Sliding Sync: `RoomResult.avatar` won't report an avatar being removed

[jmartinesp]

Description

Currently, sliding sync will only return an avatar value for a room if the sync was an initial one or if the avatar was changed (and not null).

This gives us no easy way to check an avatar has been removed other than adding m.room.avatar to the required state, which would add a probably unnecessary overhead to responses and duplicate the value in case the avatar exists.

I'm not sure if this was done on purpose or it's just a side effect of the JSON serialization assuming a null value means the field shouldn't be added. Admittedly, it can be hard to distinguish between a 'this is null because there were no changes' and 'this is null because it just changed to that' values.

Steps to reproduce

• Remove an avatar from a room.
• Observe the next sliding sync response contains the new m.room.avatar event in the timeline but no avatar value.

Homeserver

matrix.org

Synapse Version

1.133.0rc1

Installation Method

Docker (matrixdotorg/synapse)

Database

Not sure, probably doesn't apply

Workers

I don't know

Platform

Whatever matrix.org uses.

Configuration

No response

Relevant log output

I don't have any from the server, but the sliding sync response after removing an avatar in my case is:

"!ZfTsEBEUwriiEPSJGY:matrix.org": {
  "notification_count": 0,
  "highlight_count": 0,
  "timeline": [
    {
      "content": {
        "url": null
      },
      "origin_server_ts": 1750937610048,
      "sender": "@jorgem_test:matrix.org",
      "state_key": "",
      "type": "m.room.avatar",
      "unsigned": {
        "replaces_state": "$Kbso6r9Zoa6egoC0njVuBOV8iCVI2CAq8sUtM6iysj8",
        "prev_content": {
          "url": "mxc://matrix.org/RSybBEpFiNToNXHVnxCiWXUL"
        },
        "prev_sender": "@jorgem_test:matrix.org",
        "membership": "join",
        "age": 1762
      },
      "event_id": "$oQpCG5Pof4dx3KzgAdsiiy9kcW99B9A2-IQ6i_Nwmh8"
    }
  ],
  "limited": false,
  "prev_batch": "s6022399461_757284974_215780_3757974183_4245471273_266005404_1502095329_11151677813_0_502149",
  "num_live": 1
}

Anything else that would be useful to know?

This issue causes the following Rust SDK issue in turn: matrix-org/matrix-rust-sdk#5285

[anoadragon453]

Hi @jmartinesp, thanks for reporting. This is indeed a bug. MSC4186 says:

// The room avatar, if one exists. Only sent initially and when it changes.
"avatar_url": str|null,

So the homeserver should be sending down avatar_url: null when the avatar is removed.

The relevant code paths are:

• synapse/synapse/rest/client/sync.py

  Lines 1086 to 1087 in 510924a

  	if room_result.avatar:
  	serialized_rooms[room_id]["avatar"] = room_result.avatar

• synapse/synapse/handlers/sliding_sync/__init__.py

  Lines 1127 to 1128 in 510924a

  	if initial or avatar_changed:
  	meta_room_state.append((EventTypes.RoomAvatar, ""))