django/tests/sessions_tests/tests.py

import base64
import os
import shutil
import string
import tempfile
import unittest
from datetime import timedelta
from http import cookies

from django.conf import settings
from django.contrib.sessions.backends.base import UpdateError
from django.contrib.sessions.backends.cache import SessionStore as CacheSession
from django.contrib.sessions.backends.cached_db import (
    SessionStore as CacheDBSession,
)
from django.contrib.sessions.backends.db import SessionStore as DatabaseSession
from django.contrib.sessions.backends.file import SessionStore as FileSession
from django.contrib.sessions.backends.signed_cookies import (
    SessionStore as CookieSession,
)
from django.contrib.sessions.exceptions import InvalidSessionKey
from django.contrib.sessions.middleware import SessionMiddleware
from django.contrib.sessions.models import Session
from django.contrib.sessions.serializers import (
    JSONSerializer, PickleSerializer,
)
from django.core import management
from django.core.cache import caches
from django.core.cache.backends.base import InvalidCacheBackendError
from django.core.exceptions import ImproperlyConfigured, SuspiciousOperation
from django.http import HttpResponse
from django.test import (
    RequestFactory, TestCase, ignore_warnings, override_settings,
)
from django.utils import timezone

from .models import SessionStore as CustomDatabaseSession


class SessionTestsMixin:
    # This does not inherit from TestCase to avoid any tests being run with this
    # class, which wouldn't work, and to allow different TestCase subclasses to
    # be used.

    backend = None  # subclasses must specify

    def setUp(self):
        self.session = self.backend()

    def tearDown(self):
        # NB: be careful to delete any sessions created; stale sessions fill up
        # the /tmp (with some backends) and eventually overwhelm it after lots
        # of runs (think buildbots)
        self.session.delete()

    def test_new_session(self):
        self.assertIs(self.session.modified, False)
        self.assertIs(self.session.accessed, False)

    def test_get_empty(self):
        self.assertIsNone(self.session.get('cat'))

    def test_store(self):
        self.session['cat'] = "dog"
        self.assertIs(self.session.modified, True)
        self.assertEqual(self.session.pop('cat'), 'dog')

    def test_pop(self):
        self.session['some key'] = 'exists'
        # Need to reset these to pretend we haven't accessed it:
        self.accessed = False
        self.modified = False

        self.assertEqual(self.session.pop('some key'), 'exists')
        self.assertIs(self.session.accessed, True)
        self.assertIs(self.session.modified, True)
        self.assertIsNone(self.session.get('some key'))

    def test_pop_default(self):
        self.assertEqual(self.session.pop('some key', 'does not exist'),
                         'does not exist')
        self.assertIs(self.session.accessed, True)
        self.assertIs(self.session.modified, False)

    def test_pop_default_named_argument(self):
        self.assertEqual(self.session.pop('some key', default='does not exist'), 'does not exist')
        self.assertIs(self.session.accessed, True)
        self.assertIs(self.session.modified, False)

    def test_pop_no_default_keyerror_raised(self):
        with self.assertRaises(KeyError):
            self.session.pop('some key')

    def test_setdefault(self):
        self.assertEqual(self.session.setdefault('foo', 'bar'), 'bar')
        self.assertEqual(self.session.setdefault('foo', 'baz'), 'bar')
        self.assertIs(self.session.accessed, True)
        self.assertIs(self.session.modified, True)

    def test_update(self):
        self.session.update({'update key': 1})
        self.assertIs(self.session.accessed, True)
        self.assertIs(self.session.modified, True)
        self.assertEqual(self.session.get('update key', None), 1)

    def test_has_key(self):
        self.session['some key'] = 1
        self.session.modified = False
        self.session.accessed = False
        self.assertIn('some key', self.session)
        self.assertIs(self.session.accessed, True)
        self.assertIs(self.session.modified, False)

    def test_values(self):
        self.assertEqual(list(self.session.values()), [])
        self.assertIs(self.session.accessed, True)
        self.session['some key'] = 1
        self.session.modified = False
        self.session.accessed = False
        self.assertEqual(list(self.session.values()), [1])
        self.assertIs(self.session.accessed, True)
        self.assertIs(self.session.modified, False)

    def test_keys(self):
        self.session['x'] = 1
        self.session.modified = False
        self.session.accessed = False
        self.assertEqual(list(self.session.keys()), ['x'])
        self.assertIs(self.session.accessed, True)
        self.assertIs(self.session.modified, False)

    def test_items(self):
        self.session['x'] = 1
        self.session.modified = False
        self.session.accessed = False
        self.assertEqual(list(self.session.items()), [('x', 1)])
        self.assertIs(self.session.accessed, True)
        self.assertIs(self.session.modified, False)

    def test_clear(self):
        self.session['x'] = 1
        self.session.modified = False
        self.session.accessed = False
        self.assertEqual(list(self.session.items()), [('x', 1)])
        self.session.clear()
        self.assertEqual(list(self.session.items()), [])
        self.assertIs(self.session.accessed, True)
        self.assertIs(self.session.modified, True)

    def test_save(self):
        self.session.save()
        self.assertIs(self.session.exists(self.session.session_key), True)

    def test_delete(self):
        self.session.save()
        self.session.delete(self.session.session_key)
        self.assertIs(self.session.exists(self.session.session_key), False)

    def test_flush(self):
        self.session['foo'] = 'bar'
        self.session.save()
        prev_key = self.session.session_key
        self.session.flush()
        self.assertIs(self.session.exists(prev_key), False)
        self.assertNotEqual(self.session.session_key, prev_key)
        self.assertIsNone(self.session.session_key)
        self.assertIs(self.session.modified, True)
        self.assertIs(self.session.accessed, True)

    def test_cycle(self):
        self.session['a'], self.session['b'] = 'c', 'd'
        self.session.save()
        prev_key = self.session.session_key
        prev_data = list(self.session.items())
        self.session.cycle_key()
        self.assertIs(self.session.exists(prev_key), False)
        self.assertNotEqual(self.session.session_key, prev_key)
        self.assertEqual(list(self.session.items()), prev_data)

    def test_cycle_with_no_session_cache(self):
        self.session['a'], self.session['b'] = 'c', 'd'
        self.session.save()
        prev_data = self.session.items()
        self.session = self.backend(self.session.session_key)
        self.assertIs(hasattr(self.session, '_session_cache'), False)
        self.session.cycle_key()
        self.assertCountEqual(self.session.items(), prev_data)

    def test_save_doesnt_clear_data(self):
        self.session['a'] = 'b'
        self.session.save()
        self.assertEqual(self.session['a'], 'b')

    def test_invalid_key(self):
        # Submitting an invalid session key (either by guessing, or if the db has
        # removed the key) results in a new key being generated.
        try:
            session = self.backend('1')
            session.save()
            self.assertNotEqual(session.session_key, '1')
            self.assertIsNone(session.get('cat'))
            session.delete()
        finally:
            # Some backends leave a stale cache entry for the invalid
            # session key; make sure that entry is manually deleted
            session.delete('1')

    def test_session_key_empty_string_invalid(self):
        """Falsey values (Such as an empty string) are rejected."""
        self.session._session_key = ''
        self.assertIsNone(self.session.session_key)

    def test_session_key_too_short_invalid(self):
        """Strings shorter than 8 characters are rejected."""
        self.session._session_key = '1234567'
        self.assertIsNone(self.session.session_key)

    def test_session_key_valid_string_saved(self):
        """Strings of length 8 and up are accepted and stored."""
        self.session._session_key = '12345678'
        self.assertEqual(self.session.session_key, '12345678')

    def test_session_key_is_read_only(self):
        def set_session_key(session):
            session.session_key = session._get_new_session_key()
        with self.assertRaises(AttributeError):
            set_session_key(self.session)

    # Custom session expiry
    def test_default_expiry(self):
        # A normal session has a max age equal to settings
        self.assertEqual(self.session.get_expiry_age(), settings.SESSION_COOKIE_AGE)

        # So does a custom session with an idle expiration time of 0 (but it'll
        # expire at browser close)
        self.session.set_expiry(0)
        self.assertEqual(self.session.get_expiry_age(), settings.SESSION_COOKIE_AGE)

    def test_custom_expiry_seconds(self):
        modification = timezone.now()

        self.session.set_expiry(10)

        date = self.session.get_expiry_date(modification=modification)
        self.assertEqual(date, modification + timedelta(seconds=10))

        age = self.session.get_expiry_age(modification=modification)
        self.assertEqual(age, 10)

    def test_custom_expiry_timedelta(self):
        modification = timezone.now()

        # Mock timezone.now, because set_expiry calls it on this code path.
        original_now = timezone.now
        try:
            timezone.now = lambda: modification
            self.session.set_expiry(timedelta(seconds=10))
        finally:
            timezone.now = original_now

        date = self.session.get_expiry_date(modification=modification)
        self.assertEqual(date, modification + timedelta(seconds=10))

        age = self.session.get_expiry_age(modification=modification)
        self.assertEqual(age, 10)

    def test_custom_expiry_datetime(self):
        modification = timezone.now()

        self.session.set_expiry(modification + timedelta(seconds=10))

        date = self.session.get_expiry_date(modification=modification)
        self.assertEqual(date, modification + timedelta(seconds=10))

        age = self.session.get_expiry_age(modification=modification)
        self.assertEqual(age, 10)

    def test_custom_expiry_reset(self):
        self.session.set_expiry(None)
        self.session.set_expiry(10)
        self.session.set_expiry(None)
        self.assertEqual(self.session.get_expiry_age(), settings.SESSION_COOKIE_AGE)

    def test_get_expire_at_browser_close(self):
        # Tests get_expire_at_browser_close with different settings and different
        # set_expiry calls
        with override_settings(SESSION_EXPIRE_AT_BROWSER_CLOSE=False):
            self.session.set_expiry(10)
            self.assertIs(self.session.get_expire_at_browser_close(), False)

            self.session.set_expiry(0)
            self.assertIs(self.session.get_expire_at_browser_close(), True)

            self.session.set_expiry(None)
            self.assertIs(self.session.get_expire_at_browser_close(), False)

        with override_settings(SESSION_EXPIRE_AT_BROWSER_CLOSE=True):
            self.session.set_expiry(10)
            self.assertIs(self.session.get_expire_at_browser_close(), False)

            self.session.set_expiry(0)
            self.assertIs(self.session.get_expire_at_browser_close(), True)

            self.session.set_expiry(None)
            self.assertIs(self.session.get_expire_at_browser_close(), True)

    def test_decode(self):
        # Ensure we can decode what we encode
        data = {'a test key': 'a test value'}
        encoded = self.session.encode(data)
        self.assertEqual(self.session.decode(encoded), data)

    def test_decode_failure_logged_to_security(self):
        bad_encode = base64.b64encode(b'flaskdj:alkdjf').decode('ascii')
        with self.assertLogs('django.security.SuspiciousSession', 'WARNING') as cm:
            self.assertEqual({}, self.session.decode(bad_encode))
        # The failed decode is logged.
        self.assertIn('corrupted', cm.output[0])

    def test_actual_expiry(self):
        # this doesn't work with JSONSerializer (serializing timedelta)
        with override_settings(SESSION_SERIALIZER='django.contrib.sessions.serializers.PickleSerializer'):
            self.session = self.backend()  # reinitialize after overriding settings

            # Regression test for #19200
            old_session_key = None
            new_session_key = None
            try:
                self.session['foo'] = 'bar'
                self.session.set_expiry(-timedelta(seconds=10))
                self.session.save()
                old_session_key = self.session.session_key
                # With an expiry date in the past, the session expires instantly.
                new_session = self.backend(self.session.session_key)
                new_session_key = new_session.session_key
                self.assertNotIn('foo', new_session)
            finally:
                self.session.delete(old_session_key)
                self.session.delete(new_session_key)

    def test_session_load_does_not_create_record(self):
        """
        Loading an unknown session key does not create a session record.

        Creating session records on load is a DOS vulnerability.
        """
        session = self.backend('someunknownkey')
        session.load()

        self.assertIsNone(session.session_key)
        self.assertIs(session.exists(session.session_key), False)
        # provided unknown key was cycled, not reused
        self.assertNotEqual(session.session_key, 'someunknownkey')

    def test_session_save_does_not_resurrect_session_logged_out_in_other_context(self):
        """
        Sessions shouldn't be resurrected by a concurrent request.
        """
        # Create new session.
        s1 = self.backend()
        s1['test_data'] = 'value1'
        s1.save(must_create=True)

        # Logout in another context.
        s2 = self.backend(s1.session_key)
        s2.delete()

        # Modify session in first context.
        s1['test_data'] = 'value2'
        with self.assertRaises(UpdateError):
            # This should throw an exception as the session is deleted, not
            # resurrect the session.
            s1.save()

        self.assertEqual(s1.load(), {})


class DatabaseSessionTests(SessionTestsMixin, TestCase):

    backend = DatabaseSession
    session_engine = 'django.contrib.sessions.backends.db'

    @property
    def model(self):
        return self.backend.get_model_class()

    def test_session_str(self):
        "Session repr should be the session key."
        self.session['x'] = 1
        self.session.save()

        session_key = self.session.session_key
        s = self.model.objects.get(session_key=session_key)

        self.assertEqual(str(s), session_key)

    def test_session_get_decoded(self):
        """
        Test we can use Session.get_decoded to retrieve data stored
        in normal way
        """
        self.session['x'] = 1
        self.session.save()

        s = self.model.objects.get(session_key=self.session.session_key)

        self.assertEqual(s.get_decoded(), {'x': 1})

    def test_sessionmanager_save(self):
        """
        Test SessionManager.save method
        """
        # Create a session
        self.session['y'] = 1
        self.session.save()

        s = self.model.objects.get(session_key=self.session.session_key)
        # Change it
        self.model.objects.save(s.session_key, {'y': 2}, s.expire_date)
        # Clear cache, so that it will be retrieved from DB
        del self.session._session_cache
        self.assertEqual(self.session['y'], 2)

    def test_clearsessions_command(self):
        """
        Test clearsessions command for clearing expired sessions.
        """
        self.assertEqual(0, self.model.objects.count())

        # One object in the future
        self.session['foo'] = 'bar'
        self.session.set_expiry(3600)
        self.session.save()

        # One object in the past
        other_session = self.backend()
        other_session['foo'] = 'bar'
        other_session.set_expiry(-3600)
        other_session.save()

        # Two sessions are in the database before clearsessions...
        self.assertEqual(2, self.model.objects.count())
        with override_settings(SESSION_ENGINE=self.session_engine):
            management.call_command('clearsessions')
        # ... and one is deleted.
        self.assertEqual(1, self.model.objects.count())


@override_settings(USE_TZ=True)
class DatabaseSessionWithTimeZoneTests(DatabaseSessionTests):
    pass


class CustomDatabaseSessionTests(DatabaseSessionTests):
    backend = CustomDatabaseSession
    session_engine = 'sessions_tests.models'

    def test_extra_session_field(self):
        # Set the account ID to be picked up by a custom session storage
        # and saved to a custom session model database column.
        self.session['_auth_user_id'] = 42
        self.session.save()

        # Make sure that the customized create_model_instance() was called.
        s = self.model.objects.get(session_key=self.session.session_key)
        self.assertEqual(s.account_id, 42)

        # Make the session "anonymous".
        self.session.pop('_auth_user_id')
        self.session.save()

        # Make sure that save() on an existing session did the right job.
        s = self.model.objects.get(session_key=self.session.session_key)
        self.assertIsNone(s.account_id)


class CacheDBSessionTests(SessionTestsMixin, TestCase):

    backend = CacheDBSession

    def test_exists_searches_cache_first(self):
        self.session.save()
        with self.assertNumQueries(0):
            self.assertIs(self.session.exists(self.session.session_key), True)

    # Some backends might issue a warning
    @ignore_warnings(module="django.core.cache.backends.base")
    def test_load_overlong_key(self):
        self.session._session_key = (string.ascii_letters + string.digits) * 20
        self.assertEqual(self.session.load(), {})

    @override_settings(SESSION_CACHE_ALIAS='sessions')
    def test_non_default_cache(self):
        # 21000 - CacheDB backend should respect SESSION_CACHE_ALIAS.
        with self.assertRaises(InvalidCacheBackendError):
            self.backend()


@override_settings(USE_TZ=True)
class CacheDBSessionWithTimeZoneTests(CacheDBSessionTests):
    pass


# Don't need DB flushing for these tests, so can use unittest.TestCase as base class
class FileSessionTests(SessionTestsMixin, unittest.TestCase):

    backend = FileSession

    def setUp(self):
        # Do file session tests in an isolated directory, and kill it after we're done.
        self.original_session_file_path = settings.SESSION_FILE_PATH
        self.temp_session_store = settings.SESSION_FILE_PATH = tempfile.mkdtemp()
        # Reset the file session backend's internal caches
        if hasattr(self.backend, '_storage_path'):
            del self.backend._storage_path
        super().setUp()

    def tearDown(self):
        super().tearDown()
        settings.SESSION_FILE_PATH = self.original_session_file_path
        shutil.rmtree(self.temp_session_store)

    @override_settings(
        SESSION_FILE_PATH='/if/this/directory/exists/you/have/a/weird/computer',
    )
    def test_configuration_check(self):
        del self.backend._storage_path
        # Make sure the file backend checks for a good storage dir
        with self.assertRaises(ImproperlyConfigured):
            self.backend()

    def test_invalid_key_backslash(self):
        # Ensure we don't allow directory-traversal.
        # This is tested directly on _key_to_file, as load() will swallow
        # a SuspiciousOperation in the same way as an OSError - by creating
        # a new session, making it unclear whether the slashes were detected.
        with self.assertRaises(InvalidSessionKey):
            self.backend()._key_to_file("a\\b\\c")

    def test_invalid_key_forwardslash(self):
        # Ensure we don't allow directory-traversal
        with self.assertRaises(InvalidSessionKey):
            self.backend()._key_to_file("a/b/c")

    @override_settings(
        SESSION_ENGINE="django.contrib.sessions.backends.file",
        SESSION_COOKIE_AGE=0,
    )
    def test_clearsessions_command(self):
        """
        Test clearsessions command for clearing expired sessions.
        """
        storage_path = self.backend._get_storage_path()
        file_prefix = settings.SESSION_COOKIE_NAME

        def count_sessions():
            return len([
                session_file for session_file in os.listdir(storage_path)
                if session_file.startswith(file_prefix)
            ])

        self.assertEqual(0, count_sessions())

        # One object in the future
        self.session['foo'] = 'bar'
        self.session.set_expiry(3600)
        self.session.save()

        # One object in the past
        other_session = self.backend()
        other_session['foo'] = 'bar'
        other_session.set_expiry(-3600)
        other_session.save()

        # One object in the present without an expiry (should be deleted since
        # its modification time + SESSION_COOKIE_AGE will be in the past when
        # clearsessions runs).
        other_session2 = self.backend()
        other_session2['foo'] = 'bar'
        other_session2.save()

        # Three sessions are in the filesystem before clearsessions...
        self.assertEqual(3, count_sessions())
        management.call_command('clearsessions')
        # ... and two are deleted.
        self.assertEqual(1, count_sessions())


class CacheSessionTests(SessionTestsMixin, unittest.TestCase):

    backend = CacheSession

    # Some backends might issue a warning
    @ignore_warnings(module="django.core.cache.backends.base")
    def test_load_overlong_key(self):
        self.session._session_key = (string.ascii_letters + string.digits) * 20
        self.assertEqual(self.session.load(), {})

    def test_default_cache(self):
        self.session.save()
        self.assertIsNotNone(caches['default'].get(self.session.cache_key))

    @override_settings(CACHES={
        'default': {
            'BACKEND': 'django.core.cache.backends.dummy.DummyCache',
        },
        'sessions': {
            'BACKEND': 'django.core.cache.backends.locmem.LocMemCache',
            'LOCATION': 'session',
        },
    }, SESSION_CACHE_ALIAS='sessions')
    def test_non_default_cache(self):
        # Re-initialize the session backend to make use of overridden settings.
        self.session = self.backend()

        self.session.save()
        self.assertIsNone(caches['default'].get(self.session.cache_key))
        self.assertIsNotNone(caches['sessions'].get(self.session.cache_key))

    def test_create_and_save(self):
        self.session = self.backend()
        self.session.create()
        self.session.save()
        self.assertIsNotNone(caches['default'].get(self.session.cache_key))


class SessionMiddlewareTests(TestCase):
    request_factory = RequestFactory()

    @override_settings(SESSION_COOKIE_SECURE=True)
    def test_secure_session_cookie(self):
        request = self.request_factory.get('/')
        response = HttpResponse('Session test')
        middleware = SessionMiddleware()

        # Simulate a request the modifies the session
        middleware.process_request(request)
        request.session['hello'] = 'world'

        # Handle the response through the middleware
        response = middleware.process_response(request, response)
        self.assertIs(response.cookies[settings.SESSION_COOKIE_NAME]['secure'], True)

    @override_settings(SESSION_COOKIE_HTTPONLY=True)
    def test_httponly_session_cookie(self):
        request = self.request_factory.get('/')
        response = HttpResponse('Session test')
        middleware = SessionMiddleware()

        # Simulate a request the modifies the session
        middleware.process_request(request)
        request.session['hello'] = 'world'

        # Handle the response through the middleware
        response = middleware.process_response(request, response)
        self.assertIs(response.cookies[settings.SESSION_COOKIE_NAME]['httponly'], True)
        self.assertIn(
            cookies.Morsel._reserved['httponly'],
            str(response.cookies[settings.SESSION_COOKIE_NAME])
        )

    @override_settings(SESSION_COOKIE_SAMESITE='Strict')
    def test_samesite_session_cookie(self):
        request = self.request_factory.get('/')
        response = HttpResponse()
        middleware = SessionMiddleware()
        middleware.process_request(request)
        request.session['hello'] = 'world'
        response = middleware.process_response(request, response)
        self.assertEqual(response.cookies[settings.SESSION_COOKIE_NAME]['samesite'], 'Strict')

    @override_settings(SESSION_COOKIE_HTTPONLY=False)
    def test_no_httponly_session_cookie(self):
        request = self.request_factory.get('/')
        response = HttpResponse('Session test')
        middleware = SessionMiddleware()

        # Simulate a request the modifies the session
        middleware.process_request(request)
        request.session['hello'] = 'world'

        # Handle the response through the middleware
        response = middleware.process_response(request, response)
        self.assertEqual(response.cookies[settings.SESSION_COOKIE_NAME]['httponly'], '')
        self.assertNotIn(
            cookies.Morsel._reserved['httponly'],
            str(response.cookies[settings.SESSION_COOKIE_NAME])
        )

    def test_session_save_on_500(self):
        request = self.request_factory.get('/')
        response = HttpResponse('Horrible error')
        response.status_code = 500
        middleware = SessionMiddleware()

        # Simulate a request the modifies the session
        middleware.process_request(request)
        request.session['hello'] = 'world'

        # Handle the response through the middleware
        response = middleware.process_response(request, response)

        # The value wasn't saved above.
        self.assertNotIn('hello', request.session.load())

    def test_session_update_error_redirect(self):
        path = '/foo/'
        request = self.request_factory.get(path)
        response = HttpResponse()
        middleware = SessionMiddleware()

        request.session = DatabaseSession()
        request.session.save(must_create=True)
        request.session.delete()

        msg = (
            "The request's session was deleted before the request completed. "
            "The user may have logged out in a concurrent request, for example."
        )
        with self.assertRaisesMessage(SuspiciousOperation, msg):
            # Handle the response through the middleware. It will try to save
            # the deleted session which will cause an UpdateError that's caught
            # and raised as a SuspiciousOperation.
            middleware.process_response(request, response)

    def test_session_delete_on_end(self):
        request = self.request_factory.get('/')
        response = HttpResponse('Session test')
        middleware = SessionMiddleware()

        # Before deleting, there has to be an existing cookie
        request.COOKIES[settings.SESSION_COOKIE_NAME] = 'abc'

        # Simulate a request that ends the session
        middleware.process_request(request)
        request.session.flush()

        # Handle the response through the middleware
        response = middleware.process_response(request, response)

        # The cookie was deleted, not recreated.
        # A deleted cookie header looks like:
        #  Set-Cookie: sessionid=; expires=Thu, 01 Jan 1970 00:00:00 GMT; Max-Age=0; Path=/
        self.assertEqual(
            'Set-Cookie: {}=""; expires=Thu, 01 Jan 1970 00:00:00 GMT; '
            'Max-Age=0; Path=/'.format(
                settings.SESSION_COOKIE_NAME,
            ),
            str(response.cookies[settings.SESSION_COOKIE_NAME])
        )

    @override_settings(SESSION_COOKIE_DOMAIN='.example.local', SESSION_COOKIE_PATH='/example/')
    def test_session_delete_on_end_with_custom_domain_and_path(self):
        request = self.request_factory.get('/')
        response = HttpResponse('Session test')
        middleware = SessionMiddleware()

        # Before deleting, there has to be an existing cookie
        request.COOKIES[settings.SESSION_COOKIE_NAME] = 'abc'

        # Simulate a request that ends the session
        middleware.process_request(request)
        request.session.flush()

        # Handle the response through the middleware
        response = middleware.process_response(request, response)

        # The cookie was deleted, not recreated.
        # A deleted cookie header with a custom domain and path looks like:
        #  Set-Cookie: sessionid=; Domain=.example.local;
        #              expires=Thu, 01 Jan 1970 00:00:00 GMT; Max-Age=0;
        #              Path=/example/
        self.assertEqual(
            'Set-Cookie: {}=""; Domain=.example.local; expires=Thu, '
            '01 Jan 1970 00:00:00 GMT; Max-Age=0; Path=/example/'.format(
                settings.SESSION_COOKIE_NAME,
            ),
            str(response.cookies[settings.SESSION_COOKIE_NAME])
        )

    def test_flush_empty_without_session_cookie_doesnt_set_cookie(self):
        request = self.request_factory.get('/')
        response = HttpResponse('Session test')
        middleware = SessionMiddleware()

        # Simulate a request that ends the session
        middleware.process_request(request)
        request.session.flush()

        # Handle the response through the middleware
        response = middleware.process_response(request, response)

        # A cookie should not be set.
        self.assertEqual(response.cookies, {})
        # The session is accessed so "Vary: Cookie" should be set.
        self.assertEqual(response['Vary'], 'Cookie')

    def test_empty_session_saved(self):
        """
        If a session is emptied of data but still has a key, it should still
        be updated.
        """
        request = self.request_factory.get('/')
        response = HttpResponse('Session test')
        middleware = SessionMiddleware()

        # Set a session key and some data.
        middleware.process_request(request)
        request.session['foo'] = 'bar'
        # Handle the response through the middleware.
        response = middleware.process_response(request, response)
        self.assertEqual(tuple(request.session.items()), (('foo', 'bar'),))
        # A cookie should be set, along with Vary: Cookie.
        self.assertIn(
            'Set-Cookie: sessionid=%s' % request.session.session_key,
            str(response.cookies)
        )
        self.assertEqual(response['Vary'], 'Cookie')

        # Empty the session data.
        del request.session['foo']
        # Handle the response through the middleware.
        response = HttpResponse('Session test')
        response = middleware.process_response(request, response)
        self.assertEqual(dict(request.session.values()), {})
        session = Session.objects.get(session_key=request.session.session_key)
        self.assertEqual(session.get_decoded(), {})
        # While the session is empty, it hasn't been flushed so a cookie should
        # still be set, along with Vary: Cookie.
        self.assertGreater(len(request.session.session_key), 8)
        self.assertIn(
            'Set-Cookie: sessionid=%s' % request.session.session_key,
            str(response.cookies)
        )
        self.assertEqual(response['Vary'], 'Cookie')


# Don't need DB flushing for these tests, so can use unittest.TestCase as base class
class CookieSessionTests(SessionTestsMixin, unittest.TestCase):

    backend = CookieSession

    def test_save(self):
        """
        This test tested exists() in the other session backends, but that
        doesn't make sense for us.
        """
        pass

    def test_cycle(self):
        """
        This test tested cycle_key() which would create a new session
        key for the same session data. But we can't invalidate previously
        signed cookies (other than letting them expire naturally) so
        testing for this behavior is meaningless.
        """
        pass

    @unittest.expectedFailure
    def test_actual_expiry(self):
        # The cookie backend doesn't handle non-default expiry dates, see #19201
        super().test_actual_expiry()

    def test_unpickling_exception(self):
        # signed_cookies backend should handle unpickle exceptions gracefully
        # by creating a new session
        self.assertEqual(self.session.serializer, JSONSerializer)
        self.session.save()

        self.session.serializer = PickleSerializer
        self.session.load()

    @unittest.skip("Cookie backend doesn't have an external store to create records in.")
    def test_session_load_does_not_create_record(self):
        pass

    @unittest.skip("CookieSession is stored in the client and there is no way to query it.")
    def test_session_save_does_not_resurrect_session_logged_out_in_other_context(self):
        pass