[stringio] Make stringio Ractor safe

https://github.com/ruby/stringio/commit/ee3fec7512
https://github.com/ruby/stringio/commit/18dcd045ef
https://github.com/ruby/stringio/commit/18dcd045ef

ext/stringio/stringio.c

@@ -1750,6 +1750,11 @@ void
 Init_stringio(void)
 {
 #undef rb_intern
+
+#ifdef HAVE_RB_EXT_RACTOR_SAFE
+  rb_ext_ractor_safe(true);
+#endif
+
     VALUE StringIO = rb_define_class("StringIO", rb_cData);
 
     rb_define_const(StringIO, "VERSION", rb_str_new_cstr(STRINGIO_VERSION));

test/stringio/test_ractor.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+require 'test/unit'
+
+class TestStringIOInRactor < Test::Unit::TestCase
+  def setup
+    omit unless defined? Ractor
+  end
+
+  def test_ractor
+    assert_in_out_err([], <<-"end;", ["true"], [])
+      require "stringio"
+      $VERBOSE = nil
+      r = Ractor.new do
+        io = StringIO.new("")
+        io.puts "abc"
+        io.truncate(0)
+        io.puts "def"
+        "\0\0\0\0def\n" == io.string
+      end
+      puts r.take
+    end;
+  end
+end