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 <erikj@jki.re>

Co-authored-by: Erik Johnston <erikj@jki.re>

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

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+# 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).