Add test parallelization to Rails

Provides both a forked process and threaded parallelization options. To
use add `parallelize` to your test suite.

Takes a `workers` argument that controls how many times the process
is forked. For each process a new database will be created suffixed
with the worker number; test-database-0 and test-database-1
respectively.

If `ENV["PARALLEL_WORKERS"]` is set the workers argument will be ignored
and the environment variable will be used instead. This is useful for CI
environments, or other environments where you may need more workers than
you do for local testing.

If the number of workers is set to `1` or fewer, the tests will not be
parallelized.

The default parallelization method is to fork processes. If you'd like to
use threads instead you can pass `with: :threads` to the `parallelize`
method. Note the threaded parallelization does not create multiple
database and will not work with system tests at this time.

parallelize(workers: 2, with: :threads)

The threaded parallelization uses Minitest's parallel exector directly.
The processes paralleliztion uses a Ruby Drb server.

For parallelization via threads a setup hook and cleanup hook are
provided.

```
class ActiveSupport::TestCase
  parallelize_setup do |worker|
    # setup databases
  end

  parallelize_teardown do |worker|
    # cleanup database
  end

  parallelize(workers: 2)
end
```

[Eileen M. Uchitelle, Aaron Patterson]

2 files changed, 8 insertions, 0 deletions

diff --git a/railties/lib/rails/generators/rails/app/templates/test/test_helper.rb.tt b/railties/lib/rails/generators/rails/app/templates/test/test_helper.rb.tt
index 52d68cc77c..c918b57eca 100644
--- a/railties/lib/rails/generators/rails/app/templates/test/test_helper.rb.tt
+++ b/railties/lib/rails/generators/rails/app/templates/test/test_helper.rb.tt
@@ -3,6 +3,13 @@ require_relative '../config/environment'
 require 'rails/test_help'
 
 class ActiveSupport::TestCase
+  # Run tests in parallel with specified workers
+<% if defined?(JRUBY_VERSION) -%>
+  parallelize(workers: 2, with: :threads)
+<%- else -%>
+  parallelize(workers: 2)
+<% end -%>
+
 <% unless options[:skip_active_record] -%>
   # Setup all fixtures in test/fixtures/*.yml for all tests in alphabetical order.
   fixtures :all
diff --git a/railties/lib/rails/test_help.rb b/railties/lib/rails/test_help.rb
index 76c28ac85e..4bd7d74b04 100644
--- a/railties/lib/rails/test_help.rb
+++ b/railties/lib/rails/test_help.rb
@@ -22,6 +22,7 @@ if defined?(ActiveRecord::Base)
 
   module ActiveSupport
     class TestCase
+      include ActiveRecord::TestDatabases
       include ActiveRecord::TestFixtures
       self.fixture_path = "#{Rails.root}/test/fixtures/"
       self.file_fixture_path = fixture_path + "files"