Synapse omits `unsigned.membership` property on events served as part of a relationship aggregation

[richvdh]

The spec describes an unsigned.membership property, which SHOULD be populated for all events. (Added by MSC4115.)

It also describes an unsigned.[m.relations] property which aggregates relationships on the relevant event. The aggregation format for edits and thread threads both contain another event.

Synapse does not populate the unsigned.membership property for events served as part of such an aggregation.

This applies to at least /event, /context, /relations and /sync. (Note that /event also omits it on the top-level event: #17486.)

For example, if /sync returns an event which has been edited, I expect a response like this, but, in practice, the membership property on the inner event is absent:

{
  "type": "m.room.message",
  "sender": "@a:xps9320.sw1v.org",
  "content": { "msgtype": "m.text", "body": "def", "m.mentions": {} },
  "origin_server_ts": 1721927656960,
  "unsigned": {
    "membership": "join",
    "age": 1058542,
    "m.relations": {
      "m.replace": {
        "type": "m.room.message",
        "room_id": "!ScJmwYjNJCpEMjYXxe:xps9320.sw1v.org",
        "sender": "@a:xps9320.sw1v.org",
        "content": { 
          "msgtype": "m.text", "body": " * defg",
          "m.new_content": { "msgtype": "m.text", "body": "defg", "m.mentions": {}},
          "m.mentions": {},
          "m.relates_to": { "rel_type": "m.replace", "event_id": "$Gpy6Cs5oGKtUWeJ3OsFeDtTeAKkOYN2zzTlpbCNcy1E" }
        },
        "origin_server_ts": 1721927685762,
        "unsigned": {
          // THIS PROPERTY IS OMITTED!
          "membership": "join",
          "age": 1029740
        },
        "event_id": "$AJk4WFjN2Jw2-Z0FM0HyBRcFnhhY-z4UXMjjpOke_Wo"
      }
    }
  },
  "event_id": "$Gpy6Cs5oGKtUWeJ3OsFeDtTeAKkOYN2zzTlpbCNcy1E"
}