3 files changed, 25 insertions, 6 deletions
diff --git a/synapse/federation/federation_client.py b/synapse/federation/federation_client.py
index 4dca711cd2..b220ab43fc 100644
--- a/synapse/federation/federation_client.py
+++ b/synapse/federation/federation_client.py
@@ -1294,7 +1294,7 @@ class FederationClient(FederationBase):
         return resp[1]
 
     async def send_knock(self, destinations: List[str], pdu: EventBase) -> JsonDict:
-        """Attempts to send a knock event to given a list of servers. Iterates
+        """Attempts to send a knock event to a given list of servers. Iterates
         through the list until one attempt succeeds.
 
         Doing so will cause the remote server to add the event to the graph,
diff --git a/synapse/federation/federation_server.py b/synapse/federation/federation_server.py
index 907940e19e..28097664b4 100644
--- a/synapse/federation/federation_server.py
+++ b/synapse/federation/federation_server.py
@@ -824,7 +824,14 @@ class FederationServer(FederationBase):
                 context, self._room_prejoin_state_types
             )
         )
-        return {"knock_state_events": stripped_room_state}
+        return {
+            "knock_room_state": stripped_room_state,
+            # Since v1.37, Synapse incorrectly used "knock_state_events" for this field.
+            # Thus, we also populate a 'knock_state_events' with the same content to
+            # support old instances.
+            # See https://github.com/matrix-org/synapse/issues/14088.
+            "knock_state_events": stripped_room_state,
+        }
 
     async def _on_send_membership_event(
         self, origin: str, content: JsonDict, membership_type: str, room_id: str
diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 986ffed3d5..44e70c6c3c 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -781,15 +781,27 @@ class FederationHandler:
 
         # Send the signed event back to the room, and potentially receive some
         # further information about the room in the form of partial state events
-        stripped_room_state = await self.federation_client.send_knock(
-            target_hosts, event
-        )
+        knock_response = await self.federation_client.send_knock(target_hosts, event)
 
         # Store any stripped room state events in the "unsigned" key of the event.
         # This is a bit of a hack and is cribbing off of invites. Basically we
         # store the room state here and retrieve it again when this event appears
         # in the invitee's sync stream. It is stripped out for all other local users.
-        event.unsigned["knock_room_state"] = stripped_room_state["knock_state_events"]
+        stripped_room_state = (
+            knock_response.get("knock_room_state")
+            # Since v1.37, Synapse incorrectly used "knock_state_events" for this field.
+            # Thus, we also check for a 'knock_state_events' to support old instances.
+            # See https://github.com/matrix-org/synapse/issues/14088.
+            or knock_response.get("knock_state_events")
+        )
+
+        if stripped_room_state is None:
+            raise KeyError(
+                "Missing 'knock_room_state' (or legacy 'knock_state_events') field in "
+                "send_knock response"
+            )
+
+        event.unsigned["knock_room_state"] = stripped_room_state
 
         context = EventContext.for_outlier(self._storage_controllers)
         stream_id = await self._federation_event_handler.persist_events_and_notify(