we create redundant state groups when we get a no-op membership update

[richvdh]

here: if we decide that the new membership event is a duplicate, we throw it away; however, by that point we have already created a state group for it.

These state groups show up in the database with entries in state_groups and state_groups_state, but the event referred to in state_groups is not present in events or anywhere else.

I think this is probably responsible for quite a lot of the state group blowup on IRC-bridged rooms on matrix.org.

[turt2live]

I'm not totally convinced that the deduplication in that area actually works btw: #3337

[richvdh]

well, it's certainly doing something. I can well believe it's buggy (in fact there seems to be a separate layer of deduplication which looks like it is trying to do the same thing here), but that's somewhat beside the point here.