Function pick_stack_name 

pub fn pick_stack_name(
    candidates: &[String],
    service: &str,
    slot: u32,
) -> Option<String>

The first valid stack name of candidates whose instance names of service fit unshortened, else the first instance_name can shorten. Unshortened first, because that is how an existing stack (a running preview’s) was picked, so it keeps its name.