#33533: Should it be impossible for a session dict to return more than one value
for a key?
----------------------------------+--------------------------------------
Reporter: Michael | Owner: nobody
Type: Uncategorized | Status: closed
Component: contrib.sessions | Version: 4.0
Severity: Normal | Resolution: invalid
Keywords: | Triage Stage: Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Old description:
> In my project, I store the serial number of the device that logs in it's
> session. However in the code that gets the value from the session:
>
> {{{
> SESSION_SERIAL_NUMBER = 'terminal_serial_number'
>
> user._serial_number = request.session.get(SESSION_SERIAL_NUMBER, '')
> }}}
>
> Very occasionally throws an error. The part that baffles me is:
>
> > get() returned more than one Session -- it returned more than 20!
>
> How can a dict return more than one result, surely that is a bug for a
> dict to return more than one value?
>
> Note: When I query the database `django_sessions` table with the given
> `sessionid`, it only returns one row.
>
> Heres the error (from the email):
>
> {{{
> Internal Server Error: /accounts/login/
>
> ProgrammingError at /accounts/login/
> no results to fetch
> }}}
>
> And the traceback:
>
> {{{
> Traceback (most recent call last):
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/contrib/sessions/backends/base.py", line 180, in
> _get_session
> return self._session_cache
>
> During handling of the above exception ('SessionStore' object has no
> attribute '_session_cache'), another exception occurred:
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/core/handlers/exception.py", line 47, in inner
> response = get_response(request)
> File "./dist/terminals/middleware.py", line 11, in __call__
> user._serial_number = request.session.get(SESSION_SERIAL_NUMBER, '')
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/contrib/sessions/backends/base.py", line 65, in get
> return self._session.get(key, default)
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/contrib/sessions/backends/base.py", line 185, in
> _get_session
> self._session_cache = self.load()
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/contrib/sessions/backends/db.py", line 43, in load
> s = self._get_session_from_db()
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/contrib/sessions/backends/db.py", line 32, in
> _get_session_from_db
> return self.model.objects.get(
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/db/models/manager.py", line 85, in manager_method
> return getattr(self.get_queryset(), name)(*args, **kwargs)
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/db/models/query.py", line 443, in get
> raise self.model.MultipleObjectsReturned(
>
> During handling of the above exception (get() returned more than one
> Session -- it returned more than 20!), another exception occurred:
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/contrib/sessions/backends/base.py", line 180, in
> _get_session
> return self._session_cache
>
> During handling of the above exception ('SessionStore' object has no
> attribute '_session_cache'), another exception occurred:
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/db/utils.py", line 97, in inner
> return func(*args, **kwargs)
>
> The above exception (no results to fetch) was the direct cause of the
> following exception:
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/core/handlers/exception.py", line 47, in inner
> response = get_response(request)
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/utils/deprecation.py", line 126, in __call__
> response = response or self.get_response(request)
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/core/handlers/exception.py", line 49, in inner
> response = response_for_exception(request, exc)
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/core/handlers/exception.py", line 110, in
> response_for_exception
> response = handle_uncaught_exception(request,
> get_resolver(get_urlconf()), sys.exc_info())
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/core/handlers/exception.py", line 149, in
> handle_uncaught_exception
> return callback(request)
> File "./core/base/views.py", line 152, in error_500_view
> response = render(request, "core.base/500.html", context=context)
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/shortcuts.py", line 19, in render
> content = loader.render_to_string(template_name, context, request,
> using=using)
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/template/loader.py", line 62, in render_to_string
> return template.render(context, request)
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/template/backends/django.py", line 61, in render
> return self.template.render(context)
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/template/base.py", line 174, in render
> with context.bind_template(self):
> File "/usr/lib/python3.8/contextlib.py", line 113, in __enter__
> return next(self.gen)
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/debug_toolbar/panels/templates/panel.py", line 46, in
> _request_context_bind_template
> context = processor(self.request)
> File "./core/base/context_processors.py", line 13, in project
> menu = get_plug_menu(request.user)
> File "./dist/plug/menu.py", line 58, in get_plug_menu
> if user.is_anonymous or user.is_terminal:
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/utils/functional.py", line 248, in inner
> self._setup()
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/utils/functional.py", line 384, in _setup
> self._wrapped = self._setupfunc()
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/contrib/auth/middleware.py", line 25, in <lambda>
> request.user = SimpleLazyObject(lambda: get_user(request))
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/contrib/auth/middleware.py", line 11, in get_user
> request._cached_user = auth.get_user(request)
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/contrib/auth/__init__.py", line 177, in get_user
> user_id = _get_user_session_key(request)
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/contrib/auth/__init__.py", line 60, in
> _get_user_session_key
> return
> get_user_model()._meta.pk.to_python(request.session[SESSION_KEY])
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/contrib/sessions/backends/base.py", line 50, in
> __getitem__
> return self._session[key]
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/contrib/sessions/backends/base.py", line 185, in
> _get_session
> self._session_cache = self.load()
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/contrib/sessions/backends/db.py", line 43, in load
> s = self._get_session_from_db()
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/contrib/sessions/backends/db.py", line 32, in
> _get_session_from_db
> return self.model.objects.get(
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/db/models/manager.py", line 85, in manager_method
> return getattr(self.get_queryset(), name)(*args, **kwargs)
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/db/models/query.py", line 435, in get
> num = len(clone)
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/db/models/query.py", line 262, in __len__
> self._fetch_all()
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/db/models/query.py", line 1354, in _fetch_all
> self._result_cache = list(self._iterable_class(self))
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/db/models/query.py", line 51, in __iter__
> results = compiler.execute_sql(chunked_fetch=self.chunked_fetch,
> chunk_size=self.chunk_size)
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/db/models/sql/compiler.py", line 1234, in execute_sql
> return list(result)
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/db/models/sql/compiler.py", line 1678, in cursor_iter
> for rows in iter((lambda: cursor.fetchmany(itersize)), sentinel):
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/db/models/sql/compiler.py", line 1678, in <lambda>
> for rows in iter((lambda: cursor.fetchmany(itersize)), sentinel):
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/db/utils.py", line 97, in inner
> return func(*args, **kwargs)
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/db/utils.py", line 90, in __exit__
> raise dj_exc_value.with_traceback(traceback) from exc_value
> File "/home/company/.venv/project/lib/python3.8/site-
> packages/django/db/utils.py", line 97, in inner
> return func(*args, **kwargs)
>
> Exception Type: ProgrammingError at /accounts/login/
> Exception Value: no results to fetch
> Request information:
> USER: AnonymousUser
> }}}
>
> This is the middleware it sits in:
> {{{
> from dist.terminals.apps import SESSION_SERIAL_NUMBER
>
> class UserSerialNumberMiddleware:
> def __init__(self, get_response):
> self.get_response = get_response
> # One-time configuration and initialization.
>
> def __call__(self, request):
> user = request.user
> user._serial_number = request.session.get(SESSION_SERIAL_NUMBER,
> '')
> response = self.get_response(request)
> return response
> }}}
New description:
In my project, there are some methods on a custom user model that require
the `request` to calculate certain values. This simple middleware does the
trick:
{{{
class AttachRequestToUserMiddleware:
"""Adds the request to the user object, so session information can be
looked
up by the custom user model.
Must come after
django.contrib.auth.middleware.AuthenticationMiddleware which adds the
user"""
def __init__(self, get_response):
self.get_response = get_response
def __call__(self, request):
request.user.request = request
return self.get_response(request)
}}}
In production, when there are multiple workers running parrallel by uWSGI,
if one has `SESSION_SAVE_EVERY_REQUEST = True`, then if one makes async
requests from JavaScript (e.g. say a Service Worker caching pages on
install), then the way it saves/retrieves sessions on every request fails
in many spectacular ways.
Here are some example trace backs:
{{{
Django Version: 4.0.1
Python Version: 3.8.5
Exception Type: ProgrammingError
Exception Value:
no results to fetch
Exception Location: /home/michael/.venv/project/lib/python3.8/site-
packages/django/db/utils.py, line 97, in inner
/home/michael/.venv/project/lib/python3.8/site-
packages/django/db/utils.py, line 97, in inner
return func(*args, **kwargs)
Exception Type: RuntimeError
Exception Value:
generator raised StopIteration
/home/michael/.venv/project/lib/python3.8/site-
packages/django/contrib/auth/__init__.py, line 60, in
_get_user_session_key
return get_user_model()._meta.pk.to_python(request.session[SESSION_KEY])
Exception Type: MultipleObjectsReturned
Exception Value:
get() returned more than one Session -- it returned 2!
/home/michael/.venv/project/lib/python3.8/site-
packages/django/contrib/sessions/backends/base.py, line 180, in
_get_session
return self._session_cache
/home/michael/.venv/project/lib/python3.8/site-
packages/django/core/handlers/exception.py, line 47, in inner
response = get_response(request)
./core/accounts/middleware.py, line 33, in __call__
request.user.request = request
/home/michael/.venv/project/lib/python3.8/site-
packages/django/utils/functional.py, line 278, in __setattr__
self._setup()
/home/michael/.venv/project/lib/python3.8/site-
packages/django/utils/functional.py, line 384, in _setup
self._wrapped = self._setupfunc()
/home/michael/.venv/project/lib/python3.8/site-
packages/django/contrib/auth/middleware.py, line 25, in <lambda>
request.user = SimpleLazyObject(lambda: get_user(request))
/home/michael/.venv/project/lib/python3.8/site-
packages/django/contrib/auth/middleware.py, line 11, in get_user
request._cached_user = auth.get_user(request)
/home/michael/.venv/project/lib/python3.8/site-
packages/django/contrib/auth/__init__.py, line 177, in get_user
user_id = _get_user_session_key(request)
/home/michael/.venv/project/lib/python3.8/site-
packages/django/contrib/auth/__init__.py, line 60, in
_get_user_session_key
return
get_user_model()._meta.pk.to_python(request.session[SESSION_KEY])
/home/michael/.venv/project/lib/python3.8/site-
packages/django/contrib/sessions/backends/base.py, line 50, in __getitem__
return self._session[key]
/home/michael/.venv/project/lib/python3.8/site-
packages/django/contrib/sessions/backends/base.py, line 185, in
_get_session
self._session_cache = self.load()
/home/michael/.venv/project/lib/python3.8/site-
packages/django/contrib/sessions/backends/db.py, line 43, in load
s = self._get_session_from_db()
/home/michael/.venv/project/lib/python3.8/site-
packages/django/contrib/sessions/backends/db.py, line 32, in
_get_session_from_db
return self.model.objects.get(
/home/michael/.venv/project/lib/python3.8/site-
packages/django/db/models/manager.py, line 85, in manager_method
return getattr(self.get_queryset(), name)(*args, **kwargs)
/home/michael/.venv/project/lib/python3.8/site-
packages/django/db/models/query.py, line 443, in get
raise self.model.MultipleObjectsReturned(
}}}
--
Comment (by Michael):
Replying to [comment:1 Tim Graham]:
> See TicketClosingReasons/UseSupportChannels for ways to get help if you
cannot debug the issue yourself. Please only use this ticket tracker for
reporting confirmed bugs. Thanks!
Hi, I have done some more digging with some help:
https://forum.djangoproject.com/t/should-it-be-impossible-for-a-session-
dict-to-return-more-than-one-value-for-a-key/12298/13
I think the issue is this: if it receives multiple requests at the same
time (I have a service worker that requests many pages at installation so
it can cache them), and if `SESSION_SAVE_EVERY_REQUEST = True`, then the
way it handles session updates it just falls over in many spectacular
ways.
My for now my fix is to set SESSION_COOKIE_AGE to over a year, and have
`SESSION_SAVE_EVERY_REQUEST = False`
--
Ticket URL: <https://code.djangoproject.com/ticket/33533#comment:3>
Django <https://code.djangoproject.com/>
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