make rb_singleton_class ractor safe (#15591)

Since singleton classes are created lazily, we need to make sure that
we lock around their creation.  Unfortunately, that means we need to
lock around every shareable object's call to `singleton_class`,
including classes and modules.

1 files changed, 15 insertions, 0 deletions

diff --git a/test/ruby/test_class.rb b/test/ruby/test_class.rb
index 22078514ad..8f12e06685 100644
--- a/test/ruby/test_class.rb
+++ b/test/ruby/test_class.rb
@@ -930,4 +930,19 @@ CODE
       end.each(&:join)
     end;
   end
+
+  def test_safe_multi_ractor_singleton_class_access
+    assert_ractor "#{<<~"begin;"}\n#{<<~'end;'}"
+    begin;
+      class A; end
+      4.times.map do
+        Ractor.new do
+          a = A
+          100.times do
+            a = a.singleton_class
+          end
+        end
+      end.each(&:join)
+    end;
+  end
 end