* Adding samesite cookie option in django 2.1 and above
Django 2.1 release note: Added the SESSION_COOKIE_SAMESITE setting to set the SameSite cookie flag on session cookies.
...and it turns out we don't need the switch anymore, anyway.
When we upgraded to Django 1.11, this flag was added in order to
allow for a database migration that would render the user table
unwriteable for up to half an hour:
https://github.com/edx/edx-platform/pull/17561
This involved swapping out the signal handler for logins via
`user_logged_in.disconnect(django_update_last_login)`, but with
Django 2.0, that disconnect is silently failing (returning
false). Likely the disconnect is now happening too soon.
(See edx-platform/common/djangoapps/student/apps.py line 21 in 61e1eda.)
The result is that by the time the waffle switch is consulted, the
normal handler has already run, and the user's last login date has
already been updated.
For now we're just removing the test, and have filed ARCHBOM-1084 for
followup (deleting the switch and related code).
This includes an optimization to the get_bundle_version_files_cached method, which is used very often when loading blockstore data; it was previously being cached only in a process-local cache (lru_cache). My hunch is that in production, with many appservers and LMS workers and frequent deployments and a large number of bundles, the process-local cache is not being hit very often.
I also increased the MAX_BLOCKSTORE_CACHE_DELAY from 60s to 300s; this reduces the frequency with which we check if either (A) an external system modified the blockstore bundle and/or (B) we have a cache invalidation bug somewhere. I am increasing it because that check is more expensive than I thought (calling blockstore API to ascertain latest version of a particular bundle), and I haven't seen any cache invalidation errors that this would help to work around. (Plus, increasing this will make such bugs more obvious.)
Also improve usefulness of some blockstore runtime logs for debugging
Context:
Sometimes when trying to load an XBlock's XML file from Amazon S3, AWS will return a 4xx or 5xx response along with error XML like:
<Error><Code>NoSuchKey</Code><Message>The specified key does not exist.</Message><Key>foo/bar</Key>...</Error>
A bug in the get_bundle_file_data_with_cache method would cause this XML to be returned to the runtime anyways, as if it were the expected OLX. This would then (obviously) lead to strange parsing bugs, e.g. when trying to interpret <Code> as an <xblock-include>.
This fixes the bug and improves the logging, both to make this sort of issue easier to debug in the future and to return whatever detailed error code S3 provides (or Blockstore, if S3 is not being used).
