添加 CountDownLatch

添加了 CountDownLatch，一个类似 golang 中 sync.WaitGroup 的工具类

Author: zhcpku

libgo/coroutine.h

@@ -71,3 +71,21 @@ typedef ::co::CoTimer::TimerId co_timer_id;
 #define co_last_defer() ::co::GetLastDefer()
 #define co_defer_scope co_defer [&]
 
+class CountDownLatch {
+public:
+    explicit CountDownLatch(size_t n = 1) : mFlyingCount(n) {}
+
+    void Done() {
+        --mFlyingCount;
+    }
+    void Wait() {
+        while (mFlyingCount) {
+            co_yield;
+        }
+    }
+private:
+    size_t mFlyingCount;
+
+    CountDownLatch(CountDownLatch const &) = delete;
+    CountDownLatch& operator=(CountDownLatch const &) = delete;
+};