View messages from older room version after upgrade

[DanilaFe]

When upgrading rooms, messages from the old room version get swallowed. It would be nice if that didn't happen.

[gleachkr]

Seems that only a subset of state events are transferred:

https://spec.matrix.org/v1.2/client-server-api/#server-behaviour-16

So perhaps in the short term populus should look back and read the subspace state off of the previous room? The fact that m.space.child isn't copied seems to be an acknowledged problem.

• [WIP] MSC3385: Bulk small improvements to room upgrades matrix-org/matrix-spec-proposals#3385

And might already be fixed in recent synapse:

• Handle room upgrades for spaces matrix-org/synapse#10774

so hopefully this will eventually be solved upstream.

[DanilaFe]

It's unfortunate that space membership events are not preserved. Perhaps populus can do a "one-time setup" of finding and re-sending the m.space.child event to mimic server behavior if the server itself didn't send the necessary events?

[gleachkr]

OK - after a little discussion, it's clear that the problem is with annotation room upgrades. When you hit the room's beginning, it should be possible to continue to backfill from the previous room.