Any unmatched route render 404 file directly

We've had some errors reported by Render since spammy requests coming in (e.g. POSTing to non-existent routes) so as a last resort in the routes file, any unmatched routes, just render the default 404 error page directly to avoid raising any errors.

app/controllers/application_controller.rb

@@ -1,2 +1,7 @@
 class ApplicationController < ActionController::Base
+  protect_from_forgery with: :exception
+
+  def route_not_found
+    render file: Rails.public_path.join("404.html"), status: :not_found, layout: false
+  end
 end

config/routes.rb

@@ -4,7 +4,9 @@
 
   # Reveal health status on /up that returns 200 if the app boots with no exceptions, otherwise 500.
   # Can be used by load balancers and uptime monitors to verify that the app is live.
-  get "up" => "rails/health#show", as: :rails_health_check
+  get "up", to: "rails/health#show", as: :rails_health_check
 
   root to: redirect("admin")
+
+  match "*unmatched", to: "application#route_not_found", via: :all
 end

test/system/active_admin/route_not_found_test.rb

@@ -0,0 +1,9 @@
+require "application_system_test_case"
+
+class RouteNotFoundTest < ApplicationSystemTestCase
+  test "visiting non-existent route renders 404 page" do
+    visit "/does-not-exist"
+
+    assert_text "The page you were looking for doesn't exist."
+  end
+end