sync.once: add Once (#12722)

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()
+}