Make liquid template's tags to be thread safe

liquid_template_tags_thread_safe.rb

# 使用方法：
#
#   Liquid::Template.include Liquid::TemplateTagsThreadSafe
#
#   Liquid::Template.with_thread_safe do
#     Liquid::Template.register_tag('test', Liquid::Tag)
#     Liquid::Template.tags['test'] # => Liquid::Tag
#   end
#
#   Liquid::Template.tags['test'] # => nil
#
module Liquid
  module TemplateTagsThreadSafe
    def self.included(base)
      return if base.included_modules.include?(ClassMethods)

      base.class_eval do
        extend ClassMethods

        class << self
          alias_method :thread_unsafe_tags, :tags
          alias_method :tags, :thread_safe_tags
        end
      end
    end

    module ClassMethods
      def with_thread_safe(&)
        Thread.current[:liquid_template_thread_safe] = true
        copy_tags_from_thread_unsafe!
        yield
      ensure
        Thread.current[:liquid_template_thread_safe] = false
        Thread.current[:liquid_template_thread_safe_tags] = nil
      end

      def thread_safe?
        Thread.current[:liquid_template_thread_safe] == true
      end

      def copy_tags_from_thread_unsafe
        ::Liquid::Template::TagRegistry.new.tap do |registry|
          thread_unsafe_tags.each do |tag_name, klass|
            registry[tag_name] = Object.const_get(klass)
          end
        end
      end

      def copy_tags_from_thread_unsafe!
        Thread.current[:liquid_template_thread_safe_tags] = copy_tags_from_thread_unsafe
      end

      def thread_safe_tags=(tags)
        if thread_safe?
          Thread.current[:liquid_template_thread_safe_tags] = tags
        else
          thread_unsafe_tags
        end
      end

      def thread_safe_tags
        thread_safe? ? Thread.current[:liquid_template_thread_safe_tags] : thread_unsafe_tags
      end
    end
  end
end