Synapse does not report a device_id for appservice masqueraded requests to /whoami when using MSC3861

[Half-Shot]

This seems to be a bit of missing functionality in the implementation of delegated auth.

On vanilla Synapse, requesting a GET /_matrix/client/v3/account/whoami will return the masqueraded device back in the response.

$ curl -X GET -H "Authorization: Bearer my_as_token" 'http://example.com/_matrix/client/v3/account/whoami?org.matrix.msc3202.device_id=my_device&user_id=@my_user:example.com`
HTTP/1.1 200 OK

{
    "user_id": "@my_user:example.com",
    "is_guest": false,
    "device_id": "my_device"
}

whereas the same request on a MAS-enabled synapse will give no device ID at all:

$ curl https://mas-enabled-example.com/_matrix/client/v3/account/whoami\?access_token=my_as_token&org.matrix.msc3202.device_id=my_device\&user_id\=@my_user:example.com
HTTP/2 200

{
    "user_id": "@auditbot:matrix.staging.ems.element.dev",
    "is_guest": false
}

While this might seem like a fairly easy thing to workaround, the library we use to make these requests would need to specialcase around this a bit more. Ideally, the same request would give the same response.

[Half-Shot]

In the end it was a misconfiguration, I just needed unstable_features.msc3202_device_masquerading