m.room.tombstone should reference the new room's create event

[turt2live]

Background: MSC1501 adds room upgrade support, which is where m.room.tombstone comes from.

An event ID in the tombstone event makes it easier for clients to jump to the new room at the beginning of the timeline.

[richvdh]

this leads to a bit of a chicken-and-egg situation. The create event currently points to the tombstone, so has to be created after the tombstone. For the tombstone to point to the create, it would have to be created after the create event.

I suppose we could create a second tombstone event once we know the event id of the create event, but it seems a bit inelegant.

Of course, I now can't remember why we decided that the create event needed to link to the tombstone anyway. Is that actually used anywhere? Perhaps we could swap them around.

[turt2live]

It might be possible if the server reserves the event ID of either side before sending out the events. Something like:

1. Generate a placeholder ID for the tombstone event
2. Create the new room, referencing the placeholder ID
3. Populate the tombstone event with the newly created room ID/event ID
4. Send all the events to all the places

I think there's value in having both directions be strongly linked, although on retrospect having it be tombstone->new room feels more useful than the other way around.

[richvdh]

in an imminent room version, event IDs will be based on the content of the event rather than generated in advance: matrix-org/matrix-spec-proposals#1640

[turt2live]

Well that certainly makes things harder. I guess it's a question of which direction are people going to find the most value in, and doing that.

[KitsuneRal]

As a client author I really don't have a problem with having just room ids; but I'm only dealing with clients that run a /sync loop.

Even without a sync loop, one can still use a one-off /sync call without a batch token to get bottommost events in a room - coincidentally that means up to the m.room.tombstone in case of upgraded rooms. Meanwhile the mechanism to jump to the beginning of a timeline is not really there (retrieve m.room.create and then call /context separately?) So I'd probably consider swapping the reference direction too.

[richvdh]

kitsune's argument is convincing to me. Sounds like we should reverse the direction of the event id link.