Detect started threads when require only

test/ruby/test_require_lib.rb

@@ -0,0 +1,28 @@
+# frozen_string_literal: false
+require 'test/unit'
+
+class TestRequireLib < Test::Unit::TestCase
+  TEST_RATIO = ENV["TEST_REQUIRE_THREAD_RATIO"]&.tap {|s|break s.to_f} || 0.05 # testing all files needs too long time...
+
+  Dir.glob(File.expand_path('../../lib/**/*.rb', __dir__)).each do |lib|
+    # skip some problems
+    next if %r!/lib/(?:bundler|rubygems)\b! =~ lib
+    next if %r!/lib/(?:debug|mkmf)\.rb\z! =~ lib
+    # skip because "in `<module:Maker>': undefined method `add_maker' for RSS::Maker:Module (NoMethodError)"
+    next if %r!/lib/rss\b! =~ lib
+    # skip many files that almost use no threads
+    next if TEST_RATIO < rand(0.0..1.0)
+    define_method "test_thread_size:#{lib}" do
+      assert_separately(['--disable-gems', '-W0'], "#{<<~"begin;"}\n#{<<~"end;"}")
+      begin;
+        n = Thread.list.size
+        begin
+          require #{lib.dump}
+        rescue Exception
+          skip $!
+        end
+        assert_equal n, Thread.list.size
+      end;
+    end
+  end
+end