Allow 3rd party rules check_event_allowed callback to function for room version "12" creation events

changelog.d/19768.misc

@@ -0,0 +1 @@
+Allow the Third Party Rules `check_event_allowed()` callback to work with [MSC4291](https://github.com/matrix-org/matrix-spec-proposals/pull/4291) rooms. Contributed by @famedly.

synapse/handlers/message.py

@@ -2376,7 +2376,15 @@ async def _rebuild_event_after_third_party_rules(
                 original_event.room_version, third_party_result
             )
             self.validator.validate_builder(builder)
-            assert builder.room_id is not None
+
+            # The room_id should only be None for creation events using msc4291
+            # rooms(version "12" and newer)
+            if not (
+                builder.room_version.msc4291_room_ids_as_hashes
+                and builder.type == EventTypes.Create
+            ):
+                assert builder.room_id is not None
+
         except SynapseError as e:
             raise Exception(
                 "Third party rules module created an invalid event: " + e.msg,
@@ -2411,12 +2419,21 @@ async def _rebuild_event_after_third_party_rules(
         for k, v in original_event.internal_metadata.get_dict().items():
             setattr(builder.internal_metadata, k, v)
 
-        # modules can send new state events, so we re-calculate the auth events just in
-        # case.
-        prev_event_ids = await self.store.get_prev_events_for_room(builder.room_id)
+        # Creation events using msc4291 rooms will not have a room_id, and will
+        # also not have prev_events nor prev_state_events.
+        # This was asserted above, so makes an acceptable sign that room_id can not be
+        # None here(which makes mypy happy) but if it is then the lists should be empty.
+        prev_event_ids = []
+        if builder.room_id is not None:
+            # modules can send new state events, so we re-calculate the auth events just
+            # in case.
+            prev_event_ids = await self.store.get_prev_events_for_room(builder.room_id)
 
         prev_state_events = None
-        if original_event.room_version.msc4242_state_dags:
+        if (
+            original_event.room_version.msc4242_state_dags
+            and builder.room_id is not None
+        ):
             prev_state_events = list(
                 await self.store.get_state_dag_extremities(builder.room_id)
             )

tests/rest/client/test_third_party_rules.py

@@ -382,7 +382,8 @@ def test_send_event(self) -> None:
             "third_party_event_rules": {
                 "module": __name__ + ".LegacyChangeEvents",
                 "config": {},
-            }
+            },
+            "default_room_version": "12",
         }
     )
     def test_legacy_check_event_allowed(self) -> None: