debugging https://github.com/rails/rails/issues/19084, reproduction against rails master

output

$ bundle exec rails runner route_helpers_debug.rb 20 && spring stop
Threads: 20, Iterations per thread: 20
Running Mounted Engine dummy_path...
6/-/16/-/19/-/15/-/14/-/7/-/13/-/12/-/3/-/8/-/9/-/2/-/5/-/1/-/0/-/10/-/18/-/11/-/4/-/17/-/12/-/7/-/3/-/2/-/9/-/1/-/0/-/14/-/15/-/10/-/8/-/11/-/16/-/4/-/11/-/13/-
/18/-/6/-/17/-/12/-/6/-/7/-/3/-/6/-/9/-/1/-/6/-/15/-/10/-/5/-/8/-/16/-/11/-/19/-/13/-/17/-/2/-/18/-/7/-/3/-/0/-/14/-/9/-/6/-/1/-/15/-/10/-/4/-/5/-/8/-/4/-/19/-/1
3/-/17/-/2/-/18/-/12/-/2/-/0/-/3/-/14/-/9/-/6/-/15/-/14/-/16/-/11/-/10/-/5/-/8/-/10/-/13/-/19/-/7/-/17/-/12/-/13/-/0/-/2/-/13/-/3/-/6/-/15/-/16/-/13/-/11/-/4/-/1
3/-/8/-/19/-/18/-/7/-/17/-/18/-/0/-/2/-/14/-/0/-/3/-/6/-/15/-/11/-/16/-/4/-/10/-/13/-/8/-/1/-/19/-/8/-/17/-/12/-/8/-/18/-/2/-/14/-/0/-/5/-/6/-/3/-/15/-/16/-/11/-
/4/-/10/-/7/-/13/-/17/-/19/-/12/-/8/-/19/-/2/-/8/-/5/-/0/-/15/-/6/-/11/-/16/-/10/-/4/-/16/-/9/-/7/-/13/-/18/-/17/-/14/-/18/-/19/-/2/-/8/-/0/-/5/-/0/-/6/-/1/-/10/
-/4/-/1/-/16/-/7/-/12/-/17/-/14/-/18/-/19/-/2/-/3/-/15/-/5/-/8/-/0/-/11/-/9/-/6/-/16/-/1/-/7/-/4/-/10/-/12/-/1/-/7/-/14/-/18/-/19/-/3/-/2/-/15/-/5/-/2/-/0/-/17/-
/9/-/13/-/10/-/4/-/16/-/1/-/12/-/7/-/14/-/12/-/19/-/8/-/11/-/3/-/0/-/17/-/2/-/6/-/5/-/15/-/13/-/9/-/3/-/18/-/1/-/16/-/7/-/18/-/19/-/8/-/11/-/17/-/2/-/10/-/11/-/4
/-/13/-/5/-/9/-/3/-/14/-/15/-/1/-/16/-/7/-/12/-/18/-/19/-/8/-/18/-/17/-/10/-/2/-/10/-/11/-/13/-/5/-/9/-/14/-/13/-/15/-/1/-/16/-/13/-/1/-/19/-/8/-/18/-/4/-/17/-/1
8/-/10/-/11/-/5/-/7/-/12/-/14/-/15/-/13/-/1/-/13/-/2/-/8/-/4/-/2/-/3/-/18/-/2/-/11/-/5/-/7/-/12/-/16/-/19/-/7/-/16/-/1/-/10/-/9/-/3/-/4/-/8/-/18/-/2/-/8/-/14/-/5
/-/15/-/17/-/12/-/16/-/19/-/7/-/10/-/1/-/9/-/7/-/11/-/18/-/14/-/5/-/12/-/17/-/14/-/16/-/19/-/4/-/16/-/9/-/15/-/7/-/9/-/3/-/12/-/5/-/14/-/3/-/5/-/4/-/12/-/11/-/15
/-/17/-/12/-/9/-/1/-/19/-/4/-/3/-/14/-/5/-/11/-/15/-/10/-/17/-/12/-/15/-/19/-/4/-/14/-/9/-/3/-/5/-/11/-/1/-/4/-/3/-/9/-/17/-/10/-/1/-/
Finished thread 1.

Finished thread 2.

Finished thread 3.

Finished thread 4.

Finished thread 5.
9/-/
Finished thread 7.

Finished thread 8.
9/-/
Finished thread 9.

Finished thread 10.

Finished thread 11.

Finished thread 12.

Finished thread 13.

Finished thread 14.

Finished thread 15.

Finished thread 16.

Finished thread 17.

Finished thread 18.

Finished thread 19.

-------
ERRORS: 2

Exception in thread 0, iteration 14: undefined method `url_options' for #<Module:0x007f42aa9785a8>
        /home/steven/pushhealth/rails-master/actionpack/lib/action_dispatch/routing/route_set.rb:246:in `call'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_dispatch/routing/route_set.rb:195:in `call'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_dispatch/routing/route_set.rb:300:in `block (2 levels) in define_url_helper'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_dispatch/routing/mapper.rb:621:in `block (2 levels) in define_generate_prefix'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_dispatch/routing/route_set.rb:742:in `url_for'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_dispatch/routing/route_set.rb:254:in `call'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_dispatch/routing/route_set.rb:195:in `call'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_dispatch/routing/route_set.rb:300:in `block (2 levels) in define_url_helper'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_dispatch/routing/routes_proxy.rb:32:in `dummy_path'
        route_helpers_debug.rb:48:in `engine_test'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_controller/metal/implicit_render.rb:4:in `send_action'
        /home/steven/pushhealth/rails-master/actionpack/lib/abstract_controller/base.rb:189:in `process_action'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_controller/metal/rendering.rb:21:in `process_action'
        /home/steven/pushhealth/rails-master/actionpack/lib/abstract_controller/callbacks.rb:20:in `block in process_action'
        /home/steven/pushhealth/rails-master/activesupport/lib/active_support/callbacks.rb:117:in `call'
        /home/steven/pushhealth/rails-master/activesupport/lib/active_support/callbacks.rb:117:in `call'
        /home/steven/pushhealth/rails-master/activesupport/lib/active_support/callbacks.rb:558:in `block (2 levels) in compile'
        /home/steven/pushhealth/rails-master/activesupport/lib/active_support/callbacks.rb:501:in `call'
        /home/steven/pushhealth/rails-master/activesupport/lib/active_support/callbacks.rb:501:in `call'
        /home/steven/pushhealth/rails-master/activesupport/lib/active_support/callbacks.rb:90:in `run_callbacks'
        /home/steven/pushhealth/rails-master/actionpack/lib/abstract_controller/callbacks.rb:19:in `process_action'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_controller/metal/rescue.rb:29:in `process_action'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_controller/metal/instrumentation.rb:32:in `block in process_action'
        /home/steven/pushhealth/rails-master/activesupport/lib/active_support/notifications.rb:164:in `block in instrument'
        /home/steven/pushhealth/rails-master/activesupport/lib/active_support/notifications/instrumenter.rb:20:in `instrument'
        /home/steven/pushhealth/rails-master/activesupport/lib/active_support/notifications.rb:164:in `instrument'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_controller/metal/instrumentation.rb:30:in `process_action'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_controller/metal/params_wrapper.rb:249:in `process_action'
        /home/steven/pushhealth/rails-master/activerecord/lib/active_record/railties/controller_runtime.rb:18:in `process_action'
        /home/steven/pushhealth/rails-master/actionpack/lib/abstract_controller/base.rb:128:in `process'
        /home/steven/pushhealth/rails-master/actionview/lib/action_view/rendering.rb:30:in `process'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_controller/metal.rb:194:in `dispatch'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_controller/metal.rb:241:in `block in action'
        route_helpers_debug.rb:57:in `call'
        route_helpers_debug.rb:57:in `block in <top (required)>'
        route_helpers_debug.rb:15:in `call'
        route_helpers_debug.rb:15:in `block (3 levels) in run_threads'
        route_helpers_debug.rb:13:in `times'
        route_helpers_debug.rb:13:in `block (2 levels) in run_threads'

Exception in thread 6, iteration 14: undefined method `url_options' for #<Module:0x007f42adf266c8>
        /home/steven/pushhealth/rails-master/actionpack/lib/action_dispatch/routing/route_set.rb:246:in `call'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_dispatch/routing/route_set.rb:195:in `call'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_dispatch/routing/route_set.rb:300:in `block (2 levels) in define_url_helper'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_dispatch/routing/mapper.rb:621:in `block (2 levels) in define_generate_prefix'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_dispatch/routing/route_set.rb:742:in `url_for'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_dispatch/routing/route_set.rb:254:in `call'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_dispatch/routing/route_set.rb:195:in `call'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_dispatch/routing/route_set.rb:300:in `block (2 levels) in define_url_helper'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_dispatch/routing/routes_proxy.rb:32:in `dummy_path'
        route_helpers_debug.rb:48:in `engine_test'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_controller/metal/implicit_render.rb:4:in `send_action'
        /home/steven/pushhealth/rails-master/actionpack/lib/abstract_controller/base.rb:189:in `process_action'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_controller/metal/rendering.rb:21:in `process_action'
        /home/steven/pushhealth/rails-master/actionpack/lib/abstract_controller/callbacks.rb:20:in `block in process_action'
        /home/steven/pushhealth/rails-master/activesupport/lib/active_support/callbacks.rb:117:in `call'
        /home/steven/pushhealth/rails-master/activesupport/lib/active_support/callbacks.rb:117:in `call'
        /home/steven/pushhealth/rails-master/activesupport/lib/active_support/callbacks.rb:558:in `block (2 levels) in compile'
        /home/steven/pushhealth/rails-master/activesupport/lib/active_support/callbacks.rb:501:in `call'
        /home/steven/pushhealth/rails-master/activesupport/lib/active_support/callbacks.rb:501:in `call'
        /home/steven/pushhealth/rails-master/activesupport/lib/active_support/callbacks.rb:90:in `run_callbacks'
        /home/steven/pushhealth/rails-master/actionpack/lib/abstract_controller/callbacks.rb:19:in `process_action'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_controller/metal/rescue.rb:29:in `process_action'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_controller/metal/instrumentation.rb:32:in `block in process_action'
        /home/steven/pushhealth/rails-master/activesupport/lib/active_support/notifications.rb:164:in `block in instrument'
        /home/steven/pushhealth/rails-master/activesupport/lib/active_support/notifications/instrumenter.rb:20:in `instrument'
        /home/steven/pushhealth/rails-master/activesupport/lib/active_support/notifications.rb:164:in `instrument'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_controller/metal/instrumentation.rb:30:in `process_action'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_controller/metal/params_wrapper.rb:249:in `process_action'
        /home/steven/pushhealth/rails-master/activerecord/lib/active_record/railties/controller_runtime.rb:18:in `process_action'
        /home/steven/pushhealth/rails-master/actionpack/lib/abstract_controller/base.rb:128:in `process'
        /home/steven/pushhealth/rails-master/actionview/lib/action_view/rendering.rb:30:in `process'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_controller/metal.rb:194:in `dispatch'
        /home/steven/pushhealth/rails-master/actionpack/lib/action_controller/metal.rb:241:in `block in action'
        route_helpers_debug.rb:57:in `call'
        route_helpers_debug.rb:57:in `block in <top (required)>'
        route_helpers_debug.rb:15:in `call'
        route_helpers_debug.rb:15:in `block (3 levels) in run_threads'
        route_helpers_debug.rb:13:in `times'
        route_helpers_debug.rb:13:in `block (2 levels) in run_threads'

-------
ERRORS: 2
Spring stopped.

route_helpers_debug.rb

$ cat route_helpers_debug.rb
# via "rails runner"

NUM_THREADS = (ARGV[0] || 10).to_i
NUM_ITERATIONS = (ARGV[1] || NUM_THREADS).to_i

puts "Threads: #{NUM_THREADS}, Iterations per thread: #{NUM_ITERATIONS}"

def run_threads(&block)
  threads = []
  NUM_THREADS.times do |thread_num|
    threads << Thread.new(thread_num) do |num|
      Thread.current.thread_variable_set('num', num)
      NUM_ITERATIONS.times do |i|
        Thread.current.thread_variable_set('iter', i)
        print num.to_s + block.call.to_s
      end
    end
  end

  errors = []
  threads.each do |thr|
    begin
      thr.join
      print "\nFinished thread #{thr.thread_variable_get('num')}.\n"
    rescue => e
      errors << "\nException in thread #{thr.thread_variable_get('num')}, iteration #{thr.thread_variable_get('iter')}: #{e.message}\n"+e.backtrace.map{|bt| "\t#{bt}\n" }.join
    end
  end

  puts "\n-------\nERRORS: #{errors.length}"
  puts errors
  puts "\n-------\nERRORS: #{errors.length}"
end


class TestEngine < Rails::Engine; end
TestEngine.routes.draw do
  get '/' => 'dummy#index', as: :dummy
end

Rails.application.routes.draw do
  root to: 'dummy#index'
  mount TestEngine, at: '-', as: 'test_routes'
end

class TestController < ActionController::Base
  def engine_test
    render plain: test_routes.dummy_path
  end
end

Rails.logger.level = 2

puts "Running Mounted Engine dummy_path..."
run_threads do
  env = { 'REQUEST_METHOD' => 'GET', 'rack.input' => StringIO.new }
  status, headers, rack_body = TestController.action('engine_test').call(env)
  rack_body.body
end