FIX: hitting '/t/:id/posts.json' should return the first page of posts

app/controllers/topics_controller.rb

@@ -154,7 +154,7 @@ class TopicsController < ApplicationController
 
   def posts
     params.require(:topic_id)
-    params.require(:post_ids)
+    params.permit(:post_ids)
 
     @topic_view = TopicView.new(params[:topic_id], current_user, post_ids: params[:post_ids])
     render_json_dump(TopicViewPostsSerializer.new(@topic_view, scope: guardian, root: false, include_raw: !!params[:include_raw]))

config/routes.rb

@@ -544,7 +544,7 @@ Discourse::Application.routes.draw do
   get "t/:slug/:topic_id" => "topics#show", constraints: {topic_id: /\d+/}
   get "t/:slug/:topic_id/:post_number" => "topics#show", constraints: {topic_id: /\d+/, post_number: /\d+/}
   get "t/:slug/:topic_id/last" => "topics#show", post_number: 99999999, constraints: {topic_id: /\d+/}
-  get "t/:topic_id/posts" => "topics#posts", constraints: {topic_id: /\d+/}
+  get "t/:topic_id/posts" => "topics#posts", constraints: {topic_id: /\d+/}, format: :json
   post "t/:topic_id/timings" => "topics#timings", constraints: {topic_id: /\d+/}
   post "t/:topic_id/invite" => "topics#invite", constraints: {topic_id: /\d+/}
   post "t/:topic_id/move-posts" => "topics#move_posts", constraints: {topic_id: /\d+/}

spec/controllers/topics_controller_spec.rb

@@ -813,6 +813,16 @@ describe TopicsController do
     end
   end
 
+  describe '#posts' do
+    let(:topic) { Fabricate(:post).topic }
+
+    it 'returns first posts of the topic' do
+      get :posts, topic_id: topic.id, format: :json
+      expect(response).to be_success
+      expect(response.content_type).to eq('application/json')
+    end
+  end
+
   describe '#feed' do
     let(:topic) { Fabricate(:post).topic }