""" Mailu admin app utilities
"""

try:
    import cPickle as pickle
except ImportError:
    import pickle

import dns
import dns.resolver

import hmac
import secrets
import time

from multiprocessing import Value

from mailu import limiter

import flask
import flask_login
import flask_migrate
import flask_babel
import redis

from flask.sessions import SessionMixin, SessionInterface
from itsdangerous.encoding import want_bytes
from werkzeug.datastructures import CallbackDict
from werkzeug.contrib import fixers

# Login configuration
login = flask_login.LoginManager()
login.login_view = "ui.login"

@login.unauthorized_handler
def handle_needs_login():
    """ redirect unauthorized requests to login page """
    return flask.redirect(
        flask.url_for('ui.login', next=flask.request.endpoint)
    )

# DNS stub configured to do DNSSEC enabled queries
resolver = dns.resolver.Resolver()
resolver.use_edns(0, 0, 1500)
resolver.flags = dns.flags.AD | dns.flags.RD

def has_dane_record(domain, timeout=10):
    try:
        result = resolver.query(f'_25._tcp.{domain}', dns.rdatatype.TLSA,dns.rdataclass.IN, lifetime=timeout)
        if (result.response.flags & dns.flags.AD) == dns.flags.AD:
            for record in result:
                if isinstance(record, dns.rdtypes.ANY.TLSA.TLSA):
                    record.validate()
                    if record.usage in [2,3] and record.selector in [0,1] and record.mtype in [0,1,2]:
                        return True
    except dns.resolver.NoNameservers:
        # If the DNSSEC data is invalid and the DNS resolver is DNSSEC enabled
        # we will receive this non-specific exception. The safe behaviour is to
        # accept to defer the email.
        return app.config['DEFER_ON_TLS_ERROR']
    except:
        pass

# Rate limiter
limiter = limiter.LimitWraperFactory()

# Application translation
babel = flask_babel.Babel()

@babel.localeselector
def get_locale():
    """ selects locale for translation """
    translations = list(map(str, babel.list_translations()))
    flask.session['available_languages'] = translations

    try:
        language = flask.session['language']
    except KeyError:
        language = flask.request.accept_languages.best_match(translations)
        flask.session['language'] = language

    return language


# Proxy fixer
class PrefixMiddleware(object):
    """ fix proxy headers """
    def __init__(self):
        self.app = None

    def __call__(self, environ, start_response):
        prefix = environ.get('HTTP_X_FORWARDED_PREFIX', '')
        if prefix:
            environ['SCRIPT_NAME'] = prefix
        return self.app(environ, start_response)

    def init_app(self, app):
        self.app = fixers.ProxyFix(app.wsgi_app, x_for=1, x_proto=1)
        app.wsgi_app = self

proxy = PrefixMiddleware()


# Data migrate
migrate = flask_migrate.Migrate()


# session store (inspired by https://github.com/mbr/flask-kvsession)
class RedisStore:
    """ Stores session data in a redis db. """

    has_ttl = True

    def __init__(self, redisstore):
        self.redis = redisstore

    def get(self, key):
        """ load item from store. """
        value = self.redis.get(key)
        if value is None:
            raise KeyError(key)
        return value

    def put(self, key, value, ttl=None):
        """ save item to store. """
        if ttl:
            self.redis.setex(key, int(ttl), value)
        else:
            self.redis.set(key, value)

    def delete(self, key):
        """ delete item from store. """
        self.redis.delete(key)

    def list(self, prefix=None):
        """ return list of keys starting with prefix """
        if prefix:
            prefix += b'*'
        return list(self.redis.scan_iter(match=prefix))

class DictStore:
    """ Stores session data in a python dict. """

    has_ttl = False

    def __init__(self):
        self.dict = {}

    def get(self, key):
        """ load item from store. """
        return self.dict[key]

    def put(self, key, value, ttl_secs=None):
        """ save item to store. """
        self.dict[key] = value

    def delete(self, key):
        """ delete item from store. """
        try:
            del self.dict[key]
        except KeyError:
            pass

    def list(self, prefix=None):
        """ return list of keys starting with prefix """
        if prefix is None:
            return list(self.dict.keys())
        return [key for key in self.dict if key.startswith(prefix)]

class MailuSession(CallbackDict, SessionMixin):
    """ Custom flask session storage. """

    # default modified to false
    modified = False

    def __init__(self, key=None, app=None):

        self.app = app or flask.current_app

        initial = None

        key = want_bytes(key)
        if parsed := self.app.session_config.parse_key(key, self.app):
            try:
                initial = pickle.loads(app.session_store.get(key))
            except (KeyError, EOFError, pickle.UnpicklingError):
                # either the cookie was manipulated or we did not find the
                # session in the backend or the pickled data is invalid.
                # => start new session
                pass
            else:
                (self._uid, self._sid, self._created) = parsed
                self._key = key

        if initial is None:
            # start new session
            self.new = True
            self._uid = None
            self._sid = None
            self._created = self.app.session_config.gen_created()
            self._key = None

        def _on_update(obj):
            obj.modified = True

        CallbackDict.__init__(self, initial, _on_update)

    @property
    def saved(self):
        """ this reflects if the session was saved. """
        return self._key is not None

    @property
    def sid(self):
        """ this reflects the session's id. """
        if self._sid is None or self._uid is None or self._created is None:
            return None
        return b''.join([self._uid, self._sid, self._created])

    def destroy(self):
        """ destroy session for security reasons. """

        self.delete()

        self._uid = None
        self._sid = None
        self._created = None

        self.clear()

        self.modified = True
        self.new = False

    def regenerate(self):
        """ generate new id for session to avoid `session fixation`. """

        self.delete()

        self._sid = None
        self._created = self.app.session_config.gen_created()

        self.modified = True

    def delete(self):
        """ Delete stored session. """
        if self.saved:
            self.app.session_store.delete(self._key)
            self._key = None

    def save(self):
        """ Save session to store. """

        set_cookie = False

        # set uid from dict data
        if self._uid is None:
            self._uid = self.app.session_config.gen_uid(self.get('user_id', ''))

        # create new session id for new or regenerated sessions and force setting the cookie
        if self._sid is None:
            self._sid = self.app.session_config.gen_sid()
            set_cookie = True

        # get new session key
        key = self.sid

        # delete old session if key has changed
        if key != self._key:
            self.delete()

        # remember time to refresh
        self['_refresh'] = int(time.time()) + self.app.permanent_session_lifetime.total_seconds()/2

        # save session
        self.app.session_store.put(
            key,
            pickle.dumps(dict(self)),
            self.app.permanent_session_lifetime.total_seconds()
        )

        self._key = key

        self.new = False
        self.modified = False

        return set_cookie

    def needs_refresh(self):
        """ Checks if server side session needs to be refreshed. """

        return int(time.time()) > self.get('_refresh', 0)

class MailuSessionConfig:
    """ Stores sessions crypto config """

    # default size of session key parts
    uid_bits = 64 # default if SESSION_KEY_BITS is not set in config
    sid_bits = 128 # for now. must be multiple of 8!
    time_bits = 32 # for now. must be multiple of 8!

    def __init__(self, app=None):

        if app is None:
            app = flask.current_app

        bits = app.config.get('SESSION_KEY_BITS', self.uid_bits)
        if not 64 <= bits <= 256:
            raise ValueError('SESSION_KEY_BITS must be between 64 and 256!')

        uid_bytes = bits//8 + (bits%8>0)
        sid_bytes = self.sid_bits//8

        key = want_bytes(app.secret_key)

        self._hmac    = hmac.new(hmac.digest(key, b'SESSION_UID_HASH', digest='sha256'), digestmod='sha256')
        self._uid_len = uid_bytes
        self._uid_b64 = len(self._encode(bytes(uid_bytes)))
        self._sid_len = sid_bytes
        self._sid_b64 = len(self._encode(bytes(sid_bytes)))
        self._key_min = self._uid_b64 + self._sid_b64
        self._key_max = self._key_min + len(self._encode(bytes(self.time_bits//8)))

    def gen_sid(self):
        """ Generate random session id. """
        return self._encode(secrets.token_bytes(self._sid_len))

    def gen_uid(self, uid):
        """ Generate hashed user id part of session key. """
        _hmac = self._hmac.copy()
        _hmac.update(want_bytes(uid))
        return self._encode(_hmac.digest()[:self._uid_len])

    def gen_created(self, now=None):
        """ Generate base64 representation of creation time. """
        return self._encode(int(now or time.time()).to_bytes(8, byteorder='big').lstrip(b'\0'))

    def parse_key(self, key, app=None, validate=False, now=None):
        """ Split key into sid, uid and creation time. """

        if not (isinstance(key, bytes) and self._key_min <= len(key) <= self._key_max):
            return None

        uid = key[:self._uid_b64]
        sid = key[self._uid_b64:self._key_min]
        crt = key[self._key_min:]

        # validate if parts are decodeable
        created = self._decode(crt)
        if created is None or self._decode(uid) is None or self._decode(sid) is None:
            return None

        # validate creation time when requested or store does not support ttl
        if validate or not app.session_store.has_ttl:
            if now is None:
                now = int(time.time())
            created = int.from_bytes(created, byteorder='big')
            if not created < now < created + app.permanent_session_lifetime.total_seconds():
                return None

        return (uid, sid, crt)

    def _encode(self, value):
        return secrets.base64.urlsafe_b64encode(value).rstrip(b'=')

    def _decode(self, value):
        try:
            return secrets.base64.urlsafe_b64decode(value + b'='*(4-len(value)%4))
        except secrets.binascii.Error:
            return None

class MailuSessionInterface(SessionInterface):
    """ Custom flask session interface. """

    def open_session(self, app, request):
        """ Load or create session. """
        return MailuSession(request.cookies.get(app.config['SESSION_COOKIE_NAME'], None), app)

    def save_session(self, app, session, response):
        """ Save modified session. """

        # If the session is modified to be empty, remove the cookie.
        # If the session is empty, return without setting the cookie.
        if not session:
            if session.modified:
                session.delete()
                response.delete_cookie(
                    app.session_cookie_name,
                    domain=self.get_cookie_domain(app),
                    path=self.get_cookie_path(app),
                )
            return

        # Add a "Vary: Cookie" header if the session was accessed
        if session.accessed:
            response.vary.add('Cookie')

        set_cookie = session.permanent and app.config['SESSION_REFRESH_EACH_REQUEST']
        need_refresh = session.needs_refresh()

        # save modified session or refresh unmodified session
        if session.modified or need_refresh:
            set_cookie |= session.save()

        # set cookie on refreshed permanent sessions
        if need_refresh and session.permanent:
            set_cookie = True

        # set or update cookie if necessary
        if set_cookie:
            response.set_cookie(
                app.session_cookie_name,
                session.sid,
                expires=self.get_expiration_time(app, session),
                httponly=self.get_cookie_httponly(app),
                domain=self.get_cookie_domain(app),
                path=self.get_cookie_path(app),
                secure=self.get_cookie_secure(app),
                samesite=self.get_cookie_samesite(app)
            )

class MailuSessionExtension:
    """ Server side session handling """

    @staticmethod
    def cleanup_sessions(app=None):
        """ Remove invalid or expired sessions. """

        app = app or flask.current_app
        now = int(time.time())

        count = 0
        for key in app.session_store.list():
            if not app.session_config.parse_key(key, app, validate=True, now=now):
                app.session_store.delete(key)
                count += 1

        return count

    @staticmethod
    def prune_sessions(uid=None, keep=None, app=None):
        """ Remove sessions
            uid: remove all sessions (NONE) or sessions belonging to a specific user
            keep: keep listed sessions
        """

        keep = keep or set()
        app = app or flask.current_app

        prefix = None if uid is None else app.session_config.gen_uid(uid)

        count = 0
        for key in app.session_store.list(prefix):
            if key not in keep:
                app.session_store.delete(key)
                count += 1

        return count

    def init_app(self, app):
        """ Replace session management of application. """

        if app.config.get('MEMORY_SESSIONS'):
            # in-memory session store for use in development
            app.session_store = DictStore()

        else:
            # redis-based session store for use in production
            app.session_store = RedisStore(
                redis.StrictRedis().from_url(app.config['SESSION_STORAGE_URL'])
            )

            # clean expired sessions oonce on first use in case lifetime was changed
            def cleaner():
                with cleaned.get_lock():
                    if not cleaned.value:
                        cleaned.value = True
                        flask.current_app.logger.error('cleaning')
                        MailuSessionExtension.cleanup_sessions(app)

            app.before_first_request(cleaner)

        app.session_config = MailuSessionConfig(app)
        app.session_interface = MailuSessionInterface()

cleaned = Value('i', False)
session = MailuSessionExtension()