Summary

xblock-poll's celery task was broken, then fixed in #23700, then broken again by #25479. This fixes it again.

Description

XBlock-poll contains a feature to download a CSV of poll responses. It runs asynchronously in the LMS's celery workers.

However, we received a report (TNL-8370) that it is currently broken. Attempting to export the results to CSV only shows an error, which may appear in the UI or only in the browser console:

The reason for this error is that xblock-poll's celery tasks (in tasks.py) are not being registered. They were previously registered because the default LMS settings in common.py explicitly set CELERY_IMPORTS = ('poll.tasks', ...), but https://github.com/edx/edx-platform/pull/25479 removed all explicit celery imports from settings.

I think the reasoning was that all django apps would have their tasks.py auto-detected, but poll is not a django app. I tried making the main poll.py file import tasks.py, but it appears that the LMS celery workers do not load XBlock code at all unless they have some reason to, so I couldn't find any way to register these tasks only by changing the XBlock code.

I'm not sure if there's a better way to make these tasks detected, but this way definitely works.

Supporting information

• Jira ticket: https://openedx.atlassian.net/browse/TNL-8370
• Previous fix for the exact same issue: https://github.com/edx/edx-platform/pull/23700
• PR that undid the previous fix: https://github.com/edx/edx-platform/pull/25479

Testing instructions

It took me a while to figure out how to run celery workers on the docker devstack - am I missing some documentation?

First, use make lms-shell then edit /edx/etc/lms.yml. Make sure CELERY_QUEUES is as follows:

CELERY_QUEUES:
- edx.lms.core.default
- edx.lms.core.high
- edx.lms.core.high_mem

(for some reason, on my devstack, these all had wrong values like edx.lms.core.defaultedx.lms.core., while CELERY_DEFAULT_QUEUE had a correct value, resulting in the worker never seeing the tasks)

Second, add this to lms/envs/private.py:

CELERY_ALWAYS_EAGER = False
BROKER_URL = 'redis://:password@edx.devstack.redis:6379/'

Third, start Redis with make redis-up

Fourth, restart the LMS.

Fifth: from make lms-shell, start a celery worker with

DJANGO_SETTINGS_MODULE=lms.envs.devstack_with_worker celery worker --app=lms.celery:APP

Sixth: add an xblock-poll to a course, answer it so it has some data, and try to export the CSV of results. It should fail. In the logs of the celery worker you should see Received unregistered task of type 'poll.tasks.export_csv_data'..

Seventh: Check out this branch, and restart the celery worker. Try again - it should succeed.

Deadline

None, this has been an issue for months and there is a workaround that can be used in the meantime.

Other information

Nope.