sync: add sync.ManyTimes (#12729)

vlib/sync/many_times_test.v

@@ -0,0 +1,49 @@
+import sync
+
+struct Counter {
+pub mut:
+	i int
+}
+
+fn (mut c Counter) add(i int) {
+	c.i = c.i + i
+}
+
+fn run(mut m sync.ManyTimes, mut co Counter, c chan bool) {
+	m.do(fn [mut co] () {
+		co.add(5)
+	})
+	c <- true
+}
+
+fn test_many_times_once() {
+	mut co := &Counter{}
+	mut m := sync.new_many_times(1)
+	c := chan bool{}
+	n := 10
+
+	// It is executed 10 times, but only once actually.
+	for i := 0; i < n; i++ {
+		go run(mut m, mut co, c)
+	}
+	for i := 0; i < n; i++ {
+		<-c
+	}
+	assert co.i == 5
+}
+
+fn test_many_times_fifth() {
+	mut co := &Counter{}
+	mut m := sync.new_many_times(5)
+	c := chan bool{}
+	n := 10
+
+	// It is executed 10 times, but only 5 times actually.
+	for i := 0; i < n; i++ {
+		go run(mut m, mut co, c)
+	}
+	for i := 0; i < n; i++ {
+		<-c
+	}
+	assert co.i == 25
+}