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MSC 1794 - Federation v2 Invite API

This proposal adds a new /invite API to federation that supports different room versions.

Motivation

It is planned for future room versions to be able to change the format of events in various ways. To support this, all servers must know the room version whenever they need to parse an event. Currently the /invite API does not include the room version, so the target server would be unable to parse the event included in the payload.

Proposal

Add a new version of the invite API under the prefix /_matrix/federation/v2, which has a payload of:

PUT /_matrix/federation/v2/invite/<room_id>/<event_id>

{
    "room_version": <room_version>,
    "event": { ... },
    "invite_room_state": [ ... ]
}

The differences between this and v1 are:

The payload in v1 is the event, while in v2 the event is instead placed under an "event" key. This is for consistency with other APIs, and to allow extra data to be added to the request payload separately from the event.
A required field called "room_version" is added that specifies the room version.
The "invite_room_state" is moved from the unsigned section of the event to a top level key. The key invite_room_state being in the event.unsigned was a hack due to point 1. above.

The response is identical to v1, except that:

If the receiving server does not support the given room version the appropriate incompatible-room-version error is returned, in the same way as e.g. for /make_join APIs.
The response payload is no longer in the format of [200, { ... }], and is instead simply the { ... } portion. This fixes a historical accident to bring the invite API into line with the rest of the federation API.

If a call to v2 /invite results in an unrecognised request exception AND the room version is 1 or 2 then the sending server should retry the request with the v1 API.

Alternatives

Reusing V1 API

One alternative is to add a room_version query string parameter to the v1 /invite API in a similar way as for the /make_join APIs. However, older servers would ignore the query string parameter while processing an incoming /invite request, resulting in the server attempting to parse the event in the old v1 format. This would likely result in either a 400 or 500 response, which the sending server could interpret as the receiving server not supporting the room version.

This method, however, is fragile and could easily mask legitimate 400 and 500 errors that are not due to not supporting the room version.

Using V1 API for V1 room versions

Instead of all servers attempting to use the new API and falling back if the API is not found, servers could instead always use the current API for V1 and V2 rooms.

However, this would not allow us to deprecate the v1 API.

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