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]

3 files changed, 89 insertions, 5 deletions

diff --git a/railties/test/application/test_runner_test.rb b/railties/test/application/test_runner_test.rb
index a01325fdb8..fe1dd59990 100644
--- a/railties/test/application/test_runner_test.rb
+++ b/railties/test/application/test_runner_test.rb
@@ -502,10 +502,10 @@ module ApplicationTests
     end
 
     def test_output_inline_by_default
-      create_test_file :models, "post", pass: false
+      create_test_file :models, "post", pass: false, print: false
 
       output = run_test_command("test/models/post_test.rb")
-      expect = %r{Running:\n\nPostTest\nF\n\nFailure:\nPostTest#test_truth \[[^\]]+test/models/post_test.rb:6\]:\nwups!\n\nbin/rails test test/models/post_test.rb:4\n\n\n\n}
+      expect = %r{Running:\n\nF\n\nFailure:\nPostTest#test_truth \[[^\]]+test/models/post_test.rb:6\]:\nwups!\n\nbin/rails test test/models/post_test.rb:4\n\n\n\n}
       assert_match expect, output
     end
 
@@ -523,6 +523,29 @@ module ApplicationTests
         capture(:stderr) { run_test_command("test/models/post_test.rb --fail-fast", stderr: true) })
     end
 
+    def test_run_in_parallel_with_processes
+      file_name = create_parallel_processes_test_file
+
+      output = run_test_command(file_name)
+
+      assert_match %r{Finished in.*\n2 runs, 2 assertions}, output
+    end
+
+    def test_run_in_parallel_with_threads
+      app_path("/test/test_helper.rb") do |file_name|
+        file = File.read(file_name)
+        file.sub!(/parallelize\(([^\)]*)\)/, "parallelize(\\1, with: :threads)")
+        puts file
+        File.write(file_name, file)
+      end
+
+      file_name = create_parallel_threads_test_file
+
+      output = run_test_command(file_name)
+
+      assert_match %r{Finished in.*\n2 runs, 2 assertions}, output
+    end
+
     def test_raise_error_when_specified_file_does_not_exist
       error = capture(:stderr) { run_test_command("test/not_exists.rb", stderr: true) }
       assert_match(%r{cannot load such file.+test/not_exists\.rb}, error)
@@ -800,19 +823,70 @@ module ApplicationTests
         RUBY
       end
 
-      def create_test_file(path = :unit, name = "test", pass: true)
+      def create_test_file(path = :unit, name = "test", pass: true, print: true)
         app_file "test/#{path}/#{name}_test.rb", <<-RUBY
           require 'test_helper'
 
           class #{name.camelize}Test < ActiveSupport::TestCase
             def test_truth
-              puts "#{name.camelize}Test"
+              puts "#{name.camelize}Test" if #{print}
               assert #{pass}, 'wups!'
             end
           end
         RUBY
       end
 
+      def create_parallel_processes_test_file
+        app_file "test/models/parallel_test.rb", <<-RUBY
+          require 'test_helper'
+
+          class ParallelTest < ActiveSupport::TestCase
+            RD1, WR1 = IO.pipe
+            RD2, WR2 = IO.pipe
+
+            test "one" do
+              WR1.close
+              assert_equal "x", RD1.read(1) # blocks until two runs
+
+              RD2.close
+              WR2.write "y" # Allow two to run
+              WR2.close
+            end
+
+            test "two" do
+              RD1.close
+              WR1.write "x" # Allow one to run
+              WR1.close
+
+              WR2.close
+              assert_equal "y", RD2.read(1) # blocks until one runs
+            end
+          end
+        RUBY
+      end
+
+      def create_parallel_threads_test_file
+        app_file "test/models/parallel_test.rb", <<-RUBY
+          require 'test_helper'
+
+          class ParallelTest < ActiveSupport::TestCase
+            Q1 = Queue.new
+            Q2 = Queue.new
+            test "one" do
+              assert_equal "x", Q1.pop # blocks until two runs
+
+              Q2 << "y"
+            end
+
+            test "two" do
+              Q1 << "x"
+
+              assert_equal "y", Q2.pop # blocks until one runs
+            end
+          end
+        RUBY
+      end
+
       def create_env_test
         app_file "test/unit/env_test.rb", <<-RUBY
           require 'test_helper'
diff --git a/railties/test/application/test_test.rb b/railties/test/application/test_test.rb
index 0a51e98656..fb43bebfbe 100644
--- a/railties/test/application/test_test.rb
+++ b/railties/test/application/test_test.rb
@@ -7,10 +7,15 @@ module ApplicationTests
     include ActiveSupport::Testing::Isolation
 
     def setup
+      @old = ENV["PARALLEL_WORKERS"]
+      ENV["PARALLEL_WORKERS"] = "0"
+
       build_app
     end
 
     def teardown
+      ENV["PARALLEL_WORKERS"] = @old
+
       teardown_app
     end
 
diff --git a/railties/test/isolation/abstract_unit.rb b/railties/test/isolation/abstract_unit.rb
index 6568a356d6..0a4d2a9167 100644
--- a/railties/test/isolation/abstract_unit.rb
+++ b/railties/test/isolation/abstract_unit.rb
@@ -38,7 +38,12 @@ module TestHelpers
     end
 
     def app_path(*args)
-      tmp_path(*%w[app] + args)
+      path = tmp_path(*%w[app] + args)
+      if block_given?
+        yield path
+      else
+        path
+      end
     end
 
     def framework_path