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synapse/changelog.d/18746.bugfix
Kegan Dougal 76c9f09e09 Fix a bug which could corrupt auth chains (#18746)
[Complement tests](https://github.com/matrix-org/complement/pull/790)

Basically, when we use `/state_ids` in conjunction with `/event`
requests, the `/event` request can fail causing a partial state to be
returned. When we persist the state, we process dependent events first.
If we fail to process a dependent event due to missing the `auth_event`,
we didn't update the in-memory event map. This meant that we could
incorrectly persist events that depended on dropped events.

Discovered via Chaos testing.

### Pull Request Checklist

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* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
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The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
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* [x] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct (run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))
2025-08-15 10:15:02 +01:00

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Fix a bug which could corrupt auth chains making it impossible to perform state resolution.