Roll forward to cl/314747126

Signed-off-by: Alexey Polyudov <apolyudov@google.com>
Change-Id: Ie19e006429b138b3768e97dae971a43fdc5ef8bf

cpp/platform_v2/api/count_down_latch.h

@@ -0,0 +1,45 @@
+// Copyright 2020 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef PLATFORM_V2_API_COUNT_DOWN_LATCH_H_
+#define PLATFORM_V2_API_COUNT_DOWN_LATCH_H_
+
+#include <cstdint>
+
+#include "platform_v2/base/exception.h"
+#include "absl/time/time.h"
+
+namespace location {
+namespace nearby {
+namespace api {
+
+// A synchronization aid that allows one or more threads to wait until a set of
+// operations being performed in other threads completes.
+//
+// https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/CountDownLatch.html
+class CountDownLatch {
+ public:
+  virtual ~CountDownLatch() = default;
+
+  virtual Exception Await() = 0;  // throws Exception::kInterrupted
+  virtual ExceptionOr<bool> Await(
+      absl::Duration timeout) = 0;  // throws Exception::kInterrupted
+  virtual void CountDown() = 0;
+};
+
+}  // namespace api
+}  // namespace nearby
+}  // namespace location
+
+#endif  // PLATFORM_V2_API_COUNT_DOWN_LATCH_H_