Add option to set parallel test worker count to the physical core count of the machine (#34735)

* Add option to set parallel test worker count to the physical core count of the machine

Also, use the physical core count of the machine as
the default number of workers, and  generate the `test_helper.rb` file
with `parallelize(workers: :number_of_processors)`

Closes #34734

* Ensure that we always test parallel testing

Since #34734 we decided to use the physical core count of the machine as
the default number of workers in the parallel testing, we need to
ensure that some tests use at least 2 workers because we could
run those tests on VM that has only 1 physical core.
It also fixes tests failures on the CI since Travis server we are using
has only one physical core.
See https://travis-ci.org/rails/rails/jobs/469281088#L2352

1 files changed, 7 insertions, 2 deletions

diff --git a/activesupport/lib/active_support/test_case.rb b/activesupport/lib/active_support/test_case.rb
index ef12c6b9b0..7be4108ed7 100644
--- a/activesupport/lib/active_support/test_case.rb
+++ b/activesupport/lib/active_support/test_case.rb
@@ -12,6 +12,7 @@ require "active_support/testing/constant_lookup"
 require "active_support/testing/time_helpers"
 require "active_support/testing/file_fixtures"
 require "active_support/testing/parallelization"
+require "concurrent/utility/processor_counter"
 
 module ActiveSupport
   class TestCase < ::Minitest::Test
@@ -58,16 +59,20 @@ module ActiveSupport
       # If the number of workers is set to +1+ or fewer, the tests will not be
       # parallelized.
       #
+      # If +workers+ is set to +:number_of_processors+, the number of workers will be
+      # set to the actual core count on the machine you are on.
+      #
       # The default parallelization method is to fork processes. If you'd like to
       # use threads instead you can pass <tt>with: :threads</tt> 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)
+      #   parallelize(workers: :number_of_processors, with: :threads)
       #
       # The threaded parallelization uses minitest's parallel executor directly.
       # The processes parallelization uses a Ruby DRb server.
-      def parallelize(workers: 2, with: :processes)
+      def parallelize(workers: :number_of_processors, with: :processes)
+        workers = Concurrent.physical_processor_count if workers == :number_of_processors
         workers = ENV["PARALLEL_WORKERS"].to_i if ENV["PARALLEL_WORKERS"]
 
         return if workers <= 1