mongo/src/mongo/util/future.h

/**
 *    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;

    /**
     * Returns an inline Future type from this SemiFuture.
     *
     * WARNING: Do not use this unless you're extremely sure of what you're doing, as callbacks
     * chained to the resulting Future may run in unexpected places.
     */
    Future<T> unsafeToInlineFuture() && 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;
    template <typename>
    friend class SemiFuture;


    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());
    }

    /**
     * Attaches a `func` callback that will consume the result of this `Future` when it completes. A
     * `getAsync` call can be the tail end of a continuation chain, and that is only position at
     * which it can appear.
     *
     * The result argument passed to `func` is `Status` if `T` is `void`, otherwise `StatusWith<T>`.
     * The `func(result)` return type must be `void`, and not a discarded return value.
     *
     * `func` must not throw an exception. It is invoked as if by a `noexcept` function, and it will
     * `std::terminate` the process. This is because there is no appropriate context in which to
     * handle such an asynchronous exception.
     *
     * 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)));
    }

    // Calling this on a Future is never strictly necessary, since this is already an inline
    // Future, but making this public helps for writing generic utilities which need to use
    // unsafeToInlineFuture for some future types but not others.
    using SemiFuture<T>::unsafeToInlineFuture;

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>;


    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 MONGO_WARN_UNUSED_RESULT_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.
    //

    /**
     * Attach a completion callback to asynchronously consume this `ExecutorFuture`'s result.
     * \see `Future<T>::getAsync()`.
     */
    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))));
    }

    /**
     * Returns an inline Future type from this ExecutorFuture.
     *
     * WARNING: Do not use this unless you're extremely sure of what you're doing, as callbacks
     * chained to the resulting Future may run in unexpected places.
     */
    using SemiFuture<T>::unsafeToInlineFuture;

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(_exec, unique_function<Sig>(std::forward<Func>(func)));
    }

    template <typename UniqueFunc>
    MONGO_COMPILER_NOINLINE static auto _wrapCBHelper(ExecutorPtr exec, UniqueFunc&& func);

    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>;

/** Constructor tag (see the corresponding `Promise` constructor). */
struct NonNullPromiseTag {};

/**
 * 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>;
    using T_unless_void = typename SemiFuture<T>::T_unless_void;

public:
    using value_type = T;

    /**
     * Creates a null `Promise`.
     */
    Promise() = default;

    /** Makes a `Promise` that has a `Future`. */
    explicit Promise(NonNullPromiseTag) : Promise{make_intrusive<SharedStateT>()} {}

    ~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<SharedStateT>&& sharedState) {
            std::move(future).propagateResultTo(sharedState.get());
        });
    }

    /**
     * Sets the value into this Promise immediately.
     *
     * This accepts a Status for Promises<void> or a StatusWith<T> for Promise<T>.
     */
    void setFrom(StatusOrStatusWith<T> sosw) noexcept {
        setImpl([&](boost::intrusive_ptr<SharedStateT>&& sharedState) {
            sharedState->setFrom(std::move(sosw));
        });
    }

    // Use emplaceValue(Args&&...) instead.
    REQUIRES_FOR_NON_TEMPLATE(!std::is_void_v<T>)
    void setFrom(T_unless_void val) noexcept = delete;

    // Use setError(Status) instead.
    REQUIRES_FOR_NON_TEMPLATE(!std::is_void_v<T>)
    void setFrom(Status) noexcept = delete;

    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));
        });
    }

    /**
     * Create and return the Future corresponding to this Promise. Cannot be
     * called more than once (but see `SharedPromise` which supports multiple
     * `getFuture` calls).
     *
     * Caution: calling `getFuture` is only valid before the promise is
     * completed, because a Promise is nullified upon completion. This Promise
     * behavior is different from the semantics of other future libraries. The
     * PromiseAndFuture helper avoids a potential data race by eagerly calling
     * `getFuture`, returning the Promise together with its Future.
     */
    Future<T> getFuture() noexcept {
        // Copy `_sharedState` into a SharedStateHolder to make a Future,
        // exploiting the knowledge that this Promise is its sole owner.
        _sharedState->threadUnsafeIncRefCountTo(2);
        return Future<T>(future_details::SharedStateHolder<future_details::VoidToFakeVoid<T>>(
            boost::intrusive_ptr<SharedStateT>(_sharedState.get(), false)));
    }

private:
    explicit Promise(boost::intrusive_ptr<SharedStateT>&& sharedState)
        : _sharedState(std::move(sharedState)) {}

    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());
    }

    /**
     * Makes a copy of this SharedSemiFuture that resolves at the same time as the original.
     */
    SharedSemiFuture split() const noexcept {
        return toFutureImpl().share();
    }

    SemiFuture<T> semi() && noexcept {
        return SemiFuture<T>(toFutureImpl());
    }

    /**
     * Returns an inline Future type from this SharedSemiFuture.
     *
     * WARNING: Do not use this unless you're extremely sure of what you're doing, as callbacks
     * chained to the resulting Future may run in unexpected places.
     */
    Future<T> unsafeToInlineFuture() const noexcept {
        return Future<T>(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));
    }
    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 {
    using SharedStateT = future_details::SharedState<T>;
    using T_unless_void = std::conditional_t<std::is_void_v<T>, future_details::FakeVoid, T>;

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 {
        setFrom(Future<void>::makeReady().then(std::forward<Func>(func)));
    }

    /**
     * Sets the value into this SharedPromise when the passed-in Future completes, which may have
     * already happened.
     */
    void setFrom(Future<T>&& future) noexcept {
        invariant(!std::exchange(_haveCompleted, true));
        std::move(future).propagateResultTo(_sharedState.get());
    }

    /**
     * Sets the value into this SharedPromise immediately.
     *
     * This accepts a Status for SharedPromises<void> or a StatusWith<T> for SharedPromise<T>.
     */
    void setFrom(StatusOrStatusWith<T> sosw) noexcept {
        invariant(!std::exchange(_haveCompleted, true));
        _sharedState->setFrom(std::move(sosw));
    }

    // Use emplaceValue(Args&&...) instead.
    REQUIRES_FOR_NON_TEMPLATE(!std::is_void_v<T>)
    void setFrom(T_unless_void val) noexcept = delete;

    // Use setError(Status) instead.
    REQUIRES_FOR_NON_TEMPLATE(!std::is_void_v<T>)
    void setFrom(Status) noexcept = delete;

    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));
    }

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<SharedStateT> _sharedState = make_intrusive<SharedStateT>();
};

/**
 * Holds a Promise and its corresponding Future.
 * Upon construction, contains a new Promise and its Future.
 */
template <typename T>
struct PromiseAndFuture {
    Promise<T> promise{NonNullPromiseTag{}};
    Future<T> future{promise.getFuture()};
};

/** Another way to write `PromiseAndFuture<T>{}`. */
template <typename T>
PromiseAndFuture<T> makePromiseFuture() {
    return {};
}

/**
 * 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&&...>>;

/**
 * This type transform is useful for coercing (T,StatusWith<T>)->Future<T> and Status->Future<void>.
 */
template <typename T>
using FutureFor = Future<future_details::UnwrappedType<T>>;

template <typename T>
auto coerceToFuture(T&& value) {
    return FutureFor<T>(std::forward<T>(value));
}

/**
 * Makes a 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.
 *
 * Note that if func returns an unready Future, this function will not block until it is ready.
 */
TEMPLATE(typename Func)
REQUIRES(future_details::isCallable<Func, void>)
auto makeReadyFutureWith(Func&& func) -> Future<FutureContinuationResult<Func&&>> try {
    if constexpr (std::is_void_v<std::invoke_result_t<Func>>) {
        std::forward<Func>(func)();
        return Future<void>::makeReady();
    } else {
        return std::forward<Func>(func)();
    }
} catch (const DBException& ex) {
    return ex.toStatus();
}

//
// Implementations of methods that couldn't be defined in the class due to ordering requirements.
//

template <typename T>
template <typename UniqueFunc>
auto ExecutorFuture<T>::_wrapCBHelper(ExecutorPtr exec, UniqueFunc&& func) {
    return [ exec = std::move(exec), func = std::move(func) ](auto&&... args) mutable noexcept {
        using FuncR = typename UniqueFunc::result_type;
        using BoundArgs = std::tuple<std::decay_t<decltype(args)>...>;
        Promise<future_details::UnwrappedType<FuncR>> promise{NonNullPromiseTag{}};
        auto future = promise.getFuture();

        exec->schedule([
            promise = std::move(promise),
            func = std::move(func),
            boundArgs = BoundArgs{std::forward<decltype(args)>(args)...}
        ](Status execStatus) mutable noexcept {
            if (!execStatus.isOK()) {
                promise.setError(std::move(execStatus));
                return;
            }
            promise.setWith([&] {
                auto closure = [&] { return std::apply(func, std::move(boundArgs)); };
                if constexpr (future_details::isFutureLike<FuncR>) {
                    // Cheat and convert to an inline Future since we know we will schedule
                    // further user callbacks onto an executor.
                    return closure().unsafeToInlineFuture();
                } else {
                    return closure();
                }
            });
        });

        return 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