Disable parallel testing when running individual files

Setting up the parallel workers could be an overhead when running
individual files.

This patch disables that process in case the number of files to run
is less than one.

Results running a sample file:

Before:

```
actionpack $ bin/test test/controller/parameters/accessors_test.rb
Run options: --seed 48261

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

Finished in 0.211923s, 339.7460 runs/s, 552.0873 assertions/s.
72 runs, 117 assertions, 0 failures, 0 errors, 0 skips
```

After

```
actionpack $ bin/test test/controller/parameters/accessors_test.rb

Run options: --seed 5461

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

Finished in 0.008411s, 8560.2189 runs/s, 13910.3557 assertions/s.
72 runs, 117 assertions, 0 failures, 0 errors, 0 skips
```

activesupport/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Tests parallelization is now disabled when running individual files to prevent the setup overhead.
+
+    It can still be enforced if the environment variable `PARALLEL_WORKERS` is present and set to a value greater than 1.
+
+    *Ricardo Díaz*
+
 *   Fix proxying keyword arguments in `ActiveSupport::CurrentAttributes`.
 
     *Marcin Kołodziej*

activesupport/lib/active_support.rb

@@ -87,6 +87,11 @@ module ActiveSupport
   end
 
   cattr_accessor :test_order # :nodoc:
+  cattr_accessor :test_parallelization_disabled, default: false # :nodoc:
+
+  def self.disable_test_parallelization!
+    self.test_parallelization_disabled = true unless ENV["PARALLEL_WORKERS"]
+  end
 
   def self.to_time_preserves_timezone
     DateAndTime::Compatibility.preserve_timezone

activesupport/lib/active_support/test_case.rb

@@ -75,7 +75,7 @@ module ActiveSupport
         workers = Concurrent.physical_processor_count if workers == :number_of_processors
         workers = ENV["PARALLEL_WORKERS"].to_i if ENV["PARALLEL_WORKERS"]
 
-        return if workers <= 1
+        return if workers <= 1 || ActiveSupport.test_parallelization_disabled
 
         executor = case with
                    when :processes
@@ -91,6 +91,12 @@ module ActiveSupport
         Minitest.parallel_executor = executor
 
         parallelize_me!
+
+        # `Minitest::Test.parallelize_me!` will try to change `test_order` to return
+        # `:parallel`. However, since `ActiveSupport::TestCase` is already overriding it,
+        # in some inheritance situations, it will have precedence over the `Minitest` one.
+        # For this reason, it needs to be updated by hand.
+        self.test_order = :parallel
       end
 
       # Set up hook for parallel testing. This can be used if you have multiple

railties/lib/rails/test_unit/runner.rb

@@ -3,6 +3,7 @@
 require "shellwords"
 require "method_source"
 require "rake/file_list"
+require "active_support"
 require "active_support/core_ext/module/attribute_accessors"
 
 module Rails
@@ -46,7 +47,8 @@ module Rails
 
           tests = Rake::FileList[patterns.any? ? patterns : default_test_glob]
           tests.exclude(default_test_exclude_glob) if patterns.empty?
-
+          # Disable parallel testing if there's only one test file to run.
+          ActiveSupport.disable_test_parallelization! if tests.size <= 1
           tests.to_a.each { |path| require File.expand_path(path) }
         end
 

railties/test/application/test_runner_test.rb

@@ -539,6 +539,76 @@ module ApplicationTests
       assert_no_match "create_table(:users)", output
     end
 
+    def test_parallel_is_disabled_when_single_file_is_run
+      exercise_parallelization_regardless_of_machine_core_count(with: :processes, force: false)
+
+      file_name = app_file "test/unit/parallel_test.rb", <<-RUBY
+        require "test_helper"
+
+        class ParallelTest < ActiveSupport::TestCase
+          def test_verify_test_order
+            puts "Test order: \#{self.class.test_order}"
+          end
+        end
+      RUBY
+
+      output = run_test_command(file_name)
+
+      assert_match "Test order: random", output
+    end
+
+    def test_parallel_is_enabled_when_multiple_files_are_run
+      exercise_parallelization_regardless_of_machine_core_count(with: :processes, force: false)
+
+      file_1 = app_file "test/unit/parallel_test_first.rb", <<-RUBY
+        require "test_helper"
+
+        class ParallelTestFirst < ActiveSupport::TestCase
+          def test_verify_test_order
+            puts "Test order (file 1): \#{self.class.test_order}"
+          end
+        end
+      RUBY
+
+      file_2 = app_file "test/unit/parallel_test_second.rb", <<-RUBY
+        require "test_helper"
+
+        class ParallelTestSecond < ActiveSupport::TestCase
+          def test_verify_test_order
+            puts "Test order (file 2): \#{self.class.test_order}"
+          end
+        end
+      RUBY
+
+      output = run_test_command([file_1, file_2].join(" "))
+
+      assert_match "Test order (file 1): parallel", output
+      assert_match "Test order (file 2): parallel", output
+    end
+
+    def test_parallel_is_enabled_when_PARALLEL_WORKERS_is_set
+      @old = ENV["PARALLEL_WORKERS"]
+      ENV["PARALLEL_WORKERS"] = "5"
+
+      exercise_parallelization_regardless_of_machine_core_count(with: :processes, force: false)
+
+      file_name = app_file "test/unit/parallel_test.rb", <<-RUBY
+        require "test_helper"
+
+        class ParallelTest < ActiveSupport::TestCase
+          def test_verify_test_order
+            puts "Test order: \#{self.class.test_order}"
+          end
+        end
+      RUBY
+
+      output = run_test_command(file_name)
+
+      assert_match "Test order: parallel", output
+    ensure
+      ENV["PARALLEL_WORKERS"] = @old
+    end
+
     def test_run_in_parallel_with_process_worker_crash
       exercise_parallelization_regardless_of_machine_core_count(with: :processes)
 
@@ -1039,11 +1109,28 @@ module ApplicationTests
         RUBY
       end
 
-      def exercise_parallelization_regardless_of_machine_core_count(with:)
+      def exercise_parallelization_regardless_of_machine_core_count(with:, force: true)
+        file_content = ERB.new(<<-ERB, trim_mode: "-").result_with_hash(with: with.to_s, force: force)
+          ENV["RAILS_ENV"] ||= "test"
+          require_relative "../config/environment"
+          require "rails/test_help"
+
+          class ActiveSupport::TestCase
+            <%- if force -%>
+            # Force parallelization, even with single files
+            ActiveSupport.test_parallelization_disabled = false
+            <%- end -%>
+
+            # Run tests in parallel with specified workers
+            parallelize(workers: 2, with: :<%= with %>)
+
+            # Setup all fixtures in test/fixtures/*.yml for all tests in alphabetical order.
+            fixtures :all
+          end
+        ERB
+
         app_path("test/test_helper.rb") do |file_name|
-          file = File.read(file_name)
-          file.sub!(/parallelize\(([^)]*)\)/, "parallelize(workers: 2, with: :#{with})")
-          File.write(file_name, file)
+          File.write(file_name, file_content)
         end
       end