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/**
* Copyright (C) 2022-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 "mongo/base/error_codes.h"
#include "mongo/stdx/type_traits.h"
#include "mongo/util/future.h"
namespace mongo {
namespace packaged_task_detail {
/**
* SigHelper is a family of types helpful for deducing the type signature of the callable wrapped by
* a PackagedTask.
*/
template <typename>
struct SigHelper {};
// Function Type
template <typename Ret, typename... Args>
struct SigHelper<Ret (*)(Args...)> : stdx::type_identity<Ret(Args...)> {};
// Member Function Pointers
template <typename Class, typename Ret, typename... Args>
struct SigHelper<Ret (Class::*)(Args...)> : stdx::type_identity<Ret(Args...)> {};
template <typename Class, typename Ret, typename... Args>
struct SigHelper<Ret (Class::*)(Args...)&> : stdx::type_identity<Ret(Args...)> {};
template <typename Class, typename Ret, typename... Args>
struct SigHelper<Ret (Class::*)(Args...) const> : stdx::type_identity<Ret(Args...)> {};
template <typename Class, typename Ret, typename... Args>
struct SigHelper<Ret (Class::*)(Args...) const&> : stdx::type_identity<Ret(Args...)> {};
template <typename T>
using getCallOperator = decltype(&T::operator());
template <typename T>
constexpr bool hasCallOperator = stdx::is_detected_v<getCallOperator, T>;
template <typename Callable>
using SigFor = typename std::conditional_t<hasCallOperator<Callable>,
SigHelper<decltype(&Callable::operator())>,
SigHelper<Callable>>::type;
} // namespace packaged_task_detail
/**
* A PackagedTask wraps anything Callable, but packages the return value of the Callable in a Future
* that can be accessed before the Callable is run. Construct a PackagedTask by giving it a
* Callable. Once the PackagedTask is constructed, you can extract a Future that will contain the
* result of running the packaged task. The PackagedTask can be invoked as if it was the Callable
* that it wraps.
*/
template <typename Sig>
class PackagedTask;
template <typename R, typename... Args>
class PackagedTask<R(Args...)> {
using ReturnType = FutureContinuationResult<unique_function<R(Args...)>, Args...>;
public:
template <typename F>
explicit PackagedTask(F&& f) : _f(std::forward<F>(f)) {}
PackagedTask(const PackagedTask&) = delete;
PackagedTask& operator=(const PackagedTask&) = delete;
PackagedTask(PackagedTask&&) = default;
PackagedTask& operator=(PackagedTask&&) = default;
/**
* Invokes the Callable wrapped by this PackagedTask. This can only be called once, as a
* PackagedTask produces at most one result obtained from running the wrapped Callable at most
* one time. It is invalid to call this more than once.
*/
void operator()(Args... args) {
_p.setWith([&] { return _f(std::forward<Args>(args)...); });
}
/**
* Returns a Future that represents the (possibly-deferred) result of the wrapped task. Because
* running the task will produce exactly one result, it is safe to call getFuture() at most once
* on any PackagedTask; subsequent calls will throw a DBException set with
* ErrorCodes::FutureAlreadyRetrieved.
*/
Future<ReturnType> getFuture() {
if (_futureExtracted) {
iasserted(ErrorCodes::FutureAlreadyRetrieved,
"Attempted to extract more than one future from a PackagedTask");
}
_futureExtracted = true;
return std::move(_fut);
}
private:
unique_function<R(Args...)> _f;
Promise<ReturnType> _p{NonNullPromiseTag{}};
Future<ReturnType> _fut{_p.getFuture()};
bool _futureExtracted{false};
};
template <typename F, typename Sig = packaged_task_detail::SigFor<F>>
PackagedTask(F&& f)->PackagedTask<Sig>;
template <typename R, typename... Args>
PackagedTask(R (*)(Args...))->PackagedTask<R(Args...)>;
} // namespace mongo
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