bug: Backfilled join events are never synced #2211

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opened 2026-09-04 13:43:46 +00:00 by ginger · 1 comment

ginger commented

2026-09-04 13:43:46 +00:00

Owner

What is the bug?

See conversation starting here. Someone did not properly leave all their rooms before switching from conduit to continuwuity, and experienced !779 -like symptoms when rejoining rooms that their server was previously in.

Backfilled events in general don't come down sync, but it might be possible to special case joins for the syncing user. However, I would assume that the new join/no-op event sent by the join-assisting server would not be backfilled.

Steps to reproduce

Join a room that your server is already a member of from the perspective of the join-assisting server.

Server name and version

kanp.ai 26.8.1

Additional context

No response

### What is the bug? See conversation starting [here](https://matrix.to/#/!c10y-N9TQGHzj6Ne3OKqn-MEO8EhfRWrD3-CD-W9AEM/$G-1bAtFGJ-PPwP8AL9w_A3CbClj13SrRc6v49mjH1dE?via=continuwuity.org&via=matrix.org&via=federated.nexus). Someone did not properly leave all their rooms before switching from conduit to continuwuity, and experienced !779 -like symptoms when rejoining rooms that their server was previously in. Backfilled events in general don't come down sync, but it might be possible to special case joins for the syncing user. However, I would assume that the new join/no-op event sent by the join-assisting server would not be backfilled. ### Steps to reproduce - Join a room that your server is already a member of from the perspective of the join-assisting server. ### Server name and version kanp.ai 26.8.1 ### Additional context _No response_

ginger added the

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2026-09-04 13:43:46 +00:00

nex commented

2026-09-04 16:45:01 +00:00

Owner

sus. This needs more information because new joins are inherently not backfilled as they're new events:

 		info!("Appending new room join event");
 		self.services
 			.timeline
 			.append_pdu(
 				&parsed_join_pdu,
 				join_event,
 				std::iter::once(parsed_join_pdu.event_id.as_ref()),
 				&state_lock,
 				room_id,
 			)
 			.await?;

sus. This needs more information because new joins are inherently not backfilled as they're new events: https://forgejo.ellis.link/continuwuation/continuwuity/src/commit/37d4cc5a3806e0f2c95b22b73d8749c06ce92167/src/service/rooms/membership/mod.rs#L641-L651