MSC4319: Room member events in stripped state

[zecakeh]

This was spawned by the discussion in matrix-org/matrix-spec#2181.

Server Implementations:

• Synapse:
  • Own non-stripped m.room.member in invite_state: https://github.com/element-hq/synapse/blob/458e6410e8388fbc6b60b5575850405939bc53eb/synapse/rest/client/sync.py#L450-L462
  • Stripped inviter's m.room.member in invite_state: https://github.com/element-hq/synapse/blob/a6e326582f735e03e420c3b86475388fb5f2656c/synapse/handlers/message.py#L1835-L1839
  • Own non-stripped m.room.member in knock_state: https://github.com/element-hq/synapse/blob/458e6410e8388fbc6b60b5575850405939bc53eb/synapse/rest/client/sync.py#L486-L514
• Dendrite:
  • Own non-stripped m.room.member in invite_state: https://github.com/element-hq/dendrite/blob/8d2da78744387e55c28bb8925ae0cc70dd3e02e3/syncapi/types/types.go#L530-L574
  • Does not seem to add the inviter in invite_state.
  • Does not have support for knock_state.
• Conduit (and forks):
  • Own stripped m.room.member in invite_state (common to all forks): https://gitlab.com/famedly/conduit/-/blob/ed5b0514f52f848e53785dee5fe8de63780cf092/src/api/server_server.rs#L2160-2172
  • Stripped inviter's m.room.member in invite_state (Conduit): https://gitlab.com/famedly/conduit/-/merge_requests/766
  • Own non-stripped m.room.member in knock_state (Conduit): https://gitlab.com/famedly/conduit/-/blob/ed5b0514f52f848e53785dee5fe8de63780cf092/src/api/server_server.rs#L2160-2172

Most clients already rely on being able to access the m.room.member events of the knock or invite process, and of the inviter.

The following client implementations also rely on being able to access the origin_server_ts:

• Cinny: https://github.com/cinnyapp/cinny/blob/67b05eeb09c8019fc67564f65ebca3d4b37653c6/src/app/pages/client/inbox/Invites.tsx#L88
• Neochat: https://invent.kde.org/network/neochat/-/blob/c5caffcdf9b250eb5a6c57a40ab951fa188563d1/src/libneochat/neochatroom.cpp?page=2#L1661
• Ruma: client-api,events: Remove StrippedState and add optional fields to StrippedStateEvent instead ruma/ruma#2187

Rendered

[turt2live]

Alternatively, because clients want access to the origin_server_ts and generally speaking we don't want different formats for data in a list, this MSC could do two things:

1. Add origin_server_ts to all stripped state events
2. Add the inviting m.room.member event to the list of suggested state

[zecakeh]

I also found a use case for accessing unsigned of an invite m.room.member event, for clients to be able to know if the invite is after a knock by looking into the prev_content.

[turt2live]

I'm not seeing why that would be important to them, or why that needs to come from unsigned instead of somewhere else

[zecakeh]

To auto-accept an invite following a knock for example. If it's a new device there is no other place where this information can be found.

[zecakeh]

I changed the stripped state event format to include origin_server_ts and unsigned (since this one is already optional in other event formats anyway).

[Gnuxie]

Clients have depended on synapse's behaviour of including the full client formatted event for the m.room.member invite event.

the-draupnir-project/Draupnir#945

In draupnir this dependency is more than just a parsing issue. The matrix-protection-suite provides a handle called handleExternalMembership which invitations are fed into from the matrix-protection-suite's conception of the timeline, which is closer to appservice push than /sync. As appservice push also pushed invitations this way.

Deduplication and buffering is then coordinated using the event id as it is identical to any other client formatted event and not special cased. So this is a deep dependency.

https://github.com/Gnuxie/matrix-protection-suite/blob/a85102c0aaa6ba73beae8fdb35600eec0999eada/src/Protection/Protection.ts#L176
https://github.com/the-draupnir-project/Draupnir/blob/main/src/protections/invitation/JoinRoomsOnInviteProtection.tsx#L86-L96
https://github.com/the-draupnir-project/Draupnir/blob/f1dad52288ff32a096cbd3ff5be110648574c51f/src/Draupnir.ts#L355
https://github.com/Gnuxie/matrix-protection-suite/blob/453e55b182282aad59db430f53549d89541a8e21/src/ClientManagement/StandardClientRooms.ts#L95-L126
https://github.com/the-draupnir-project/Draupnir/blob/f1dad52288ff32a096cbd3ff5be110648574c51f/src/DraupnirBotMode.ts#L97-L99
https://github.com/the-draupnir-project/Draupnir/blob/f1dad52288ff32a096cbd3ff5be110648574c51f/src/appservice/AppService.ts#L355

[Gnuxie]

The change complicates clients that allow either /sync or appservice push to be their event source. Since now invitations sourced from /sync will be a different format to those provided in appservice push. See #4319 (comment).

[tulir]

Additionally, the appservice event can't be stripped, because then you'd have two different data formats in the events array of /transactions

In this specific context, the AS API is closer to S-S than C-S /sync, as both AS and S-S send full invite events with stripped state in unsigned -> invite_room_state, while /sync flattens invite event + stripped state into an invite_state array