fix(state): reload before change_state so a stale snapshot can't revert a sibling State's write - #4694
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…rt a sibling State's write Under shared_state, molecule builds one State object per Config sharing the same state.yml. change_state wrote back its whole in-memory _data on every call, so a State object constructed before another one's write still had the old values in memory and clobbered them on its next change_state call. change_state now reloads _data from disk before applying its key, so it always merges onto the latest persisted state instead of its own snapshot. reset() is untouched; it intentionally overwrites the whole file. Fixes ansible#4689 Signed-off-by: Amir Fathi <amirfathi.me@gmail.com>
📝 WalkthroughWalkthrough
ChangesShared state write preservation
Estimated code review effort: 2 (Simple) | ~10 minutes Suggested reviewers: Merge Risk: 🟡 Moderate · up to This change preserves sequential updates from sibling State instances, but concurrent workers can still lose state changes when writes overlap. Shared-state parallel use remains unsafe until the full update transaction is serialized. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
✨ Finishing Touches🧪 Generate unit tests (beta)
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In `@src/molecule/state.py`:
- Line 210: Serialize the full state update transaction around self._get_data(),
the assignment, and _write_state_file so concurrent workers cannot overwrite
each other with stale snapshots. Update the relevant `@marshal` flow and use a
process- or file-level lock held across all three operations; locking only the
read or write is insufficient.
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src/molecule/state.pytests/unit/test_state.py
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| if key not in VALID_KEYS: | ||
| raise InvalidState | ||
| # Reload first so a sibling State object's write under shared_state isn't reverted. | ||
| self._data = self._get_data() |
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🗄️ Data Integrity & Integration | 🟠 Major | 🏗️ Heavy lift
Serialize the reload-and-write transaction.
self._get_data() and the write performed by @marshal are separate operations. If two workers overlap under --workers, both can reload the same snapshot, and the later atomic write can discard the earlier update. Hold a process or file lock across reload, assignment, and _write_state_file, or serialize shared-state writers. A lock around only the read or only the write is insufficient.
The new test covers sequential sibling writes, not overlapping writes. The PR context confirms that concurrent writers can still interleave under --workers.
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In `@src/molecule/state.py` at line 210, Serialize the full state update
transaction around self._get_data(), the assignment, and _write_state_file so
concurrent workers cannot overwrite each other with stale snapshots. Update the
relevant `@marshal` flow and use a process- or file-level lock held across all
three operations; locking only the read or write is insufficient.
After applying the fix, consider running `coderabbit review --agent` for local
review. Visit https://docs.coderabbit.ai/cli.
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Fair point, and it's the same gap the PR body already calls out: this closes the sequential-write race (two State objects for the same file, one built before the other's write lands), not the fully concurrent case across separate --workers processes. A real fix there needs a lock held across _get_data, the assignment, and _write_state_file together, which is a bigger change than this issue's repro needs. Leaving it as a follow-up rather than folding it in here.
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@AmirF194, understood. The current change fixes the sequential stale-snapshot case from #4689.
The concurrent --workers update race remains a separate concern. Would you like me to open a follow-up GitHub issue for locking the complete reload, update, and write operation?
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No need, thanks. It's already noted as out of scope in the PR body, and I'll open an issue myself if the cross-process locking work happens. |
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I reviewed this because I filed #4666, which covers the same read-modify-write mechanism from the parallel side, and I wrote the I verified the test is a genuine red/green rather than a test that passes either way. Reverting only the two production lines in With the change restored, 14/14 pass. A few notes, none of them blocking. The deferred lock has an issue already. You flagged in the description that two concurrent writers under Reads still serve the stale snapshot. The properties (
Two things I checked and found clean, in case they come up:
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Fixes #4689.
Confirmed the root cause you found:
change_statemutates its own_datadict andmarshalwrites that whole dict back, so aStateobject constructed before create runs still hascreated: falsein memory and stomps it back onto disk whenpreparecallschange_state("prepared", True)later.Went with option 2 from the issue:
change_statenow reloads_datafrom disk right before applying the key it's changing, so it always merges onto whatever the last writer actually persisted instead of its own possibly-stale snapshot. Leftreset()alone since it's meant to blow away the whole file (used bydestroy), not merge.One thing I didn't add: a lock around the reload-then-write. This closes the sequential-construction race in your repro (default_config's State and the scenario's State disagreeing because one was built before the other's write), but two genuinely concurrent writers under
--workerscould still interleave between the reload and the write.atomic_write_fileguarantees no reader sees a torn file, not that two writers can't race. That's tracked as defect 2 in #4666, so no new issue needed for it.Added a unit test that builds two
Stateobjects against the same state file the wayshared_statedoes: the first setscreated, then the second (still holding the older snapshot) setsprepared. On main the second write revertscreatedback to false; with the change both keys stick.Summary by CodeRabbit
Bug Fixes
Tests