Minitest Fork Executor

minitest-fork_executor helps you avoid leaking state between individual test_* methods and *Test classes by running each test method in a separate process.

Installation

Install via Ruby Gems:

gem install minitest-fork_executor

or add to Gemfile:

gem 'minitest-fork_executor', group: :test

Then configure Minitest to use it by adding the following to test_helper.rb or a similar file:

Minitest.parallel_executor = Minitest::ForkExecutor.new

Thats it! From now on, each test method will be run in a separate process.

Caveat

When minitest-fork_executor some exceptions raised during test case execution may be reported as a different exception class. This is by design and was required to overcome limitations imposed by certain implementation choices.

minitest-fork_executor run each test method in a separate process. The result, including number of assertions and exceptions (if any) raised during execution, must be passed to the parent process for reporting. We use Marshal to communicate between processes using a pipe object.

Problems araise when the test method raises an exception that references an object unsupported by Marshal. For example, a database exception may contain a reference to the database connection socket, which cannot be marshalled. To overcome that limitation, we convert non-marshallable exceptions into a different class, called Minitest::ForkExecutor::UnmarshallableError that is guaranteed to be marshallable and which tries to retain as much original information as possible.

Rationale

The idea for for a new forker came when I was working on active_record_doctor. I had just set up a test suite against multiple versions of Ruby and Rails but couldn't get rid of random test case failures. Further troubleshooting indicated the problem lied in insufficient garbage collection, most likely caused by a bug deep in the guts of Active Support. Resolving the problem for a specific Ruby/Rails version combination would make the problem appear in different versions.

Submitting a patch to Active Support wouldn't be a complete solution because Rails 4.2, which I wanted to support, had already reached end-of-life. That, plus my limited time budget for open source work made minitest-fork_executor an economical idea.

Prior Art

minitest-parallel_fork is a similar gem but works at a class-level instead of test-case level. It means test_* methods defined on the same test class can still leak state. The problem could be avoided by splitting each test class into multiple single-method test classes (one for each test_* method) however I decided against that solution in order to maintain higher test class cohesion.

Author

This gem is developed and maintained by Greg Navis.