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// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
// This source code is licensed under both the GPLv2 (found in the
// COPYING file in the root directory) and Apache 2.0 License
// (found in the LICENSE.Apache file in the root directory).
/*
* Copyright (c) 2016-present, Facebook, Inc.
* All rights reserved.
*
* This source code is licensed under both the BSD-style license (found in the
* LICENSE file in the root directory of this source tree) and the GPLv2 (found
* in the COPYING file in the root directory of this source tree).
*/
#pragma once
#include <atomic>
#include <cassert>
#include <condition_variable>
#include <cstddef>
#include <functional>
#include <mutex>
#include <queue>
#include "rocksdb/rocksdb_namespace.h"
namespace ROCKSDB_NAMESPACE {
/// Unbounded thread-safe work queue.
//
// This file is an excerpt from Facebook's zstd repo at
// https://github.com/facebook/zstd/. The relevant file is
// contrib/pzstd/utils/WorkQueue.h.
template <typename T>
class WorkQueue {
// Protects all member variable access
std::mutex mutex_;
std::condition_variable readerCv_;
std::condition_variable writerCv_;
std::condition_variable finishCv_;
std::queue<T> queue_;
bool done_;
std::size_t maxSize_;
// Must have lock to call this function
bool full() const {
if (maxSize_ == 0) {
return false;
}
return queue_.size() >= maxSize_;
}
public:
/**
* Constructs an empty work queue with an optional max size.
* If `maxSize == 0` the queue size is unbounded.
*
* @param maxSize The maximum allowed size of the work queue.
*/
WorkQueue(std::size_t maxSize = 0) : done_(false), maxSize_(maxSize) {}
/**
* Push an item onto the work queue. Notify a single thread that work is
* available. If `finish()` has been called, do nothing and return false.
* If `push()` returns false, then `item` has not been copied from.
*
* @param item Item to push onto the queue.
* @returns True upon success, false if `finish()` has been called. An
* item was pushed iff `push()` returns true.
*/
template <typename U>
bool push(U&& item) {
{
std::unique_lock<std::mutex> lock(mutex_);
while (full() && !done_) {
writerCv_.wait(lock);
}
if (done_) {
return false;
}
queue_.push(std::forward<U>(item));
}
readerCv_.notify_one();
return true;
}
/**
* Attempts to pop an item off the work queue. It will block until data is
* available or `finish()` has been called.
*
* @param[out] item If `pop` returns `true`, it contains the popped item.
* If `pop` returns `false`, it is unmodified.
* @returns True upon success. False if the queue is empty and
* `finish()` has been called.
*/
bool pop(T& item) {
{
std::unique_lock<std::mutex> lock(mutex_);
while (queue_.empty() && !done_) {
readerCv_.wait(lock);
}
if (queue_.empty()) {
assert(done_);
return false;
}
item = queue_.front();
queue_.pop();
}
writerCv_.notify_one();
return true;
}
/**
* Sets the maximum queue size. If `maxSize == 0` then it is unbounded.
*
* @param maxSize The new maximum queue size.
*/
void setMaxSize(std::size_t maxSize) {
{
std::lock_guard<std::mutex> lock(mutex_);
maxSize_ = maxSize;
}
writerCv_.notify_all();
}
/**
* Promise that `push()` won't be called again, so once the queue is empty
* there will never any more work.
*/
void finish() {
{
std::lock_guard<std::mutex> lock(mutex_);
assert(!done_);
done_ = true;
}
readerCv_.notify_all();
writerCv_.notify_all();
finishCv_.notify_all();
}
/// Blocks until `finish()` has been called (but the queue may not be empty).
void waitUntilFinished() {
std::unique_lock<std::mutex> lock(mutex_);
while (!done_) {
finishCv_.wait(lock);
}
}
};
} // namespace ROCKSDB_NAMESPACE
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