Support sending no state_events_at_start in the MSC2716 /batch_send endpoint

changelog.d/11188.bugfix

@@ -0,0 +1 @@
+Allow an empty list of `state_events_at_start` to be sent when using the [MSC2716](https://github.com/matrix-org/matrix-doc/pull/2716) `/batch_send` endpoint and the author of the historical messages is already part of the current room state at the given `?prev_event_id`.

synapse/rest/client/room_batch.py

@@ -131,20 +131,22 @@ async def on_POST(
             prev_event_ids_from_query
         )
 
+        state_event_ids_at_start = []
         # Create and persist all of the state events that float off on their own
         # before the batch. These will most likely be all of the invite/member
         # state events used to auth the upcoming historical messages.
-        state_event_ids_at_start = (
-            await self.room_batch_handler.persist_state_events_at_start(
-                state_events_at_start=body["state_events_at_start"],
-                room_id=room_id,
-                initial_auth_event_ids=auth_event_ids,
-                app_service_requester=requester,
+        if body["state_events_at_start"]:
+            state_event_ids_at_start = (
+                await self.room_batch_handler.persist_state_events_at_start(
+                    state_events_at_start=body["state_events_at_start"],
+                    room_id=room_id,
+                    initial_auth_event_ids=auth_event_ids,
+                    app_service_requester=requester,
+                )
             )
-        )
-        # Update our ongoing auth event ID list with all of the new state we
-        # just created
-        auth_event_ids.extend(state_event_ids_at_start)
+            # Update our ongoing auth event ID list with all of the new state we
+            # just created
+            auth_event_ids.extend(state_event_ids_at_start)
 
         inherited_depth = await self.room_batch_handler.inherit_depth_from_prev_ids(
             prev_event_ids_from_query
@@ -197,8 +199,11 @@ async def on_POST(
 
         # Also connect the historical event chain to the end of the floating
         # state chain, which causes the HS to ask for the state at the start of
-        # the batch later.
-        prev_event_ids = [state_event_ids_at_start[-1]]
+        # the batch later. If there is no state chain to connect to, just make
+        # the insertion event float itself.
+        prev_event_ids = []
+        if len(state_event_ids_at_start):
+            prev_event_ids = [state_event_ids_at_start[-1]]
 
         # Create and persist all of the historical events as well as insertion
         # and batch meta events to make the batch navigable in the DAG.