From 26f5effe45169cd843c45fc3325dbd649593f1f1 Mon Sep 17 00:00:00 2001 From: David Robertson Date: Mon, 23 Jan 2023 02:51:54 +0000 Subject: [PATCH] MSC3943: Partial joins to nameless rooms should include heroes' memberships (#3943) * Draft * Assign MSC number * Fix underscores inside backticks * typo fix Co-authored-by: Erik Johnston Co-authored-by: Erik Johnston --- ...43-heroes-in-partial-send-join-response.md | 70 +++++++++++++++++++ 1 file changed, 70 insertions(+) create mode 100644 proposals/3943-heroes-in-partial-send-join-response.md diff --git a/proposals/3943-heroes-in-partial-send-join-response.md b/proposals/3943-heroes-in-partial-send-join-response.md new file mode 100644 index 00000000..e8decc5f --- /dev/null +++ b/proposals/3943-heroes-in-partial-send-join-response.md @@ -0,0 +1,70 @@ +# MSC3943: Partial joins to nameless rooms should include heroes' memberships. + +This is an addendum to [MSC3706](https://github.com/matrix-org/matrix-spec-proposals/pull/3706) which ensures that partial-joining a nameless room has a reasonable client UX. + +## Background + +[MSC3706](https://github.com/matrix-org/matrix-spec-proposals/pull/3706) 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 state[^1] of the room to be joined; the full state is lazily loaded by the joining server. + +[^1]: More precisely: the current state of the room to be joined _from the perspective of the resident 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](https://github.com/matrix-org/synapse/issues/12995)._ + +Clients need to present a display name for the room, using `m.room.name` or `m.room.canonical_alias` state events[^2] as described [in the spec](https://spec.matrix.org/v1.5/client-server-api/#calculating-the-display-name-for-a-room). If neither event exists, then the client falls back to a name calculated based on the membership events for the "heroes" of the room. + +[^2]: More precisely: using state events with (type, state\_key) pair equal to `('m.room.name', '')` or `('m.room.canonical_alias', '')`. + +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](https://github.com/matrix-org/matrix-spec-proposals/pull/3706).