Keep confirm_human from overwriting concurrent session writes - #204
Keep confirm_human from overwriting concurrent session writes#204PetrDlouhy wants to merge 5 commits into
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confirm_human() writes its session flag and then replays the
participant's enrollments and goals - a counter round trip each - while
the session is only saved when the response is returned. On a busy site
that replay takes seconds, and the save then writes back the whole
session dict as it looked when the request loaded it. Anything a
concurrent request wrote to the same session in the meantime is silently
lost.
The way this was found: python-social-auth stores its OAuth `state` in
the session on /login/<backend>/. When a confirm_human ping (fired from
every page load for new visitors) overlapped the login, the state was
erased and the provider callback failed with AuthStateMissing. Measured
on a production-like Heroku app:
POST /experiments/confirm_human/ 13.241 service=3998ms -> saved 17.239
POST /login/facebook/ 13.299 service=1618ms -> saved 14.917
GET /complete/facebook/ 18.851 AuthStateMissing
The view now re-reads the stored session after confirm_human() has run,
writes only the keys that actually changed, and keeps the middleware from
saving the stale snapshot. Cookie-backed sessions are left alone: they
have no server-side store to race on, and suppressing the middleware save
would drop the response cookie that is their persistence.
The regression test injects a concurrent write inside confirm_human(),
which is exactly where such requests land; it fails on master with the
symptom above (the concurrent key reads back as None) and passes with
this change.
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Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: ⚪ Minimal · up to This change prevents concurrent server-side session writes from being overwritten while preserving cookie-backed session persistence; no actionable merge-blocking risk remains after normal checks and review. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
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🧪 Generate unit tests (beta)
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Inline comments:
In `@experiments/views.py`:
- Around line 74-75: Update the response/session handling around fresh.save() so
SessionMiddleware cannot persist the stale request snapshot when
SESSION_SAVE_EVERY_REQUEST is enabled; resetting session.modified alone is
insufficient. Suppress middleware persistence for that response while preserving
the merged state written by fresh.save(), and add a regression test covering
this setting and concurrent-key preservation.
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Fix all unresolved CodeRabbit comments on this PR:
- Push a commit to this branch (recommended)
- Create a new PR with the fixes
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experiments/tests/test_views.pyexperiments/views.py
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With SESSION_SAVE_EVERY_REQUEST=True the session middleware saves even an unmodified session, so resetting session.modified was not enough: the middleware would write this request's stale snapshot over the merged state, re-introducing the lost-update the previous commit fixed (spotted by CodeRabbit on the PR). Point the request's session object at the merged contents instead - then whatever the middleware decides to persist is the merged state, never the stale snapshot. The regression test drives the same concurrent write under override_settings(SESSION_SAVE_EVERY_REQUEST=True); it fails on the previous commit and passes here. Also adds the docstrings the review tooling flagged.
Two time bombs, both visible on any PR today:
* The workflow has run Python 3.12 since the Django 5.0 update, but
[gh-actions] in tox.ini never learned the mapping - so on 3.12
tox-gh-actions falls back to the bare 'py' env, which has no Django
pin. That installed whatever was newest; since Django 6 released,
'makemigrations --check' demands a BigAutoField migration and the job
fails before a single test runs. Mapping 3.12 to the py312 envs runs
the pinned Django 4.2/5.0 matrix instead.
* Python 3.7 is no longer available on the ubuntu-latest runner images
('Version 3.7 with arch x64 not found'), so that job cannot even set
up. Dropped from the workflow; the py37 tox envs remain for anyone
running tox locally on an interpreter that has it.
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Status after review:
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Two real incompatibilities surfaced once the resolver was allowed to install modern Django: * Django 6.1 gave SessionBase a __bool__, so an *empty* session is now falsy. Two truthiness checks changed meaning: _get_participant demoted every fresh visitor to a DummyUser (no enrollment, nothing counted), and _session_key returned None for them, keying every such visitor's enrollments and counters to the same identifier - the test suite's MultipleObjectsReturned came from exactly that collision. Both are now identity checks. * Django >= 6 defaults DEFAULT_AUTO_FIELD to BigAutoField, which made makemigrations demand an id migration from every project. The app now pins its historical AutoField in the AppConfig, so existing installations are not asked to alter their tables. The tox envlist grows django5.1/5.2 (py310-313) and django6.0/6.1 (py312-313), the workflow matrix gains Python 3.13, and [gh-actions] learns the 3.13 mapping. Verified locally on Django 5.0.14, 5.1.15, 5.2.17, 6.0.8 and 6.1: makemigrations --check clean and the full suite OK on each.
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To get a green run here without waiting: #205's branch is merged into this one — base Until #205 merges, this PR's diff therefore also shows the CI/Django-support commits; once The merged branch runs the full suite green locally on Django 5.0.14, 5.2.17 and 6.1 |
1 participant
The bug
confirm_human()writes its session flag, then replays the participant's enrollments andgoals — a counter round trip each — while the session is only saved when the response is
returned. On a busy site that replay takes seconds, and the end-of-request save writes
back the whole session dict as it looked when the request loaded it. Anything a
concurrent request wrote to the same session in the meantime is silently lost.
How it was found: python-social-auth stores its OAuth
statein the session on/login/<backend>/. The confirm-human ping fires from every page load for new visitors,so it routinely overlaps a login. When it does, the state is erased and the provider
callback fails with
AuthStateMissing("Session value state missing"). Heroku routertimings from the incident:
The ping loaded the session 58 ms before the login request and wrote its stale snapshot
back 2.3 s after the login had saved the state.
The fix
After
confirm_human()has run, the view re-reads the stored session, writes only thekeys that actually changed, and sets
session.modified = Falseso the middleware doesnot save the stale snapshot. The race window shrinks from the seconds the replay takes to
the microseconds of the merge.
Cookie-backed sessions (
signed_cookies) are deliberately left on the old path: they haveno server-side store to race on, and suppressing the middleware save would drop the
response cookie that is their persistence.
Testing
New
experiments/tests/test_views.py. The regression test injects a concurrent sessionwrite inside
confirm_human()— exactly where such requests land. On master it fails withthe production symptom (
None != 'state-written-by-a-concurrent-request'); with thischange the concurrent write survives and the human flag is still set. Full suite: 95
tests, OK (Django 5.0 / Python 3.12 / live Redis).
Downstream context: BlenderKit currently works around this with a wrapped view
(BlenderKit/BlenderKit-server#3601);
that wrapper gets deleted once this lands, since the project pins this repo's
master.Summary by CodeRabbit
Bug Fixes
Tests