sync: add implementation for WaitGroup.go/1, add test (#24797)

vlib/sync/waitgroup.c.v

@@ -82,3 +82,14 @@ pub fn (mut wg WaitGroup) wait() {
 	C.atomic_fetch_add_u32(voidptr(&wg.wait_count), 1)
 	wg.sem.wait() // blocks until task_count becomes 0
 }
+
+// go starts `f` in a new thread, arranging to call wg.add(1) before that,
+// and wg.done() in the same thread. The function `f` should not panic.
+// Calls to wg.go() should happen before the call to wg.wait().
+pub fn (mut wg WaitGroup) go(f fn ()) {
+	wg.add(1)
+	spawn fn (mut wg WaitGroup, f fn ()) {
+		f()
+		wg.done()
+	}(mut wg, f)
+}

vlib/sync/waitgroup_test.v

@@ -39,3 +39,16 @@ fn test_waitgroup_no_use() {
 	wg.wait()
 	done = true
 }
+
+fn test_waitgroup_go() {
+	mut counter := 0
+	p := unsafe { &counter }
+	mut wg := new_waitgroup()
+	for i in 0 .. 10 {
+		wg.go(fn [p] () {
+			unsafe { (*p)++ }
+		})
+	}
+	wg.wait()
+	assert counter == 10
+}