import sync

struct One {
pub mut:
    i int
}

fn (mut o One) add(i int) {
    o.i = o.i + i
}

fn run(mut once sync.Once, mut o One, c chan bool) {
    once.do(fn [mut o] () {
        o.add(5)
    })
    c <- true
}

fn test_once() {
    mut o := &One{}
    mut once := sync.new_once()
    c := chan bool{}
    n := 10

    // It is executed 10 times, but only once actually.
    for i := 0; i < n; i++ {
        spawn run(mut once, mut o, c)
    }
    for i := 0; i < n; i++ {
        <-c
    }
    assert o.i == 5
}