Database corruption when servers backfill over federation

[kegsay]

Consider:

• Bob on his server invites Alice to a room.
• Alice joins the room first over federation. Proxy does the right thing and sets her membership to join. There is no timeline though due to not having backfilled.
• Alice's client backfills in the room which pulls in the invite event, but the SS proxy doesn't see it as it's backfill, not /sync.
• Charlie joins the same room via SS, which makes the SS proxy see 50 timeline events, which includes the invite. As the proxy has never seen this invite event before, it assumes it is newer than the join event and inserts it, corrupting state.

To fix this:

• SELECT nids for all events prior to insertion.
• Find the lowest-index which has a NID and drop all events earlier than this as they are backfilled.
• Discard the state block when this happens, as it cannot be trusted to do the right thing now that we are dropping parts of the timeline.

[DMRobertson]

• Find the lowest-index which has a NID and drop all events earlier than this as they are backfilled.

earlier in what sense---origin server TS?

[DMRobertson]

Answer: earlier meaning lower index in the timeline given to use in the sync v2 poll payload.