"""
A protocol is an endpoint for EWS service connections. It contains all necessary information to make HTTPS connections.

Protocols should be accessed through an Account, and are either created from a default Configuration or autodiscovered
when creating an Account.
"""
import datetime
import logging
from multiprocessing.pool import ThreadPool
import os
from threading import Lock
from queue import LifoQueue, Empty, Full

from cached_property import threaded_cached_property
import requests.adapters
import requests.sessions
import requests.utils
from oauthlib.oauth2 import BackendApplicationClient, WebApplicationClient
from requests_oauthlib import OAuth2Session

from .credentials import OAuth2AuthorizationCodeCredentials, OAuth2Credentials
from .errors import TransportError, SessionPoolMinSizeReached
from .properties import FreeBusyViewOptions, MailboxData, TimeWindow, TimeZone
from .services import GetServerTimeZones, GetRoomLists, GetRooms, ResolveNames, GetUserAvailability, \
    GetSearchableMailboxes, ExpandDL, ConvertId
from .transport import get_auth_instance, get_service_authtype, NTLM, GSSAPI, SSPI, OAUTH2, DEFAULT_HEADERS
from .version import Version, API_VERSIONS

log = logging.getLogger(__name__)


def close_connections():
    CachingProtocol.clear_cache()


class BaseProtocol:
    """Base class for Protocol which implements the bare essentials"""

    # The maximum number of sessions (== TCP connections, see below) we will open to this service endpoint. Keep this
    # low unless you have an agreement with the Exchange admin on the receiving end to hammer the server and
    # rate-limiting policies have been disabled for the connecting user.
    SESSION_POOLSIZE = 4
    # We want only 1 TCP connection per Session object. We may have lots of different credentials hitting the server and
    # each credential needs its own session (NTLM auth will only send credentials once and then secure the connection,
    # so a connection can only handle requests for one credential). Having multiple connections ser Session could
    # quickly exhaust the maximum number of concurrent connections the Exchange server allows from one client.
    CONNECTIONS_PER_SESSION = 1
    # Timeout for HTTP requests
    TIMEOUT = 120

    # The adapter class to use for HTTP requests. Override this if you need e.g. proxy support or specific TLS versions
    HTTP_ADAPTER_CLS = requests.adapters.HTTPAdapter

    # The User-Agent header to use for HTTP requests. Override this to set an app-specific one
    USERAGENT = None

    def __init__(self, config):
        from .configuration import Configuration
        if not isinstance(config, Configuration):
            raise ValueError("'config' %r must be a Configuration instance" % config)
        if not config.service_endpoint:
            raise AttributeError("'config.service_endpoint' must be set")
        self.config = config
        self._session_pool_size = self.SESSION_POOLSIZE

        # Autodetect authentication type if necessary
        if self.config.auth_type is None:
            self.config.auth_type = self.get_auth_type()

        # Try to behave nicely with the remote server. We want to keep the connection open between requests.
        # We also want to re-use sessions, to avoid the NTLM auth handshake on every request. We must know the
        # authentication method to create a session pool.
        self._session_pool = self._create_session_pool()
        self._session_pool_lock = Lock()

    @property
    def service_endpoint(self):
        return self.config.service_endpoint

    @property
    def auth_type(self):
        return self.config.auth_type

    @property
    def credentials(self):
        return self.config.credentials

    @credentials.setter
    def credentials(self, value):
        # We are updating credentials, but that doesn't automatically propagate to the session objects. The simplest
        # solution is to just kill the session pool and rebuild it.
        with self._session_pool_lock:
            self.config._credentials = value
            self.close()
            self._session_pool = self._create_session_pool()

    @property
    def retry_policy(self):
        return self.config.retry_policy

    @property
    def server(self):
        return self.config.server

    def __getstate__(self):
        # The session pool and lock cannot be pickled
        state = self.__dict__.copy()
        del state['_session_pool']
        del state['_session_pool_lock']
        return state

    def __setstate__(self, state):
        # Restore the session pool and lock
        self.__dict__.update(state)
        self._session_pool = self._create_session_pool()
        self._session_pool_lock = Lock()

    def __del__(self):
        # pylint: disable=bare-except
        try:
            self.close()
        except Exception:  # nosec
            # __del__ should never fail
            pass

    def close(self):
        log.debug('Server %s: Closing sessions', self.server)
        while True:
            try:
                self._session_pool.get(block=False).close()
            except Empty:
                break

    @classmethod
    def get_adapter(cls):
        # We want just one connection per session. No retries, since we wrap all requests in our own retry handler
        return cls.HTTP_ADAPTER_CLS(
            pool_block=True,
            pool_connections=cls.CONNECTIONS_PER_SESSION,
            pool_maxsize=cls.CONNECTIONS_PER_SESSION,
            max_retries=0,
        )

    def get_auth_type(self):
        # Autodetect and return authentication type
        raise NotImplementedError()

    @classmethod
    def get_useragent(cls):
        if not cls.USERAGENT:
            # import here to avoid a cyclic import
            from exchangelib import __version__
            cls.USERAGENT = "exchangelib/%s (%s)" % (__version__, requests.utils.default_user_agent())
        return cls.USERAGENT

    def _create_session_pool(self):
        # Create a pool to reuse sessions containing connections to the server
        session_pool = LifoQueue(maxsize=self._session_pool_size)
        for _ in range(self._session_pool_size):
            session_pool.put(self.create_session(), block=False)
        return session_pool

    @property
    def session_pool_size(self):
        return self._session_pool_size

    def decrease_poolsize(self):
        """Decreases the session pool size in response to error messages from the server requesting to rate-limit
        requests. We decrease by one session per call.
        """
        # Take a single session from the pool and discard it. We need to protect this with a lock while we are changing
        # the pool size variable, to avoid race conditions. We must keep at least one session in the pool.
        if self._session_pool_size <= 1:
            raise SessionPoolMinSizeReached('Session pool size cannot be decreased further')
        with self._session_pool_lock:
            if self._session_pool_size <= 1:
                log.debug('Session pool size was decreased in another thread')
                return
            log.warning('Lowering session pool size from %s to %s', self._session_pool_size,
                        self._session_pool_size - 1)
            self.get_session().close()
            self._session_pool_size -= 1

    def get_session(self):
        _timeout = 60  # Rate-limit messages about session starvation
        while True:
            try:
                log.debug('Server %s: Waiting for session', self.server)
                session = self._session_pool.get(timeout=_timeout)
                log.debug('Server %s: Got session %s', self.server, session.session_id)
                return session
            except Empty:
                # This is normal when we have many worker threads starving for available sessions
                log.debug('Server %s: No sessions available for %s seconds', self.server, _timeout)

    def release_session(self, session):
        # This should never fail, as we don't have more sessions than the queue contains
        log.debug('Server %s: Releasing session %s', self.server, session.session_id)
        try:
            self._session_pool.put(session, block=False)
        except Full:
            log.debug('Server %s: Session pool was already full %s', self.server, session.session_id)

    def retire_session(self, session):
        # The session is useless. Close it completely and place a fresh session in the pool
        log.debug('Server %s: Retiring session %s', self.server, session.session_id)
        session.close()
        del session
        self.release_session(self.create_session())

    def renew_session(self, session):
        # The session is useless. Close it completely and place a fresh session in the pool
        log.debug('Server %s: Renewing session %s', self.server, session.session_id)
        session.close()
        del session
        return self.create_session()

    def refresh_credentials(self, session):
        # Credentials need to be refreshed, probably due to an OAuth
        # access token expiring. If we've gotten here, it's because the
        # application didn't provide an OAuth client secret, so we can't
        # handle token refreshing for it.
        with self.credentials.lock:
            if hash(self.credentials) == session.credentials_hash:
                # Credentials have not been refreshed by another thread:
                # they're the same as the session was created with. If
                # this isn't the case, we can just go ahead with a new
                # session using the already-updated credentials.
                self.credentials.refresh()
        return self.renew_session(session)

    def create_session(self):
        with self.credentials.lock:
            if self.auth_type is None:
                raise ValueError('Cannot create session without knowing the auth type')
            if isinstance(self.credentials, OAuth2Credentials):
                session = self.create_oauth2_session()
            elif self.credentials:
                if self.auth_type == NTLM and self.credentials.type == self.credentials.EMAIL:
                    username = '\\' + self.credentials.username
                else:
                    username = self.credentials.username
                session = self.raw_session()
                session.auth = get_auth_instance(auth_type=self.auth_type, username=username,
                                                 password=self.credentials.password)
            else:
                if self.auth_type not in (GSSAPI, SSPI):
                    raise ValueError('Auth type %r requires credentials' % self.auth_type)
                session = self.raw_session()
                session.auth = get_auth_instance(auth_type=self.auth_type)
            # Keep track of the credentials used to create this session. If
            # and when we need to renew credentials (for example, refreshing
            # an OAuth access token), this lets us easily determine whether
            # the credentials have already been refreshed in another thread
            # by the time this session tries.
            session.credentials_hash = hash(self.credentials)
        # Add some extra info
        session.session_id = sum(map(ord, str(os.urandom(100))))  # Used for debugging messages in services
        session.protocol = self
        log.debug('Server %s: Created session %s', self.server, session.session_id)
        return session

    def create_oauth2_session(self):
        if self.auth_type != OAUTH2:
            raise ValueError('Auth type must be %r for credentials type OAuth2Credentials' % OAUTH2)

        has_token = False
        scope = ['https://outlook.office365.com/.default']
        session_params = {}
        token_params = {}

        if isinstance(self.credentials, OAuth2AuthorizationCodeCredentials):
            # Ask for a refresh token
            scope.append('offline_access')

            # We don't know (or need) the Microsoft tenant ID. Use
            # common/ to let Microsoft select the appropriate tenant
            # for the provided authorization code or refresh token.
            #
            # Suppress looks-like-password warning from Bandit.
            token_url = 'https://login.microsoftonline.com/common/oauth2/v2.0/token'  # nosec

            client_params = {}
            has_token = self.credentials.access_token is not None
            if has_token:
                session_params['token'] = self.credentials.access_token
            elif self.credentials.authorization_code is not None:
                token_params['code'] = self.credentials.authorization_code
                self.credentials.authorization_code = None

            if self.credentials.client_id is not None and self.credentials.client_secret is not None:
                # If we're given a client ID and secret, we have enough
                # to refresh access tokens ourselves. In other cases the
                # session will raise TokenExpiredError and we'll need to
                # ask the calling application to refresh the token (that
                # covers cases where the caller doesn't have access to
                # the client secret but is working with a service that
                # can provide it refreshed tokens on a limited basis).
                session_params.update({
                    'auto_refresh_kwargs': {
                        'client_id': self.credentials.client_id,
                        'client_secret': self.credentials.client_secret,
                    },
                    'auto_refresh_url': token_url,
                    'token_updater': self.credentials.on_token_auto_refreshed,
                })
            client = WebApplicationClient(self.credentials.client_id, **client_params)
        else:
            token_url = 'https://login.microsoftonline.com/%s/oauth2/v2.0/token' % self.credentials.tenant_id
            client = BackendApplicationClient(client_id=self.credentials.client_id)

        session = self.raw_session(client, session_params)
        if not has_token:
            # Fetch the token explicitly -- it doesn't occur implicitly
            token = session.fetch_token(token_url=token_url, client_id=self.credentials.client_id,
                                        client_secret=self.credentials.client_secret, scope=scope,
                                        **token_params)
            # Allow the credentials object to update its copy of the new
            # token, and give the application an opportunity to cache it
            self.credentials.on_token_auto_refreshed(token)
        session.auth = get_auth_instance(auth_type=OAUTH2, client=client)

        return session

    @classmethod
    def raw_session(cls, oauth2_client=None, oauth2_session_params=None):
        if oauth2_client:
            session = OAuth2Session(client=oauth2_client, **(oauth2_session_params or {}))
        else:
            session = requests.sessions.Session()
        session.headers.update(DEFAULT_HEADERS)
        session.headers["User-Agent"] = cls.get_useragent()
        session.mount('http://', adapter=cls.get_adapter())
        session.mount('https://', adapter=cls.get_adapter())
        return session

    def __repr__(self):
        return self.__class__.__name__ + repr((self.service_endpoint, self.credentials, self.auth_type))


class CachingProtocol(type):
    _protocol_cache = {}
    _protocol_cache_lock = Lock()

    def __call__(cls, *args, **kwargs):
        # Cache Protocol instances that point to the same endpoint and use the same credentials. This ensures that we
        # re-use thread and connection pools etc. instead of flooding the remote server. This is a modified Singleton
        # pattern.
        #
        # We ignore auth_type from kwargs in the cache key. We trust caller to supply the correct auth_type - otherwise
        # __init__ will guess the correct auth type.

        # We may be using multiple different credentials and changing our minds on TLS verification. This key
        # combination should be safe.
        _protocol_cache_key = kwargs['config'].service_endpoint, kwargs['config'].credentials

        protocol = cls._protocol_cache.get(_protocol_cache_key)
        if isinstance(protocol, Exception):
            # The input data leads to a TransportError. Re-throw
            raise protocol
        if protocol is not None:
            return protocol

        # Acquire lock to guard against multiple threads competing to cache information. Having a per-server lock is
        # probably overkill although it would reduce lock contention.
        log.debug('Waiting for _protocol_cache_lock')
        with cls._protocol_cache_lock:
            protocol = cls._protocol_cache.get(_protocol_cache_key)
            if isinstance(protocol, Exception):
                # Someone got ahead of us while holding the lock, but the input data leads to a TransportError. Re-throw
                raise protocol
            if protocol is not None:
                # Someone got ahead of us while holding the lock
                return protocol
            log.debug("Protocol __call__ cache miss. Adding key '%s'", str(_protocol_cache_key))
            try:
                protocol = super().__call__(*args, **kwargs)
            except TransportError as e:
                # This can happen if, for example, autodiscover supplies us with a bogus EWS endpoint
                log.warning('Failed to create cached protocol with key %s: %s', _protocol_cache_key, e)
                cls._protocol_cache[_protocol_cache_key] = e
                raise e
            cls._protocol_cache[_protocol_cache_key] = protocol
        return protocol

    @classmethod
    def clear_cache(mcs):
        for key, protocol in mcs._protocol_cache.items():
            if isinstance(protocol, Exception):
                continue
            service_endpoint = key[0]
            log.debug("Service endpoint '%s': Closing sessions", service_endpoint)
            protocol.close()
        mcs._protocol_cache.clear()


class Protocol(BaseProtocol, metaclass=CachingProtocol):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self._api_version_hint = None
        self._version_lock = Lock()

    def get_auth_type(self):
        # Autodetect authentication type. We also set version hint here.
        name = str(self.credentials) if self.credentials and str(self.credentials) else 'DUMMY'
        auth_type, api_version_hint = get_service_authtype(
            service_endpoint=self.service_endpoint, retry_policy=self.retry_policy, api_versions=API_VERSIONS, name=name
        )
        self._api_version_hint = api_version_hint
        return auth_type

    @property
    def version(self):
        # Make sure only one thread does the guessing.
        if not self.config.version or not self.config.version.build:
            with self._version_lock:
                if not self.config.version or not self.config.version.build:
                    # Version.guess() needs auth objects and a working session pool
                    self.config.version = Version.guess(self, api_version_hint=self._api_version_hint)
        return self.config.version

    @threaded_cached_property
    def thread_pool(self):
        # Used by services to process service requests that are able to run in parallel. Thread pool should be
        # larger than the connection pool so we have time to process data without idling the connection.
        # Create the pool as the last thing here, since we may fail in the version or auth type guessing, which would
        # leave open threads around to be garbage collected.
        thread_poolsize = 4 * self._session_pool_size
        return ThreadPool(processes=thread_poolsize)

    def close(self):
        log.debug('Server %s: Closing thread pool', self.server)
        # Close the thread pool before closing the session pool to ensure all sessions are released.
        if "thread_pool" in self.__dict__:
            # Calling thread_pool.join() in Python 3.8 will hang forever. This is seen when running a test case that
            # uses the thread pool, e.g.: python tests/__init__.py MessagesTest.test_export_with_error
            # I don't know yet why this is happening.
            self.thread_pool.terminate()
            del self.__dict__["thread_pool"]
        super().close()

    def get_timezones(self, timezones=None, return_full_timezone_data=False):
        """ Get timezone definitions from the server

        :param timezones: A list of EWSDateTime instances. If None, fetches all timezones from server
        :param return_full_timezone_data: If true, also returns periods and transitions
        :return: A list of (tz_id, name, periods, transitions) tuples
        """
        return GetServerTimeZones(protocol=self).call(
            timezones=timezones, return_full_timezone_data=return_full_timezone_data
        )

    def get_free_busy_info(self, accounts, start, end, merged_free_busy_interval=30, requested_view='DetailedMerged'):
        """ Returns free/busy information for a list of accounts

        :param accounts: A list of (account, attendee_type, exclude_conflicts) tuples, where account is an Account
               object, attendee_type is a MailboxData.attendee_type choice, and exclude_conflicts is a boolean.
        :param start: The start datetime of the request
        :param end: The end datetime of the request
        :param merged_free_busy_interval: The interval, in minutes, of merged free/busy information
        :param requested_view: The type of information returned. Possible values are defined in the
               FreeBusyViewOptions.requested_view choices.
        :return: A generator of FreeBusyView objects
        """
        from .account import Account
        for account, attendee_type, exclude_conflicts in accounts:
            if not isinstance(account, Account):
                raise ValueError("'accounts' item %r must be an 'Account' instance" % account)
            if attendee_type not in MailboxData.ATTENDEE_TYPES:
                raise ValueError("'accounts' item %r must be one of %s" % (attendee_type, MailboxData.ATTENDEE_TYPES))
            if not isinstance(exclude_conflicts, bool):
                raise ValueError("'accounts' item %r must be a 'bool' instance" % exclude_conflicts)
        if start >= end:
            raise ValueError("'start' must be less than 'end' (%s -> %s)" % (start, end))
        if not isinstance(merged_free_busy_interval, int):
            raise ValueError("'merged_free_busy_interval' value %r must be an 'int'" % merged_free_busy_interval)
        if requested_view not in FreeBusyViewOptions.REQUESTED_VIEWS:
            raise ValueError(
                "'requested_view' value %r must be one of %s" % (requested_view, FreeBusyViewOptions.REQUESTED_VIEWS))
        _, _, periods, transitions, transitions_groups = list(self.get_timezones(
            timezones=[start.tzinfo],
            return_full_timezone_data=True
        ))[0]
        return GetUserAvailability(self).call(
                timezone=TimeZone.from_server_timezone(
                    periods=periods,
                    transitions=transitions,
                    transitionsgroups=transitions_groups,
                    for_year=start.year
                ),
                mailbox_data=[MailboxData(
                    email=account.primary_smtp_address,
                    attendee_type=attendee_type,
                    exclude_conflicts=exclude_conflicts
                ) for account, attendee_type, exclude_conflicts in accounts],
                free_busy_view_options=FreeBusyViewOptions(
                    time_window=TimeWindow(start=start, end=end),
                    merged_free_busy_interval=merged_free_busy_interval,
                    requested_view=requested_view,
                ),
        )

    def get_roomlists(self):
        return GetRoomLists(protocol=self).call()

    def get_rooms(self, roomlist):
        from .properties import RoomList
        return GetRooms(protocol=self).call(roomlist=RoomList(email_address=roomlist))

    def resolve_names(self, names, return_full_contact_data=False, search_scope=None, shape=None):
        """ Resolve accounts on the server using partial account data, e.g. an email address or initials

        :param names: A list of identifiers to query
        :param return_full_contact_data: If True, returns full contact data
        :param search_scope: The scope to perform the search. Must be one of SEARCH_SCOPE_CHOICES
        :param shape:
        :return: A list of Mailbox items or, if return_full_contact_data is True, tuples of (Mailbox, Contact) items
        """
        from .items import SHAPE_CHOICES, SEARCH_SCOPE_CHOICES
        if search_scope:
            if search_scope not in SEARCH_SCOPE_CHOICES:
                raise ValueError("'search_scope' %s must be one if %s" % (search_scope, SEARCH_SCOPE_CHOICES))
        if shape:
            if shape not in SHAPE_CHOICES:
                raise ValueError("'shape' %s must be one if %s" % (shape, SHAPE_CHOICES))
        return list(ResolveNames(protocol=self).call(
            unresolved_entries=names, return_full_contact_data=return_full_contact_data, search_scope=search_scope,
            contact_data_shape=shape,
        ))

    def expand_dl(self, distribution_list):
        """ Expand distribution list into it's members

        :param distribution_list: SMTP address of the distribution list to expand, or a DLMailbox representing the list
        :return: List of Mailbox items that are members of the distribution list
        """
        from .properties import DLMailbox
        if isinstance(distribution_list, str):
            distribution_list = DLMailbox(email_address=distribution_list, mailbox_type='PublicDL')
        return list(ExpandDL(protocol=self).call(distribution_list=distribution_list))

    def get_searchable_mailboxes(self, search_filter=None, expand_group_membership=False):
        """This method is only available to users who have been assigned the Discovery Management RBAC role. See
        https://docs.microsoft.com/en-us/exchange/permissions-exo/permissions-exo

        :param search_filter: Is set, must be a single email alias
        :param expand_group_membership: If True, returned distribution lists are expanded
        :return: a list of SearchableMailbox, FailedMailbox or Exception instances
        """
        return list(GetSearchableMailboxes(protocol=self).call(
            search_filter=search_filter,
            expand_group_membership=expand_group_membership,
        ))

    def convert_ids(self, ids, destination_format):
        """Converts item and folder IDs between multiple formats

        :param ids: a list of AlternateId, AlternatePublicFolderId or AlternatePublicFolderItemId instances
        :param destination_format: A string
        :return: a generator of AlternateId, AlternatePublicFolderId or AlternatePublicFolderItemId instances
        """
        from .properties import ID_FORMATS, AlternateId, AlternatePublicFolderId, AlternatePublicFolderItemId
        if destination_format not in ID_FORMATS:
            raise ValueError("'destination_format' %r must be one of %s" % (destination_format, ID_FORMATS))
        cls_map = {cls.response_tag(): cls for cls in (
            AlternateId, AlternatePublicFolderId, AlternatePublicFolderItemId
        )}
        for i in ConvertId(protocol=self).call(items=ids, destination_format=destination_format):
            if isinstance(i, Exception):
                yield i
            else:
                id_cls = cls_map[i.tag]
                yield id_cls.from_xml(i, account=None)

    def __getstate__(self):
        # The lock and thread pool cannot be pickled
        state = super().__getstate__()
        del state['_version_lock']
        try:
            del state['thread_pool']
        except KeyError:
            # thread_pool is a cached property and may not exist
            pass
        return state

    def __setstate__(self, state):
        # Restore the lock. The thread pool is a cached property and will be recreated automatically.
        self.__dict__.update(state)
        self._version_lock = Lock()

    def __str__(self):
        # Don't trigger version guessing here just for the sake of printing
        if self.config.version:
            fullname, api_version, build = self.version.fullname, self.version.api_version, self.version.build
        else:
            fullname, api_version, build = '[unknown]', '[unknown]', '[unknown]'

        return '''\
EWS url: %s
Product name: %s
EWS API version: %s
Build number: %s
EWS auth: %s''' % (self.service_endpoint, fullname, api_version, build, self.auth_type)


class NoVerifyHTTPAdapter(requests.adapters.HTTPAdapter):
    """An HTTP adapter that ignores TLS validation errors. Use at own risk."""
    def cert_verify(self, conn, url, verify, cert):
        # pylint: disable=unused-argument
        # We're overiding a method so we have to keep the signature
        super().cert_verify(conn=conn, url=url, verify=False, cert=cert)


class RetryPolicy:
    """Stores retry logic used when faced with errors from the server"""
    @property
    def fail_fast(self):
        # Used to choose the error handling policy. When True, a fault-tolerant policy is used. False, a fail-fast
        # policy is used.
        raise NotImplementedError()

    @property
    def back_off_until(self):
        raise NotImplementedError()

    @back_off_until.setter
    def back_off_until(self, value):
        raise NotImplementedError()


class FailFast(RetryPolicy):
    """Fail immediately on server errors"""
    @property
    def fail_fast(self):
        return True

    @property
    def back_off_until(self):
        return None


class FaultTolerance(RetryPolicy):
    """Enables fault-tolerant error handling. Tells internal methods to do an exponential back off when requests start
    failing, and wait up to max_wait seconds before failing.
    """
    def __init__(self, max_wait=3600):
        self.max_wait = max_wait
        self._back_off_until = None
        self._back_off_lock = Lock()

    def __getstate__(self):
        # Locks cannot be pickled
        state = self.__dict__.copy()
        del state['_back_off_lock']
        return state

    def __setstate__(self, state):
        # Restore the lock
        self.__dict__.update(state)
        self._back_off_lock = Lock()

    @property
    def fail_fast(self):
        return False

    @property
    def back_off_until(self):
        """Returns the back off value as a datetime. Resets the current back off value if it has expired."""
        if self._back_off_until is None:
            return None
        with self._back_off_lock:
            if self._back_off_until is None:
                return None
            if self._back_off_until < datetime.datetime.now():
                self._back_off_until = None  # The back off value has expired. Reset
                return None
            return self._back_off_until

    @back_off_until.setter
    def back_off_until(self, value):
        with self._back_off_lock:
            self._back_off_until = value

    def back_off(self, seconds):
        if seconds is None:
            seconds = 60  # Back off 60 seconds if we didn't get an explicit suggested value
        value = datetime.datetime.now() + datetime.timedelta(seconds=seconds)
        with self._back_off_lock:
            self._back_off_until = value
