folly/folly/futures/Barrier.h

/*
 * Copyright (c) Meta Platforms, Inc. and affiliates.
 *
 * 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.
 */

#pragma once

#include <atomic>
#include <cstdint>

#include <folly/futures/Future.h>
#include <folly/futures/Promise.h>

namespace folly {
namespace futures {

// A folly::Future-istic Barrier synchronization primitive
//
// The barrier is initialized with a count N.
//
// The first N-1 calls to wait() return uncompleted futures.
//
// The Nth call to wait() completes the previous N-1 futures successfully,
// returns a future that is already completed successfully, and resets the
// barrier; the barrier may be reused immediately, as soon as at least one
// of the future completions has been observed.
//
// Of these N futures, exactly one is completed with true, while the others are
// completed with false; it is unspecified which future completes with true.
// (This may be used to elect a "leader" among a group of threads.)
//
// If the barrier is destroyed, any futures already returned by wait() will
// complete with an error.
class Barrier { … };

} // namespace futures
} // namespace folly