The Mocking will continue, until CI improves

One might think, this blog is exclusively about weird language behavior and yelling at computers… Well, welcome to another episode of Jackass!

Today's opponent is Ruby, or maybe minitest , or maybe Mocha. I'm not exactly sure, but it was a rather amusing exercise and I like to share my nightmares ;)

It all started with the classical "you're using old and unmaintained software, please switch to something new".

The first attempt was to switch from the ci_reporter_minitest plugin to the minitest-ci plugin. While the change worked great for Foreman itself, it broke the reporting in Katello - the tests would run but no junit.xml was generated and Jenkins rightfully complained that it got no test results.

While investigating what the hell was wrong, we realized that Katello was already using a minitest reporting plugin: minitest-reporters. Loading two different reporting plugins seemed like a good source for problems, so I tried using the same plugin for Foreman too.

Guess what? After a bit of massaging (mostly to disable the second minitest-reporters initialization in Katello) reporting of test results from Katello started to work like a charm. But now the Foreman tests started to fail. Not fail to report, fail to actually run. WTH‽

The failure was quite interesting too:

test/unit/parameter_filter_test.rb:5:in `block in <class:ParameterFilterTest>':
  Mocha methods cannot be used outside the context of a test (Mocha::NotInitializedError)

Yes, this is a single test file failing, all others were fine.

The failing code doesn't look problematic on first glance:

require 'test_helper'

class ParameterFilterTest < ActiveSupport::TestCase
  let(:klass) do
    mock('Example').tap do |k|

  test 'something' do

The failing line (5) is mock('Example').tap … and for some reason Mocha thinks it's not initialized here.

This certainly has something to do with how the various reporting plugins inject themselves, but I really didn't want to debug how to run two reporting plugins in parallel (which, as you remember, didn't expose this behavior). So the only real path forward was to debug what's happening here.

Calling the test on its own, with one of the working reporter was the first step:

$ bundle exec rake test TEST=test/unit/parameter_filter_test.rb TESTOPTS=-v

#<Mocha::Mock:0x0000557bf1f22e30>#test_0001_permits plugin-added attribute = 0.04 s = .
#<Mocha::Mock:0x0000557bf12cf750>#test_0002_permits plugin-added attributes from blocks = 0.49 s = .

Wait, what? #<Mocha::Mock:…>? Shouldn't this read more like ParameterFilterTest::… as it happens for every single other test in our test suite? It definitely should! That's actually great, as it tells us that there is really something wrong with the test and the change of the reporting plugin just makes it worse.

What comes next is sheer luck. Well, that, and years of experience in yelling at computers.

We use let(:klass) to define an object called klass and this object is a Mocha::Mock that we'll use in our tests later. Now klass is a very common term in Ruby when talking about classes and needing to store them — mostly because one can't use class which is a keyword. Is something else in the stack using klass and our let is overriding that, making this whole thing explode?

It was! The moment we replaced klass with klass1 (silly, I know, but there also was a klass2 in that code, so it did fit), things started to work nicely.

I really liked Tomer's comment in the PR: "no idea why, but I am not going to dig into mocha to figure that out."

Turns out, I couldn't let (HAH!) the code rest and really wanted to understand what happened there.

What I didn't want to do is to debug the whole Foreman test stack, because it is massive.

So I started to write a minimal reproducer for the issue.

All starts with a Gemfile, as we need a few dependencies:

gem 'rake'
gem 'mocha'
gem 'minitest', '~> 5.1', '< 5.11'

Then a Rakefile:

require 'rake/testtask' do |t|
  t.libs << 'test'
  t.test_files = FileList["test/**/*_test.rb"]

task :default => :test

And a test! I took the liberty to replace ActiveSupport::TestCase with Minitest::Test, as the test won't be using any Rails features and I wanted to keep my environment minimal.

require 'minitest/autorun'
require 'minitest/spec'
require 'mocha/minitest'

class ParameterFilterTest < Minitest::Test
  extend Minitest::Spec::DSL

  let(:klass) do
    mock('Example').tap do |k|

  def test_lol
    assert klass

Well, damn, this passed! Is it Rails after all that breaks stuff? Let's add it to the Gemfile!

$ vim Gemfile
$ bundle install
$ bundle exec rake test TESTOPTS=-v

#<Mocha::Mock:0x0000564bbfe17e98>#test_lol = 0.00 s = .

Wait, I didn't change anything and it's already failing?! Fuck! I mean, cool!

But the test isn't minimal yet. What can we reduce? let is just a fancy, lazy def, right? So instead of let(:klass) we should be able to write def klass and achieve a similar outcome and drop that Minitest::Spec.

require 'minitest/autorun'
require 'mocha/minitest'

class ParameterFilterTest < Minitest::Test
  def klass

  def test_lol
    assert klass
$ bundle exec rake test TESTOPTS=-v

/home/evgeni/Devel/minitest-wtf/test/parameter_filter_test.rb:5:in `klass': Mocha methods cannot be used outside the context of a test (Mocha::NotInitializedError)
    from /home/evgeni/Devel/minitest-wtf/vendor/bundle/ruby/3.0.0/gems/railties- `format_line'
    from /home/evgeni/Devel/minitest-wtf/vendor/bundle/ruby/3.0.0/gems/railties- `record'
    from /home/evgeni/Devel/minitest-wtf/vendor/bundle/ruby/3.0.0/gems/minitest-5.10.3/lib/minitest.rb:682:in `block in record'
    from /home/evgeni/Devel/minitest-wtf/vendor/bundle/ruby/3.0.0/gems/minitest-5.10.3/lib/minitest.rb:681:in `each'
    from /home/evgeni/Devel/minitest-wtf/vendor/bundle/ruby/3.0.0/gems/minitest-5.10.3/lib/minitest.rb:681:in `record'
    from /home/evgeni/Devel/minitest-wtf/vendor/bundle/ruby/3.0.0/gems/minitest-5.10.3/lib/minitest.rb:324:in `run_one_method'
    from /home/evgeni/Devel/minitest-wtf/vendor/bundle/ruby/3.0.0/gems/minitest-5.10.3/lib/minitest.rb:311:in `block (2 levels) in run'
    from /home/evgeni/Devel/minitest-wtf/vendor/bundle/ruby/3.0.0/gems/minitest-5.10.3/lib/minitest.rb:310:in `each'
    from /home/evgeni/Devel/minitest-wtf/vendor/bundle/ruby/3.0.0/gems/minitest-5.10.3/lib/minitest.rb:310:in `block in run'
    from /home/evgeni/Devel/minitest-wtf/vendor/bundle/ruby/3.0.0/gems/minitest-5.10.3/lib/minitest.rb:350:in `on_signal'
    from /home/evgeni/Devel/minitest-wtf/vendor/bundle/ruby/3.0.0/gems/minitest-5.10.3/lib/minitest.rb:337:in `with_info_handler'
    from /home/evgeni/Devel/minitest-wtf/vendor/bundle/ruby/3.0.0/gems/minitest-5.10.3/lib/minitest.rb:309:in `run'
    from /home/evgeni/Devel/minitest-wtf/vendor/bundle/ruby/3.0.0/gems/minitest-5.10.3/lib/minitest.rb:159:in `block in __run'
    from /home/evgeni/Devel/minitest-wtf/vendor/bundle/ruby/3.0.0/gems/minitest-5.10.3/lib/minitest.rb:159:in `map'
    from /home/evgeni/Devel/minitest-wtf/vendor/bundle/ruby/3.0.0/gems/minitest-5.10.3/lib/minitest.rb:159:in `__run'
    from /home/evgeni/Devel/minitest-wtf/vendor/bundle/ruby/3.0.0/gems/minitest-5.10.3/lib/minitest.rb:136:in `run'
    from /home/evgeni/Devel/minitest-wtf/vendor/bundle/ruby/3.0.0/gems/minitest-5.10.3/lib/minitest.rb:63:in `block in autorun'
rake aborted!

Oh nice, this is even better! Instead of the mangled class name, we now get the very same error the Foreman tests aborted with, plus a nice stack trace! But wait, why is it pointing at railties? We're not loading that! Anyways, lets look at railties-, line 68

def format_line(result)
  klass = result.respond_to?(:klass) ? result.klass : result.class
  "%s#%s = %.2f s = %s" % [klass,, result.time, result.result_code]

Heh, this is touching result.klass, which we just messed up. Nice!

But quickly back to railties… What if we only add that to the Gemfile, not full blown Rails?

gem 'railties'
gem 'rake'
gem 'mocha'
gem 'minitest', '~> 5.1', '< 5.11'

Yepp, same failure. Also happens with require => false added to the line, so it seems railties somehow injects itself into rake even if nothing is using it?! "Cool"!

By the way, why are we still pinning minitest to < 5.11? Oh right, this was the original reason to look into that whole topic. And, uh, it's pointing at klass there already! 4 years ago!

So lets remove that boundary and funny enough, now tests are passing again, even if we use klass!

Minitest 5.11 changed how Minitest::Test is structured, and seems not to rely on klass at that point anymore. And I guess Rails also changed a bit since the original pin was put in place four years ago.

I didn't want to go another rabbit hole, finding out what changed in Rails, but I did try with 5.0 (well, to be precise, and the output with newer (>= 5.11) Minitest was interesting:

$ bundle exec rake test TESTOPTS=-v

Minitest::Result#test_lol = 0.00 s = .

It's leaking Minitest::Result as klass now, instead of Mocha::Mock. So probably something along these lines was broken 4 years ago and triggered this pin.

What do we learn from that?

  • klass is cursed and shouldn't be used in places where inheritance and tooling might decide to use it for some reason
  • inheritance is cursed - why the heck are implementation details of Minitest leaking inside my tests?!
  • tooling is cursed - why is railties injecting stuff when I didn't ask it to?!
  • dependency pinning is cursed - at least if you pin to avoid an issue and then forget about said issue for four years
  • I like cursed things!


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