Declare as Ractor-safe

Fixes https://bugs.ruby-lang.org/issues/17659

ext/io/wait/wait.c

@@ -273,6 +273,10 @@ io_wait(int argc, VALUE *argv, VALUE io)
 void
 Init_wait(void)
 {
+#ifdef HAVE_RB_EXT_RACTOR_SAFE
+    RB_EXT_RACTOR_SAFE(true);
+#endif
+
     rb_define_method(rb_cIO, "nread", io_nread, 0);
     rb_define_method(rb_cIO, "ready?", io_ready_p, 0);
 

test/io/wait/test_ractor.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+require 'test/unit'
+require 'rbconfig'
+require 'io/wait'
+
+class TestIOWaitInRactor < Test::Unit::TestCase
+  def setup
+    omit unless defined? Ractor
+  end
+
+  def test_ractor
+    ext = "/io/wait.#{RbConfig::CONFIG['DLEXT']}"
+    path = $".find {|path| path.end_with?(ext)}
+    assert_in_out_err(%W[-r#{path}], <<-"end;", ["true"], [])
+      $VERBOSE = nil
+      r = Ractor.new do
+        $stdout.equal?($stdout.wait_writable)
+      end
+      puts r.take
+    end;
+  end
+end