Constant RESYNC_ERR_ALREADY_EXITED 

pub const RESYNC_ERR_ALREADY_EXITED: &str = "resync_controller: controller already exited (respawn suppressed)";

Sentinel error string resync_controller returns when it declines to respawn a STATUS_DONE controller because its caller passed respawn_if_done: false (see that parameter’s own doc comment). Callers that want to treat “already exited” differently from a genuine failure (e.g. try_attach_to_existing_shell falling through to a fresh shell instead of surfacing a 500) match on this exact string rather than parsing every other possible error message resync_controller can return.