executor.diff

diff -r old/lib/minitest.rb new/lib/minitest.rb
--- old/lib/minitest.rb
+++ new/lib/minitest.rb
@@ -36,7 +36,7 @@
   warn "DEPRECATED: use MT_CPU instead of N for parallel test runs" if ENV["N"] && ENV["N"].to_i > 0
   n_threads = (ENV["MT_CPU"] || ENV["N"] || Etc.nprocessors).to_i
 
-  self.parallel_executor = Parallel::Executor.new n_threads
+  self.parallel_executor = Parallel::Executor.new n_threads if n_threads > 1
 
   ##
   # Filter object for backtraces.
@@ -291,7 +291,7 @@
     rescue Interrupt
       warn "Interrupted. Exiting..."
     end
-    self.parallel_executor.shutdown
+    self.parallel_executor.shutdown if parallel_executor.respond_to? :shutdown
 
     # might have been removed/replaced during init_plugins:
     summary = reporter.reporters.grep(SummaryReporter).first
diff -r old/lib/minitest/test.rb new/lib/minitest/test.rb
--- old/lib/minitest/test.rb
+++ new/lib/minitest/test.rb
@@ -58,6 +58,7 @@
     # tests are awesome.
 
     def self.parallelize_me!
+      return unless Minitest.parallel_executor
       include Minitest::Parallel::Test
       extend Minitest::Parallel::Test::ClassMethods
     end
diff -r old/test/minitest/test_minitest_test.rb new/test/minitest/test_minitest_test.rb
--- old/test/minitest/test_minitest_test.rb
+++ new/test/minitest/test_minitest_test.rb
@@ -662,7 +662,7 @@
       2 runs, 2 assertions, 0 failures, 0 errors, 0 skips
     EOM
 
-    skip if Minitest.parallel_executor.size < 2 # locks up test runner if 1 CPU
+    skip unless Minitest.parallel_executor # locks up test runner if 1 CPU
 
     assert_report expected do |reporter|
       reporter.extend Module.new {