[ruby/psych] Make Ractor-ready.

Config is Ractor-local.

Benchmarking reveals that using `Ractor.local_storage` for storing cache
is similar to accessing a constant (~15% slower).

ext/psych/lib/psych.rb

@@ -632,10 +632,30 @@ module Psych
   private_class_method :warn_with_uplevel, :parse_caller
 
   class << self
+    if defined?(Ractor)
+      require 'forwardable'
+      extend Forwardable
+
+      class Config
+        attr_accessor :load_tags, :dump_tags, :domain_types
+        def initialize
+          @load_tags = {}
+          @dump_tags = {}
+          @domain_types = {}
+        end
+      end
+
+      def config
+        Ractor.current[:PsychConfig] ||= Config.new
+      end
+
+      def_delegators :config, :load_tags, :dump_tags, :domain_types, :load_tags=, :dump_tags=, :domain_types=
+    else
       attr_accessor :load_tags
       attr_accessor :dump_tags
       attr_accessor :domain_types
     end
+  end
   self.load_tags = {}
   self.dump_tags = {}
   self.domain_types = {}

ext/psych/lib/psych/visitors/visitor.rb

@@ -8,12 +8,26 @@ module Psych
 
       private
 
-      DISPATCH = Hash.new do |hash, klass|
-        hash[klass] = "visit_#{klass.name.gsub('::', '_')}"
+      # @api private
+      def self.dispatch_cache
+        Hash.new do |hash, klass|
+          hash[klass] = :"visit_#{klass.name.gsub('::', '_')}"
+        end
+      end
+
+      if defined?(Ractor)
+        def dispatch
+          Ractor.current[:Psych_Visitors_Visitor] ||= Visitor.dispatch_cache
+        end
+      else
+        DISPATCH = dispatch_cache
+        def dispatch
+          DISPATCH
+        end
       end
 
       def visit target
-        send DISPATCH[target.class], target
+        send dispatch[target.class], target
       end
     end
   end

ext/psych/psych.c

@@ -22,6 +22,9 @@ VALUE mPsych;
 
 void Init_psych(void)
 {
+    #ifdef HAVE_RB_EXT_RACTOR_SAFE
+	RB_EXT_RACTOR_SAFE(true);
+    #endif
     mPsych = rb_define_module("Psych");
 
     rb_define_singleton_method(mPsych, "libyaml_version", libyaml_version, 0);

test/psych/test_ractor.rb

@@ -0,0 +1,47 @@
+# frozen_string_literal: true
+require_relative 'helper'
+
+class TestPsychRactor < Test::Unit::TestCase
+  def test_ractor_round_trip
+    assert_ractor(<<~RUBY, require_relative: 'helper')
+      obj = {foo: [42]}
+      obj2 = Ractor.new(obj) do |obj|
+        Psych.load(Psych.dump(obj))
+      end.take
+      assert_equal obj, obj2
+    RUBY
+  end
+
+  def test_not_shareable
+    # There's no point in making these frozen / shareable
+    # and the C-ext disregards begin frozen
+    assert_ractor(<<~RUBY, require_relative: 'helper')
+      parser = Psych::Parser.new
+      emitter = Psych::Emitter.new(nil)
+      assert_raise(Ractor::Error) { Ractor.make_shareable(parser) }
+      assert_raise(Ractor::Error) { Ractor.make_shareable(emitter) }
+    RUBY
+  end
+
+  def test_ractor_config
+    assert_ractor(<<~RUBY, require_relative: 'helper')
+      r = Ractor.new do
+        Psych.add_builtin_type 'omap' do |type, val|
+          val * 2
+        end
+        Psych.load('--- !!omap hello')
+      end.take
+      assert_equal 'hellohello', r
+      assert_equal 'hello', Psych.load('--- !!omap hello')
+    RUBY
+  end
+
+  def test_ractor_constants
+    assert_ractor(<<~RUBY, require_relative: 'helper')
+      r = Ractor.new do
+        Psych.libyaml_version.join('.') == Psych::LIBYAML_VERSION
+      end.take
+      assert_equal true, r
+    RUBY
+  end
+end