sync.once: add Once (#12722)

vlib/sync/once_test.v

@@ -0,0 +1,33 @@
+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++ {
+		go run(mut once, mut o, c)
+	}
+	for i := 0; i < n; i++ {
+		<-c
+	}
+	assert o.i == 5
+}