KeyError in generate_room_entry

[clokep]

See https://sentry.matrix.org/sentry/synapse-matrixorg/issues/196208/ for background.

This code is breaking because the current state doesn't include a m.room.create event:

synapse/synapse/handlers/room_list.py

Lines 303 to 305 in 5a9cdaa

	# Return whether this room is open to federation users or not
	create_event = current_state[EventTypes.Create, ""]
	result["m.federate"] = create_event.content.get("m.federate", True)

Looking in the database a the room it does include a create event, but only in the events table, not the current_state_events (which is made up purely of membership events). It seems there might be a bug here that server left the room but didn't properly clean-up the current state events.

Some conversation around this: https://matrix.to/#/!XaqDhxuTIlvldquJaV:matrix.org/$dLy7NInKFWgQW5zeIfxlEjPHvCGWOuxNV1WhpRy6Epk?via=matrix.org&via=vector.modular.im&via=maunium.net

[clokep]

According to @erikjohnston:

I thought we had a BG update to delete entries in current state evnets if the server had left

It seems this is the delete_old_current_state_events update, maybe there's a bug in that method:

synapse/synapse/storage/databases/main/state.py

Lines 344 to 348 in 8a4a418

	async def _background_remove_left_rooms(self, progress, batch_size):
	"""Background update to delete rows from `current_state_events` and
	`event_forward_extremities` tables of rooms that the server is no
	longer joined to.
	"""