matrix-doc/proposals/3943-heroes-in-partial-send-join-response.md

MSC3943: Partial joins to nameless rooms should include heroes' memberships.

This is an addendum to MSC3706 which ensures that partial-joining a nameless room has a reasonable client UX.

Background

MSC3706 extends the federation /send_join endpoint to optionally return a "partial state" response. In this mode of operation, the joining server is given only a subset of the current state¹ of the room to be joined; the full state is lazily loaded by the joining server.

MSC3706 specifies this subset

state: if partial state is being returned, then state events with event type m.room.member are omitted from the response. All other room state is returned as normal.

because

Currently, m.room.member events are by far the biggest problem.

but also notes that

In future, the list of event types to omit could be expanded. (Some rooms may have large numbers of other state events).

Problem

Originally noted in synapse#12995.

Clients need to present a display name for the room, using m.room.name or m.room.canonical_alias state events² as described in the spec. If neither event exists, then the client falls back to a name calculated based on the membership events for the "heroes" of the room.

The problem is that the partial-joining server does not have the heroes' membership events until it fetches the full state of the room; in this situation, the room has no defined display name. This results in a poor end-user experience.

Proposal

When serving a partial join, if the room to be joined has no m.room.name or m.room.canonical_alias events in its current state, the resident server SHOULD

• determine the heroes for the room (as specced today),
• include the heroes' current membership events in the response state field, and
• include the heroes' current membership events' auth chains in the response auth_chain field.

Potential issues

This enlarges the state and the auth_chain returned in a partial state response. The effect on state is limited as there are at most 5 heroes in any given room. The auth_chain for the heroes' membership events may be arbitrarily long.

Alternatives

• Have the resident server compute a dummy room name and express this as a new field in the /send\_join response. This is hard to internationalise and complicates the data model for minimal gain.
• Require clients to use the room ID or alias which initiated the join as a room displayname.
  • What if the join was actioned by a different client device, or by the server directly?
• Change the spec's display name rules to fallback to the room ID if there are no membership events available.
  • Poor end-user experience: room IDs are opaque.
  • This would also gives rooms that contain only an m.room.create event a defined display name. But that's not especially useful.

Security considerations

None at present. Or better put: this doesn't introduce any new concerns.

• The joining server is already privvy to these additional membership events. It will receive them when it receives the full state from the resident server.
• The joining server already trusts that the resident server is sending an accurate partial and full state.

Unstable prefix

Not required.

Dependencies

MSC3706.

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1. More precisely: the current state of the room to be joined from the perspective of the resident server. ↩︎

2. More precisely: using state events with (type, state_key) pair equal to ('m.room.name', '') or ('m.room.canonical_alias', ''). ↩︎