Function configure_once 

fn configure_once<F>(socket_path: &str, plan: &F) -> Result<usize, ConfigErr>
where
    F: Fn(&[Screen]) -> Result<Vec<(ZwlrOutputHeadV1, Plan)>, String>,

One attempt: a connection of its own, the heads and the serial that stamps them, and plan applied against that state. A retry reconnects rather than waiting for a fresh serial on the one it has, because the change that canceled it is often a head arriving or leaving, and the heads a kept connection holds do not survive that.