Forbid repeated upgrade of the same room

[KitsuneRal]

As of this writing, the spec allows upgrading a room that already has a tombstone event. I don't think there was a provision to use the upgrade mechanism to fork rooms; so let's just disallow upgrading a room if it already has m.room.tombstone in its state.

[turt2live]

based on some internal troubleshooting over the last couple weeks, I'm not entirely convinced that we should do this. Although it's not great that people can rewrite the upgrade chain, the benefit is that when someone screws up the upgrade and it needs to be fixed it is incredibly useful to overwrite their disaster.

[anoadragon453]

How exactly is the update screwed up though? Is it a bug or just the user messing with things?

Additionally, we could support rewriting the chain by redacting the tombstone event.

[turt2live]

I've seen both cases equally. There's not much difference in redacting a tombstone and overwriting it.

[anoadragon453]

I'd prefer redacting, assuming clients would handle it properly.

[anoadragon453]

Have we discussed whether sending any events after a tombstone is allowed?

[turt2live]

It's normally prevented by the power level change, but not strictly enforced. Reason being the rooms we want to upgrade are great at resetting state, I think

[anoadragon453]

Is the server supposed to drop everyone to PL 0 in the room?

[turt2live]

Nope, just raise the requirement to send events.

[anoadragon453]

It raises it to 50, but that's going to prevent new events from being sent by mods/admins.

[turt2live]

Correct. It's just meant to stop clients which don't support upgrades from using the old room

[ShadowJonathan]

FWIW there is a sytest test asserting this: "Cannot send tombstone event that points to the same room" in 30rooms/60version_upgrade