Function refresh_task 

async fn refresh_task(
    federated_token_file: PathBuf,
    exchange: Arc<dyn Fn(String, Vec<String>) -> BoxFuture<'static, Result<AccessToken>> + Send + Sync>,
    slot: Arc<RwLock<AccessToken>>,
    scopes: Vec<String>,
    refresh_buffer: Duration,
    retry_interval: Duration,
)

Keeps slot holding an unexpired token by fetching a replacement within refresh_buffer of the current token’s expiry. A failed fetch leaves the current token in place and is retried after retry_interval.