Investigate anonymous user id discrepancy

[sampaccoud]

Our Sentry is getting spammed by the same repeating error:

Stored anonymous user id <SimpleLazyObject: <User: insider>> for user None in course u'rrrssstttuuuvvvwwwxxxyyyzzz' doesn't match computed id u'aaabbbcccdddeeefffggghhhiii'

We had to silence it fast in order to avoid bursting our events quota on Sentry, so we commented out the piece of code that logs the error: see PR openfun/edx-platform#56.

However, eventhough these errors do not seem to affect our users, we should take the time to understand it and fix it at its root.

[rmoch]

@sampaccoud we can close this one, right ?

[sampaccoud]

Not really because there is really something wrong in the way edX handles anonymous user ids:
https://github.com/edx/edx-platform/blob/master/common/djangoapps/student/models.py#L195

The anonymous id changes every time you modify your Django SECRET_KEY setting.

I think when the Django secret key changes, existing user anonymous ids should not change... so the anonymous_id_for_user method should return the existing anonymous id and not the digest computed with the current Django secret key.

If edX wanted the anonymous id to change with the secret key, they shouldn't have persisted it to the database... What they do is not coherent. Raising a warning does not solve it and adds to the confusion.