Take VM lock in `class_switch_superclass` (#15356)

Safe multi-ractor subclass list mutation

We need to lock around mutation and accesses of a class's subclasses
list. Unfortunately we also need to do this when creating singleton
classes, as the singleton class does need to go into `super`'s
subclasses list for CC invalidation purposes.

1 files changed, 13 insertions, 0 deletions

diff --git a/test/ruby/test_class.rb b/test/ruby/test_class.rb
index 10b7655e9a..1faecd0cb2 100644
--- a/test/ruby/test_class.rb
+++ b/test/ruby/test_class.rb
@@ -917,4 +917,17 @@ CODE
       end
     end;
   end
+
+  def test_safe_multi_ractor_subclasses_list_mutation
+    assert_ractor "#{<<~"begin;"}\n#{<<~'end;'}"
+    begin;
+      4.times.map do
+        Ractor.new do
+          20_000.times do
+            Object.new.singleton_class
+          end
+        end
+      end.each(&:join)
+    end;
+  end
 end