Very old events appear out of order in the live timeline after federation backfill

[matrixbot]

Created by @ richvdh:sw1v.org.

when synapse decides to backfill get_missing_events events over federation, these end up being sent to clients over /sync, which leads to very confusing timelines with old messages

[ara4n]

element-hq/element-web#897 might be related?

[richvdh]

yes, they sound like duplicates

[richvdh]

... so let's close this one.

[richvdh]

Hrm, I'm not convinced they are dups

[ara4n]

This is getting really nasty with federation glitches recently. It'd almost be better to drop events with ancient timestamps than append them to the live timeline.

[jfrederickson]

I'd say drop events with ancient timestamps if the surrounding events aren't in the current timeline view, otherwise insert them in the correct place in the timeline.

Possible gotcha: a malicious homeserver could forge an event that appears to have been sent in the past. (But maybe that's already possible and they look like older events after a refresh?)

[ara4n]

in terms of UI for when old messages flood in: how about showing up a /!\ status prompt saying "you received old messages from <a href="permalink">timestamp</a>"?

[erdnaxeli]

Put the event at the correct location and add a banner would be better. But as a user I probably don't care about this event. This is instant messaging, I will not react to a message 3 pages back (it's not really about time but the quantity of messages).

[t3chguy]

Riot likely does not have events that far back to be able to put it at the correct location

[erdnaxeli]

Sometime it does, see the attached screenshot of what happened today to me. I suppose when it don't, Riot should just drop the event.

[turt2live]

This is effectively impossible for the client to fix on its own - it will need some sort of spec fix, possibly from Sliding Sync or being able to identify gaps in the timeline.

[MadLittleMods]

Related to:

• MSC for gappy timelines: MSC3871: Gappy timelines matrix-org/matrix-spec-proposals#3871
• Messages can appear out of order if /sync is delayed but message PUTs are 200ing #1934
• Message ordering can break if /sync is stuck as messages aren't always queued element-web#639
• Order of events after a federation outage is confusing element-web#20024
• Vector should display consistent ordering on timelines. #1900