ensure_for_user create IntegrityError race condition

[awseeley]

Hi - we are experiencing a race condition where concurrent requests can cause user.agentsettings = self.create(user=user) to throw an integrity error due to the user ID already existing in the database.

This shows up in our sentry error logs as follows:
duplicate key value violates unique constraint "django_agent_trust_agentsettings_user_id_key"
DETAIL: Key (user_id)=(1) already exists.

This was caused by the following commit which removed the get_or_create
71de1ac#diff-36c097cacb670d42993df6c72f8f2aae021eb52fb353e9cab14ca71f2afb54ac
#2

Timeline of this exception occurring as follows:

1. Request A sent in no User<->Django Agent Trust relationship found exists on the request.user object so it attempts to create this relationship
2. Request B sent in no User<->Django Agent Trust relationship found exists on the request.user object so it attempts to create this relationship
3. Request A successfully creates this
4. Request B fails to create as already done in step Bump django from 2.2.4 to 2.2.8 #3, throws an integrity error on user_id already existing.

I see two options to address this.

1. Revert the problem commit, although it does have to attempt a retrive from the database first the performance overheads are very minimal in my view.
2. Add a try catch on the create. If there is an IntegrityError exception then pass. As follows:

def ensure_for_user(self, user): """ Loads a user's AgentSettings instance, creating a default if necessary. """ try: user.agentsettings except self.model.DoesNotExist: try: user.agentsettings = self.create(user=user) except IntegrityError: pass

[psagers]

get_or_create actually has the same race condition; I think catching the IntegrityError is really the only thing one can do. As far as I know, the ORM doesn't provide the kind of clean ON CONFLICT semantics that some of the underlying SQL implementations provide.

[psagers]

What do you think about #7?

[awseeley]

Agreed - yep #7 looks good to me!

[psagers]

Version 1.0.2