Include `event_id` when `/state/:type[/:key]?format=event`

[MadLittleMods]

Problem

Currently when you GET /_matrix/client/v3/rooms/{room_id}/state/m.room.create?format=event (notice the event format), it doesn't include event_id which is kinda annoying to work with and reference.

Example:

{
    "content": {
        "creator": "@foo:bar",
        "room_version": "6"
    },
    "origin_server_ts": 1606978812896,
    "room_id": "!abc",
    "sender": "@foo:bar",
    "state_key": "",
    "type": "m.room.create",
    "unsigned": {
        "age_ts": 1606978812896
    }
}

Proposal

Add event_id to the output. There is no spec to go off of but seems like a non-breaking change we can do.

---

Spawning from a need in matrix-org/matrix-viewer#167 to get the event_id of the m.room.create event in the room.

Is there a better way to do this? Perhaps /_matrix/client/v3/rooms/{room_id}/messages?dir=f&limit=1

Dev notes

• Relevant spec issue for the ?format query parameter: Spec ?format=event|content on GET /state/:type[/:key] matrix-spec#1047
• ?format introduced to Synapse in Allow clients to ask for the whole of a single state event #1094

Relevant code:

synapse/synapse/rest/client/room.py

Lines 258 to 262 in d935b80

	if format == "event":
	event = format_event_for_client_v2(data.get_dict())
	return 200, event
	elif format == "content":
	return 200, data.get_dict()["content"]

Currently, we use format_event_for_client_v2(data.get_dict()) against the EventBase PDU JSON dictionary which doesn't include event_id. We could instead use serialize_event(data, self.clock.time_msec()) which would get us something more workable.

We could also alternatively, add event_id to the dictionary returned by EventBase.get_dict() but that probably has too many downstream side-effects like messing with the PDU JSON serialization stuff.

synapse/synapse/events/__init__.py

Lines 362 to 366 in d935b80

	def get_dict(self) -> JsonDict:
	d = dict(self._dict)
	d.update({"signatures": self.signatures, "unsigned": dict(self.unsigned)})
	return d

---

Separately, it would be good to standardize with the ?event_format=client|federation parameter used with other endpoints

[H-Shay]

We discussed this in the backend meeting and the consensus was that it should be okay to go ahead and do this, with the caveat that the spec issue should be updated when it's done.

[DMRobertson]

(The rationale for it being this way is that the event ID is not part of the event (since room v2); it's a form of derived data.)

[MadLittleMods]

Makes sense for how it could have ended up this way 👍

For the /state client API, I think it makes less sense since it's going to be hard and cumbersome to use Canonical JSON to calculate the event ID yourself on all platforms. Looks like there is a spec'ed ClientEvent format .