From 76c9f09e099aeccf6bcfcc1ef7a96e7230b100dd Mon Sep 17 00:00:00 2001
From: Kegan Dougal <7190048+kegsay@users.noreply.github.com>
Date: Fri, 15 Aug 2025 10:15:02 +0100
Subject: [PATCH] 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

<!-- Please read
https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->

* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
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))
---
 changelog.d/18746.bugfix             | 1 +
 synapse/handlers/federation_event.py | 3 +++
 2 files changed, 4 insertions(+)
 create mode 100644 changelog.d/18746.bugfix

diff --git a/changelog.d/18746.bugfix b/changelog.d/18746.bugfix
new file mode 100644
index 000000000..b80a880c0
--- /dev/null
+++ b/changelog.d/18746.bugfix
@@ -0,0 +1 @@
+Fix a bug which could corrupt auth chains making it impossible to perform state resolution.
diff --git a/synapse/handlers/federation_event.py b/synapse/handlers/federation_event.py
index 2ef7e77b1..511394c66 100644
--- a/synapse/handlers/federation_event.py
+++ b/synapse/handlers/federation_event.py
@@ -1728,6 +1728,9 @@ class FederationEventHandler:
                             event,
                             auth_event_id,
                         )
+                        # Drop the event from the auth_map too, else we may incorrectly persist
+                        # events which depend on this dropped event.
+                        auth_map.pop(event.event_id, None)
                         return
                     auth.append(ae)