Handling of missing auth events does not update state group

[richvdh]

When we receive an event whose auth chain includes events which are missing from the state group at the relevant point on the DAG, we include those missing events when authing the new event. (see https://github.com/matrix-org/synapse/blob/09eb08f/synapse/handlers/federation.py#L1510).

However, it appears we do not persist those events to the state group, which means that the room state stays out of sync, and leads to further rejections down the line. (For instance, if the new event was a join, and the invite_rules was missing from the state group, then the join will later get rejected during conflict resolution).

The real question is why the auth events are missing in the state group (which is probably due to #1935), but given we have this self-healing mechanism, it would be nice if it worked.

[richvdh]

I'm reasonably sure that this got fixed a while ago.