SimpleCov showing incorrect coverage result when parallelize is enabled in Rails 6.0.0 beta3

[alkesh26]

Rails version: 6.0.0 beta3
Ruby version: 2.6.1

Issue
We have configured our test helper file as below

require "simplecov"

SimpleCov.start do
  add_filter "/test/"

  add_group "Models", "app/models"
end

ENV["RAILS_ENV"] ||= "test"
require_relative "../config/environment"
require "rails/test_help"

class ActiveSupport::TestCase
  parallelize(workers: :number_of_processors)
  
  fixtures :all
end

On executing rake test the coverage report is incorrect even when test cases are written of the method. So this is our user model

class User < ApplicationRecord
  devise :database_authenticatable, :registerable, :trackable,
         :recoverable, :rememberable, :validatable

  has_one_attached :avatar
  validates :email, uniqueness: true

  def admin?
    self.role == "admin"
  end

  def name
    "#{first_name} #{last_name}"
  end
end

and the test cases we have are

  def test_user_admin
    user = users :admin
    assert user.admin?
  end

  def test_user_is_not_an_admin
    user = users :albert
    assert_not user.admin?
  end

  def test_should_return_first_name_and_last_name_as_name
    user = users :albert
    assert_equal "Albert Smith", user.name
  end

But the coverage report when parallelize is enabled is as below

and when it is commented out the test results are all green

I tried below approaches but all of them give correct report only when parallelize is commented:

1. Simplecov with minitest Unit tests? #235 (comment)
2. Created a .simplecov file in root directory and executed rake.
3. https://github.com/colszowka/simplecov/issues?utf8=✓&q=is%3Aissue+is%3Aopen+parallel followed the issues and tried other approaches.

But I am still not able to get the correct coverage result when parallelize is enabled.

Many thanks in advance.

[oystersauce8]

@alkesh26 I don't have a solution for you but have link to my app and reproducible steps:

1. Uncomment parallelize(workers: :number_of_processors)
2. Run tests rm -rf coverage/; RAILS_ENV=test ./bin/rails test; open coverage/index.html
3. Coverage of app/lib/watermelon/example.rb shows 0%
4. Comment out parallelize(workers: :number_of_processors)
5. Run tests rm -rf coverage/; RAILS_ENV=test ./bin/rails test; open coverage/index.html
6. Coverage of app/lib/watermelon/example.rb shows 70%

[PragTob]

Yes this is a biggie. Sorry for taking so long to respond, I took a bit of a break from maintaining simplecov.

Making simplecov completely work with rails and its new parallelization will be a bigger undertaking I fear. PRs are welcome. I'll see if I can allocate the time for it but I'm unsure.

Thank you for reporting 💚

[bbascarevic]

As a workaround we can piggy-back on the already existing feature of result merging, we just need to trick SimpleCov that each parallel run is another Command. We set SimpleCov.use_merging to true in the root process, and then in each fork we change the command name so forks would not overwrite each others' results.

# test_helper.rb

require 'simplecov'

SimpleCov.use_merging(true)  # Important!
SimpleCov.start('rails') do
  ... # Whatever you use here...
end

module ActiveSupport
  class TestCase
    parallelize

    parallelize_setup do |worker|
      SimpleCov.command_name("#{SimpleCov.command_name}-#{worker}")
      SimpleCov.pid = Process.pid
      SimpleCov.at_exit {} # This will suppress formatters running at the end of each fork
    end
    # ...
  end
end

[chrisdebruin]

@bbascarevic-tti doesn't work for me, Are you running test with PARALLEL_WORKERS=x ?
I' am using parallelize(workers: :number_of_processors) and it reports wrong coverage.

[brandoncordell]

@bbascarevic-tti @chrisdebruin This doesn't work for me either. I'm also using parallelize(workers: :number_of_processors).

[michaelherold]

The solution by @bbascarevic-tti is almost there. Here's what I ended up with; it seems to be working as expected:

if ENV['COVERAGE']
  require 'simplecov'

  SimpleCov.start 'rails'
end

ENV['RAILS_ENV'] ||= 'test'
require_relative '../config/environment'
require 'rails/test_help'

module ActiveSupport
  class TestCase
    # Run tests in parallel with specified workers
    parallelize(workers: :number_of_processors)

    if ENV['COVERAGE']
      parallelize_setup do |worker|
        SimpleCov.command_name "#{SimpleCov.command_name}-#{worker}"
      end

      parallelize_teardown do |worker|
        SimpleCov.result
      end
    end

    # Setup all fixtures in test/fixtures/*.yml for all tests in alphabetical order.
    fixtures :all

    # Add more helper methods to be used by all tests here...
  end
end

There are a few changes from @bbascarevic-tti's solution:

1. Do not set SimpleCov.pid to Process.pid in the child process. This causes SimpleCov to report a failure for every single child process because SimpleCov then thinks that the child process is the coordinator.
2. Do not specify SimpleCov.use_merging since it defaults to true.
3. Don't bother wiping out the SimpleCov.at_exit hook - these are not run in the workers anyway.
4. Set a parallelize_teardown hook to call SimpleCov.result. This complex method generates the result and, if SimpleCov.use_merging is true (which it is by default), stores the result for later merging. This allows the results of all workers to be merged into a single result.

Here's an example output:

↪ rm -rf coverage/ && DISABLE_SPRING=1 COVERAGE=1 bin/rails test
Run options: --seed 52156

# Running:

......................

Finished in 0.393015s, 55.9775 runs/s, 101.7772 assertions/s.
22 runs, 40 assertions, 0 failures, 0 errors, 0 skips
Coverage report generated for MiniTest, MiniTest-0, MiniTest-1, MiniTest-2, MiniTest-3 to /home/herold/project/coverage. 58 / 148 LOC (39.19%) covered.

I've run the wipe / re-run without Spring many times and receive a consistent result so I don't believe there are any race conditions in the process model. When you mix in Spring, you get the lovely problems inherent in such a venture, so YMMV if you choose to try it with Spring.

Threads

Note that I've only been able to produce completely consistent results using process parallelization. When I tested out with: :threads, I noticed that the coverage numbers would non-deterministically jump between 58 / 148 and 49 / 148. There were also errors due to thread deadlocks in the database too though, so I think threads on MRI just aren't fully baked (which is fine since you don't get true parallelism anyway).

Next Steps

I haven't been able to figure out a way to upstream this workflow into SimpleCov, so if someone has any bright ideas, please feel free to do that! The way parallel_tests sets the environment variables and gives you access to both the current worker ID and the total number of workers makes sense. Perhaps Rails could use similar functionality to pass the total number of workers into the parallelize_setup and parallelize_teardown hooks?

[brandoncordell]

@michaelherold I think you're close! I'm running into an issue though and maybe it's just general SimpleCov config and not due to parallelize?

Regular tests

rails test
Running via Spring preloader in process 77422
Run options: --seed 42107

# Running:

.........................

Finished in 12.725153s, 1.9646 runs/s, 2.9076 assertions/s.
25 runs, 37 assertions, 0 failures, 0 errors, 0 skips
Coverage report generated for MiniTest, MiniTest-0, MiniTest-1 to  
/Users/Bcordell/Code/<redacted>/coverage. 177 / 232 LOC (76.29%) covered.

System tests run right afterwards

rails test:system                                                                                        

# Running:

Capybara starting Puma...
* Version 3.12.1 , codename: Llamas in Pajamas
* Min threads: 0, max threads: 4
* Listening on tcp://127.0.0.1:63394
Capybara starting Puma...
* Version 3.12.1 , codename: Llamas in Pajamas
* Min threads: 0, max threads: 4
* Listening on tcp://127.0.0.1:63392
..

Finished in 14.470213s, 0.1382 runs/s, 0.0691 assertions/s.
2 runs, 1 assertions, 0 failures, 0 errors, 0 skips
Coverage report generated for MiniTest, MiniTest-0, MiniTest-1 to 
/Users/Bcordell/Code/<redacted>/coverage. 122 / 293 LOC (41.64%) covered.

Regular tests again run right after the system tests

Regular tests

rails test
Running via Spring preloader in process 77422
Run options: --seed 42107

# Running:

.........................

Finished in 12.725153s, 1.9646 runs/s, 2.9076 assertions/s.
25 runs, 37 assertions, 0 failures, 0 errors, 0 skips
Coverage report generated for MiniTest, MiniTest-0, MiniTest-1 to  
/Users/Bcordell/Code/<redacted>/coverage. 177 / 232 LOC (76.29%) covered.

It seems like it's not merging my system tests when I run rails test:system.

Edit

I just had to put SimpleCov.command = 'test:system' inside of ApplicationSystemTestCase and the fix is working perfectly!

[PragTob]

Hey everyone,

thanks all of you for your work on this and exchanging ideas helping each other and laying important ground work 👏

I don't have a lot of time to invest into this right now (sadly), however I submitted a simplecov project to RubyTogether and this is on the list of things to work on/fix should the project get accepted.

[PragTob]

And as a small update, I did get the ruby together fund and hence an improved rails support especially its new test parallelization is towards the top of the list (after branch coverage and some friends though)