sync: only release semaphore in WaitGroup when there are waiters (#10967)

vlib/sync/waitgroup_test.v

@@ -0,0 +1,41 @@
+module sync
+
+import time
+
+fn test_waitgroup_reuse() {
+	mut wg := new_waitgroup()
+
+	wg.add(1)
+	wg.done()
+
+	wg.add(1)
+	mut executed := false
+	go fn (mut wg WaitGroup, executed voidptr) {
+		defer {
+			wg.done()
+		}
+		unsafe {
+			*(&bool(executed)) = true
+		}
+		time.sleep(100 * time.millisecond)
+		assert wg.wait_count == 1
+	}(mut wg, voidptr(&executed))
+
+	wg.wait()
+	assert executed
+	assert wg.wait_count == 0
+}
+
+fn test_waitgroup_no_use() {
+	mut done := false
+	go fn (done voidptr) {
+		time.sleep(1 * time.second)
+		if *(&bool(done)) == false {
+			panic('test_waitgroup_no_use did not complete in time')
+		}
+	}(voidptr(&done))
+
+	mut wg := new_waitgroup()
+	wg.wait()
+	done = true
+}