FIX: Show 404 html on /posts/:id/raw and /p/:id (#16131)

It returned a blank page before.

app/controllers/posts_controller.rb

@@ -31,7 +31,7 @@ class PostsController < ApplicationController
   MARKDOWN_TOPIC_PAGE_SIZE ||= 100
 
   def markdown_id
-    markdown Post.find(params[:id].to_i)
+    markdown Post.find_by(id: params[:id].to_i)
   end
 
   def markdown_num
@@ -58,14 +58,6 @@ class PostsController < ApplicationController
     end
   end
 
-  def markdown(post)
-    if post && guardian.can_see?(post)
-      render plain: post.raw
-    else
-      raise Discourse::NotFound
-    end
-  end
-
   def latest
     params.permit(:before)
     last_post_id = params[:before].to_i
@@ -168,10 +160,12 @@ class PostsController < ApplicationController
   end
 
   def short_link
-    post = Post.find(params[:post_id].to_i)
+    post = Post.find_by(id: params[:post_id].to_i)
+    raise Discourse::NotFound unless post
+
     # Stuff the user in the request object, because that's what IncomingLink wants
     if params[:user_id]
-      user = User.find(params[:user_id].to_i)
+      user = User.find_by(id: params[:user_id].to_i)
       request['u'] = user.username_lower if user
     end
 
@@ -629,6 +623,14 @@ class PostsController < ApplicationController
 
   protected
 
+  def markdown(post)
+    if post && guardian.can_see?(post)
+      render plain: post.raw
+    else
+      raise Discourse::NotFound
+    end
+  end
+
   # We can't break the API for making posts. The new, queue supporting API
   # doesn't return the post as the root JSON object, but as a nested object.
   # If a param is present it uses that result structure.

spec/requests/posts_controller_spec.rb

@@ -1931,6 +1931,12 @@ describe PostsController do
       expect(response.status).to eq(200)
       expect(response.body).to eq("123456789")
     end
+
+    it "renders a 404 page" do
+      get "/posts/0/raw"
+      expect(response.status).to eq(404)
+      expect(response.body).to include(I18n.t("page_not_found.title"))
+    end
   end
 
   describe '#markdown_num' do
@@ -1968,6 +1974,12 @@ describe PostsController do
       get "/p/#{post.id}.json"
       expect(response).to be_forbidden
     end
+
+    it "renders a 404 page" do
+      get "/p/0"
+      expect(response.status).to eq(404)
+      expect(response.body).to include(I18n.t("page_not_found.title"))
+    end
   end
 
   describe '#user_posts_feed' do