```ruby
require 'rack/protection'

module Rack
  module Protection
    ##
    # Prevented attack::   CSRF
    # Supported browsers:: all
    # More infos::         http://en.wikipedia.org/wiki/Cross-site_request_forgery
    #
    # Only accepts unsafe HTTP requests if a given access token matches the token
    # included in the session.
    #
    # Compatible with Rails and rack-csrf.
    #
    # Options:
    #
    # authenticity_param: Defines the param's name that should contain the token on a request.
    #
    class AuthenticityToken < Base
      default_options :authenticity_param => 'authenticity_token'

      def accepts?(env)
        session = session env
        token   = session[:csrf] ||= session['_csrf_token'] || random_string
        safe?(env) ||
          env['HTTP_X_CSRF_TOKEN'] == token ||
          Request.new(env).params[options[:authenticity_param]] == token
      end
    end
  end
end
```

Add allow_if to authenticity token
user
```ruby
require 'rack/protection'

module Rack
  module Protection
    ##
    # Prevented attack::   CSRF
    # Supported browsers:: all
    # More infos::         http://en.wikipedia.org/wiki/Cross-site_request_forgery
    #
    # Only accepts unsafe HTTP requests if a given access token matches the token
    # included in the session.
    #
    # Compatible with Rails and rack-csrf.
    #
    # Options:
    #
    # authenticity_param: Defines the param's name that should contain the token on a request.
    #
    class AuthenticityToken < Base
      default_options :authenticity_param => 'authenticity_token',
                      :allow_if => nil

      def accepts?(env)
        session = session env
        token   = session[:csrf] ||= session['_csrf_token'] || random_string

        safe?(env) ||
          env['HTTP_X_CSRF_TOKEN'] == token ||
          Request.new(env).params[options[:authenticity_param]] == token ||
          ( options[:allow_if] && options[:allow_if].call(env) )
      end
    end
  end
end
```
assistant