sync.once: add Once (#12722)

cmd/tools/vtest-self.v

@@ -115,6 +115,7 @@ const (
 		'vlib/vweb/request_test.v',
 		'vlib/net/http/request_test.v',
 		'vlib/vweb/route_test.v',
+		'vlib/sync/once_test.v',
 	]
 	skip_on_non_windows           = [
 		'do_not_remove',

vlib/sync/once.v

@@ -0,0 +1,33 @@
+module sync
+
+import sync.atomic2
+
+pub struct Once {
+mut:
+	m RwMutex
+pub:
+	count u64
+}
+
+// new_once return a new Once struct.
+pub fn new_once() &Once {
+	mut once := &Once{}
+	once.m.init()
+	return once
+}
+
+// do execute the function only once.
+pub fn (mut o Once) do(f fn ()) {
+	if atomic2.load_u64(&o.count) < 1 {
+		o.do_slow(f)
+	}
+}
+
+fn (mut o Once) do_slow(f fn ()) {
+	o.m.@lock()
+	if o.count < 1 {
+		atomic2.store_u64(&o.count, 1)
+		f()
+	}
+	o.m.unlock()
+}

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
+}