jekyll plugin that render an external page completely and include it

render_partial.rb

module Jekyll

  class RenderPartialTag < Liquid::Tag
    def initialize(tag_name, file, tokens)
      super
      @file = file.strip
    end

    def render(context)
      site = context.registers[:site]
      includes_dir = File.join(site.source, '_includes')

      if File.symlink?(includes_dir)
        return "Includes directory '#{includes_dir}' cannot be a symlink"
      end

      if @file !~ /^[a-zA-Z0-9_\/\.-]+$/ || @file =~ /\.\// || @file =~ /\/\./
        return "Include file '#{@file}' contains invalid characters or sequences"
      end

      Dir.chdir(includes_dir) do
        choices = Dir['**/*'].reject { |x| File.symlink?(x) }
        if choices.include?(@file)
          partial = Page.new(site, site.source, '_includes', @file)
          partial.render(site.layouts, site.site_payload)
          partial.output
        else
          "Included file '#{@file}' not found in _includes directory"
        end
      end
    end
  end

end

Liquid::Template.register_tag('render_partial', Jekyll::RenderPartialTag)