Spec unsigned.membership property, per MSC4115

changelogs/client_server/newsfragments/1847.feature

@@ -0,0 +1 @@
+Add the new `unsigned.membership` property to events served over the client-server API, as per [MSC4115](https://github.com/matrix-org/matrix-spec-proposals/pull/4115).

data/api/client-server/definitions/client_event_without_room_id.yaml

@@ -90,6 +90,7 @@ properties:
           "origin_server_ts": 1632491098485,
           "unsigned": {
             "age": 1257,
+            "membership": "leave"
           }
         }
       transaction_id:
@@ -112,3 +113,20 @@ properties:
             this.
         title: EventContent
         type: object
+      membership:
+        description: |
+          The room membership of the user making the request, at the time of the event.
+
+          This property is the value of the `membership` property of the
+          requesting user's [`m.room.member`](#mroommember) state at the point
+          of the event, including any changes caused by the event. If the user
+          had yet to join the room at the time of the event (i.e, they have no
+          `m.room.member` state), this property is set to `leave`.
+
+          It is recommended that homeservers SHOULD populate this property
+          wherever practical, but they MAY omit it if necessary (for example,
+          if calculating the value is expensive, servers might choose to only
+          implement it in encrypted rooms).
+        type: string
+        example: join
+        x-addedInMatrixVersion: "1.11"

data/event-schemas/examples/core/room_event.json

@@ -5,6 +5,7 @@
     "sender": "@example:example.org",
     "origin_server_ts": 1432735824653,
     "unsigned": {
-      "age": 1234
+      "age": 1234,
+      "membership": "join"
     }
 }