Function spawn 

pub fn spawn(
    bind_addr: SocketAddr,
    promote: Option<PromotionRequest>,
    rotate_requested: Option<Arc<AtomicBool>>,
    ack_policy: Option<Arc<AtomicU8>>,
    shutdown: Arc<AtomicBool>,
    authorized_keys: Arc<AuthorizedKeys>,
) -> Result<(JoinHandle<()>, SocketAddr), Box<dyn Error>>

Spawn the admin listener on a dedicated thread.

The listener is bound (and set non-blocking) here, before the thread starts, so a taken or misconfigured admin port fails startup loudly — bound inside the thread it died with an error! nobody joins on, leaving PROMOTE/ROTATE/ACK-POLICY silently dead on a healthy-looking node. Returns the bound address alongside the handle (they differ from bind_addr when it carries port 0).

Any of promote / rotate_requested / ack_policy may be None to disable the corresponding command on this node. The listener still accepts connections and authenticates them — a disabled command is rejected at the command-dispatch step, not at connect time, so operator tooling sees a structured ERR rather than a TCP RST.