Function get_keys_helper 

pub async fn get_keys_helper<F>(
    services: &Services,
    sender_user: Option<&UserId>,
    device_keys_input: &BTreeMap<OwnedUserId, Vec<OwnedDeviceId>>,
    allowed_signatures: F,
    include_display_names: bool,
) -> Result<Response>
where
    F: Fn(&UserId) -> bool + Send + Sync,

Queries device and cross-signing keys for the requested users.

The signature predicate filters returned signatures, and the display-name switch selects configured device names rather than device IDs.