CSAPI `GET /rooms/{roomId}/event/{eventId}` does not pull events over federation, breaking pinned messages (it's not clear how clients should fetch `m.room.pinned_events`)

[reivilibre]

Link to problem area:

• https://spec.matrix.org/v1.12/client-server-api/#mroompinned_events
• https://spec.matrix.org/v1.12/client-server-api/#get_matrixclientv3roomsroomideventeventid

Issue

Pinned events currently don't work in Element Web if the user is on a (Synapse) homeserver that doesn't have the event already in the local homeserver database.

The root cause appears to be because Element Web is fetching pinned events using the CSAPI GET /rooms/{roomId}/event/{eventId} endpoint, which does not pull events over federation. (The end result is that new users do not see the pinned events, assuming they aren't using a homeserver that was already in the room before. This degrades their utility as a replacement for the room topic.) Two problems here:

• The spec doesn't prescribe any behaviour for clients to fetch the events to display the pinned messages; I don't think you can blame Element Web from trying the obvious here.
• The spec doesn't mention whether GET /rooms/{roomId}/event/{eventId} should or should not pull over federation.

Even if this endpoint should pull over federation, it's not clear who to pull from, given we have no hint as to which server has that event or not.

[Giwayume]

The roomId has the origin homeserver name on it, like !ABCDEFGHIJKLMNO123:matrix.org, unless I'm missing something, why wouldn't it be:

1. Pull from local homeserver if it has it.
2. Otherwise, pull from origin homeserver.
3. All else fails, 404.

[turt2live]

The server name in the room ID has no meaning: https://spec.matrix.org/v1.17/appendices/#room-ids

[Giwayume]

When I say "origin homeserver" that's the homeserver of the user who created the room, which every homeserver should have knowledge of. I haven't seen that part of the spec, but even if the domain is missing from the room ID, origin homeserver is discoverable.