From 696acd35151ff32bb69e555baee4e584c504d4d6 Mon Sep 17 00:00:00 2001
From: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>
Date: Thu, 17 Feb 2022 11:59:26 +0000
Subject: [PATCH] `send_join` response: get create event from `state`, not
 `auth_chain` (#12005)

msc3706 proposes changing the `/send_join` response:

> Any events returned within `state` can be omitted from `auth_chain`.

Currently, we rely on `m.room.create` being returned in `auth_chain`, but since
the `m.room.create` event must necessarily be part of the state, the above
change will break this.

In short, let's look for `m.room.create` in `state` rather than `auth_chain`.
---
 changelog.d/12005.misc               | 1 +
 synapse/handlers/federation_event.py | 2 +-
 2 files changed, 2 insertions(+), 1 deletion(-)
 create mode 100644 changelog.d/12005.misc

diff --git a/changelog.d/12005.misc b/changelog.d/12005.misc
new file mode 100644
index 0000000000..45e21dbe59
--- /dev/null
+++ b/changelog.d/12005.misc
@@ -0,0 +1 @@
+Preparation for faster-room-join work: when parsing the `send_join` response, get the `m.room.create` event from `state`, not `auth_chain`.
diff --git a/synapse/handlers/federation_event.py b/synapse/handlers/federation_event.py
index 6dc27a38f3..7683246bef 100644
--- a/synapse/handlers/federation_event.py
+++ b/synapse/handlers/federation_event.py
@@ -420,7 +420,7 @@ class FederationEventHandler:
             SynapseError if the response is in some way invalid.
         """
         create_event = None
-        for e in auth_events:
+        for e in state:
             if (e.type, e.state_key) == (EventTypes.Create, ""):
                 create_event = e
                 break