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Add tests for room creators being able to join v12 room that everyone has left #843

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20 changes: 20 additions & 0 deletions tests/v12_test.go
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Expand Up @@ -52,6 +52,26 @@ func TestMSC4289PrivilegedRoomCreators(t *testing.T) {
)
}

@MadLittleMods MadLittleMods Feb 16, 2026

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Presumably v12 is special because historically Synapse would use the domain in the room ID as a last-ditch server to join via?

-- @kegsay, #843 (comment)

I wouldn't expect this to work in other room versions (unless you backdate events to rollback state).

But for v12, the room creator has ultimate power to do whatever and so should be able to send a m.room.mebership to join again.

@tulir tulir Jul 13, 2026

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Isn't the reason for empty rooms being unjoinable that you can end up with two independent branches if users from different servers join the empty room? You have no way to know if other users rejoined the same empty room on their own servers. It was never about permissions, so I don't see how this is any different in v12 than previous room versions.

@MadLittleMods MadLittleMods Jul 13, 2026

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@tulir Unclear. Could that just be a different variant of a fork? That sounds like a plausible technical reason but I've never heard it before. Any prior art/spec about this?

Feels like something that needs to be clarified.

@tulir tulir Jul 13, 2026 •
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Not sure if it has ever been explicitly specified. Might have always been an implicit constraint of servers being expected to use the make/send_join handshake for rooms they're not participating in, which you obviously can't do if there are no servers participating in the room. Though looking at the spec, even that isn't actually specified, https://spec.matrix.org/v1.19/server-server-api/#joining-rooms just says "When the homeserver does not yet know about the room it cannot do this directly." (this = just send a m.room.member event). You could interpret that to mean rooms you've left can be rejoined directly as long as the homeserver hasn't deleted the data from the db, but that'd be bad in terms of causing forks.

However, if we were to go with allowing joining empty rooms, I don't see any technical reason why it would have to be limited to room creators. Being a creator doesn't allow bypassing join rules, so you can't join invite-only rooms that way. In public rooms, anyone could send a join event like that, not only creators. The only reason to limit it would be to avoid forks, but if that's the justification, it should probably be limited to the primary creator and not include additional_creators.

Definitely need some spec clarifications, I think it'd be best to just define empty rooms are unjoinable by declaring that servers must use the make/send_join handshake unless they're actually participating in the room (at least 1 local member joined) and explicitly specifying that the handshake requirement leads to empty rooms being permanently unjoinable

@MadLittleMods MadLittleMods Jul 13, 2026

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However, if we were to go with allowing joining empty rooms, I don't see any technical reason why it would have to be limited to room creators. Being a creator doesn't allow bypassing join rules, so you can't join invite-only rooms that way.

hmmm, I guess a point against this is literally in MSC4289,

The room creator has always been able to leave an invite-only room and then rejoin it without an invite.

-- Security considerations in MSC4289

But the MSC doesn't mention join rules at all or how this is surmised.

In public rooms, anyone could send a join event like that, not only creators. The only reason to limit it would be to avoid forks, but if that's the justification, it should probably be limited to the primary creator and not include additional_creators.

I'm not sure we really need to prevent forks. Kinda feels the same as the normal disappearing in a submarine for months type of thing.

@tulir tulir Jul 13, 2026

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But the MSC doesn't mention join rules at all or how this is surmised.

I think that part just means backdating/referencing old events, since auth rules don't allow it even in v12. Has to be considered from a security perspective, but not something that can be used as an actual feature.

I'm not sure we really need to prevent forks

I guess if we don't care about that at all, then we could just allow anyone to join empty public rooms (that their server still has in the local database)

IMO we should just forbid joining empty rooms entirely, but if other people think allowing it is better, we can write the spec that way too. I'll make a spec issue and/or PR

@tulir tulir Jul 13, 2026

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Spec issue is matrix-org/matrix-spec#2415, might make a PR for the forbid joining empty rooms option later

t.Run("Room creator can re-join room after everyone left", func(t *testing.T) {
roomID := alice.MustCreateRoom(t, map[string]interface{}{
"room_version": roomVersion12,
})
alice.MustLeaveRoom(t, roomID)
alice.MustJoinRoom(t, roomID, nil)
})

t.Run("Additional room creator can re-join room after everyone left", func(t *testing.T) {
roomID := alice.MustCreateRoom(t, map[string]interface{}{
"room_version": roomVersion12,
"creation_content": map[string]any{
"additional_creators": []string{bob.UserID},
},
})
alice.MustLeaveRoom(t, roomID)
bob.MustLeaveRoom(t, roomID)
bob.MustJoinRoom(t, roomID, nil)
})

t.Run("PL event is missing creator in users map", func(t *testing.T) {
roomID := alice.MustCreateRoom(t, map[string]interface{}{
"room_version": roomVersion12,
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