Strip invite/knock event itself and avoid mutating event unsigned

Make sure we don't run into
https://github.com/element-hq/synapse/issues/14919
(https://github.com/matrix-org/synapse/issues/14919)

synapse/events/utils.py

@@ -836,3 +836,21 @@ def maybe_upsert_event_field(
             del container[key]
 
     return upsert_okay
+
+
+def strip_event(event: EventBase) -> JsonDict:
+    """
+    Used for "stripped state" events which provide a simplified view of the state of a
+    room intended to help a potential joiner identify the room (relevant when the user
+    is invited or knocked).
+
+    Stripped state events can only have the `sender`, `type`, `state_key` and `content`
+    properties present.
+    """
+
+    return {
+        "type": event.type,
+        "state_key": event.state_key,
+        "content": event.content,
+        "sender": event.sender,
+    }

synapse/handlers/sliding_sync.py

@@ -25,8 +25,10 @@ from immutabledict import immutabledict
 
 from synapse.api.constants import AccountDataTypes, Direction, EventTypes, Membership
 from synapse.events import EventBase
+from synapse.events.utils import strip_event
 from synapse.storage.roommember import RoomsForUser
 from synapse.types import (
+    JsonDict,
     PersistedEventPosition,
     Requester,
     RoomStreamToken,
@@ -793,7 +795,7 @@ class SlidingSyncHandler:
             )
 
         # Figure out any stripped state events for invite/knocks
-        stripped_state: List[EventBase] = []
+        stripped_state: List[JsonDict] = []
         if rooms_for_user_membership_at_to_token.membership in {
             Membership.INVITE,
             Membership.KNOCK,
@@ -804,15 +806,15 @@ class SlidingSyncHandler:
 
             stripped_state = []
             if invite_or_knock_event.membership == Membership.INVITE:
-                stripped_state = invite_or_knock_event.unsigned.get(
-                    "invite_room_state", []
+                stripped_state.extend(
+                    invite_or_knock_event.unsigned.get("invite_room_state", [])
                 )
             elif invite_or_knock_event.membership == Membership.KNOCK:
-                stripped_state = invite_or_knock_event.unsigned.get(
-                    "knock_room_state", []
+                stripped_state.extend(
+                    invite_or_knock_event.unsigned.get("knock_room_state", [])
                 )
 
-            stripped_state.append(invite_or_knock_event)
+            stripped_state.append(strip_event(invite_or_knock_event))
 
         return SlidingSyncResult.RoomResult(
             # TODO: Dummy value

synapse/rest/client/sync.py

@@ -1017,18 +1017,10 @@ class SlidingSyncRestServlet(RestServlet):
 
             # Stripped state only applies to invite/knock rooms
             if room_result.stripped_state:
-                serialized_stripped_state = (
-                    await self.event_serializer.serialize_events(
-                        room_result.stripped_state,
-                        time_now,
-                        config=serialize_options,
-                    )
-                )
-
                 # TODO: Would be good to rename this to `stripped_state` so it can be
                 # shared between invite and knock rooms, see
                 # https://github.com/matrix-org/matrix-spec-proposals/pull/3575#discussion_r1117629919
-                serialized_rooms[room_id]["invite_state"] = serialized_stripped_state
+                serialized_rooms[room_id]["invite_state"] = room_result.stripped_state
 
         return serialized_rooms
 

synapse/storage/databases/main/events_worker.py

@@ -55,7 +55,7 @@ from synapse.api.room_versions import (
 )
 from synapse.events import EventBase, make_event_from_dict
 from synapse.events.snapshot import EventContext
-from synapse.events.utils import prune_event
+from synapse.events.utils import prune_event, strip_event
 from synapse.logging.context import (
     PreserveLoggingContext,
     current_context,
@@ -1025,15 +1025,7 @@ class EventsWorkerStore(SQLBaseStore):
 
         state_to_include = await self.get_events(selected_state_ids.values())
 
-        return [
-            {
-                "type": e.type,
-                "state_key": e.state_key,
-                "content": e.content,
-                "sender": e.sender,
-            }
-            for e in state_to_include.values()
-        ]
+        return [strip_event(e) for e in state_to_include.values()]
 
     def _maybe_start_fetch_thread(self) -> None:
         """Starts an event fetch thread if we are not yet at the maximum number."""

synapse/types/handlers/__init__.py

@@ -31,7 +31,7 @@ else:
     from pydantic import Extra
 
 from synapse.events import EventBase
-from synapse.types import JsonMapping, StreamToken, UserID
+from synapse.types import JsonDict, JsonMapping, StreamToken, UserID
 from synapse.types.rest.client import SlidingSyncBody
 
 
@@ -193,7 +193,7 @@ class SlidingSyncResult:
         required_state: List[EventBase]
         timeline: List[EventBase]
         is_dm: bool
-        stripped_state: Optional[List[EventBase]]
+        stripped_state: Optional[List[JsonDict]]
         prev_batch: StreamToken
         limited: bool
         joined_count: int