Synapse does not correctly send device-list-un-tracked indications in `/sync` responses when lazy-loading is enabled

[richvdh]

• Alice is in a room, and uses lazy-loading of memberships
• Charlie joins the room. Alice sees the join event in her sync, together with a device_lists update:

  {
    "next_batch":"s7_2_0_1_1_1_1_4_0_1",
    "device_lists":{"changed":["@user-3-charlie:hs1"]},
    "rooms":{"join":{"!RcwoXhPrnmxBAlwqGb:hs1":{
      "timeline":{
        "events":[
          {"type":"m.room.member","sender":"@user-3-charlie:hs1","content":{"membership":"join","displayname":"user-3-charlie"},"state_key":"@user-3-charlie:hs1","origin_server_ts":1708524078957,"unsigned":{"age":22},"event_id":"$qq0CJ2AtNKVNGaYZkRiNpnirejAH8zvkWZ2DQh_ftBc"}
        ],
        "prev_batch":"s6_2_0_1_1_1_1_4_0_1",
        "limited":false
      },
      "state":{"events":[]}
    }}}
  }

• Charlie sends a message, and then leaves the room. Alice syncs again. The server returns both of Charlie's events, and another device_lists update:

  {
    "next_batch":"s9_4_0_1_1_1_1_4_0_1",
    "device_lists":{"left":["@user-3-charlie:hs1"]},
    "rooms":{"join":{"!RcwoXhPrnmxBAlwqGb:hs1":{
      "timeline":{
        "events":[
          {"type":"m.room.message","sender":"@user-3-charlie:hs1","content":{"body":"test0","msgtype":"m.text"},"origin_server_ts":1708524078982,"unsigned":{"age":56},"event_id":"$mxxcBMt7WBKGVC84XMerO1O5Kd1GOoATGaChzHhCOqY"},
          {"type":"m.room.member","sender":"@user-3-charlie:hs1","content":{"membership":"leave"},"state_key":"@user-3-charlie:hs1","origin_server_ts":1708524079008,"unsigned":{"replaces_state":"$qq0CJ2AtNKVNGaYZkRiNpnirejAH8zvkWZ2DQh_ftBc","prev_content":{"membership":"join","displayname":"user-3-charlie"},"prev_sender":"@user-3-charlie:hs1","age":30},"event_id":"$1vNTMX633F6JSBoZCb981mGIBE0QTyJ4C4bnAtYt0Jc"}
        ],
        "prev_batch":"s7_4_0_1_1_1_1_4_0_1",
        "limited":false
      },
      "state":{"events":[]}
    }}}
  }

• So far so good. However, the sync response gets dropped, and Alice syncs again using the same sync token. Charlie's join event is now (redundantly) included in the state section of the response, but more worryingly, Charlie is now listed under "changed" rather than "left" in the device lists section:

  {
    "next_batch":"s9_4_0_1_1_1_1_4_0_1",
    "device_lists":{"changed":["@user-3-charlie:hs1"]},
    "rooms":{"join":{"!RcwoXhPrnmxBAlwqGb:hs1":{
      "timeline":{
        "events":[
          {"type":"m.room.message","sender":"@user-3-charlie:hs1","content":{"body":"test0","msgtype":"m.text"},"origin_server_ts":1708524078982,"unsigned":{"age":64},"event_id":"$mxxcBMt7WBKGVC84XMerO1O5Kd1GOoATGaChzHhCOqY"},
          {"type":"m.room.member","sender":"@user-3-charlie:hs1","content":{"membership":"leave"},"state_key":"@user-3-charlie:hs1","origin_server_ts":1708524079008,"unsigned":{"replaces_state":"$qq0CJ2AtNKVNGaYZkRiNpnirejAH8zvkWZ2DQh_ftBc","prev_content":{"membership":"join","displayname":"user-3-charlie"},"prev_sender":"@user-3-charlie:hs1","age":38},"event_id":"$1vNTMX633F6JSBoZCb981mGIBE0QTyJ4C4bnAtYt0Jc"}
        ],
        "prev_batch":"s7_4_0_1_1_1_1_4_0_1",
        "limited":false
      },
      "state":{"events":[
        {"type":"m.room.member","sender":"@user-3-charlie:hs1","content":{"membership":"join","displayname":"user-3-charlie"},"state_key":"@user-3-charlie:hs1","origin_server_ts":1708524078957,"unsigned":{"age":89},"event_id":"$qq0CJ2AtNKVNGaYZkRiNpnirejAH8zvkWZ2DQh_ftBc"}
      ]}
    }}}
  }

[richvdh]

Test case at matrix-org/complement#714