From 73451b12cf748b0b0716a87a2a709bb73614dbdc Mon Sep 17 00:00:00 2001
From: Eric Eastwood <erice@element.io>
Date: Sat, 7 Aug 2021 01:35:57 -0500
Subject: [PATCH 1/3] Only return state events that the AS passed in via
 state_events_at_start

As discovered by @Half-Shot in
https://github.com/matrix-org/matrix-doc/pull/2716#discussion_r684158448

Part of MSC2716
---
 synapse/rest/client/v1/room.py | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/synapse/rest/client/v1/room.py b/synapse/rest/client/v1/room.py
index 502a91758813..86d143ae0d1c 100644
--- a/synapse/rest/client/v1/room.py
+++ b/synapse/rest/client/v1/room.py
@@ -438,6 +438,7 @@ async def on_POST(self, request, room_id):
         prev_state_ids = list(prev_state_map.values())
         auth_event_ids = prev_state_ids
 
+        state_events_at_start = []
         for state_event in body["state_events_at_start"]:
             assert_params_in_dict(
                 state_event, ["type", "origin_server_ts", "content", "sender"]
@@ -500,6 +501,7 @@ async def on_POST(self, request, room_id):
                 )
                 event_id = event.event_id
 
+            state_events_at_start.append(event_id)
             auth_event_ids.append(event_id)
 
         events_to_create = body["events"]
@@ -647,7 +649,7 @@ async def on_POST(self, request, room_id):
             event_ids.append(base_insertion_event.event_id)
 
         return 200, {
-            "state_events": auth_event_ids,
+            "state_events": state_events_at_start,
             "events": event_ids,
             "next_chunk_id": insertion_event["content"][
                 EventContentFields.MSC2716_NEXT_CHUNK_ID

From 7aa2e848affe93c089fd610ec8127f9b76c834e9 Mon Sep 17 00:00:00 2001
From: Eric Eastwood <erice@element.io>
Date: Sat, 7 Aug 2021 01:46:10 -0500
Subject: [PATCH 2/3] Add changelog

---
 changelog.d/10552.msc | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 changelog.d/10552.msc

diff --git a/changelog.d/10552.msc b/changelog.d/10552.msc
new file mode 100644
index 000000000000..fc5f6aea5ffb
--- /dev/null
+++ b/changelog.d/10552.msc
@@ -0,0 +1 @@
+Update `/batch_send` endpoint to only return `state_events` created by the `state_events_from_before` passed in.

From 7554c0e78df4a28f52304f8d92607b20a5927c8b Mon Sep 17 00:00:00 2001
From: Eric Eastwood <erice@element.io>
Date: Sat, 7 Aug 2021 19:23:12 -0500
Subject: [PATCH 3/3] Fix changelog extension

---
 changelog.d/{10552.msc => 10552.misc} | 0
 1 file changed, 0 insertions(+), 0 deletions(-)
 rename changelog.d/{10552.msc => 10552.misc} (100%)

diff --git a/changelog.d/10552.msc b/changelog.d/10552.misc
similarity index 100%
rename from changelog.d/10552.msc
rename to changelog.d/10552.misc