Clear featured-list caches when featured learning paths are edited

[mbertrand]

What are the relevant tickets?

Closes https://github.com/mitodl/hq/issues/11979

Description (What does it do?)

Channel managers couldn't see featured-list edits for up to 24h (Redis view cache) plus 30min (Fastly page cache). Now:

• Editing a unit channel's featured learning path through the API (path update/delete, item add/reorder/remove, or the bulk add-to-list action) clears the featured_resources Redis cache, hard-purges /c/unit/<name> in Fastly, and soft-purges the homepage, once the change commits.
• The clear runs synchronously in the request rather than via celery, so it can't sit behind ETL/indexing work on the default queue and is guaranteed done by the time the save returns. It's best-effort: a Redis or Fastly failure just logs and doesn't break the save, and each Fastly purge is independent of the others.
• Frontend learning-path mutations also invalidate the featured React Query cache, so editors see the change without a full reload.
• call_fastly_purge_api gains a soft=True kwarg (sends Fastly-Soft-Purge: 1).

How can this be tested?

1. Seed featured lists if you haven't already:

   docker compose run --rm web python manage.py populate_featured_lists

2. In a private/incognito window (anonymous), load a unit channel page — e.g. http://open.odl.local:8062/c/unit/mitx — and note the courses in the featured carousel. Load the homepage too. This primes the anonymous Redis cache for /api/v1/featured/.

3. In a window where you're logged in as an admin user, go to http://open.odl.local:8062/learningpaths, open "MITx Featured Resources", and remove a course from the list (or add one via a course card's add-to-list dialog).

   docker compose logs -f web | grep -i "Skipping Fastly"

   As soon as the edit saves you should see:

   Skipping Fastly purge for /c/unit/mitx (dev environment)
   Skipping Fastly purge for / (dev environment)
   

   The Fastly purges are skipped locally (no FASTLY_API_KEY); on RC/prod they become real PURGE requests against /c/unit/<name> (hard) and / (soft).

4. Reload the incognito channel page and homepage: the featured carousel reflects the edit. Without this PR, the anonymous featured API response stays cached for REDIS_VIEW_CACHE_DURATION (24h default in deployed envs) and the pages keep rendering the old list.

Additional Context

• Fastly URL purges are exact-match, so query-string variants of the channel page (?topic=...) stay cached up to s-maxage (30 min). The base URL the editor refreshes is hard-purged. Per-channel surrogate keys are the upgrade path if that ever matters.
• Edits via Django admin/shell don't trigger invalidation — the hooks are on the viewsets deliberately, since model signals would fire on every ETL run.

[github-actions]

OpenAPI Changes

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[Copilot]

Pull request overview

Adds cache invalidation when featured learning paths are edited.

Changes:

• Adds Fastly soft-purge support and backend invalidation hooks.
• Invalidates featured React Query caches after mutations.
• Adds backend/frontend tests and null-safe item counts.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
main/utils.py	Adds soft-purge headers.
main/utils_test.py	Tests soft-purge behavior.
learning_resources/views.py	Adds featured-cache invalidation hooks.
learning_resources/views_learningpath_test.py	Tests backend invalidation paths.
frontends/ol-components/src/components/LearningResourceCard/LearningResourceListCard.tsx	Handles missing item counts.
frontends/api/src/hooks/learningResources/index.ts	Invalidates featured queries after bulk edits.
frontends/api/src/hooks/learningResources/index.test.ts	Tests bulk-edit invalidation.
frontends/api/src/hooks/learningPaths/index.ts	Invalidates featured queries after path mutations.
frontends/api/src/hooks/learningPaths/index.test.ts	Tests path-mutation invalidation.

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[mbertrand]

Intentional — the celery version sat on the default queue behind ETL backpopulates and bulk indexing, so a purge could wait minutes, which defeats the point of the issue (editor confirms their change within seconds). The work on the request path is small: a tuned Redis pattern delete plus a couple of Fastly purges that normally return in tens of ms. Worst case with Fastly down is ~5s per purge before the save returns (still succeeds, just logs). The PR description was stale from the earlier approach — updated it to match.

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated no new comments.

Suppressed comments (1)

learning_resources/views.py:495

• If Redis raises here, clear_featured_caches exits before any Fastly purge runs; the outer callback catches the exception, but all channel and homepage objects remain stale. Handle/log the Redis failure locally so the independent best-effort Fastly purges still execute.

    clear_views_cache(key_prefix="featured_resources")

[shanbady]

looks good. LGTM 👍