Add spec for rb_gc_register_address() called before the assignment

* See oracle/truffleruby#2721

optional/capi/ext/gc_spec.c

@@ -7,6 +7,8 @@ extern "C" {
 
 VALUE registered_tagged_value;
 VALUE registered_reference_value;
+VALUE registered_before_rb_gc_register_address;
+VALUE registered_before_rb_global_variable;
 
 static VALUE registered_tagged_address(VALUE self) {
   return registered_tagged_value;
@@ -16,6 +18,14 @@ static VALUE registered_reference_address(VALUE self) {
   return registered_reference_value;
 }
 
+static VALUE get_registered_before_rb_gc_register_address(VALUE self) {
+  return registered_before_rb_gc_register_address;
+}
+
+static VALUE get_registered_before_rb_global_variable(VALUE self) {
+  return registered_before_rb_global_variable;
+}
+
 static VALUE gc_spec_rb_gc_enable(VALUE self) {
   return rb_gc_enable();
 }
@@ -50,9 +60,16 @@ void Init_gc_spec(void) {
 
   rb_gc_register_address(&registered_tagged_value);
   rb_gc_register_address(&registered_reference_value);
+  rb_gc_register_address(&registered_before_rb_gc_register_address);
+  rb_global_variable(&registered_before_rb_global_variable);
+
+  registered_before_rb_gc_register_address = rb_str_new_cstr("registered before rb_gc_register_address()");
+  registered_before_rb_global_variable = rb_str_new_cstr("registered before rb_global_variable()");
 
   rb_define_method(cls, "registered_tagged_address", registered_tagged_address, 0);
   rb_define_method(cls, "registered_reference_address", registered_reference_address, 0);
+  rb_define_method(cls, "registered_before_rb_gc_register_address", get_registered_before_rb_gc_register_address, 0);
+  rb_define_method(cls, "registered_before_rb_global_variable", get_registered_before_rb_global_variable, 0);
   rb_define_method(cls, "rb_gc_enable", gc_spec_rb_gc_enable, 0);
   rb_define_method(cls, "rb_gc_disable", gc_spec_rb_gc_disable, 0);
   rb_define_method(cls, "rb_gc", gc_spec_rb_gc, 0);

optional/capi/gc_spec.rb

@@ -7,15 +7,28 @@
     @f = CApiGCSpecs.new
   end
 
-  it "correctly gets the value from a registered address" do
-    @f.registered_tagged_address.should == 10
-    @f.registered_tagged_address.should equal(@f.registered_tagged_address)
-    @f.registered_reference_address.should == "Globally registered data"
-    @f.registered_reference_address.should equal(@f.registered_reference_address)
+  describe "rb_gc_register_address" do
+    it "correctly gets the value from a registered address" do
+      @f.registered_tagged_address.should == 10
+      @f.registered_tagged_address.should equal(@f.registered_tagged_address)
+      @f.registered_reference_address.should == "Globally registered data"
+      @f.registered_reference_address.should equal(@f.registered_reference_address)
+    end
+
+    it "keeps the value alive even if the value is assigned after rb_gc_register_address() is called" do
+      GC.start
+      @f.registered_before_rb_gc_register_address.should == "registered before rb_gc_register_address()"
+    end
   end
 
-  describe "rb_gc_enable" do
+  describe "rb_global_variable" do
+    it "keeps the value alive even if the value is assigned after rb_global_variable() is called" do
+      GC.start
+      @f.registered_before_rb_global_variable.should == "registered before rb_global_variable()"
+    end
+  end
 
+  describe "rb_gc_enable" do
     after do
       GC.enable
     end