fix: ensure navigation sidebar serves fresh data after course publish

[wgu-taylor-payne]

Summary

After a course publish, there is a ~30-second window where the navigation sidebar endpoint serves stale block structure data. Worse, the stale response is cached for 1 hour, extending the staleness far beyond the initial window.

This PR adds a synchronous staleness check before reading the block structure on a navigation cache miss.

Problem

This issue was brought to light while testing an internal Open edX instance. A unit was deleted in Studio, a refresh in the course in the learning MFE still showed the unit in the course outline.

In a Verawood sandbox, I deleted a unit in Studio, waited 30+ seconds and then refreshed and the unit was removed from the outline in the learning MFE. I deleted another unit in Studio and refreshed the course page within a few seconds, and the deleted unit still showed. Refreshing again before an hour passed, it still showed. Refreshing after an hour, it was no longer present in the outline.

Three components interact to create a race condition:

1. Block structure rebuild is delayed 30s after publish

The course_published signal handler queues the rebuild task with a 30s countdown:

update_course_in_cache_v2.apply_async(
    kwargs=dict(course_id=str(course_key)),
    countdown=settings.BLOCK_STRUCTURES_SETTINGS['COURSE_PUBLISH_TASK_DELAY'],
)

2. Navigation cache key includes course version — causing a miss on publish

The cache key uses course_version, so after a publish the version changes and the previous cached response no longer matches. This is a cache miss.

3. On cache miss, stale block structure data is read and cached for 1 hour

The if not course_blocks branch calls get_course_outline_block_tree(), which reads from the (still stale) block structure cache, then stores the result for 1 hour.

Timeline:

1. T+0s: Course published — new version, block structure rebuild queued with 30s delay
2. T+1s: Learner hits navigation → cache miss (new version) → reads stale block structure → caches stale data for 1 hour
3. T+30s: Rebuild task runs, block structure updated (but navigation cache already poisoned)
4. T+1s → T+1h: All requests served stale data from navigation cache

Fix

Before reading the block structure on a cache miss, call update_collected_if_needed(). This compares the cached block structure version against the modulestore version and synchronously rebuilds only if stale.

Performance impact

• Cache hit (common case): No change.
• Cache miss, block structure current: One lightweight is_up_to_date check (DB read + version comparison).
• Cache miss, block structure stale (the bug): Synchronous rebuild — same work the delayed Celery task would do, just done eagerly instead of serving stale data.

Testing

To run the automated test:

tutor dev run lms pytest --ds=lms.envs.test \
  lms/djangoapps/course_home_api/outline/tests/test_view.py::SidebarBlocksTestViews::test_navigation_serves_fresh_data_after_publish

Manual steps:

• In a production like environment (i.e. Celery used to process async tasks)
• Delete a unit in Studio
• Refresh the course in the learning MFE within 30 seconds
• Verify that the unit is no longer listed in the course outline

AI Usage

Used Kiro with model set to auto to aid in the discovery of the root cause of the error, talk through potential fixes, come up with a test that addresses the issue being fixed, and write this PR summary.

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[ormsbee]

I don't think this is operationally feasible for large courses and high traffic. Let's talk more about other possible mitigations.

[ormsbee]

Going through the collection phase of a large course can be extremely expensive, which is why it's done asynchronously in celery tasks or management commands (it can often exceed the 30s timeout that many sites use for giving up on web worker requests). Placing it in the GET here also risks causing a stampede if it is a popular course that many concurrent users are trying to access, as parallel workers try to recompute the same collection phase data.

[wgu-taylor-payne]

Thank you, I appreciate this feedback. I'll look into another way of preventing the stale cache.

[ormsbee]

For instance, I think a course's navigation being incorrect for a minute after a deletion is a bad, but not necessarily release-blocking bug (FYI @crathbun428 and @jmakowski1123, who can weigh in here). If the wrong navigation is getting cached for an hour, then maybe that's the part that we should focus on for this fix.

[jmakowski1123]

I agree with Dave, I would not classify a 30-60sec cache as a blocker. But an hour is a bigger problem.