Add missing lock in rb_gc_impl_undefine_finalizer

The table is global so accesses must be synchronized.

gc/default/default.c

@@ -2774,7 +2774,11 @@ rb_gc_impl_undefine_finalizer(void *objspace_ptr, VALUE obj)
     GC_ASSERT(!OBJ_FROZEN(obj));
 
     st_data_t data = obj;
+
+    int lev = rb_gc_vm_lock();
     st_delete(finalizer_table, &data, 0);
+    rb_gc_vm_unlock(lev);
+
     FL_UNSET(obj, FL_FINALIZE);
 }
 

gc/mmtk/mmtk.c

@@ -966,7 +966,11 @@ rb_gc_impl_undefine_finalizer(void *objspace_ptr, VALUE obj)
     struct objspace *objspace = objspace_ptr;
 
     st_data_t data = obj;
+
+    int lev = rb_gc_vm_lock();
     st_delete(objspace->finalizer_table, &data, 0);
+    rb_gc_vm_unlock(lev);
+
     FL_UNSET(obj, FL_FINALIZE);
 }
 

test/objspace/test_ractor.rb

@@ -14,4 +14,23 @@ class TestObjSpaceRactor < Test::Unit::TestCase
       end
     RUBY
   end
+
+  def test_undefine_finalizer
+    assert_ractor(<<~'RUBY', require: 'objspace')
+      def fin
+        ->(id) { }
+      end
+      ractors = 5.times.map do
+        Ractor.new do
+          10_000.times do
+            o = Object.new
+            ObjectSpace.define_finalizer(o, fin)
+            ObjectSpace.undefine_finalizer(o)
+          end
+        end
+      end
+
+      ractors.each(&:take)
+    RUBY
+  end
 end