Fix HTTP 200 response code on paths to remote accounts that don't exist (#40341)

app/controllers/concerns/web_app_controller_concern.rb

@@ -27,10 +27,7 @@ module WebAppControllerConcern
   end
 
   def redirect_unauthenticated_to_permalinks!
-    return if user_signed_in? && current_account.moved_to_account_id.nil?
-
-    permalink_redirector = PermalinkRedirector.new(request.original_fullpath)
-    return if permalink_redirector.redirect_path.blank?
+    return if (user_signed_in? && current_account.moved_to_account_id.nil?) || permalink_redirector.redirect_path.blank?
 
     expires_in(15.seconds, public: true, stale_while_revalidate: 30.seconds, stale_if_error: 1.day) unless user_signed_in?
 
@@ -72,4 +69,8 @@ module WebAppControllerConcern
   def set_referer_header
     response.set_header('Referrer-Policy', Setting.allow_referrer_origin ? 'strict-origin-when-cross-origin' : 'same-origin')
   end
+
+  def permalink_redirector
+    @permalink_redirector ||= PermalinkRedirector.new(request.original_fullpath)
+  end
 end

app/controllers/remote_accounts_controller.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+class RemoteAccountsController < HomeController
+  before_action :require_resource_exists!
+
+  private
+
+  def require_resource_exists!
+    # The page should still render normally, but the HTTP code should be modified
+    response.status = 404 if permalink_redirector.object.nil?
+  end
+end