id
       end
 
       it 'returns a success' do