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|
/**
* 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
// Keeping this first to ensure it compiles by itself
#include "mongo/util/future_impl.h"
#include <boost/intrusive_ptr.hpp>
#include <type_traits>
#include "mongo/base/status.h"
#include "mongo/base/status_with.h"
#include "mongo/stdx/type_traits.h"
#include "mongo/util/assert_util.h"
#include "mongo/util/debug_util.h"
#include "mongo/util/interruptible.h"
#include "mongo/util/intrusive_counter.h"
#include "mongo/util/out_of_line_executor.h"
namespace mongo {
/**
* SemiFuture<T> is logically a possibly-deferred StatusWith<T> (or Status when T is void).
*
* Unlike Future<T> it only supports blocking operations, not directly chained continuations. You
* are only allowed to chain continuations by passing an executor to thenRunOn(). This is intended
* to protect the promise-completer's execution context from needing to perform arbitrary operations
* requested by other subsystem's continuations.
*
* SemiFutures can't convert to or be assigned to Futures since that would allow adding
* continuations which would defeat the purpose of SemiFuture.
*
* A future may be passed between threads, but only one thread may use it at a time.
*
* TODO decide if destroying a Future before extracting the result should cancel work or should
* cancellation be explicit. For now avoid unnecessarily throwing away active Futures since the
* behavior may change. End all Future chains with either a blocking call to get()/getNoThrow() or a
* non-blocking call to getAsync().
*
* SemiFuture<void> is the same as the generic SemiFuture<T> with the following exceptions:
* - Anything mentioning StatusWith<T> will use Status instead.
* - Anything returning references to T will just return void since there are no void references.
* - Anything taking a T argument will receive no arguments.
*/
template <typename T>
class MONGO_WARN_UNUSED_RESULT_CLASS SemiFuture {
using Impl = future_details::FutureImpl<T>;
using T_unless_void = std::conditional_t<std::is_void_v<T>, future_details::FakeVoid, T>;
public:
static_assert(!std::is_same<T, Status>::value,
"Future<Status> is banned. Use Future<void> instead.");
static_assert(!isStatusWith<T>, "Future<StatusWith<T>> is banned. Just use Future<T> instead.");
static_assert(!future_details::isFutureLike<T>,
"Future of Future types is banned. Just use Future<T> instead.");
static_assert(!std::is_reference<T>::value, "Future<T&> is banned.");
static_assert(!std::is_const<T>::value, "Future<const T> is banned.");
static_assert(!std::is_array<T>::value, "Future<T[]> is banned.");
using value_type = T;
/**
* For non-void T: Constructs a SemiFuture in a moved-from state that can only be assigned to
* or destroyed.
*
* For void T: Constructs a ready Semifuture for parity with SemiFuture<T>(T)
*/
SemiFuture() = default;
SemiFuture& operator=(SemiFuture&&) = default;
SemiFuture(SemiFuture&&) = default;
SemiFuture(const SemiFuture&) = delete;
SemiFuture& operator=(const SemiFuture&) = delete;
/**
* For non-void T: This must be passed a not-OK Status.
*
* For void T: This behaves like the StatusWith constructor and accepts any Status.
*/
/* implicit */ SemiFuture(Status status) : SemiFuture(Impl::makeReady(std::move(status))) {}
// These should not be used with T=void.
/* implicit */ SemiFuture(T_unless_void val) : SemiFuture(Impl::makeReady(std::move(val))) {
static_assert(!std::is_void_v<T>);
}
/* implicit */ SemiFuture(StatusWith<T_unless_void> sw)
: SemiFuture(Impl::makeReady(std::move(sw))) {
static_assert(!std::is_void_v<T>);
}
/**
* Make a ready SemiFuture<T> from a value for cases where you don't need to wait
* asynchronously.
*
* Calling this is faster than getting a SemiFuture out of a Promise, and is effectively free.
* It is
* fast enough that you never need to avoid returning a SemiFuture from an API, even if the
* result
* is ready 99.99% of the time.
*
* As an example, if you are handing out results from a batch, you can use this when for each
* result while you have a batch, then use a Promise to return a not-ready SemiFuture when you
* need
* to get another batch.
*/
static SemiFuture<T> makeReady(T_unless_void val) {
return SemiFuture(Impl::makeReady(std::move(val)));
}
static SemiFuture<T> makeReady(Status status) {
return SemiFuture(Impl::makeReady(std::move(status)));
}
static SemiFuture<T> makeReady(StatusWith<T_unless_void> val) {
return SemiFuture(Impl::makeReady(std::move(val)));
}
REQUIRES_FOR_NON_TEMPLATE(std::is_void_v<T>)
static SemiFuture<void> makeReady() {
return SemiFuture(Impl::makeReady());
}
/**
* A no-op so that you can always do `return makesFutureOrSemiFuture().semi()` when you want to
* protect your execution context.
*/
SemiFuture<T> semi() && noexcept {
return std::move(*this);
}
/**
* Convert this SemiFuture to a SharedSemiFuture.
*/
SharedSemiFuture<T> share() && noexcept {
return std::move(_impl).share();
}
/**
* If this returns true, get() is guaranteed not to block and callbacks will be immediately
* invoked. You can't assume anything if this returns false since it may be completed
* immediately after checking (unless you have independent knowledge that this SemiFuture can't
* complete in the background).
*
* Callers must still call get() or similar, even on SemiFuture<void>, to ensure that they are
* correctly sequenced with the completing task, and to be informed about whether the Promise
* completed successfully.
*
* This is generally only useful as an optimization to avoid prep work, such as setting up
* timeouts, that is unnecessary if the SemiFuture is ready already.
*/
bool isReady() const {
return _impl.isReady();
}
/**
* Returns when the Semifuture isReady().
*
* Throws if the interruptible passed is interrupted (explicitly or via deadline).
*/
void wait(Interruptible* interruptible = Interruptible::notInterruptible()) const {
return _impl.wait(interruptible);
}
/**
* Returns Status::OK() when the Semifuture isReady().
*
* Returns a non-okay status if the interruptible is interrupted.
*/
Status waitNoThrow(Interruptible* interruptible = Interruptible::notInterruptible()) const
noexcept {
return _impl.waitNoThrow(interruptible);
}
/**
* Gets the value out of this SemiFuture, blocking until it is ready.
*
* get() methods throw on error, while getNoThrow() returns a !OK status.
*
* These methods can be called multiple times, except for the rvalue overloads.
*
* Note: It is impossible to differentiate interruptible interruption from an error propagating
* down the Semifuture chain with these methods. If you need to distinguish the two cases, call
* wait() first.
*/
T get(Interruptible* interruptible = Interruptible::notInterruptible()) && {
return std::move(_impl).get(interruptible);
}
future_details::AddRefUnlessVoid<T> get(
Interruptible* interruptible = Interruptible::notInterruptible()) & {
return _impl.get(interruptible);
}
future_details::AddRefUnlessVoid<const T> get(
Interruptible* interruptible = Interruptible::notInterruptible()) const& {
return _impl.get(interruptible);
}
StatusOrStatusWith<T> getNoThrow(
Interruptible* interruptible = Interruptible::notInterruptible()) &&
noexcept {
return std::move(_impl).getNoThrow(interruptible);
}
StatusOrStatusWith<T> getNoThrow(
Interruptible* interruptible = Interruptible::notInterruptible()) const& noexcept {
return _impl.getNoThrow(interruptible);
}
/**
* Ignores the return value of a future, transforming it down into a SemiFuture<void>.
*
* This only ignores values, not errors. Those remain propagated to an onError handler.
*/
SemiFuture<void> ignoreValue() && noexcept {
return SemiFuture<void>(std::move(this->_impl).ignoreValue());
}
/**
* Returns a future that allows you to add continuations that are guaranteed to run on the
* provided executor.
*
* Be sure to read the ExecutorFuture class comment.
*/
ExecutorFuture<T> thenRunOn(ExecutorPtr exec) && noexcept;
private:
friend class Promise<T>;
friend class SharedPromise<T>;
template <typename>
friend class Future;
template <typename>
friend class ExecutorFuture;
template <typename>
friend class future_details::FutureImpl;
template <typename>
friend class SharedSemiFuture;
Future<T> unsafeToInlineFuture() && noexcept;
explicit SemiFuture(future_details::SharedStateHolder<T_unless_void>&& impl)
: _impl(std::move(impl)) {}
explicit SemiFuture(Impl&& impl) : _impl(std::move(impl)) {}
operator Impl &&() && {
return std::move(_impl);
}
template <typename U>
void propagateResultTo(U&& arg) && {
std::move(_impl).propagateResultTo(std::forward<U>(arg));
}
Impl _impl;
};
// Deduction Guides
TEMPLATE(typename T)
REQUIRES(!isStatusOrStatusWith<T> && !future_details::isFutureLike<T>)
SemiFuture(T)->SemiFuture<T>;
template <typename T>
SemiFuture(StatusWith<T>)->SemiFuture<T>;
/**
* Future<T> is a SemiFuture<T> (which is logically a possibly deferred StatusOrStatusWith<T>),
* extended with the ability to chain additional continuations that will be invoked when the result
* is ready.
*
* All comments on SemiFuture<T> apply to Future<T> as well.
*/
template <typename T>
class MONGO_WARN_UNUSED_RESULT_CLASS Future : private SemiFuture<T> {
using Impl = typename SemiFuture<T>::Impl;
using T_unless_void = typename SemiFuture<T>::T_unless_void;
public:
/**
* Re-export the API of SemiFuture. The API of Future is a superset, except you can't convert
* from a SemiFuture to a Future.
*/
using value_type = T;
using SemiFuture<T>::SemiFuture; // Constructors.
using SemiFuture<T>::share;
using SemiFuture<T>::isReady;
using SemiFuture<T>::wait;
using SemiFuture<T>::waitNoThrow;
using SemiFuture<T>::get;
using SemiFuture<T>::getNoThrow;
using SemiFuture<T>::semi;
using SemiFuture<T>::thenRunOn;
/**
* Re-export makeReady, but return a Future<T>
*/
static Future<T> makeReady(T_unless_void val) {
return Future(Impl::makeReady(std::move(val)));
}
static Future<T> makeReady(Status status) {
return Future(Impl::makeReady(std::move(status)));
}
static Future<T> makeReady(StatusWith<T_unless_void> val) {
return Future(Impl::makeReady(std::move(val)));
}
REQUIRES_FOR_NON_TEMPLATE(std::is_void_v<T>)
static Future<void> makeReady() {
return Future(Impl::makeReady());
}
Future<void> ignoreValue() && noexcept {
return Future<void>(std::move(this->_impl).ignoreValue());
}
/**
* This ends the Future continuation chain by calling a callback on completion. Use this to
* escape back into a callback-based API.
*
* For now, the callback must not fail, since there is nowhere to propagate the error to.
* TODO decide how to handle func throwing.
*/
TEMPLATE(typename Func)
REQUIRES(future_details::isCallableExactR<void, Func, StatusOrStatusWith<T>>)
void getAsync(Func&& func) && noexcept {
std::move(this->_impl).getAsync(std::forward<Func>(func));
}
//
// The remaining methods are all continuation-based and take a callback and return a Future-like
// type based on the return type of the callback, except for the "tap" methods that always
// return Future<T>. When passed a callback that returns a FutureLike<U> type, the return type
// of the method will be either Future<U> if FutureLike is Future, otherwise SemiFuture<U>. The
// result of the callback will be automatically unwrapped and connected to the returned
// FutureLike<U> rather than producing a Future<FutureLike<U>>. When the callback returns a
// non-FutureLike type U, the return type of the method will be Future<U>, with the adjustment
// for Status/StatusWith described below.
//
// Each method has a comment indicating the supported signatures for that callback, and a
// description of when the callback is invoked and how the impacts the returned Future. It may
// be helpful to think of Future continuation chains as a pipeline of stages that take input
// from earlier stages and produce output for later stages.
//
// Be aware that the callback may be invoked inline at the call-site or at the producer when
// setting the value. Therefore, you should avoid doing blocking work inside of a callback.
// Additionally, avoid acquiring any locks or mutexes that the caller already holds, otherwise
// you risk a deadlock. If either of these concerns apply to your callback, it should schedule
// itself on an executor, rather than doing work in the callback.
// TODO make this easier to do by having executor APIs return Futures.
//
// Error handling in callbacks: all exceptions thrown propagate to the returned Future
// automatically. Callbacks that return Status or StatusWith<T> behave as-if they were wrapped
// in something that called uassertStatusOK() on the return value. There is no way to
// distinguish between a function throwing or returning a !OK status.
//
/**
* Callbacks passed to then() are only called if the input Future completes successfully.
* Otherwise the error propagates automatically, bypassing the callback.
*
* The callback takes a T and can return anything (see above for how Statusy and Futurey returns
* are handled.)
*/
TEMPLATE(typename Func)
REQUIRES(future_details::isCallable<Func, T>)
/*see above*/ auto then(Func&& func) && noexcept {
return wrap<Func, T>(std::move(this->_impl).then(std::forward<Func>(func)));
}
/**
* Callbacks passed to onCompletion() are called if the input Future completes with or without
* an error.
*
* The callback takes a StatusOrStatusWith<T> and can return anything (see above for how Statusy
* and Futurey returns are handled.)
*/
TEMPLATE(typename Func)
REQUIRES(future_details::isCallable<Func, StatusOrStatusWith<T>>)
/*see above*/ auto onCompletion(Func&& func) && noexcept {
return wrap<Func, Status>(std::move(this->_impl).onCompletion(std::forward<Func>(func)));
}
/**
* Callbacks passed to onError() are only called if the input Future completes with an error.
* Otherwise, the successful result propagates automatically, bypassing the callback.
*
* The callback can either produce a replacement value (which must be a T), return a replacement
* Future<T> (such as by retrying), or return/throw a replacement error.
*
* Note that this will only catch errors produced by earlier stages; it is not registering a
* general error handler for the entire chain.
*
* The callback takes a non-OK Status and returns a possibly-wrapped T (see above for how
* Statusy and Futurey returns are handled.)
*/
TEMPLATE(typename Func)
REQUIRES(future_details::isCallableR<T, Func, Status>)
/*see above*/ auto onError(Func&& func) && noexcept {
return wrap<Func, Status>(std::move(this->_impl).onError(std::forward<Func>(func)));
}
/**
* Same as the other two onErrors but only calls the callback if the code matches the template
* parameter. Otherwise lets the error propagate unchanged.
*
* The callback takes a non-OK Status and returns a possibly-wrapped T (see above for how
* Statusy and Futurey returns are handled.)
*/
TEMPLATE(ErrorCodes::Error code, typename Func)
REQUIRES(future_details::isCallableR<T, Func, Status>)
/*see above*/ auto onError(Func&& func) && noexcept {
return wrap<Func, Status>(
std::move(this->_impl).template onError<code>(std::forward<Func>(func)));
}
/**
* Similar to the first two onErrors, but only calls the callback if the category matches
* the template parameter. Otherwise lets the error propagate unchanged.
*
* The callback takes a non-OK Status and returns a possibly-wrapped T (see above for how
* Statusy and Futurey returns are handled.)
*/
TEMPLATE(ErrorCategory category, typename Func)
REQUIRES(future_details::isCallableR<T, Func, Status>)
/*see above*/ auto onErrorCategory(Func&& func) && noexcept {
return wrap<Func, Status>(
std::move(this->_impl).template onErrorCategory<category>(std::forward<Func>(func)));
}
//
// The tap/tapError/tapAll family of functions take callbacks to observe the flow through a
// future chain without affecting the propagating result, except possibly if they throw. If the
// naming seems odd, you can think of it like a "wire tap" in that it allows you to observe a
// conversation between two parties (the promise-producer and future-consumer) without adding
// messages of your own. This is why all callbacks are required to return void.
//
// TODO decide what to do if callback throws:
// - transition the future chain to failure
// - ignore
// - fatal (current impl)
//
/**
* Callback is called if the input completes successfully.
*
* This can be used to inform some outside system of the result.
*
* The callback takes a const T& and must return void.
*/
TEMPLATE(typename Func)
REQUIRES(future_details::isCallableExactR<void, Func, const T>)
Future<T> tap(Func&& func) && noexcept {
return Future<T>(std::move(this->_impl).tap(std::forward<Func>(func)));
}
/**
* Callback is called if the input completes with an error.
*
* This can be used to log.
*
* The callback takes a non-OK Status and must return void.
*/
TEMPLATE(typename Func)
REQUIRES(future_details::isCallableExactR<void, Func, const Status>)
Future<T> tapError(Func&& func) && noexcept {
return Future<T>(std::move(this->_impl).tapError(std::forward<Func>(func)));
}
/**
* Callback is called when the input completes, regardless of success or failure.
*
* This can be used for cleanup. Some other libraries name the equivalent method finally to
* match the common semantic from other languages.
*
* The callback takes a StatusOrStatusWith<T> and must return void.
*/
TEMPLATE(typename Func)
REQUIRES(future_details::isCallableExactR<void, Func, const StatusOrStatusWith<T>>)
Future<T> tapAll(Func&& func) && noexcept {
return Future<T>(std::move(this->_impl).tapAll(std::forward<Func>(func)));
}
private:
template <typename>
friend class ExecutorFuture;
template <typename>
friend class Future;
template <typename>
friend class future_details::FutureImpl;
friend class Promise<T>;
friend class SharedPromise<T>;
using SemiFuture<T>::unsafeToInlineFuture;
template <typename Func, typename Arg, typename U>
static auto wrap(future_details::FutureImpl<U>&& impl) {
using namespace future_details;
return FutureContinuationKind<NormalizedCallResult<Func, Arg>>(std::move(impl));
}
};
// Deduction Guides
TEMPLATE(typename T)
REQUIRES(!isStatusOrStatusWith<T> && !future_details::isFutureLike<T>)
Future(T)->Future<T>;
template <typename T>
Future(StatusWith<T>)->Future<T>;
/**
* An ExecutorFuture is like a Future that ensures that all callbacks are run on a supplied
* executor.
*
* IMPORTANT: Executors are allowed to refuse work by invoking their task callbacks with a non-OK
* Status. In that event, callbacks passed to continuation functions WILL NOT RUN. Instead, the
* error status will propagate down the future chain until it would run a callback on an executor
* that doesn't refuse the work, or it is extracted by calling a blocking get() method. Destructors
* for these callbacks can run in any context, so be suspicious of callbacks that capture Promises
* because they will propagate out BrokenPromise if the executor refuses work.
*/
template <typename T>
class ExecutorFuture : private SemiFuture<T> {
using Impl = typename SemiFuture<T>::Impl;
using T_unless_void = typename SemiFuture<T>::T_unless_void;
public:
/**
* Default construction is disallowed to ensure that every ExecutorFuture has an associated
* Executor (unless it has been moved-from).
*/
ExecutorFuture() = delete;
ExecutorFuture(ExecutorPtr exec, Status status)
: SemiFuture<T>(std::move(status)), _exec(std::move(exec)) {}
// These should not be used with T=void.
ExecutorFuture(ExecutorPtr exec, T_unless_void val)
: SemiFuture<T>(std::move(val)), _exec(std::move(exec)) {
static_assert(!std::is_void_v<T>);
}
ExecutorFuture(ExecutorPtr exec, StatusWith<T_unless_void> sw)
: SemiFuture<T>(std::move(sw)), _exec(std::move(exec)) {
static_assert(!std::is_void_v<T>);
}
REQUIRES_FOR_NON_TEMPLATE(std::is_void_v<T>)
explicit ExecutorFuture(ExecutorPtr exec) : SemiFuture<void>(), _exec(std::move(exec)) {}
/**
* Re-export the accessor API of SemiFuture. The access API of ExecutorFuture is a superset, but
* you can't create an ExecutorFuture without supplying an executor.
*/
using value_type = T;
using SemiFuture<T>::share;
using SemiFuture<T>::isReady;
using SemiFuture<T>::wait;
using SemiFuture<T>::waitNoThrow;
using SemiFuture<T>::get;
using SemiFuture<T>::getNoThrow;
using SemiFuture<T>::semi;
using SemiFuture<T>::thenRunOn;
ExecutorFuture<void> ignoreValue() && noexcept {
return ExecutorFuture<void>(std::move(_exec), std::move(this->_impl).ignoreValue());
}
//
// Provide the callback-taking API from Future (except for the taps). All callbacks will be run
// on the executor associated with this ExecutorFuture. See class comment for how we handle
// executors that refuse work.
//
// All methods that return non-void will return an ExecutorFuture bound to the same executor as
// this.
//
// There is no tap support because we can't easily be both non-intrusive in the value flow and
// schedule on an executor that is allowed to fail. In particular, the inability to copy
// move-only values means that we would need to refer directly into the internal SharedState
// objects and keep them alive longer that we otherwise would. If there is a real need for this,
// it should be doable, but will be fairly complicated.
//
TEMPLATE(typename Func)
REQUIRES(future_details::isCallableExactR<void, Func, StatusOrStatusWith<T>>)
void getAsync(Func&& func) && noexcept {
static_assert(std::is_void_v<decltype(func(std::declval<StatusOrStatusWith<T>>()))>,
"func passed to getAsync must return void");
// Can't use wrapCB since we don't want to return a future, just schedule a non-chainable
// callback.
std::move(this->_impl).getAsync([
exec = std::move(_exec), // Unlike wrapCB this can move because we won't need it later.
func = std::forward<Func>(func)
](StatusOrStatusWith<T> arg) mutable noexcept {
exec->schedule([ func = std::move(func),
arg = std::move(arg) ](Status execStatus) mutable noexcept {
if (execStatus.isOK())
func(std::move(arg));
});
});
}
TEMPLATE(typename Func)
REQUIRES(future_details::isCallable<Func, T>)
auto then(Func&& func) && noexcept {
return mongo::ExecutorFuture(
std::move(_exec), std::move(this->_impl).then(wrapCB<T>(std::forward<Func>(func))));
}
TEMPLATE(typename Func)
REQUIRES(future_details::isCallable<Func, StatusOrStatusWith<T>>)
auto onCompletion(Func&& func) && noexcept {
return mongo::ExecutorFuture(
std::move(_exec),
std::move(this->_impl)
.onCompletion(wrapCB<StatusOrStatusWith<T>>(std::forward<Func>(func))));
}
TEMPLATE(typename Func)
REQUIRES(future_details::isCallableR<T, Func, Status>)
ExecutorFuture<T> onError(Func&& func) && noexcept {
return mongo::ExecutorFuture(
std::move(_exec),
std::move(this->_impl).onError(wrapCB<Status>(std::forward<Func>(func))));
}
TEMPLATE(ErrorCodes::Error code, typename Func)
REQUIRES(future_details::isCallableR<T, Func, Status>)
ExecutorFuture<T> onError(Func&& func) && noexcept {
return mongo::ExecutorFuture(
std::move(_exec),
std::move(this->_impl)
.template onError<code>(wrapCB<Status>(std::forward<Func>(func))));
}
TEMPLATE(ErrorCategory category, typename Func)
REQUIRES(future_details::isCallableR<T, Func, Status>)
ExecutorFuture<T> onErrorCategory(Func&& func) && noexcept {
return mongo::ExecutorFuture(
std::move(_exec),
std::move(this->_impl)
.template onErrorCategory<category>(wrapCB<Status>(std::forward<Func>(func))));
}
private:
// This *must* take exec by ref to ensure it isn't moved from while evaluating wrapCB above.
ExecutorFuture(ExecutorPtr&& exec, Impl&& impl) : SemiFuture<T>(std::move(impl)), _exec(exec) {
dassert(_exec);
}
/**
* Wraps func in a callback that takes the argument it would and returns an appropriately typed
* Future<U>, then schedules a task on _exec to complete the associated promise with the result
* of calling func with that argument.
*/
template <typename RawArg, typename Func>
auto wrapCB(Func&& func) {
// Have to take care to never put void in argument position, since that is a hard error.
using Result = typename std::conditional_t<std::is_void_v<RawArg>,
std::invoke_result<Func>,
std::invoke_result<Func, RawArg>>::type;
using DummyArg = std::conditional_t<std::is_void_v<RawArg>, //
future_details::FakeVoid,
RawArg>;
using Sig = std::conditional_t<std::is_void_v<RawArg>, //
Result(),
Result(DummyArg)>;
return wrapCBHelper(unique_function<Sig>(std::forward<Func>(func)));
}
template <typename Sig>
MONGO_COMPILER_NOINLINE auto wrapCBHelper(unique_function<Sig>&& func);
using SemiFuture<T>::unsafeToInlineFuture;
template <typename>
friend class ExecutorFuture;
template <typename>
friend class SemiFuture;
template <typename>
friend class SharedSemiFuture;
template <typename>
friend class future_details::FutureImpl;
ExecutorPtr _exec;
};
// Deduction Guides
TEMPLATE(typename T)
REQUIRES(!isStatusOrStatusWith<T> && !future_details::isFutureLike<T>)
ExecutorFuture(ExecutorPtr, T)->ExecutorFuture<T>;
template <typename T>
ExecutorFuture(ExecutorPtr, future_details::FutureImpl<T>)->ExecutorFuture<T>;
template <typename T>
ExecutorFuture(ExecutorPtr, StatusWith<T>)->ExecutorFuture<T>;
ExecutorFuture(ExecutorPtr)->ExecutorFuture<void>;
/**
* This class represents the producer side of a Future.
*
* This is a single-shot class: you may either set a value or error at most once. If no value or
* error has been set at the time this Promise is destroyed, a error will be set with
* ErrorCode::BrokenPromise. This should generally be considered a programmer error, and should not
* be relied upon. We may make it debug-fatal in the future.
*
* Only one thread can use a given Promise at a time, but another thread may be using the associated
* Future object.
*
* If the result is ready when producing the Future, it is more efficient to use
* makeReadyFutureWith() or Future<T>::makeReady() than to use a Promise<T>.
*
* A default constructed `Promise` is in a null state. Null `Promises` can only be assigned over
* and destroyed. It is a programmer error to call any methods on a null `Promise`. Any methods
* that complete a `Promise` leave it in the null state.
*/
template <typename T>
class Promise {
using SharedStateT = future_details::SharedState<T>;
public:
using value_type = T;
/**
* Creates a null `Promise`.
*/
Promise() = default;
~Promise() {
breakPromiseIfNeeded();
}
Promise(const Promise&) = delete;
Promise& operator=(const Promise&) = delete;
/**
* Breaks this `Promise`, if not fulfilled and not in a null state.
*/
Promise& operator=(Promise&& p) noexcept {
breakPromiseIfNeeded();
_sharedState = std::move(p._sharedState);
return *this;
}
Promise(Promise&&) = default;
/**
* Sets a value or error into this Promise by calling func, which must take no arguments and
* return one of T, StatusWith<T> (or Status when T is void), or Future<T>. All errors, whether
* returned or thrown, will be correctly propagated.
*
* If the function returns a Future<T>, this Promise's Future will complete when the returned
* Future<T> completes, as-if it was passed to Promise::setFrom().
*
* If any work is needed to produce the result, prefer doing something like:
* promise.setWith([&]{ return makeResult(); });
* over code like:
* promise.emplaceValue(makeResult());
* because this method will correctly propagate errors thrown from makeResult(), rather than
* ErrorCodes::BrokenPromise.
*/
TEMPLATE(typename Func)
REQUIRES(future_details::isCallableR<T, Func, void>)
void setWith(Func&& func) noexcept {
setFrom(Future<void>::makeReady().then(std::forward<Func>(func)));
}
/**
* Sets the value into this Promise when the passed-in Future completes, which may have already
* happened. If it hasn't, it is still safe to destroy this Promise since it is no longer
* involved.
*/
void setFrom(Future<T>&& future) noexcept {
setImpl([&](boost::intrusive_ptr<future_details::SharedState<T>>&& sharedState) {
std::move(future).propagateResultTo(sharedState.get());
});
}
TEMPLATE(typename... Args)
REQUIRES(std::is_constructible_v<T, Args...> || (std::is_void_v<T> && sizeof...(Args) == 0))
void emplaceValue(Args&&... args) noexcept {
setImpl([&](boost::intrusive_ptr<SharedStateT>&& sharedState) {
sharedState->emplaceValue(std::forward<Args>(args)...);
});
}
void setError(Status status) noexcept {
invariant(!status.isOK());
setImpl([&](boost::intrusive_ptr<SharedStateT>&& sharedState) {
sharedState->setError(std::move(status));
});
}
// TODO rename to not XXXWith and handle void
void setFromStatusWith(StatusWith<T> sw) noexcept {
setImpl([&](boost::intrusive_ptr<SharedStateT>&& sharedState) {
sharedState->setFromStatusWith(std::move(sw));
});
}
static auto makePromiseFutureImpl() {
struct PromiseAndFuture {
Promise<T> promise = Promise(make_intrusive<SharedStateT>());
Future<T> future = promise.getFuture();
};
return PromiseAndFuture();
}
private:
explicit Promise(boost::intrusive_ptr<SharedStateT>&& sharedState)
: _sharedState(std::move(sharedState)) {}
// This is not public because we found it frequently was involved in races. The
// `makePromiseFuture<T>` API avoids those races entirely.
Future<T> getFuture() noexcept {
using namespace future_details;
_sharedState->threadUnsafeIncRefCountTo(2);
return Future<T>(SharedStateHolder<VoidToFakeVoid<T>>(
boost::intrusive_ptr<SharedState<T>>(_sharedState.get(), /*add ref*/ false)));
}
friend class Future<void>;
template <typename Func>
void setImpl(Func&& doSet) noexcept {
invariant(_sharedState);
// We keep `sharedState` as a stack local, to preserve ownership of the resource,
// in case the code in `doSet` unblocks a thread which winds up causing
// `~Promise` to be invoked.
auto sharedState = std::move(_sharedState);
doSet(std::move(sharedState));
// Note: `this` is potentially dead, at this point.
}
// The current promise will be broken, if not already fulfilled.
void breakPromiseIfNeeded() {
if (MONGO_unlikely(_sharedState)) {
_sharedState->setError({ErrorCodes::BrokenPromise, "broken promise"});
}
}
boost::intrusive_ptr<SharedStateT> _sharedState;
};
/**
* SharedSemiFuture<T> is logically a possibly-deferred StatusWith<T> (or Status when T is void).
*
* All methods that are present do the same as on a Future<T> so see it for documentation.
*
* Unlike Future<T> it only supports blocking operation, not chained continuations. This is intended
* to protect the promise-completer's execution context from needing to perform arbitrary operations
* requested by other subsystem's continuations.
* TODO Support continuation chaining when supplied with an executor to run them on.
*
* A SharedSemiFuture may be passed between threads, but only one thread may use it at a time.
*/
template <typename T>
class MONGO_WARN_UNUSED_RESULT_CLASS SharedSemiFuture {
using Impl = future_details::SharedStateHolder<T>;
using T_unless_void = std::conditional_t<std::is_void_v<T>, future_details::FakeVoid, T>;
public:
static_assert(!std::is_same<T, Status>::value,
"SharedSemiFuture<Status> is banned. Use SharedSemiFuture<void> instead.");
static_assert(
!isStatusWith<T>,
"SharedSemiFuture<StatusWith<T>> is banned. Just use SharedSemiFuture<T> instead.");
static_assert(
!future_details::isFutureLike<T>,
"SharedSemiFuture of Future types is banned. Just use SharedSemiFuture<T> instead.");
static_assert(!std::is_reference<T>::value, "SharedSemiFuture<T&> is banned.");
static_assert(!std::is_const<T>::value, "SharedSemiFuture<const T> is banned.");
static_assert(!std::is_array<T>::value, "SharedSemiFuture<T[]> is banned.");
static_assert(std::is_void_v<T> || std::is_copy_constructible_v<T>,
"SharedSemiFuture currently requires copyable types. Let us know if this is a "
"problem. Supporting this for blocking use cases is easy, but it will require "
"more work for async usage.");
using value_type = T;
SharedSemiFuture() = default;
/*implicit*/ SharedSemiFuture(const Future<T>& fut) = delete;
/*implicit*/ SharedSemiFuture(Future<T>&& fut) : SharedSemiFuture(std::move(fut).share()) {}
/*implicit*/ SharedSemiFuture(Status error) : _shared(Impl::makeReady(std::move(error))) {}
// These should not be used with T=void.
/*implicit*/ SharedSemiFuture(T_unless_void val) : _shared(Impl::makeReady(std::move(val))) {
static_assert(!std::is_void_v<T>);
}
/*implicit*/ SharedSemiFuture(StatusWith<T_unless_void> sw)
: _shared(Impl::makeReady(std::move(sw))) {
static_assert(!std::is_void_v<T>);
}
bool isReady() const {
return _shared.isReady();
}
void wait(Interruptible* interruptible = Interruptible::notInterruptible()) const {
_shared.wait(interruptible);
}
Status waitNoThrow(Interruptible* interruptible = Interruptible::notInterruptible()) const
noexcept {
return _shared.waitNoThrow(interruptible);
}
future_details::AddRefUnlessVoid<const T> get(
Interruptible* interruptible = Interruptible::notInterruptible()) const& {
return _shared.get(interruptible);
}
StatusOrStatusWith<T> getNoThrow(
Interruptible* interruptible = Interruptible::notInterruptible()) const& noexcept {
return _shared.getNoThrow(interruptible);
}
ExecutorFuture<T> thenRunOn(ExecutorPtr exec) const noexcept {
return ExecutorFuture<T>(std::move(exec), toFutureImpl());
}
private:
template <typename>
friend class SharedPromise;
template <typename>
friend class future_details::FutureImpl;
friend class SharedSemiFuture<void>;
template <typename>
friend class ExecutorFuture;
future_details::FutureImpl<T> toFutureImpl() const noexcept {
static_assert(std::is_void_v<T> || std::is_copy_constructible_v<T>);
return future_details::FutureImpl<T>(_shared.addChild());
}
// These are needed to support chaining where a SharedSemiFuture is returned from a
// continuation.
explicit operator future_details::FutureImpl<T>() const noexcept {
return toFutureImpl();
}
template <typename U>
void propagateResultTo(U&& arg) const noexcept {
toFutureImpl().propagateResultTo(std::forward<U>(arg));
}
Future<T> unsafeToInlineFuture() const noexcept {
return Future<T>(toFutureImpl());
}
explicit SharedSemiFuture(boost::intrusive_ptr<future_details::SharedState<T>> ptr)
: _shared(std::move(ptr)) {}
explicit SharedSemiFuture(future_details::SharedStateHolder<T>&& holder)
: _shared(std::move(holder)) {}
future_details::SharedStateHolder<T> _shared;
};
// Deduction Guides
TEMPLATE(typename T)
REQUIRES(!isStatusOrStatusWith<T> && !future_details::isFutureLike<T>)
SharedSemiFuture(T)->SharedSemiFuture<T>;
template <typename T>
SharedSemiFuture(StatusWith<T>)->SharedSemiFuture<T>;
/**
* This class represents the producer of SharedSemiFutures.
*
* This is a single-shot class: you may either set a value or error at most once. However you may
* extract as many futures as you want and they will all be completed at the same time. Any number
* of threads can extract a future at the same time. It is also safe to extract a future
* concurrently with completing the promise. If you extract a future after the promise has been
* completed, a ready future will be returned. You must still ensure that all calls to getFuture()
* complete prior to destroying the Promise.
*
* If no value or error has been set at the time this Promise is destroyed, an error will be set
* with ErrorCode::BrokenPromise. This should generally be considered a programmer error, and should
* not be relied upon. We may make it debug-fatal in the future.
*
* Unless otherwise specified, all methods behave the same as on Promise<T>.
*/
template <typename T>
class SharedPromise {
public:
using value_type = T;
/**
* Creates a `SharedPromise` ready for use.
*/
SharedPromise() = default;
~SharedPromise() {
if (MONGO_unlikely(!_haveCompleted)) {
_sharedState->setError({ErrorCodes::BrokenPromise, "broken promise"});
}
}
SharedPromise(const SharedPromise&) = delete;
SharedPromise(SharedPromise&&) = delete;
SharedPromise& operator=(const SharedPromise&) = delete;
SharedPromise& operator=(SharedPromise&& p) noexcept = delete;
/**
* Returns a future associated with this promise. All returned futures will be completed when
* the promise is completed.
*/
SharedSemiFuture<T> getFuture() const {
return SharedSemiFuture<T>(_sharedState);
}
TEMPLATE(typename Func)
REQUIRES(future_details::isCallableR<T, Func, void>)
void setWith(Func&& func) noexcept {
invariant(!std::exchange(_haveCompleted, true));
setFrom(Future<void>::makeReady().then(std::forward<Func>(func)));
}
void setFrom(Future<T>&& future) noexcept {
invariant(!std::exchange(_haveCompleted, true));
std::move(future).propagateResultTo(_sharedState.get());
}
TEMPLATE(typename... Args)
REQUIRES(std::is_constructible_v<T, Args...> || (std::is_void_v<T> && sizeof...(Args) == 0))
void emplaceValue(Args&&... args) noexcept {
invariant(!std::exchange(_haveCompleted, true));
_sharedState->emplaceValue(std::forward<Args>(args)...);
}
void setError(Status status) noexcept {
invariant(!status.isOK());
invariant(!std::exchange(_haveCompleted, true));
_sharedState->setError(std::move(status));
}
// TODO rename to not XXXWith and handle void
void setFromStatusWith(StatusWith<T> sw) noexcept {
invariant(!std::exchange(_haveCompleted, true));
_sharedState->setFromStatusWith(std::move(sw));
}
private:
friend class Future<void>;
// This is slightly different from whether the SharedState is in kFinished, because this
// SharedPromise may have been completed with a Future that isn't ready yet.
bool _haveCompleted = false;
const boost::intrusive_ptr<future_details::SharedState<T>> _sharedState =
make_intrusive<future_details::SharedState<T>>();
};
/**
* Makes a ready Future with the return value of a nullary function. This has the same semantics as
* Promise::setWith, and has the same reasons to prefer it over Future<T>::makeReady(). Also, it
* deduces the T, so it is easier to use.
*/
TEMPLATE(typename Func)
REQUIRES(future_details::isCallable<Func, void>)
auto makeReadyFutureWith(Func&& func) {
return Future<void>::makeReady().then(std::forward<Func>(func));
}
/**
* Returns a bound Promise and Future in a struct with friendly names (promise and future) that also
* works well with C++17 structured bindings.
*/
template <typename T>
inline auto makePromiseFuture() {
return Promise<T>::makePromiseFutureImpl();
}
/**
* This metafunction allows APIs that take callbacks and return Future to avoid doing their own type
* calculus. This results in the base value_type that would result from passing Func to a
* Future<T>::then(), with the same normalizing of T/StatusWith<T>/Future<T> returns. This is
* primarily useful for implementations of executors rather than their users.
*
* This returns the unwrapped T rather than Future<T> so it will be easy to create a Promise<T>.
*
* Examples:
*
* FutureContinuationResult<std::function<void()>> == void
* FutureContinuationResult<std::function<Status()>> == void
* FutureContinuationResult<std::function<Future<void>()>> == void
*
* FutureContinuationResult<std::function<int()>> == int
* FutureContinuationResult<std::function<StatusWith<int>()>> == int
* FutureContinuationResult<std::function<Future<int>()>> == int
*
* FutureContinuationResult<std::function<int(bool)>, bool> == int
*
* FutureContinuationResult<std::function<int(bool)>, NotBool> SFINAE-safe substitution failure.
*/
template <typename Func, typename... Args>
using FutureContinuationResult =
future_details::UnwrappedType<std::invoke_result_t<Func, Args&&...>>;
//
// Implementations of methods that couldn't be defined in the class due to ordering requirements.
//
template <typename T>
template <typename Sig>
MONGO_COMPILER_NOINLINE auto ExecutorFuture<T>::wrapCBHelper(unique_function<Sig>&& func) {
using namespace future_details;
return [
func = std::move(func),
exec = _exec // can't move this!
](auto&&... args) mutable noexcept
->Future<UnwrappedType<decltype(func(std::forward<decltype(args)>(args)...))>> {
auto [promise, future] = makePromiseFuture<
UnwrappedType<decltype(func(std::forward<decltype(args)>(args)...))>>();
exec->schedule([
promise = std::move(promise),
func = std::move(func),
argsT =
std::tuple<std::decay_t<decltype(args)>...>(std::forward<decltype(args)>(args)...)
](Status execStatus) mutable noexcept {
if (execStatus.isOK()) {
promise.setWith([&] {
return [&](auto nullary) {
// Using a lambda taking a nullary lambda here to work around an MSVC2017
// bug that caused it to not ignore the other side of the constexpr-if.
// TODO Make this less silly once we upgrade to 2019.
if constexpr (!isFutureLike<decltype(nullary())>) {
return nullary();
} else {
// Cheat and convert to an inline Future since we know we will schedule
// further user callbacks onto an executor.
return nullary().unsafeToInlineFuture();
}
}([&] { return std::apply(func, std::move(argsT)); });
});
} else {
promise.setError(std::move(execStatus));
}
});
return std::move(future);
};
}
template <typename T>
inline ExecutorFuture<T> SemiFuture<T>::thenRunOn(ExecutorPtr exec) && noexcept {
return ExecutorFuture<T>(std::move(exec), std::move(_impl));
}
template <typename T>
Future<T> SemiFuture<T>::unsafeToInlineFuture() && noexcept {
return Future<T>(std::move(_impl));
}
template <typename T>
inline SharedSemiFuture<future_details::FakeVoidToVoid<T>>
future_details::FutureImpl<T>::share() && noexcept {
using Out = SharedSemiFuture<FakeVoidToVoid<T>>;
if (_immediate)
return Out(SharedStateHolder<FakeVoidToVoid<T>>::makeReady(std::move(*_immediate)));
return Out(SharedStateHolder<FakeVoidToVoid<T>>(std::move(_shared)));
}
} // namespace mongo
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