text: /**
   *    Copyright (C) 2018-present MongoDB, Inc.
   *
   *    This program is free software: you can redistribute it and/or modify
   *    it under the terms of the Server Side Public License, version 1,
   *    as published by MongoDB, Inc.
   *
   *    This program is distributed in the hope that it will be useful,
   *    but WITHOUT ANY WARRANTY; without even the implied warranty of
   *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   *    Server Side Public License for more details.
   *
   *    You should have received a copy of the Server Side Public License
   *    along with this program. If not, see
   *    <http://www.mongodb.com/licensing/server-side-public-license>.
   *
   *    As a special exception, the copyright holders give permission to link the
   *    code of portions of this program with the OpenSSL library under certain
   *    conditions as described in each individual source file and distribute
   *    linked combinations including the program with the OpenSSL library. You
   *    must comply with the Server Side Public License in all respects for
   *    all of the code used other than as permitted herein. If you modify file(s)
   *    with this exception, you may extend this exception to your version of the
   *    file(s), but you are not obligated to do so. If you do not wish to do so,
   *    delete this exception statement from your version. If you delete this
   *    exception statement from all source files in the program, then also delete
   *    it in the license file.
   */
  
  #pragma once
  
  #include <boost/optional.hpp>
  
  #include "mongo/db/operation_context.h"
  #include "mongo/platform/mutex.h"
  #include "mongo/stdx/condition_variable.h"
  #include "mongo/util/assert_util.h"
  #include "mongo/util/duration.h"
  #include "mongo/util/hierarchical_acquisition.h"
  #include "mongo/util/time_support.h"
  
  namespace mongo {
  
  /**
   * Allows waiting for a result returned from an asynchronous operation.
   */
  template <class T>
  class Notification {
  public:
      Notification() = default;
  
      /**
       * Creates a notification object, which has already been set. Calls to any of the getters will
       * return immediately.
       */
      explicit Notification(T value) : _value(value) {}
  
      /**
       * Returns true if the notification has been set (i.e., the call to get/waitFor would not
       * block).
       */
      explicit operator bool() const {
          stdx::unique_lock<Latch> lock(_mutex);
          return !!_value;
      }
  
      /**
       * If the notification has been set, returns immediately. Otherwise blocks until it becomes set.
       * If the wait is interrupted, throws an exception.
       */
      T& get(OperationContext* opCtx) {
          stdx::unique_lock<Latch> lock(_mutex);
          opCtx->waitForConditionOrInterrupt(_condVar, lock, [this]() -> bool { return !!_value; });
          return _value.get();
      }
  
      /**
       * If the notification has been set, returns immediately. Otherwise blocks until it becomes set.
       * This variant of get cannot be interrupted.
       */
      T& get() {
          stdx::unique_lock<Latch> lock(_mutex);
          while (!_value) {
              _condVar.wait(lock);
          }
  
          return _value.get();
      }
  
      /**
       * Sets the notification result and wakes up any threads, which might be blocked in the wait
       * call. Must only be called once for the lifetime of the notification.
       */
      void set(T value) {
          stdx::lock_guard<Latch> lock(_mutex);
          invariant(!_value);
          _value = std::move(value);
          _condVar.notify_all();
      }
  
      /**
       * If the notification is set, returns immediately. Otherwise, blocks until it either becomes
       * set or the waitTimeout expires, whichever comes first. Returns true if the notification is
       * set (in which case a subsequent call to get is guaranteed to not block) or false otherwise.
       * If the wait is interrupted, throws an exception.
       */
      bool waitFor(OperationContext* opCtx, Milliseconds waitTimeout) {
          stdx::unique_lock<Latch> lock(_mutex);
          return opCtx->waitForConditionOrInterruptFor(
              _condVar, lock, waitTimeout, [&]() { return !!_value; });
      }
  
  private:
      mutable Mutex _mutex =
          MONGO_MAKE_LATCH(HierarchicalAcquisitionLevel(0), "Notification::_mutex");
      stdx::condition_variable _condVar;
  
      // Protected by mutex and only moves from not-set to set once
      boost::optional<T> _value{boost::none};
  };
  
  template <>
  class Notification<void> {
  public:
      explicit operator bool() const {
          return _notification.operator bool();
      }
  
      void get(OperationContext* opCtx) {
          _notification.get(opCtx);
      }
  
      void get() {
          _notification.get();
      }
  
      void set() {
          _notification.set(true);
      }
  
      bool waitFor(OperationContext* opCtx, Milliseconds waitTimeout) {
          return _notification.waitFor(opCtx, waitTimeout);
      }
  
  private:
      Notification<bool> _notification;
  };
  
  }  // namespace mongo
  