fix: CSRF handshake and Referer header for Django 4+ HTTPS session auth

[blarghmatey]

Summary

xqueue-watcher authenticates against a DRF-backed xqueue endpoint via session
cookies. Two bugs in _login() caused persistent 403 responses for
put_result in production after upgrading to Django 4+/5+:

1. No CSRF token on put_result POST

_login() posted credentials directly without first obtaining a CSRF cookie.
The session therefore had no X-CSRFToken header, and DRF's
SessionAuthentication.enforce_csrf() rejected every subsequent put_result
POST with 403.

Fix: GET /xqueue/login/ before posting credentials.
openedx/edx-submissions#352
exposes a GET handler on the login endpoint for this purpose — it calls
get_token(request) which ensures Django sets the csrftoken cookie in the
response. After login, the CSRF token is persisted in session.headers so
every subsequent mutating request automatically includes X-CSRFToken.

Older deployments that only accept POST will return 405; we log and proceed
since the login POST is AllowAny and therefore CSRF-exempt.

Stale session cookies are cleared before the GET so the request arrives as an
anonymous user and receives a fresh cookie.

2. Missing Referer header on HTTPS POST (Django 4+ regression)

Django 4+ adds an extra check in CsrfViewMiddleware.process_view(): for
HTTPS POST requests with no HTTP_ORIGIN header it calls _check_referer()
which raises REASON_NO_REFERER if HTTP_REFERER is absent — before the
CSRF token is even checked. requests.Session does not add Referer
automatically, so all put_result POSTs failed this check even when the CSRF
token was correct.

Fix: after a successful login, store Referer: <xqueue_server> in the
session headers. All subsequent POSTs to the same server pass Django's
_check_referer() without any per-call plumbing.

The two fixes together fully resolve the production failure mode:

GET  /xqueue/login/           → 200 + csrftoken cookie
POST /xqueue/login/           → 200 (Referer + X-CSRFToken on this request)
# session headers now contain: Referer, X-CSRFToken
POST /xqueue/put_result/      → 200 ✓

Also in this PR

• _request() now treats 401 and 403 as authentication failures and
  re-authenticates, matching the behaviour for 301/302 redirects. DRF returns
  these directly rather than redirecting to the login page.
• A reauthenticated guard prevents infinite login storms on persistent
  failures.
• New unit tests cover every part of the CSRF handshake, the Referer
  persistence, and the 403→reauth→retry cycle. All 21 tests pass.

Related

• fix: use CsrfExemptSessionAuthentication in XQueueViewSet edx-submissions#352 — adds GET to the login endpoint so the CSRF
  cookie can be obtained before the POST (server-side counterpart to this fix)

Test plan

• All 21 unit tests pass: python -m pytest tests/test_xqueue_client.py -v
• Deploy to a Django 4+/5+ xqueue deployment and confirm put_result no longer returns 403

🤖 Generated with Claude Code

[openedx-webhooks]

Thanks for the pull request, @blarghmatey!

This repository is currently unmaintained.

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