path: root/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

#include <boost/intrusive_ptr.hpp>
#include <boost/optional.hpp>
#include <type_traits>

#include "mongo/base/checked_cast.h"
#include "mongo/base/static_assert.h"
#include "mongo/base/status.h"
#include "mongo/base/status_with.h"
#include "mongo/platform/atomic_word.h"
#include "mongo/stdx/condition_variable.h"
#include "mongo/stdx/mutex.h"
#include "mongo/stdx/type_traits.h"
#include "mongo/stdx/utility.h"
#include "mongo/util/assert_util.h"
#include "mongo/util/debug_util.h"
#include "mongo/util/functional.h"
#include "mongo/util/interruptible.h"
#include "mongo/util/intrusive_counter.h"
#include "mongo/util/scopeguard.h"

namespace mongo {

namespace future_details {
template <typename T>
class Promise;

template <typename T>
class Future;
template <>
class Future<void>;

template <typename T>
class SharedPromise;

template <typename T>
class SharedSemiFuture;
template <>
class SharedSemiFuture<void>;

// Using extern constexpr to prevent the compiler from allocating storage as a poor man's c++17
// inline constexpr variable.
// TODO delete extern in c++17 because inline is the default for constexper variables.
template <typename T>
extern constexpr bool isFuture = false;
template <typename T>
extern constexpr bool isFuture<Future<T>> = true;

template <typename T>
extern constexpr bool isFutureLike = false;
template <typename T>
extern constexpr bool isFutureLike<Future<T>> = true;
template <typename T>
extern constexpr bool isFutureLike<SharedSemiFuture<T>> = true;

// This is used to "normalize" void since it can't be used as an argument and it becomes Status
// rather than StatusWith<void>.
struct FakeVoid {};

template <typename T>
using VoidToFakeVoid = std::conditional_t<std::is_void<T>::value, FakeVoid, T>;

// call(func, FakeVoid) -> func(Status::OK())
// This simulates the implicit Status/T overloading you get by taking a StatusWith<T> that doesn't
// work for Status/void and Status.
// TODO replace this dispatch with constexpr if in c++17
template <typename Func>
inline auto callVoidOrStatus(Func&& func, std::true_type useStatus) {
    return func(Status::OK());
}

template <typename Func>
inline auto callVoidOrStatus(Func&& func, std::false_type useStatus) {
    return func();
}

/**
 * call() normalizes arguments to hide the FakeVoid shenanigans from users of Futures.
 * In the future it may also expand tuples to argument lists.
 */
template <typename Func, typename Arg>
inline auto call(Func&& func, Arg&& arg) {
    return func(std::forward<Arg>(arg));
}

template <typename Func>
inline auto call(Func&& func) {
    return func();
}

template <typename Func>
inline auto call(Func&& func, FakeVoid) {
    auto useStatus = std::integral_constant<bool,
                                            (!stdx::is_invocable<Func>() &&
                                             stdx::is_invocable<Func, Status>())>();
    return callVoidOrStatus(func, useStatus);
}

template <typename Func>
inline auto call(Func&& func, StatusWith<FakeVoid> sw) {
    return func(sw.getStatus());
}

/**
 * statusCall() normalizes return values so everything returns StatusWith<T>. Exceptions are
 * converted to !OK statuses. void and Status returns are converted to StatusWith<FakeVoid>
 */
template <
    typename Func,
    typename... Args,
    typename RawResult = decltype(call(std::declval<Func>(), std::declval<Args>()...)),
    typename = std::enable_if_t<!std::is_void<RawResult>::value &&
                                !std::is_same<RawResult, Status>::value>,
    typename Result = std::conditional_t<isStatusWith<RawResult>, RawResult, StatusWith<RawResult>>>
inline Result statusCall(Func&& func, Args&&... args) noexcept {
    try {
        return call(func, std::forward<Args>(args)...);
    } catch (const DBException& ex) {
        return ex.toStatus();
    }
}

template <typename Func,
          typename... Args,
          typename RawResult = decltype(call(std::declval<Func>(), std::declval<Args>()...)),
          typename = std::enable_if_t<std::is_void<RawResult>::value>>
inline StatusWith<FakeVoid> statusCall(Func&& func, Args&&... args) noexcept {
    try {
        call(func, std::forward<Args>(args)...);
        return FakeVoid{};
    } catch (const DBException& ex) {
        return ex.toStatus();
    }
}

template <typename Func,
          typename... Args,
          typename RawResult = decltype(call(std::declval<Func>(), std::declval<Args>()...)),
          typename = std::enable_if_t<std::is_same<RawResult, Status>::value>,
          typename = void,
          typename = void>
inline StatusWith<FakeVoid> statusCall(Func&& func, Args&&... args) noexcept {
    try {
        auto status = call(func, std::forward<Args>(args)...);
        if (status.isOK())
            return FakeVoid{};
        return std::move(status);
    } catch (const DBException& ex) {
        return ex.toStatus();
    }
}

/**
 * throwingCall() normalizes return values so everything returns T or FakeVoid. !OK Statuses are
 * converted exceptions. void and Status returns are converted to FakeVoid.
 *
 * This is equivalent to uassertStatusOK(statusCall(func, args...)), but avoids catching just to
 * rethrow.
 */
template <
    typename Func,
    typename... Args,
    typename Result = decltype(call(std::declval<Func>(), std::declval<Args>()...)),
    typename = std::enable_if_t<!std::is_void<Result>::value && !isStatusOrStatusWith<Result>>>
inline Result throwingCall(Func&& func, Args&&... args) {
    return call(func, std::forward<Args>(args)...);
}

template <typename Func,
          typename... Args,
          typename Result = decltype(call(std::declval<Func>(), std::declval<Args>()...)),
          typename = std::enable_if_t<std::is_void<Result>::value>>
inline FakeVoid throwingCall(Func&& func, Args&&... args) {
    call(func, std::forward<Args>(args)...);
    return FakeVoid{};
}

template <typename Func,
          typename... Args,
          typename Result = decltype(call(std::declval<Func>(), std::declval<Args>()...)),
          typename = std::enable_if_t<std::is_same<Result, Status>::value>,
          typename = void>
inline FakeVoid throwingCall(Func&& func, Args&&... args) {
    uassertStatusOK(call(func, std::forward<Args>(args)...));
    return FakeVoid{};
}

template <typename Func,
          typename... Args,
          typename StatusWithResult = decltype(call(std::declval<Func>(), std::declval<Args>()...)),
          typename = std::enable_if_t<isStatusWith<StatusWithResult>>,
          typename = void,
          typename = void>
inline typename StatusWithResult::value_type throwingCall(Func&& func, Args&&... args) noexcept {
    return uassertStatusOK(call(func, std::forward<Args>(args)...));
}

template <typename Func, typename... Args>
using RawNormalizedCallResult =
    decltype(throwingCall(std::declval<Func>(), std::declval<Args>()...));

template <typename Func, typename... Args>
using NormalizedCallResult =
    std::conditional_t<std::is_same<RawNormalizedCallResult<Func, Args...>, FakeVoid>::value,
                       void,
                       RawNormalizedCallResult<Func, Args...>>;

template <typename T>
struct FutureContinuationResultImpl {
    using type = T;
};
template <typename T>
struct FutureContinuationResultImpl<Future<T>> {
    using type = T;
};
template <typename T>
struct FutureContinuationResultImpl<StatusWith<T>> {
    using type = T;
};
template <>
struct FutureContinuationResultImpl<Status> {
    using type = void;
};

template <typename T>
struct SharedStateImpl;

template <typename T>
using SharedState = SharedStateImpl<VoidToFakeVoid<T>>;

/**
 * SSB is SharedStateBase, and this is its current state.
 *
 * Legal transitions on future side:
 *      kInit -> kWaiting
 *      kInit -> kHaveContinuation
 *      kWaiting -> kHaveContinuation
 *
 * Legal transitions on promise side:
 *      kInit -> kFinished
 *      kWaiting -> kFinished
 *      kHaveContinuation -> kFinished
 *
 * Note that all and only downward transitions are legal.
 *
 * Each thread must change the state *after* it is set up all data that it is releasing to the other
 * side. This must be done with an exchange() or compareExchange() so that you know what to do if
 * the other side finished its transition before you.
 */
enum class SSBState : uint8_t {
    // Initial state: Promise hasn't been completed and has nothing to do when it is.
    kInit,

    // Promise hasn't been completed. Someone has constructed the condvar and may be waiting on it.
    // We do not transition back to kInit if they give up on waiting. There is also no continuation
    // registered in this state.
    kWaiting,

    // Promise hasn't been completed. Someone has registered a callback to be run when it is.
    //
    // There is no-one currently waiting on the condvar. TODO This assumption will need to change
    // when we add continuation support to SharedSemiFuture.
    kHaveContinuation,

    // The promise has been completed with a value or error. This is the terminal state. This should
    // stay last since we have code like assert(state < kFinished).
    kFinished,
};

class SharedStateBase : public RefCountable {
public:
    SharedStateBase(const SharedStateBase&) = delete;
    SharedStateBase(SharedStateBase&&) = delete;
    SharedStateBase& operator=(const SharedStateBase&) = delete;
    SharedStateBase& operator=(SharedStateBase&&) = delete;

    virtual ~SharedStateBase() = default;

    // Only called by future side, but may be called multiple times if waiting times out and is
    // retried.
    void wait(Interruptible* interruptible) {
        if (state.load(std::memory_order_acquire) == SSBState::kFinished)
            return;

        stdx::unique_lock<stdx::mutex> lk(mx);
        if (!cv) {
            cv.emplace();

            auto oldState = SSBState::kInit;
            if (MONGO_unlikely(!state.compare_exchange_strong(
                    oldState, SSBState::kWaiting, std::memory_order_acq_rel))) {
                // transitionToFinished() transitioned after we did our initial check.
                dassert(oldState == SSBState::kFinished);
                return;
            }
        } else {
            // Someone has already created the cv and put us in the waiting state. The promise may
            // also have completed after we checked above, so we can't assume we aren't at
            // kFinished.
            dassert(state.load() != SSBState::kInit);
        }

        interruptible->waitForConditionOrInterrupt(*cv, lk, [&] {
            // The mx locking above is insufficient to establish an acquire if state transitions to
            // kFinished before we get here, but we aquire mx before the producer does.
            return state.load(std::memory_order_acquire) == SSBState::kFinished;
        });
    }

    // Remaining methods only called from promise side.
    void transitionToFinished() noexcept {
        auto oldState = state.exchange(SSBState::kFinished, std::memory_order_acq_rel);
        if (oldState == SSBState::kInit)
            return;

        dassert(oldState == SSBState::kWaiting || oldState == SSBState::kHaveContinuation);

        DEV {
            // If you hit this limit one of two things has probably happened
            //
            // 1. The justForContinuation optimization isn't working.
            // 2. You may be creating a variable length chain.
            //
            // If those statements don't mean anything to you, please ask an editor of this file.
            // If they don't work here anymore, I'm sorry.
            const size_t kMaxDepth = 32;

            size_t depth = 0;
            for (auto ssb = continuation.get(); ssb;
                 ssb = ssb->state.load(std::memory_order_acquire) == SSBState::kHaveContinuation
                     ? ssb->continuation.get()
                     : nullptr) {
                depth++;

                invariant(depth < kMaxDepth);
            }
        }

        if (oldState == SSBState::kHaveContinuation) {
            invariant(callback);
            callback(this);
        } else if (cv) {
            stdx::unique_lock<stdx::mutex> lk(mx);
            // This must be done inside the lock to correctly synchronize with wait().
            cv->notify_all();
        }
    }

    void setError(Status statusArg) noexcept {
        invariant(!statusArg.isOK());
        dassert(state.load() < SSBState::kFinished, statusArg.toString());
        status = std::move(statusArg);
        transitionToFinished();
    }

    //
    // Concurrency Rules for members: Each non-atomic member is initially owned by either the
    // Promise side or the Future side, indicated by a P/F comment. The general rule is that members
    // representing the propagating data are owned by Promise, while members representing what
    // to do with the data are owned by Future. The owner may freely modify the members it owns
    // until it releases them by doing a release-store to state of kFinished from Promise or
    // kWaiting from Future. Promise can acquire access to all members by doing an acquire-load of
    // state and seeing kWaiting (or Future with kFinished). Transitions should be done via
    // acquire-release exchanges to combine both actions.
    //
    // Future::propagateResults uses an alternative mechanism to transfer ownership of the
    // continuation member. The logical Future-side does a release-store of true to
    // isJustForContinuation, and the Promise-side can do an acquire-load seeing true to get access.
    //


    std::atomic<SSBState> state{SSBState::kInit};  // NOLINT

    // This is used to prevent infinite chains of SharedStates that just propagate results.
    std::atomic<bool> isJustForContinuation{false};  // NOLINT

    // This is likely to be a different derived type from this, since it is the logical output of
    // callback.
    boost::intrusive_ptr<SharedStateBase> continuation;  // F

    // Takes this as argument and usually writes to continuation.
    unique_function<void(SharedStateBase* input)> callback;  // F


    // These are only used to signal completion to blocking waiters. Benchmarks showed that it was
    // worth deferring the construction of cv, so it can be avoided when it isn't necessary.
    stdx::mutex mx;                                // F (not that it matters)
    boost::optional<stdx::condition_variable> cv;  // F

    Status status = Status::OK();  // P

protected:
    SharedStateBase() = default;
};

template <typename T>
struct SharedStateImpl final : SharedStateBase {
    MONGO_STATIC_ASSERT(!std::is_void<T>::value);

    // Remaining methods only called by promise side.
    void fillFrom(SharedState<T>&& other) {
        dassert(state.load() < SSBState::kFinished);
        dassert(other.state.load() == SSBState::kFinished);
        if (other.status.isOK()) {
            data = std::move(other.data);
        } else {
            status = std::move(other.status);
        }
        transitionToFinished();
    }

    template <typename... Args>
    void emplaceValue(Args&&... args) noexcept {
        dassert(state.load() < SSBState::kFinished);
        try {
            data.emplace(std::forward<Args>(args)...);
        } catch (const DBException& ex) {
            status = ex.toStatus();
        }
        transitionToFinished();
    }

    void setFromStatusWith(StatusWith<T> sw) {
        if (sw.isOK()) {
            emplaceValue(std::move(sw.getValue()));
        } else {
            setError(std::move(sw.getStatus()));
        }
    }

    boost::optional<T> data;  // P
};
}  // namespace future_details

// These are in the future_details namespace to get access to its contents, but they are part of the
// public API.
using future_details::Promise;
using future_details::Future;
using future_details::SharedPromise;
using future_details::SharedSemiFuture;

/**
 * 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 future_details::Promise {
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>
    void setWith(Func&& func) noexcept;

    /**
     * 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;

    template <typename... Args>
    void emplaceValue(Args&&... args) noexcept {
        setImpl([&](boost::intrusive_ptr<SharedState<T>>&& sharedState) {
            sharedState->emplaceValue(std::forward<Args>(args)...);
        });
    }

    void setError(Status status) noexcept {
        invariant(!status.isOK());
        setImpl([&](boost::intrusive_ptr<SharedState<T>>&& sharedState) {
            sharedState->setError(std::move(status));
        });
    }

    // TODO rename to not XXXWith and handle void
    void setFromStatusWith(StatusWith<T> sw) noexcept {
        setImpl([&](boost::intrusive_ptr<SharedState<T>>&& sharedState) {
            sharedState->setFromStatusWith(std::move(sw));
        });
    }

    static auto makePromiseFutureImpl() {
        struct PromiseAndFuture {
            Promise<T> promise{make_intrusive<SharedState<T>>()};
            Future<T> future = promise.getFuture();
        };
        return PromiseAndFuture();
    }

private:
    explicit Promise(boost::intrusive_ptr<SharedState<T>>&& 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;

    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<SharedState<T>> _sharedState;
};

/**
 * Future<T> is logically a possibly-deferred StatusWith<T> (or Status when T is void).
 *
 * As is usual for rvalue-qualified methods, you may call at most one of them on a given Future.
 *
 * 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().
 */
template <typename T>
class MONGO_WARN_UNUSED_RESULT_CLASS future_details::Future {
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(!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;

    /**
     * Constructs a Future in a moved-from state that can only be assigned to or destroyed.
     */
    Future() = default;

    Future& operator=(Future&&) = default;
    Future(Future&&) = default;

    Future(const Future&) = delete;
    Future& operator=(const Future&) = delete;

    /* implicit */ Future(T val) : Future(makeReady(std::move(val))) {}
    /* implicit */ Future(Status status) : Future(makeReady(std::move(status))) {}
    /* implicit */ Future(StatusWith<T> sw) : Future(makeReady(std::move(sw))) {}

    /**
     * Make a ready Future<T> from a value for cases where you don't need to wait asynchronously.
     *
     * Calling this is faster than getting a Future out of a Promise, and is effectively free. It is
     * fast enough that you never need to avoid returning a Future 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 Future when you need
     * to get another batch.
     */
    static Future<T> makeReady(T val) {  // TODO emplace?
        Future out;
        out._immediate = std::move(val);
        return out;
    }

    static Future<T> makeReady(Status status) {
        invariant(!status.isOK());
        auto out = Future<T>(make_intrusive<SharedState<T>>());
        out._shared->setError(std::move(status));
        return out;
    }

    static Future<T> makeReady(StatusWith<T> val) {
        if (val.isOK())
            return makeReady(std::move(val.getValue()));
        return makeReady(val.getStatus());
    }

    /**
     * Convert this Future to a SharedSemiFuture.
     */
    SharedSemiFuture<T> share() && noexcept;

    /**
     * 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 Future can't
     * complete in the background).
     *
     * Callers must still call get() or similar, even on Future<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 Future is ready already.
     */
    bool isReady() const {
        return _immediate || _shared->state.load(std::memory_order_acquire) == SSBState::kFinished;
    }

    /**
     * Returns when the future isReady().
     *
     * Throws if the interruptible passed is interrupted (explicitly or via deadline).
     */
    void wait(Interruptible* interruptible = Interruptible::notInterruptible()) const {
        if (_immediate) {
            return;
        }

        _shared->wait(interruptible);
    }

    /**
     * Returns Status::OK() when the future isReady().
     *
     * Returns a non-okay status if the interruptible is interrupted.
     */
    Status waitNoThrow(Interruptible* interruptible = Interruptible::notInterruptible()) const
        noexcept {
        if (_immediate) {
            return Status::OK();
        }

        try {
            _shared->wait(interruptible);
        } catch (const DBException& ex) {
            return ex.toStatus();
        }

        return Status::OK();
    }

    /**
     * Gets the value out of this Future, 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 future chain with these methods.  If you need to distinguish the two cases, call
     * wait() first.
     */
    T get(Interruptible* interruptible = Interruptible::notInterruptible()) && {
        return std::move(getImpl(interruptible));
    }
    T& get(Interruptible* interruptible = Interruptible::notInterruptible()) & {
        return getImpl(interruptible);
    }
    const T& get(Interruptible* interruptible = Interruptible::notInterruptible()) const& {
        return const_cast<Future*>(this)->getImpl(interruptible);
    }
    StatusWith<T> getNoThrow(Interruptible* interruptible = Interruptible::notInterruptible()) &&
        noexcept {
        if (_immediate) {
            return std::move(*_immediate);
        }

        try {
            _shared->wait(interruptible);
        } catch (const DBException& ex) {
            return ex.toStatus();
        }

        if (!_shared->status.isOK())
            return std::move(_shared->status);
        return std::move(*_shared->data);
    }
    StatusWith<T> getNoThrow(
        Interruptible* interruptible = Interruptible::notInterruptible()) const& noexcept {
        if (_immediate) {
            return *_immediate;
        }

        try {
            _shared->wait(interruptible);
        } catch (const DBException& ex) {
            return ex.toStatus();
        }

        if (!_shared->status.isOK())
            return _shared->status;
        return *_shared->data;
    }

    /**
     * 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>  // StatusWith<T> -> void
        void getAsync(Func&& func) && noexcept {
        static_assert(std::is_void<decltype(call(func, std::declval<StatusWith<T>>()))>::value,
                      "func passed to getAsync must return void");

        return generalImpl(
            // on ready success:
            [&](T&& val) { call(func, std::move(val)); },
            // on ready failure:
            [&](Status&& status) { call(func, std::move(status)); },
            // on not ready yet:
            [&] {
                _shared->callback = [func = std::forward<Func>(func)](SharedStateBase *
                                                                      ssb) mutable noexcept {
                    const auto input = checked_cast<SharedState<T>*>(ssb);
                    if (input->status.isOK()) {
                        call(func, std::move(*input->data));
                    } else {
                        call(func, std::move(input->status));
                    }
                };
            });
    }

    //
    // The remaining methods are all continuation based and take a callback and return a Future.
    // 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 that return Future<T> are automatically unwrapped and connected to the returned
    // Future<T>, rather than producing a Future<Future<T>>.
    //

    /**
     * Callbacks passed to then() are only called if the input Future completes successfully.
     * Otherwise the error propagates automatically, bypassing the callback.
     */
    template <typename Func,  // T -> Result or T -> StatusWith<Result>
              typename Result = NormalizedCallResult<Func, T>,
              typename = std::enable_if_t<!isFuture<Result>>>
        Future<Result> then(Func&& func) && noexcept {
        return generalImpl(
            // on ready success:
            [&](T&& val) { return Future<Result>::makeReady(statusCall(func, std::move(val))); },
            // on ready failure:
            [&](Status&& status) { return Future<Result>::makeReady(std::move(status)); },
            // on not ready yet:
            [&] {
                return makeContinuation<Result>([func = std::forward<Func>(func)](
                    SharedState<T> * input, SharedState<Result> * output) mutable noexcept {
                    if (!input->status.isOK())
                        return output->setError(std::move(input->status));

                    output->setFromStatusWith(statusCall(func, std::move(*input->data)));
                });
            });
    }

    /**
     * Same as above then() but for case where func returns a Future that needs to be unwrapped.
     */
    template <typename Func,  // T -> Future<UnwrappedResult>
              typename RawResult = NormalizedCallResult<Func, T>,
              typename = std::enable_if_t<isFuture<RawResult>>,
              typename UnwrappedResult = typename RawResult::value_type>
        Future<UnwrappedResult> then(Func&& func) && noexcept {
        return generalImpl(
            // on ready success:
            [&](T&& val) {
                try {
                    return Future<UnwrappedResult>(throwingCall(func, std::move(val)));
                } catch (const DBException& ex) {
                    return Future<UnwrappedResult>::makeReady(ex.toStatus());
                }
            },
            // on ready failure:
            [&](Status&& status) { return Future<UnwrappedResult>::makeReady(std::move(status)); },
            // on not ready yet:
            [&] {
                return makeContinuation<UnwrappedResult>([func = std::forward<Func>(func)](
                    SharedState<T> * input,
                    SharedState<UnwrappedResult> * output) mutable noexcept {
                    if (!input->status.isOK())
                        return output->setError(std::move(input->status));

                    try {
                        throwingCall(func, std::move(*input->data)).propagateResultTo(output);
                    } catch (const DBException& ex) {
                        output->setError(ex.toStatus());
                    }
                });
            });
    }

    /**
     * Callbacks passed to onCompletion() are called if the input Future completes with or without
     * an error.
     *
     * 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.
     */
    template <
        // T -> Result, T -> StatusWith<Result>, Status -> Result or Status -> StatusWith<Result>
        typename Func,
        typename Result = NormalizedCallResult<Func, Status>,
        typename = std::enable_if_t<!isFuture<Result>>>
        Future<Result> onCompletion(Func&& func) && noexcept {
        static_assert(std::is_same<Result, NormalizedCallResult<Func, T>>::value,
                      "func passed to Future<T>::onCompletion must return the same type for "
                      "arguments of Status and T");

        return generalImpl(
            // on ready success:
            [&](T&& val) {
                return Future<Result>::makeReady(
                    statusCall(std::forward<Func>(func), std::move(val)));
            },
            // on ready failure:
            [&](Status&& status) {
                return Future<Result>::makeReady(
                    statusCall(std::forward<Func>(func), std::move(status)));
            },
            // on not ready yet:
            [&] {
                return makeContinuation<Result>([func = std::forward<Func>(func)](
                    SharedState<T> * input, SharedState<Result> * output) mutable noexcept {
                    if (!input->status.isOK())
                        return output->setFromStatusWith(
                            statusCall(func, std::move(input->status)));

                    output->setFromStatusWith(statusCall(func, std::move(*input->data)));
                });
            });
    }

    /**
     * Same as above onCompletion() but for the case where func returns a Future that needs to be
     * unwrapped.
     */
    template <typename Func,  // T -> Future<UnwrappedResult> or Status -> Future<UnwrappedResult>
              typename RawResult = NormalizedCallResult<Func, Status>,
              typename = std::enable_if_t<isFuture<RawResult>>,
              typename UnwrappedResult = typename RawResult::value_type>
        Future<UnwrappedResult> onCompletion(Func&& func) && noexcept {
        static_assert(std::is_same<UnwrappedResult,
                                   typename NormalizedCallResult<Func, T>::value_type>::value,
                      "func passed to Future<T>::onCompletion must return the same type for "
                      "arguments of Status and T");

        return generalImpl(
            // on ready success:
            [&](T&& val) {
                try {
                    return Future<UnwrappedResult>(
                        throwingCall(std::forward<Func>(func), std::move(val)));
                } catch (const DBException& ex) {
                    return Future<UnwrappedResult>::makeReady(ex.toStatus());
                }
            },
            // on ready failure:
            [&](Status&& status) {
                try {
                    return Future<UnwrappedResult>(
                        throwingCall(std::forward<Func>(func), std::move(status)));
                } catch (const DBException& ex) {
                    return Future<UnwrappedResult>::makeReady(ex.toStatus());
                }
            },
            // on not ready yet:
            [&] {
                return makeContinuation<UnwrappedResult>([func = std::forward<Func>(func)](
                    SharedState<T> * input,
                    SharedState<UnwrappedResult> * output) mutable noexcept {
                    if (!input->status.isOK()) {
                        try {
                            throwingCall(func, std::move(input->status)).propagateResultTo(output);
                        } catch (const DBException& ex) {
                            output->setError(ex.toStatus());
                        }

                        return;
                    }

                    try {
                        throwingCall(func, std::move(*input->data)).propagateResultTo(output);
                    } catch (const DBException& ex) {
                        output->setError(ex.toStatus());
                    }
                });
            });
    }

    /**
     * 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.
     */
    template <typename Func,  // Status -> T or Status -> StatusWith<T>
              typename Result = RawNormalizedCallResult<Func, Status>,
              typename = std::enable_if_t<!isFuture<Result>>>
        Future<T> onError(Func&& func) && noexcept {
        static_assert(
            std::is_same<Result, T>::value,
            "func passed to Future<T>::onError must return T, StatusWith<T>, or Future<T>");

        return generalImpl(
            // on ready success:
            [&](T&& val) { return Future<T>::makeReady(std::move(val)); },
            // on ready failure:
            [&](Status&& status) {
                return Future<T>::makeReady(statusCall(func, std::move(status)));
            },
            // on not ready yet:
            [&] {
                return makeContinuation<T>([func = std::forward<Func>(func)](
                    SharedState<T> * input, SharedState<T> * output) mutable noexcept {
                    if (input->status.isOK())
                        return output->emplaceValue(std::move(*input->data));

                    output->setFromStatusWith(statusCall(func, std::move(input->status)));
                });
            });
    }

    /**
     * Same as above onError() but for case where func returns a Future that needs to be unwrapped.
     */
    template <typename Func,  // Status -> Future<T>
              typename Result = RawNormalizedCallResult<Func, Status>,
              typename = std::enable_if_t<isFuture<Result>>,
              typename = void>
        Future<T> onError(Func&& func) && noexcept {
        static_assert(
            std::is_same<Result, Future<T>>::value ||
                (std::is_same<T, FakeVoid>::value && std::is_same<Result, Future<void>>::value),
            "func passed to Future<T>::onError must return T, StatusWith<T>, or Future<T>");

        return generalImpl(
            // on ready success:
            [&](T&& val) { return Future<T>::makeReady(std::move(val)); },
            // on ready failure:
            [&](Status&& status) {
                try {
                    return Future<T>(throwingCall(func, std::move(status)));
                } catch (const DBException& ex) {
                    return Future<T>::makeReady(ex.toStatus());
                }
            },
            // on not ready yet:
            [&] {
                return makeContinuation<T>([func = std::forward<Func>(func)](
                    SharedState<T> * input, SharedState<T> * output) mutable noexcept {
                    if (input->status.isOK())
                        return output->emplaceValue(std::move(*input->data));

                    try {
                        throwingCall(func, std::move(input->status)).propagateResultTo(output);
                    } catch (const DBException& ex) {
                        output->setError(ex.toStatus());
                    }
                });
            });
    }

    /**
     * Same as the other two onErrors but only calls the callback if the code matches the template
     * parameter. Otherwise lets the error propagate unchanged.
     */
    template <ErrorCodes::Error code, typename Func>
        Future<T> onError(Func&& func) && noexcept {
        using Result = RawNormalizedCallResult<Func, Status>;
        static_assert(
            std::is_same<Result, T>::value || std::is_same<Result, Future<T>>::value ||
                (std::is_same<T, FakeVoid>::value && std::is_same<Result, Future<void>>::value),
            "func passed to Future<T>::onError must return T, StatusWith<T>, or Future<T>");

        if (_immediate || (isReady() && _shared->status.isOK()))
            return std::move(*this);  // Avoid copy/moving func if we know we won't call it.

        // TODO in C++17 with constexpr if this can be done cleaner and more efficiently by not
        // throwing.
        return std::move(*this).onError([func =
                                             std::forward<Func>(func)](Status && status) mutable {
            if (status != code)
                uassertStatusOK(status);
            return throwingCall(func, std::move(status));
        });
    }

    /**
     * Similar to the first two onErrors, but only calls the callback if the category matches
     * the template parameter. Otherwise lets the error propagate unchanged.
     */
    template <ErrorCategory category, typename Func>
        Future<T> onErrorCategory(Func&& func) && noexcept {
        using Result = RawNormalizedCallResult<Func, Status>;
        static_assert(
            std::is_same<Result, T>::value || std::is_same<Result, Future<T>>::value ||
                (std::is_same<T, FakeVoid>::value && std::is_same<Result, Future<void>>::value),
            "func passed to Future<T>::onErrorCategory must return T, StatusWith<T>, or Future<T>");

        if (_immediate || (isReady() && _shared->status.isOK()))
            return std::move(*this);

        return std::move(*this).onError([func =
                                             std::forward<Func>(func)](Status && status) mutable {
            if (!ErrorCodes::isA<category>(status.code()))
                uassertStatusOK(status);
            return throwingCall(func, std::move(status));
        });
    }

    /**
     * TODO do we need a version of then/onError like onCompletion() that handles both success and
     * Failure, but doesn't end the chain like getAsync()? Right now we don't, and we can add one if
     * we do.
     */

    //
    // 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.
     */
    template <typename Func>  // T -> void
        Future<T> tap(Func&& func) && noexcept {
        static_assert(std::is_void<decltype(call(func, std::declval<const T&>()))>::value,
                      "func passed to tap must return void");

        return tapImpl(std::forward<Func>(func),
                       [](Func && func, const T& val) noexcept { call(func, val); },
                       [](Func && func, const Status& status) noexcept {});
    }

    /**
     * Callback is called if the input completes with an error.
     *
     * This can be used to log.
     */
    template <typename Func>  // Status -> void
        Future<T> tapError(Func&& func) && noexcept {
        static_assert(std::is_void<decltype(call(func, std::declval<const Status&>()))>::value,
                      "func passed to tapError must return void");

        return tapImpl(std::forward<Func>(func),
                       [](Func && func, const T& val) noexcept {},
                       [](Func && func, const Status& status) noexcept { call(func, status); });
    }

    /**
     * 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.
     *
     * Warning: If func takes a StatusWith<T>, it requires copying the value on success. If that is
     * too expensive, it can be avoided by either providing a function object with separate
     * Status/const T& overloads, or by using a generic lambda if you don't need to consult the
     * value for your cleanup.
     */
    template <typename Func>  // StatusWith<T> -> void, or Status/const T& overloads.
        Future<T> tapAll(Func&& func) && noexcept {
        static_assert(std::is_void<decltype(call(func, std::declval<const T&>()))>::value,
                      "func passed to tapAll must return void");
        static_assert(std::is_void<decltype(call(func, std::declval<const Status&>()))>::value,
                      "func passed to tapAll must return void");

        return tapImpl(std::forward<Func>(func),
                       [](Func && func, const T& val) noexcept { call(func, val); },
                       [](Func && func, const Status& status) noexcept { call(func, status); });
    }

    /**
     * Ignores the return value of a future, transforming it down into a Future<void>.
     *
     * This only ignores values, not errors.  Those remain propogated until an onError handler.
     *
     * Equivalent to then([](auto&&){});
     */
    Future<void> ignoreValue() && noexcept;

private:
    template <typename T2>
    friend class Future;
    friend class Promise<T>;

    T& getImpl(Interruptible* interruptible) {
        if (_immediate) {
            return *_immediate;
        }

        _shared->wait(interruptible);
        uassertStatusOK(_shared->status);
        return *(_shared->data);
    }

    // All callbacks are called immediately so they are allowed to capture everything by reference.
    // All callbacks should return the same return type.
    template <typename SuccessFunc, typename FailFunc, typename NotReady>
    auto generalImpl(SuccessFunc&& success, FailFunc&& fail, NotReady&& notReady) noexcept {
        if (_immediate) {
            return success(std::move(*_immediate));
        }

        auto oldState = _shared->state.load(std::memory_order_acquire);
        dassert(oldState != SSBState::kHaveContinuation);
        if (oldState == SSBState::kFinished) {
            if (_shared->status.isOK()) {
                return success(std::move(*_shared->data));
            } else {
                return fail(std::move(_shared->status));
            }
        }

        // This is always done after notReady, which never throws. It is in an ON_BLOCK_EXIT to
        // support both void- and value-returning notReady implementations since we can't assign
        // void to a variable.
        ON_BLOCK_EXIT([&] {
            // oldState could be either kInit or kWaiting, depending on whether we've failed a call
            // to wait().
            if (MONGO_unlikely(!_shared->state.compare_exchange_strong(
                    oldState, SSBState::kHaveContinuation, std::memory_order_acq_rel))) {
                dassert(oldState == SSBState::kFinished);
                _shared->callback(_shared.get());
            }
        });

        return notReady();
    }

    // success and fail may be called from a continuation so they shouldn't capture anything.
    template <typename Callback, typename SuccessFunc, typename FailFunc>
    Future<T> tapImpl(Callback&& cb, SuccessFunc&& success, FailFunc&& fail) noexcept {
        // Make sure they don't capture anything.
        MONGO_STATIC_ASSERT(std::is_empty<SuccessFunc>::value);
        MONGO_STATIC_ASSERT(std::is_empty<FailFunc>::value);

        return generalImpl(
            [&](T&& val) {
                success(std::forward<Callback>(cb), stdx::as_const(val));
                return Future<T>::makeReady(std::move(val));
            },
            [&](Status&& status) {
                fail(std::forward<Callback>(cb), stdx::as_const(status));
                return Future<T>::makeReady(std::move(status));
            },
            [&] {
                return makeContinuation<T>([ success, fail, cb = std::forward<Callback>(cb) ](
                    SharedState<T> * input, SharedState<T> * output) mutable noexcept {
                    if (input->status.isOK()) {
                        success(std::forward<Callback>(cb), stdx::as_const(*input->data));
                    } else {
                        fail(std::forward<Callback>(cb), stdx::as_const(input->status));
                    }

                    output->fillFrom(std::move(*input));
                });
            });
    }

    void propagateResultTo(SharedState<T>* output) noexcept {
        generalImpl(
            // on ready success:
            [&](T&& val) { output->emplaceValue(std::move(val)); },
            // on ready failure:
            [&](Status&& status) { output->setError(std::move(status)); },
            // on not ready yet:
            [&] {
                // If the output is just for continuation, bypass it and just directly fill in the
                // SharedState that it would write to. The concurrency situation is a bit subtle
                // here since we are the Future-side of shared, but the Promise-side of output.
                // The rule is that p->isJustForContinuation must be acquire-read as true before
                // examining p->continuation, and p->continuation must be written before doing the
                // release-store of true to p->isJustForContinuation.
                if (output->isJustForContinuation.load(std::memory_order_acquire)) {
                    _shared->continuation = std::move(output->continuation);
                } else {
                    _shared->continuation = output;
                }
                _shared->isJustForContinuation.store(true, std::memory_order_release);

                _shared->callback = [](SharedStateBase * ssb) noexcept {
                    const auto input = checked_cast<SharedState<T>*>(ssb);
                    const auto output = checked_cast<SharedState<T>*>(ssb->continuation.get());
                    output->fillFrom(std::move(*input));
                };
            });
    }

    template <typename Result, typename OnReady>
    inline Future<Result> makeContinuation(OnReady&& onReady) {
        invariant(!_shared->callback && !_shared->continuation);

        auto continuation = make_intrusive<SharedState<Result>>();
        continuation->threadUnsafeIncRefCountTo(2);
        _shared->continuation.reset(continuation.get(), /*add ref*/ false);
        _shared->callback = [onReady = std::forward<OnReady>(onReady)](SharedStateBase *
                                                                       ssb) mutable noexcept {
            const auto input = checked_cast<SharedState<T>*>(ssb);
            const auto output = checked_cast<SharedState<Result>*>(ssb->continuation.get());
            onReady(input, output);
        };
        return Future<VoidToFakeVoid<Result>>(std::move(continuation));
    }

    explicit Future(boost::intrusive_ptr<SharedState<T>> ptr) : _shared(std::move(ptr)) {}

    // At most one of these will be active.
    boost::optional<T> _immediate;
    boost::intrusive_ptr<SharedState<T>> _shared;
};

/**
 * The void specialization of Future<T>. See the general Future<T> for detailed documentation.
 * It should be the same as the generic Future<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 <>
class MONGO_WARN_UNUSED_RESULT_CLASS future_details::Future<void> {
public:
    using value_type = void;

    /* implicit */ Future() : Future(makeReady()) {}
    /* implicit */ Future(Status status) : Future(makeReady(std::move(status))) {}

    static Future<void> makeReady() {
        return Future<FakeVoid>::makeReady(FakeVoid{});
    }

    static Future<void> makeReady(Status status) {
        if (status.isOK())
            return makeReady();
        return Future<FakeVoid>::makeReady(std::move(status));
    }

    SharedSemiFuture<void> share() && noexcept;

    bool isReady() const {
        return _inner.isReady();
    }

    void wait(Interruptible* interruptible = Interruptible::notInterruptible()) const {
        _inner.wait(interruptible);
    }

    Status waitNoThrow(Interruptible* interruptible = Interruptible::notInterruptible()) const
        noexcept {
        return _inner.waitNoThrow(interruptible);
    }

    void get(Interruptible* interruptible = Interruptible::notInterruptible()) const {
        _inner.get(interruptible);
    }

    Status getNoThrow(Interruptible* interruptible = Interruptible::notInterruptible()) const
        noexcept {
        return _inner.getNoThrow(interruptible).getStatus();
    }

    template <typename Func>  // Status -> void
        void getAsync(Func&& func) && noexcept {
        return std::move(_inner).getAsync(std::forward<Func>(func));
    }

    template <typename Func>  // () -> T or StatusWith<T> or Future<T>
        auto then(Func&& func) && noexcept {
        return std::move(_inner).then(std::forward<Func>(func));
    }

    template <typename Func>  // Status -> T or StatusWith<T> or Future<T>
        auto onCompletion(Func&& func) && noexcept {
        return std::move(_inner).onCompletion(std::forward<Func>(func));
    }

    template <typename Func>  // Status -> T or StatusWith<T> or Future<T>
        Future<void> onError(Func&& func) && noexcept {
        return std::move(_inner).onError(std::forward<Func>(func));
    }

    template <ErrorCodes::Error code, typename Func>  // Status -> T or StatusWith<T> or Future<T>
        Future<void> onError(Func&& func) && noexcept {
        return std::move(_inner).onError<code>(std::forward<Func>(func));
    }

    template <typename Func>  // () -> void
        Future<void> tap(Func&& func) && noexcept {
        return std::move(_inner).tap(std::forward<Func>(func));
    }

    template <typename Func>  // Status -> void
        Future<void> tapError(Func&& func) && noexcept {
        return std::move(_inner).tapError(std::forward<Func>(func));
    }

    template <typename Func>  // Status -> void
        Future<void> tapAll(Func&& func) && noexcept {
        return std::move(_inner).tapAll(std::forward<Func>(func));
    }

    Future<void> ignoreValue() && noexcept {
        return std::move(*this);
    }

private:
    template <typename T>
    friend class Future;
    friend class Promise<void>;

    explicit Future(boost::intrusive_ptr<SharedState<FakeVoid>> ptr) : _inner(std::move(ptr)) {}
    /*implicit*/ Future(Future<FakeVoid>&& inner) : _inner(std::move(inner)) {}
    /*implicit*/ operator Future<FakeVoid>() && {
        return std::move(_inner);
    }

    void propagateResultTo(SharedState<void>* output) noexcept {
        _inner.propagateResultTo(output);
    }

    static Future<void> makeReady(StatusWith<FakeVoid> status) {
        return Future<FakeVoid>::makeReady(std::move(status));
    }

    Future<FakeVoid> _inner;
};

/**
 * 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 future_details::SharedSemiFuture {
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(
        !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.");

    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(T val) : SharedSemiFuture(Future<T>(std::move(val))) {}
    /*implicit*/ SharedSemiFuture(Status error) : SharedSemiFuture(Future<T>(std::move(error))) {}
    /*implicit*/ SharedSemiFuture(StatusWith<T> sw) : SharedSemiFuture(Future<T>(std::move(sw))) {}

    bool isReady() const {
        return _shared->state.load(std::memory_order_acquire) == SSBState::kFinished;
    }

    void wait(Interruptible* interruptible = Interruptible::notInterruptible()) const {
        _shared->wait(interruptible);
    }

    Status waitNoThrow(Interruptible* interruptible = Interruptible::notInterruptible()) const
        noexcept {
        try {
            _shared->wait(interruptible);
        } catch (const DBException& ex) {
            return ex.toStatus();
        }

        return Status::OK();
    }

    const T& get(Interruptible* interruptible = Interruptible::notInterruptible()) const& {
        _shared->wait(interruptible);
        uassertStatusOK(_shared->status);
        return *(_shared->data);
    }

    StatusWith<T> getNoThrow(
        Interruptible* interruptible = Interruptible::notInterruptible()) const& noexcept {
        try {
            _shared->wait(interruptible);
        } catch (const DBException& ex) {
            return ex.toStatus();
        }

        if (!_shared->status.isOK())
            return _shared->status;
        return *_shared->data;
    }

private:
    template <typename T2>
    friend class SharedPromise;
    template <typename T2>
    friend class Future;
    friend class SharedSemiFuture<void>;

    explicit SharedSemiFuture(boost::intrusive_ptr<SharedState<T>> ptr) : _shared(std::move(ptr)) {}

    boost::intrusive_ptr<SharedState<T>> _shared;
};

template <>
class MONGO_WARN_UNUSED_RESULT_CLASS future_details::SharedSemiFuture<void> {
public:
    using value_type = void;

    SharedSemiFuture() = default;

    /*implicit*/ SharedSemiFuture(const Future<void>& fut) = delete;
    /*implicit*/ SharedSemiFuture(Future<void>&& fut) : SharedSemiFuture(std::move(fut).share()) {}
    /*implicit*/ SharedSemiFuture(Status err) : SharedSemiFuture(Future<void>(std::move(err))) {}

    bool isReady() const {
        return _inner.isReady();
    }

    void wait(Interruptible* interruptible = Interruptible::notInterruptible()) const {
        _inner.wait(interruptible);
    }

    Status waitNoThrow(Interruptible* interruptible = Interruptible::notInterruptible()) const
        noexcept {
        return _inner.waitNoThrow(interruptible);
    }

    void get(Interruptible* interruptible = Interruptible::notInterruptible()) const {
        _inner.get(interruptible);
    }

    Status getNoThrow(Interruptible* interruptible = Interruptible::notInterruptible()) const
        noexcept {
        return _inner.getNoThrow(interruptible).getStatus();
    }

private:
    friend class SharedPromise<void>;
    friend class Future<void>;

    explicit SharedSemiFuture(boost::intrusive_ptr<SharedState<FakeVoid>> ptr)
        : _inner(std::move(ptr)) {}

    /*implicit*/ SharedSemiFuture(SharedSemiFuture<FakeVoid>&& inner) : _inner(std::move(inner)) {}
    /*implicit*/ operator SharedSemiFuture<FakeVoid>() && {
        return std::move(_inner);
    }

    SharedSemiFuture<FakeVoid> _inner;
};

/**
 * 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 future_details::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>
    void setWith(Func&& func) noexcept {
        invariant(!haveCompleted());
        setFrom(Future<void>::makeReady().then(std::forward<Func>(func)));
    }

    void setFrom(Future<T>&& future) noexcept {
        invariant(!haveCompleted());
        std::move(future).propagateResultTo(_sharedState.get());
    }

    template <typename... Args>
    void emplaceValue(Args&&... args) noexcept {
        invariant(!haveCompleted());
        _sharedState->emplaceValue(std::forward<Args>(args)...);
    }

    void setError(Status status) noexcept {
        invariant(!status.isOK());
        invariant(!haveCompleted());
        _sharedState->setError(std::move(status));
    }

    // TODO rename to not XXXWith and handle void
    void setFromStatusWith(StatusWith<T> sw) noexcept {
        invariant(!haveCompleted());
        _sharedState->setFromStatusWith(std::move(sw));
    }

private:
    friend class Future<void>;

    bool haveCompleted() const noexcept {
        // This can be relaxed because it is only called from the Promise thread which is also the
        // only thread that will transition this from returning false to true. Additionally it isn't
        // used to establish synchronization with any other thread.
        return _sharedState->state.load(std::memory_order_relaxed) == SSBState::kFinished;
    }

    const boost::intrusive_ptr<SharedState<T>> _sharedState = make_intrusive<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>
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 = typename future_details::FutureContinuationResultImpl<
    std::invoke_result_t<Func, Args&&...>>::type;

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

template <typename T>
inline Future<T> Promise<T>::getFuture() noexcept {
    using namespace future_details;
    _sharedState->threadUnsafeIncRefCountTo(2);
    return Future<T>(boost::intrusive_ptr<SharedState<T>>(_sharedState.get(), /*add ref*/ false));
}

template <typename T>
inline void Promise<T>::setFrom(Future<T>&& future) noexcept {
    using namespace future_details;
    setImpl([&](boost::intrusive_ptr<SharedState<T>>&& sharedState) {
        future.propagateResultTo(sharedState.get());
    });
}

template <typename T>
template <typename Func>
inline void Promise<T>::setWith(Func&& func) noexcept {
    setFrom(Future<void>::makeReady().then(std::forward<Func>(func)));
}

template <typename T>
    inline Future<void> Future<T>::ignoreValue() && noexcept {
    return std::move(*this).then([](auto&&) {});
}

template <typename T>
    inline SharedSemiFuture<T> Future<T>::share() && noexcept {
    using namespace future_details;
    if (!_immediate)
        return SharedSemiFuture<T>(std::move(_shared));

    auto shared = make_intrusive<SharedState<T>>();
    shared->emplaceValue(std::move(*_immediate));
    return SharedSemiFuture<T>(std::move(shared));
}

inline SharedSemiFuture<void> Future<void>::share() && noexcept {
    return std::move(_inner).share();
}

}  // namespace mongo