for https://bugs.ruby-lang.org/issues/11328

README

1. ruby extconf.rb
2. make
3. ruby test11328.rb
4. watch memory usage of 3. process

You should reproduce Makefile (step 1) if you try another version of ruby.

bug11328.c

#include "ruby/ruby.h"

static VALUE
bug_str_buf_cat(VALUE mod, VALUE str1, VALUE str2)
{
	(void)mod;
	rb_str_cat(str1, RSTRING_PTR(str2), RSTRING_LEN(str2));
	return str1;
}

void
Init_bug11328(void)
{
	VALUE mBug11328 = rb_define_module("Bug11328");
	rb_define_singleton_method(mBug11328, "str_buf_cat", bug_str_buf_cat, 2);
}

extconf.rb

require "mkmf"

create_makefile("bug11328")

test11328.rb

require "./bug11328"

while true
  str = "hello"
  str2 = "foo"
  10000.times do
    Bug11328.str_buf_cat(str, str2)
  end
  GC.start
end