caching on `have_seen_event` is somewhat broken

As discovered in https://github.com/matrix-org/synapse/pull/13863#discussion_r977053643 and discussed in #synapse-dev. Part of https://github.com/matrix-org/synapse/issues/13856

The cache on have_seen_event uses a TreeCache, which means the cache is structured as {room_id: {event_id: result}}. However, _have_seen_events_dict tries to use it with @cachedList, which stores entries as {(room_id, event_id): result}. As a result, caching is inefficient, and cache invalidation is unreliable.

[erikjohnston]

I think that the usage of @cachedList is wrong. It expects to be given the same number of arguments as the underlying cached function, just with one of the arguments being an iterable

https://github.com/matrix-org/synapse/blob/cfe486b0410c045e4db12c694ed5388365f5b54b/synapse/storage/databases/main/events_worker.py#L1485-L1488

I think that should actually be:

 @cachedList(cached_method_name="have_seen_event", list_name="event_ids") 
 async def _have_seen_events_dict( 
     self, room_id: str, event_ids: Collection[str],
 ) -> Dict[str, bool]: 

[richvdh]

oh right, that’s quite a lot easier.