api_guard.rb

# Guard API with OAuth 2.0 Access Token

require 'rack/oauth2'

module API
  module APIGuard
    extend ActiveSupport::Concern

    included do |base|
      # OAuth2 Resource Server Authentication
      use Rack::OAuth2::Server::Resource::Bearer, 'The API' do |request|
        # The authenticator only fetches the raw token string

        # Must yield access token to store it in the env
        request.access_token
      end

      helpers HelperMethods

      install_error_responders(base)
    end

    # Helper Methods for Grape Endpoint
    module HelperMethods
      # Invokes the doorkeeper guard.
      #
      # If token is presented and valid, then it sets @current_user.
      #
      # If the token does not have sufficient scopes to cover the requred scopes,
      # then it raises InsufficientScopeError.
      #
      # If the token is expired, then it raises ExpiredError.
      #
      # If the token is revoked, then it raises RevokedError.
      #
      # If the token is not found (nil), then it returns nil
      #
      # Arguments:
      #
      #   scopes: (optional) scopes required for this guard.
      #           Defaults to empty array.
      #
      def doorkeeper_guard(scopes: [])
        access_token = find_access_token
        return nil unless access_token

        case validate_access_token(access_token, scopes)
        when Oauth2::AccessTokenValidationService::INSUFFICIENT_SCOPE
          raise InsufficientScopeError.new(scopes)

        when Oauth2::AccessTokenValidationService::EXPIRED
          raise ExpiredError

        when Oauth2::AccessTokenValidationService::REVOKED
          raise RevokedError

        when Oauth2::AccessTokenValidationService::VALID
          @current_user = User.find(access_token.resource_owner_id)
        end
      end

      def current_user
        @current_user
      end

      private

      def find_access_token
        @access_token ||= Doorkeeper.authenticate(doorkeeper_request, Doorkeeper.configuration.access_token_methods)
      end

      def doorkeeper_request
        @doorkeeper_request ||= ActionDispatch::Request.new(env)
      end

      def validate_access_token(access_token, scopes)
        Oauth2::AccessTokenValidationService.validate(access_token, scopes: scopes)
      end
    end

    module ClassMethods
      private

      def install_error_responders(base)
        error_classes = [ MissingTokenError, TokenNotFoundError,
                          ExpiredError, RevokedError, InsufficientScopeError]

        base.send :rescue_from, *error_classes, oauth2_bearer_token_error_handler
      end

      def oauth2_bearer_token_error_handler
        Proc.new do |e|
          response =
            case e
            when MissingTokenError
              Rack::OAuth2::Server::Resource::Bearer::Unauthorized.new

            when TokenNotFoundError
              Rack::OAuth2::Server::Resource::Bearer::Unauthorized.new(
                :invalid_token,
                "Bad Access Token.")

            when ExpiredError
              Rack::OAuth2::Server::Resource::Bearer::Unauthorized.new(
                :invalid_token,
                "Token is expired. You can either do re-authorization or token refresh.")

            when RevokedError
              Rack::OAuth2::Server::Resource::Bearer::Unauthorized.new(
                :invalid_token,
                "Token was revoked. You have to re-authorize from the user.")

            when InsufficientScopeError
              # FIXME: ForbiddenError (inherited from Bearer::Forbidden of Rack::Oauth2)
              # does not include WWW-Authenticate header, which breaks the standard.
              Rack::OAuth2::Server::Resource::Bearer::Forbidden.new(
                :insufficient_scope,
                Rack::OAuth2::Server::Resource::ErrorMethods::DEFAULT_DESCRIPTION[:insufficient_scope],
                { scope: e.scopes })
            end

          response.finish
        end
      end
    end

    #
    # Exceptions
    #

    class MissingTokenError < StandardError; end

    class TokenNotFoundError < StandardError; end

    class ExpiredError < StandardError; end

    class RevokedError < StandardError; end

    class InsufficientScopeError < StandardError
      attr_reader :scopes
      def initialize(scopes)
        @scopes = scopes
      end
    end
  end
end