Allow 3rd party rules check_event_allowed callback to function for room version "12" creation events (#19768)

changelog.d/19768.bugfix

@@ -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

@@ -2449,11 +2449,15 @@ class EventCreationHandler:
                 original_event.room_version, third_party_result
             )
             self.validator.validate_builder(builder)
-            assert builder.room_id is not None
+
         except SynapseError as e:
-            raise Exception(
-                "Third party rules module created an invalid event: " + e.msg,
-            )
+            # Prepend the error message with some context. This will be raised as a
+            # `400` since assumption of the validator is that it came directly from a
+            # client. Change this to a `500`, as the module will have changed something
+            # and that is a fault of the server
+            e.msg = "Third party rules module created an invalid event: " + e.msg
+            e.code = HTTPStatus.INTERNAL_SERVER_ERROR
+            raise
 
         immutable_fields = [
             # changing the room is going to break things: we've already checked that the
@@ -2484,12 +2488,25 @@ class EventCreationHandler:
         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 msc4242 and msc4291 rooms will not have a room_id, and
+        # will also not have prev_events nor prev_state_events(below).
+        prev_event_ids = []
+        if builder.room_id is not None and not (
+            builder.type == EventTypes.Create
+            and original_event.room_version.msc4291_room_ids_as_hashes
+        ):
+            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:
+        prev_state_events: list[str] | None = (
+            [] if original_event.room_version.msc4242_state_dags else None
+        )
+        if (
+            original_event.room_version.msc4242_state_dags
+            and builder.type != EventTypes.Create
+            and builder.room_id is not None
+        ):
+            # modules can send new state events, so we re-calculate the auth events just
+            # in case.
             prev_state_events = list(
                 await self.store.get_state_dag_extremities(builder.room_id)
             )