path: root/tests/sessions_tests/tests.py

import base64
import os
import shutil
import string
import sys
import tempfile
import unittest
from datetime import timedelta

from django.conf import settings
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
from django.http import HttpResponse
from django.test import (
    RequestFactory, TestCase, ignore_warnings, override_settings,
)
from django.test.utils import patch_logger
from django.utils import six, timezone
from django.utils.encoding import force_text
from django.utils.six.moves import http_cookies

from .custom_db_backend import SessionStore as CustomDatabaseSession


class SessionTestsMixin(object):
    # 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.assertFalse(self.session.modified)
        self.assertFalse(self.session.accessed)

    def test_get_empty(self):
        self.assertEqual(self.session.get('cat'), None)

    def test_store(self):
        self.session['cat'] = "dog"
        self.assertTrue(self.session.modified)
        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.assertTrue(self.session.accessed)
        self.assertTrue(self.session.modified)
        self.assertEqual(self.session.get('some key'), None)

    def test_pop_default(self):
        self.assertEqual(self.session.pop('some key', 'does not exist'),
                         'does not exist')
        self.assertTrue(self.session.accessed)
        self.assertFalse(self.session.modified)

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

    def test_update(self):
        self.session.update({'update key': 1})
        self.assertTrue(self.session.accessed)
        self.assertTrue(self.session.modified)
        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.assertTrue(self.session.accessed)
        self.assertFalse(self.session.modified)

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

    def test_iterkeys(self):
        self.session['x'] = 1
        self.session.modified = False
        self.session.accessed = False
        i = six.iterkeys(self.session)
        self.assertTrue(hasattr(i, '__iter__'))
        self.assertTrue(self.session.accessed)
        self.assertFalse(self.session.modified)
        self.assertEqual(list(i), ['x'])

    def test_itervalues(self):
        self.session['x'] = 1
        self.session.modified = False
        self.session.accessed = False
        i = six.itervalues(self.session)
        self.assertTrue(hasattr(i, '__iter__'))
        self.assertTrue(self.session.accessed)
        self.assertFalse(self.session.modified)
        self.assertEqual(list(i), [1])

    def test_iteritems(self):
        self.session['x'] = 1
        self.session.modified = False
        self.session.accessed = False
        i = six.iteritems(self.session)
        self.assertTrue(hasattr(i, '__iter__'))
        self.assertTrue(self.session.accessed)
        self.assertFalse(self.session.modified)
        self.assertEqual(list(i), [('x', 1)])

    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.assertTrue(self.session.accessed)
        self.assertTrue(self.session.modified)

    def test_save(self):
        if (hasattr(self.session, '_cache') and 'DummyCache' in
                settings.CACHES[settings.SESSION_CACHE_ALIAS]['BACKEND']):
            raise unittest.SkipTest("Session saving tests require a real cache backend")
        self.session.save()
        self.assertTrue(self.session.exists(self.session.session_key))

    def test_delete(self):
        self.session.save()
        self.session.delete(self.session.session_key)
        self.assertFalse(self.session.exists(self.session.session_key))

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

    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.assertNotEqual(self.session.session_key, prev_key)
        self.assertEqual(list(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')
            try:
                session.save()
            except AttributeError:
                self.fail(
                    "The session object did not save properly. "
                    "Middleware may be saving cache items without namespaces."
                )
            self.assertNotEqual(session.session_key, '1')
            self.assertEqual(session.get('cat'), None)
            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()
        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.assertFalse(self.session.get_expire_at_browser_close())

            self.session.set_expiry(0)
            self.assertTrue(self.session.get_expire_at_browser_close())

            self.session.set_expiry(None)
            self.assertFalse(self.session.get_expire_at_browser_close())

        with override_settings(SESSION_EXPIRE_AT_BROWSER_CLOSE=True):
            self.session.set_expiry(10)
            self.assertFalse(self.session.get_expire_at_browser_close())

            self.session.set_expiry(0)
            self.assertTrue(self.session.get_expire_at_browser_close())

            self.session.set_expiry(None)
            self.assertTrue(self.session.get_expire_at_browser_close())

    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')
        with patch_logger('django.security.SuspiciousSession', 'warning') as calls:
            self.assertEqual({}, self.session.decode(bad_encode))
            # check that the failed decode is logged
            self.assertEqual(len(calls), 1)
            self.assertIn('corrupted', calls[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.
        """
        if self.backend is CookieSession:
            raise unittest.SkipTest("Cookie backend doesn't have an external store to create records in.")
        session = self.backend('someunknownkey')
        session.load()

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


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(force_text(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.custom_db_backend'

    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.assertEqual(s.account_id, None)


class CacheDBSessionTests(SessionTestsMixin, TestCase):

    backend = CacheDBSession

    @unittest.skipIf('DummyCache' in
        settings.CACHES[settings.SESSION_CACHE_ALIAS]['BACKEND'],
        "Session saving tests require a real cache backend")
    def test_exists_searches_cache_first(self):
        self.session.save()
        with self.assertNumQueries(0):
            self.assertTrue(self.session.exists(self.session.session_key))

    # 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.
        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(FileSessionTests, self).setUp()

    def tearDown(self):
        super(FileSessionTests, self).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
        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 IOError - by creating
        # a new session, making it unclear whether the slashes were detected.
        self.assertRaises(InvalidSessionKey,
                          self.backend()._key_to_file, "a\\b\\c")

    def test_invalid_key_forwardslash(self):
        # Ensure we don't allow directory-traversal
        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.assertNotEqual(caches['default'].get(self.session.cache_key), None)

    @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.assertEqual(caches['default'].get(self.session.cache_key), None)
        self.assertNotEqual(caches['sessions'].get(self.session.cache_key), None)


class SessionMiddlewareTests(TestCase):

    @override_settings(SESSION_COOKIE_SECURE=True)
    def test_secure_session_cookie(self):
        request = RequestFactory().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.assertTrue(
            response.cookies[settings.SESSION_COOKIE_NAME]['secure'])

    @override_settings(SESSION_COOKIE_HTTPONLY=True)
    def test_httponly_session_cookie(self):
        request = RequestFactory().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.assertTrue(
            response.cookies[settings.SESSION_COOKIE_NAME]['httponly'])
        self.assertIn(http_cookies.Morsel._reserved['httponly'],
            str(response.cookies[settings.SESSION_COOKIE_NAME]))

    @override_settings(SESSION_COOKIE_HTTPONLY=False)
    def test_no_httponly_session_cookie(self):
        request = RequestFactory().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.assertFalse(response.cookies[settings.SESSION_COOKIE_NAME]['httponly'])

        self.assertNotIn(http_cookies.Morsel._reserved['httponly'],
                         str(response.cookies[settings.SESSION_COOKIE_NAME]))

    def test_session_save_on_500(self):
        request = RequestFactory().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)

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

    def test_session_delete_on_end(self):
        request = RequestFactory().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)

        # Check that 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,
                '""' if sys.version_info >= (3, 5) else '',
            ),
            str(response.cookies[settings.SESSION_COOKIE_NAME])
        )

    @override_settings(SESSION_COOKIE_DOMAIN='.example.local')
    def test_session_delete_on_end_with_custom_domain(self):
        request = RequestFactory().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)

        # Check that the cookie was deleted, not recreated.
        # A deleted cookie header with a custom domain looks like:
        #  Set-Cookie: sessionid=; Domain=.example.local;
        #              expires=Thu, 01-Jan-1970 00:00:00 GMT; Max-Age=0; Path=/
        self.assertEqual(
            'Set-Cookie: {}={}; Domain=.example.local; expires=Thu, '
            '01-Jan-1970 00:00:00 GMT; Max-Age=0; Path=/'.format(
                settings.SESSION_COOKIE_NAME,
                '""' if sys.version_info >= (3, 5) else '',
            ),
            str(response.cookies[settings.SESSION_COOKIE_NAME])
        )

    def test_flush_empty_without_session_cookie_doesnt_set_cookie(self):
        request = RequestFactory().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 = RequestFactory().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(CookieSessionTests, self).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()