Separate WaitGroup (#572)

* Separate WaitGroup

* Update core/utils/wait_group.hpp

Co-authored-by: Andrei Lobov <andrei.lobov@arangodb.com>

* Exact count

* fix lints

---------

Co-authored-by: Andrei Lobov <andrei.lobov@arangodb.com>

core/index/index_writer.hpp

@@ -45,6 +45,7 @@
 #include "utils/object_pool.hpp"
 #include "utils/string.hpp"
 #include "utils/thread_utils.hpp"
+#include "utils/wait_group.hpp"
 
 #include <absl/container/flat_hash_map.h>
 
@@ -813,35 +814,7 @@ class IndexWriter : private util::noncopyable {
     std::deque<PendingSegmentContext> pending_segments_;
     // entries from 'pending_segments_' that are available for reuse
     Freelist pending_freelist_;
-    // TODO(MBkkt) Considered to replace with YACLib in ArangoDB 3.11+ or ...
-    struct WaitGroup {
-      std::mutex& Mutex() noexcept { return m_; }
-
-      void Add() noexcept { counter_.fetch_add(1, std::memory_order_relaxed); }
-
-      void Done() noexcept {
-        if (counter_.fetch_sub(1, std::memory_order_acq_rel) == 1) {
-          std::lock_guard lock{m_};
-          cv_.notify_one();
-        }
-      }
-
-      void Wait() noexcept {
-        if (counter_.fetch_sub(1, std::memory_order_acq_rel) != 1) {
-          std::unique_lock lock{m_};
-          while (counter_.load(std::memory_order_acquire) != 0) {
-            cv_.wait(lock);
-          }
-        }
-        // We can put acquire here and remove above, but is it worth?
-        counter_.store(1, std::memory_order_relaxed);
-      }
-
-     private:
-      std::atomic_size_t counter_{1};
-      std::mutex m_;
-      std::condition_variable cv_;
-    } pending_;
+    WaitGroup pending_;
 
     // set of segments to be removed from the index upon commit
     ConsolidatingSegments segment_mask_;

core/utils/wait_group.hpp

@@ -0,0 +1,76 @@
+////////////////////////////////////////////////////////////////////////////////
+/// DISCLAIMER
+///
+/// Copyright 2016 by EMC Corporation, All Rights Reserved
+///
+/// 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
+///
+///     http://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.
+///
+/// Copyright holder is EMC Corporation
+///
+/// @author Valery Mironov
+////////////////////////////////////////////////////////////////////////////////
+
+#pragma once
+
+#include <atomic>
+#include <condition_variable>
+#include <mutex>
+
+namespace irs {
+
+// TODO(MBkkt) Considered to replace with YACLib
+struct WaitGroup {
+  explicit WaitGroup(size_t counter = 0) noexcept : counter_{2 * counter + 1} {}
+
+  void Add(size_t counter = 1) noexcept {
+    counter_.fetch_add(2 * counter, std::memory_order_relaxed);
+  }
+
+  void Done(size_t counter = 1) noexcept {
+    if (counter_.fetch_sub(2 * counter, std::memory_order_acq_rel) ==
+        2 * counter) {
+      std::lock_guard lock{m_};
+      cv_.notify_one();
+    }
+  }
+
+  // Multiple parallel Wait not supported, if needed check YACLib
+  void Wait(size_t counter = 0) noexcept {
+    if (counter_.fetch_sub(1, std::memory_order_acq_rel) != 1) {
+      std::unique_lock lock{m_};
+      while (counter_.load(std::memory_order_acquire) != 0) {
+        cv_.wait(lock);
+      }
+    }
+    // We can put acquire here and remove above, but is it worth?
+    Reset(counter);
+  }
+
+  // It shouldn't used for synchronization
+  size_t Count() const noexcept {
+    return counter_.load(std::memory_order_relaxed) / 2;
+  }
+
+  void Reset(size_t counter) noexcept {
+    counter_.store(2 * counter + 1, std::memory_order_relaxed);
+  }
+
+  std::mutex& Mutex() noexcept { return m_; }
+
+ private:
+  std::atomic_size_t counter_;
+  std::condition_variable cv_;
+  std::mutex m_;
+};
+
+}  // namespace irs