Add C extension spec for rb_define_finalizer and rb_undefine_finalizer

optional/capi/ext/finalizer_spec.c

@@ -0,0 +1,25 @@
+#include "ruby.h"
+#include "rubyspec.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+static VALUE define_finalizer(VALUE self, VALUE obj, VALUE finalizer) {
+  return rb_define_finalizer(obj, finalizer);
+}
+
+static VALUE undefine_finalizer(VALUE self, VALUE obj) {
+  return rb_undefine_finalizer(obj);
+}
+
+void Init_finalizer_spec(void) {
+  VALUE cls = rb_define_class("CApiFinalizerSpecs", rb_cObject);
+
+  rb_define_method(cls, "rb_define_finalizer", define_finalizer, 2);
+  rb_define_method(cls, "rb_undefine_finalizer", undefine_finalizer, 1);
+}
+
+#ifdef __cplusplus
+}
+#endif

optional/capi/finalizer_spec.rb

@@ -0,0 +1,40 @@
+require_relative "spec_helper"
+
+extension_path = load_extension("finalizer")
+
+describe "CApiFinalizerSpecs" do
+  before :each do
+    @s = CApiFinalizerSpecs.new
+  end
+
+  describe "rb_define_finalizer" do
+    it "defines a finalizer on the object" do
+      code = <<~RUBY
+        require #{extension_path.dump}
+
+        obj = Object.new
+        finalizer = Proc.new { puts "finalizer run" }
+        CApiFinalizerSpecs.new.rb_define_finalizer(obj, finalizer)
+      RUBY
+
+      ruby_exe(code).should == "finalizer run\n"
+    end
+  end
+
+  describe "rb_undefine_finalizer" do
+    ruby_bug "#20981", "3.4.0"..."3.4.2" do
+      it "removes finalizers from the object" do
+        code = <<~RUBY
+          require #{extension_path.dump}
+
+          obj = Object.new
+          finalizer = Proc.new { puts "finalizer run" }
+          ObjectSpace.define_finalizer(obj, finalizer)
+          CApiFinalizerSpecs.new.rb_undefine_finalizer(obj)
+        RUBY
+
+        ruby_exe(code).should.empty?
+      end
+    end
+  end
+end