Thinking out loud: You can't just lock the session when you think you have
a critical section of code in a view because the session data is stored in
one field in the database, and is *completely overwritten* on write.
Consider this case:
Thread A: Read session data
Thread B: Open transaction, read session (with lock) data, add
session['foo'] = 1, write session back to database, close transaction.
Thread A: session['bar'] = 1, save session (completely overwriting Thread
B's changes so 'foo' doesn't exist anymore)
So any view that will write to the session *must* lock it before reading.
And like Stephen is saying, you don't want to do that on every request. I
suppose you could keep session reads as-is. But any time you need to do a
write, you must explicitly wrap it in a transaction, and refresh the data
from the database (while locking other writers):
...
# freely read any session data (but it could be stale)
...
# now we need to write something to the session
with transaction.atomic():
# the session data needs to be refresh if it was updated by another
transaction after it was first read by this thread
request.session.lock_and_refresh()
if request.session.get('has_commented', False):
request.session['has_commented'] = True
...
request.session.save()
Session implementations could provide a context manager (if they support
atomic writes) so it's not so ugly:
with request.session.atomic():
if request.session.get("has_commented", False):
request.session['has_commented'] = True
...
But EVERY session write must follow that pattern, or it's all for naught. I
don't think Django core uses sessions much, so updating core doesn't sound
bad. It's *everyone *downstream. That seems totally impractical. I do think
there are some *very *subtle bugs that would be resolved by adopting
something like this. But it may not be worth the trouble.
On Sunday, February 7, 2021 at 11:13:51 AM UTC-8 Adam Johnson wrote:
> Stephen - you're right that a constraint is the best way to enforce
> consistency in the DB. That doesn't fit every use case for sessions though
> - people use Django's built-ins with many different kinds of data stores
> that don't support locking or constraint semantics, such as remote API's.
>
> Matt - I think we could remove that example.
>
> As for a new session locking method, I'm not sure how feasible it is *in
> general* since the session API is designed to back onto any kind of data
> store. If you had an example implementation it would be good to see and
> know how well it performs in a real world setting.
>
> On Sun, 7 Feb 2021 at 18:58, Stephen J. Butler <stephen...@gmail.com>
> wrote:
>
>> The way I'd solve this example is to create a unique constraint/index in
>> the DB. The session check gives a quick pre-check, but in the event of a
>> race the DB enforces absolute consistency.
>>
>> If the constraint isn't possible or is too complex, then your "SELECT...
>> FOR UPDATE" belongs on the check for whether someone is allowed to comment,
>> not on the session. Select for update is a kind of locking, and the rule
>> for locking is to do it as little as possible, in as isolated section of
>> code as possible. So you do it around the code that checks if a user can
>> create a comment and then creates it (which will probably be a small
>> percent of your requests) vs. doing it in the session logic which gets run
>> for 100% of requests (but is unneeded 99.99% of the time).
>>
>> On Sun, Feb 7, 2021 at 12:08 PM Matt <ma...@satchamo.com> wrote:
>>
>>> The "post_comment" example of sessions appears to be incorrect:
>>> https://docs.djangoproject.com/en/3.1/topics/http/sessions/#examples
>>>
>>> Imagine two HTTP requests coming in at the same time, both seeing that
>>> "has_commented" is False, and then both create a comment and set the
>>> session variable to True.
>>>
>>> I just experienced this issue myself, which is why I bring it up.
>>>
>>> I would offer to update the documentation...but I'm struggling to come
>>> up with a way to actually make this pattern work with sessions! Assuming
>>> there are other views that update other variables in the session, it seems
>>> like you must lock the session when you read it (i.e. "SELECT...FOR
>>> UPDATE"). So anything that reads or writes to the session must be wrapped
>>> in an atomic block.
>>>
>>> I think it might be useful to add a method to support locking the
>>> session before any reads/writes. Or maybe we should just update the docs to
>>> warn people about how race prone sessions are?
>>>
>>> Any thoughts?
>>>
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>
> --
> Adam
>
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