[pathname] Make Pathname Ractor safe (#3940)

ext/pathname/pathname.c

@@ -1512,6 +1512,10 @@ path_f_pathname(VALUE self, VALUE str)
 void
 Init_pathname(void)
 {
+#ifdef HAVE_RB_EXT_RACTOR_SAFE
+    rb_ext_ractor_safe(true);
+#endif
+
     InitVM(pathname);
 
     rb_cPathname = rb_define_class("Pathname", rb_cObject);

test/pathname/test_ractor.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+require "test/unit"
+require "pathname"
+
+class TestPathnameRactor < Test::Unit::TestCase
+  def setup
+    skip unless defined? Ractor
+  end
+
+  def test_ractor_shareable
+    assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}")
+    begin;
+      $VERBOSE = nil
+      require "pathname"
+      r = Ractor.new Pathname("a") do |x|
+        x.join(Pathname("b"), Pathname("c"))
+      end
+      assert_equal(Pathname("a/b/c"), r.take)
+    end;
+  end
+end
+