Switch to `chunk` events so we can auth via power_levels (MSC2716) (#10432)

Previously, we were using `content.chunk_id` to connect one
chunk to another. But these events can be from any `sender`
and we can't tell who should be able to send historical events.
We know we only want the application service to do it but these
events have the sender of a real historical message, not the
application service user ID as the sender. Other federated homeservers
also have no indicator which senders are an application service on
the originating homeserver.

So we want to auth all of the MSC2716 events via power_levels
and have them be sent by the application service with proper
PL levels in the room.

1 files changed, 13 insertions, 4 deletions

diff --git a/synapse/rest/client/v1/room.py b/synapse/rest/client/v1/room.py
index 31a1193cd3..c95c5ae234 100644
--- a/synapse/rest/client/v1/room.py
+++ b/synapse/rest/client/v1/room.py
@@ -553,9 +553,18 @@ class RoomBatchSendEventRestServlet(TransactionRestServlet):
             ]
 
         # Connect this current chunk to the insertion event from the previous chunk
-        last_event_in_chunk["content"][
-            EventContentFields.MSC2716_CHUNK_ID
-        ] = chunk_id_to_connect_to
+        chunk_event = {
+            "type": EventTypes.MSC2716_CHUNK,
+            "sender": requester.user.to_string(),
+            "room_id": room_id,
+            "content": {EventContentFields.MSC2716_CHUNK_ID: chunk_id_to_connect_to},
+            # Since the chunk event is put at the end of the chunk,
+            # where the newest-in-time event is, copy the origin_server_ts from
+            # the last event we're inserting
+            "origin_server_ts": last_event_in_chunk["origin_server_ts"],
+        }
+        # Add the chunk event to the end of the chunk (newest-in-time)
+        events_to_create.append(chunk_event)
 
         # Add an "insertion" event to the start of each chunk (next to the oldest-in-time
         # event in the chunk) so the next chunk can be connected to this one.
@@ -567,7 +576,7 @@ class RoomBatchSendEventRestServlet(TransactionRestServlet):
             # the first event we're inserting
             origin_server_ts=events_to_create[0]["origin_server_ts"],
         )
-        # Prepend the insertion event to the start of the chunk
+        # Prepend the insertion event to the start of the chunk (oldest-in-time)
         events_to_create = [insertion_event] + events_to_create
 
         event_ids = []