Never return negative bump stamp

[erikjohnston]

Fixes #17737

[sandhose]

Looks sane, even though I must admit, I don't know much about all this

[sandhose]

"new message" or "new event"? Because if it's new events, the join event will anyway generate a new positive stream ordering? (I basically know nothing about all this, I'm trying to understand :D)

[erikjohnston]

Technically its a new "bump event", which are things like messages, stickers, etc. The idea is that we only want to "bump" the room to the top of the room list (on the client) for events of interest, and not e.g. display name changes.

synapse/synapse/types/handlers/__init__.py

Lines 28 to 36 in 5173741

	SLIDING_SYNC_DEFAULT_BUMP_EVENT_TYPES = {
	EventTypes.Create,
	EventTypes.Message,
	EventTypes.Encrypted,
	EventTypes.Sticker,
	EventTypes.CallInvite,
	EventTypes.PollStart,
	EventTypes.LiveLocationShareStart,
	}

the join event will anyway generate a new positive stream ordering?

Yeah, the membership events should always be created with a positive stream ordering. It's only if the room gets locally deleted and then rejoins the room that the local membership events would get persisted with a negative stream ordering.

[spaetz]

Given the nature of #community-moderation-effort-bl:neko.dev, it basically never receives any of the above events, so it is plausible that there is a huge backfill without any bump event.