import time
import json
import logging
import urlparse

import jwt

from cachetools import lru_cache
from cachetools.ttl import TTLCache
from cryptography.hazmat.backends import default_backend
from cryptography.hazmat.primitives.serialization import load_der_public_key
from jwkest.jwk import KEYS

from oauth.base import OAuthService, OAuthExchangeCodeException, OAuthGetUserInfoException
from oauth.login import OAuthLoginException
from util.security.jwtutil import decode, InvalidTokenError

logger = logging.getLogger(__name__)


OIDC_WELLKNOWN = ".well-known/openid-configuration"
PUBLIC_KEY_CACHE_TTL = 3600 # 1 hour
ALLOWED_ALGORITHMS = ['RS256']
JWT_CLOCK_SKEW_SECONDS = 30

class DiscoveryFailureException(Exception):
  """ Exception raised when OIDC discovery fails. """
  pass

class PublicKeyLoadException(Exception):
  """ Exception raised if loading the OIDC public key fails. """
  pass


class OIDCLoginService(OAuthService):
  """ Defines a generic service for all OpenID-connect compatible login services. """
  def __init__(self, config, key_name, client=None):
    super(OIDCLoginService, self).__init__(config, key_name)

    self._public_key_cache = TTLCache(1, PUBLIC_KEY_CACHE_TTL, missing=self._load_public_key)
    self._id = key_name[0:key_name.find('_')].lower()
    self._http_client = client or config['HTTPCLIENT']
    self._mailing = config.get('FEATURE_MAILING', False)

  def service_id(self):
    return self._id

  def service_name(self):
    return self.config.get('SERVICE_NAME', self.service_id())

  def get_icon(self):
    return self.config.get('SERVICE_ICON', 'fa-user-circle')

  def get_login_scopes(self):
    default_scopes = ['openid']

    if self.user_endpoint() is not None:
      default_scopes.append('profile')

    if self._mailing:
      default_scopes.append('email')

    return self._oidc_config().get('scopes_supported', default_scopes)

  def authorize_endpoint(self):
    return self._oidc_config().get('authorization_endpoint', '') + '?response_type=code&'

  def token_endpoint(self):
    return self._oidc_config().get('token_endpoint')

  def user_endpoint(self):
    return self._oidc_config().get('userinfo_endpoint')

  def validate(self):
    return bool(self.user_endpoint())

  def validate_client_id_and_secret(self, http_client, app_config):
    # TODO: find a way to verify client secret too.
    check_auth_url = http_client.get(self.get_auth_url())
    if check_auth_url.status_code // 100 != 2:
      raise Exception('Got non-200 status code for authorization endpoint')

  def requires_form_encoding(self):
    return True

  def get_public_config(self):
    return {
      'CLIENT_ID': self.client_id(),
      'OIDC': True,
    }

  def exchange_code_for_login(self, app_config, http_client, code, redirect_suffix):
    # Exchange the code for the access token and id_token
    try:
      json_data = self.exchange_code(app_config, http_client, code,
                                     redirect_suffix=redirect_suffix,
                                     form_encode=self.requires_form_encoding())
    except OAuthExchangeCodeException as oce:
      raise OAuthLoginException(oce.message)

    # Make sure we received both.
    access_token = json_data.get('access_token', None)
    if access_token is None:
      logger.debug('Missing access_token in response: %s', json_data)
      raise OAuthLoginException('Missing `access_token` in OIDC response')

    id_token = json_data.get('id_token', None)
    if id_token is None:
      logger.debug('Missing id_token in response: %s', json_data)
      raise OAuthLoginException('Missing `id_token` in OIDC response')

    # Decode the id_token.
    try:
      decoded_id_token = self._decode_user_jwt(id_token)
    except InvalidTokenError as ite:
      logger.exception('Got invalid token error on OIDC decode: %s', ite.message)
      raise OAuthLoginException('Could not decode OIDC token')
    except PublicKeyLoadException as pke:
      logger.exception('Could not load public key during OIDC decode: %s', pke.message)
      raise OAuthLoginException('Could find public OIDC key')

    # Retrieve the user information.
    try:
      user_info = self.get_user_info(http_client, access_token)
    except OAuthGetUserInfoException as oge:
      raise OAuthLoginException(oge.message)

    # Verify subs.
    if user_info['sub'] != decoded_id_token['sub']:
      raise OAuthLoginException('Mismatch in `sub` returned by OIDC user info endpoint')

    # Check if we have a verified email address.
    email_address = user_info.get('email') if user_info.get('email_verified') else None
    if self._mailing:
      if email_address is None:
        raise OAuthLoginException('A verified email address is required to login with this service')

    # Check for a preferred username.
    lusername = user_info.get('preferred_username') or user_info.get('sub')
    return decoded_id_token['sub'], lusername, email_address

  @property
  def _issuer(self):
    return self.config.get('OIDC_ISSUER', self.config['OIDC_SERVER'])

  @lru_cache(maxsize=1)
  def _oidc_config(self):
    if self.config.get('OIDC_SERVER'):
      return self._load_oidc_config_via_discovery(self.config.get('DEBUGGING', False))
    else:
      return {}

  def _load_oidc_config_via_discovery(self, is_debugging):
    """ Attempts to load the OIDC config via the OIDC discovery mechanism. If is_debugging is True,
        non-secure connections are alllowed. Raises an DiscoveryFailureException on failure.
    """
    oidc_server = self.config['OIDC_SERVER']
    if not oidc_server.startswith('https://') and not is_debugging:
      raise DiscoveryFailureException('OIDC server must be accessed over SSL')

    discovery_url = urlparse.urljoin(oidc_server, OIDC_WELLKNOWN)
    discovery = self._http_client.get(discovery_url, timeout=5, verify=is_debugging is False)
    if discovery.status_code // 100 != 2:
      logger.debug('Got %s response for OIDC discovery: %s', discovery.status_code, discovery.text)
      raise DiscoveryFailureException("Could not load OIDC discovery information")

    try:
      return json.loads(discovery.text)
    except ValueError:
      logger.exception('Could not parse OIDC discovery for url: %s', discovery_url)
      raise DiscoveryFailureException("Could not parse OIDC discovery information")

  def _decode_user_jwt(self, token):
    """ Decodes the given JWT under the given provider and returns it. Raises an InvalidTokenError
        exception on an invalid token or a PublicKeyLoadException if the public key could not be
        loaded for decoding.
    """
    # Find the key to use.
    headers = jwt.get_unverified_header(token)
    kid = headers.get('kid', None)
    if kid is None:
      raise InvalidTokenError('Missing `kid` header')

    try:
      return decode(token, self._get_public_key(kid), algorithms=ALLOWED_ALGORITHMS,
                    audience=self.client_id(),
                    issuer=self._issuer,
                    leeway=JWT_CLOCK_SKEW_SECONDS,
                    options=dict(require_nbf=False))
    except InvalidTokenError:
      # Public key may have expired. Try to retrieve an updated public key and use it to decode.
      return decode(token, self._get_public_key(kid, force_refresh=True),
                    algorithms=ALLOWED_ALGORITHMS,
                    audience=self.client_id(),
                    issuer=self._issuer,
                    leeway=JWT_CLOCK_SKEW_SECONDS,
                    options=dict(require_nbf=False))

  def _get_public_key(self, kid, force_refresh=False):
    """ Retrieves the public key for this handler with the given kid. Raises a
        PublicKeyLoadException on failure. """

    # If force_refresh is true, we expire all the items in the cache by setting the time to
    # the current time + the expiration TTL.
    if force_refresh:
      self._public_key_cache.expire(time=time.time() + PUBLIC_KEY_CACHE_TTL)

    # Retrieve the public key from the cache. If the cache does not contain the public key,
    # it will internally call _load_public_key to retrieve it and then save it.
    return self._public_key_cache[kid]

  def _load_public_key(self, kid):
    """ Loads the public key for this handler from the OIDC service. Raises PublicKeyLoadException
        on failure.
    """
    keys_url = self._oidc_config()['jwks_uri']

    # Load the keys.
    try:
      keys = KEYS()
      keys.load_from_url(keys_url, verify=not self.config.get('DEBUGGING', False))
    except Exception as ex:
      logger.exception('Exception loading public key')
      raise PublicKeyLoadException(ex.message)

    # Find the matching key.
    keys_found = keys.by_kid(kid)
    if len(keys_found) == 0:
      raise PublicKeyLoadException('Public key %s not found' % kid)

    rsa_keys = [key for key in keys_found if key.kty == 'RSA']
    if len(rsa_keys) == 0:
      raise PublicKeyLoadException('No RSA form of public key %s not found' % kid)

    matching_key = rsa_keys[0]
    matching_key.deserialize()

    # Reload the key so that we can give a key *instance* to PyJWT to work around its weird parsing
    # issues.
    return load_der_public_key(matching_key.key.exportKey('DER'), backend=default_backend())