future.hh

/*
 * This file is open source software, licensed to you under the terms
 * of the Apache License, Version 2.0 (the "License").  See the NOTICE file
 * distributed with this work for additional information regarding copyright
 * ownership.  You may not use this file except in compliance with the License.
 *
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
/*
 * Copyright (C) 2015 Cloudius Systems, Ltd.
 */

#pragma once

#include <seastar/core/apply.hh>
#include <seastar/core/task.hh>
#include <seastar/core/preempt.hh>
#include <seastar/core/thread_impl.hh>
#include <stdexcept>
#include <atomic>
#include <memory>
#include <type_traits>
#include <assert.h>
#include <cstdlib>
#include <seastar/core/function_traits.hh>
#include <seastar/util/alloc_failure_injector.hh>
#include <seastar/util/attribute-compat.hh>
#include <seastar/util/gcc6-concepts.hh>
#include <seastar/util/noncopyable_function.hh>

namespace seastar {





template <class... T>
class promise;

template <class... T>
class future;

template <typename... T>
class shared_future;

struct future_state_base;

template <typename... T, typename... A>
future<T...> make_ready_future(A&&... value);

template <typename... T>
future<T...> make_exception_future(std::exception_ptr&& value) noexcept;

template <typename... T>
future<T...> make_exception_future(const std::exception_ptr& ex) noexcept {
    return make_exception_future<T...>(std::exception_ptr(ex));
}

template <typename... T>
future<T...> make_exception_future(std::exception_ptr& ex) noexcept {
    return make_exception_future<T...>(static_cast<const std::exception_ptr&>(ex));
}

void engine_exit(std::exception_ptr eptr = {});

void report_failed_future(const std::exception_ptr& ex) noexcept;

void report_failed_future(const future_state_base& state) noexcept;


struct broken_promise : std::logic_error {
    broken_promise();
};

namespace internal {
template <class... T>
class promise_base_with_type;

template <typename... T>
future<T...> current_exception_as_future() noexcept;

extern template
future<> current_exception_as_future() noexcept;

// It doesn't seem to be possible to use std::tuple_element_t with an empty tuple. There is an static_assert in it that
// fails the build even if it is in the non enabled side of std::conditional.
template <typename... T>
struct get0_return_type {
    using type = void;
    static type get0(std::tuple<T...> v) { }
};

template <typename T0, typename... T>
struct get0_return_type<T0, T...> {
    using type = T0;
    static type get0(std::tuple<T0, T...> v) { return std::get<0>(std::move(v)); }
};

template <typename T, bool is_trivial_class>
struct uninitialized_wrapper_base;

template <typename T>
struct uninitialized_wrapper_base<T, false> {
    union any {
        any() {}
        ~any() {}
        T value;
    } _v;

public:
    template<typename... U>
    void uninitialized_set(U&&... vs) {
        new (&_v.value) T(std::forward<U>(vs)...);
    }
    T& uninitialized_get() {
        return _v.value;
    }
    const T& uninitialized_get() const {
        return _v.value;
    }
};

template <typename T> struct uninitialized_wrapper_base<T, true> : private T {
    template<typename... U>
    void uninitialized_set(U&&... vs) {
        new (this) T(std::forward<U>(vs)...);
    }
    T& uninitialized_get() {
        return *this;
    }
    const T& uninitialized_get() const {
        return *this;
    }
};

template <typename T>
constexpr bool can_inherit =
#ifdef _LIBCPP_VERSION
// We expect std::tuple<> to be trivially constructible and
// destructible. That is not the case with libc++
// (https://bugs.llvm.org/show_bug.cgi?id=41714).  We could avoid this
// optimization when using libc++ and relax the asserts, but
// inspection suggests that std::tuple<> is trivial, it is just not
// marked as such.
        std::is_same<std::tuple<>, T>::value ||
#endif
        (std::is_trivially_destructible<T>::value && std::is_trivially_constructible<T>::value &&
                std::is_class<T>::value && !std::is_final<T>::value);

// The objective is to avoid extra space for empty types like std::tuple<>. We could use std::is_empty_v, but it is
// better to check that both the constructor and destructor can be skipped.
template <typename T>
struct uninitialized_wrapper
    : public uninitialized_wrapper_base<T, can_inherit<T>> {};

static_assert(std::is_empty<uninitialized_wrapper<std::tuple<>>>::value, "This should still be empty");

template <typename T>
struct is_trivially_move_constructible_and_destructible {
    static constexpr bool value = std::is_trivially_move_constructible<T>::value && std::is_trivially_destructible<T>::value;
};

template <bool... v>
struct all_true : std::false_type {};

template <>
struct all_true<> : std::true_type {};

template <bool... v>
struct all_true<true, v...> : public all_true<v...> {};
}

//
// A future/promise pair maintain one logical value (a future_state).
// There are up to three places that can store it, but only one is
// active at any time.
//
// - in the promise _local_state member variable
//
//   This is necessary because a promise is created first and there
//   would be nowhere else to put the value.
//
// - in the future _state variable
//
//   This is used anytime a future exists and then has not been called
//   yet. This guarantees a simple access to the value for any code
//   that already has a future.
//
// - in the task associated with the .then() clause (after .then() is called,
//   if a value was not set)
//
//
// The promise maintains a pointer to the state, which is modified as
// the state moves to a new location due to events (such as .then() or
// get_future being called) or due to the promise or future being
// moved around.
//

// non templated base class to reduce code duplication
struct future_state_base {
    static_assert(std::is_nothrow_copy_constructible<std::exception_ptr>::value,
                  "std::exception_ptr's copy constructor must not throw");
    static_assert(std::is_nothrow_move_constructible<std::exception_ptr>::value,
                  "std::exception_ptr's move constructor must not throw");
    static_assert(sizeof(std::exception_ptr) == sizeof(void*), "exception_ptr not a pointer");
    enum class state : uintptr_t {
         invalid = 0,
         future = 1,
         // the substate is intended to decouple the run-time prevention
         // for duplicative result extraction (calling e.g. then() twice
         // ends up in abandoned()) from the wrapped object's destruction
         // handling which is orchestrated by future_state. Instead of
         // creating a temporary future_state just for the sake of setting
         // the "invalid" in the source instance, result_unavailable can
         // be set to ensure future_state_base::available() returns false.
         result_unavailable = 2,
         result = 3,
         exception_min = 4,  // or anything greater
    };
    union any {
        any() { st = state::future; }
        any(state s) { st = s; }
        void set_exception(std::exception_ptr&& e) {
            new (&ex) std::exception_ptr(std::move(e));
            assert(st >= state::exception_min);
        }
        any(std::exception_ptr&& e) {
            set_exception(std::move(e));
        }
        ~any() {}
        std::exception_ptr take_exception() {
            std::exception_ptr ret(std::move(ex));
            // Unfortunately in libstdc++ ~exception_ptr is defined out of line. We know that it does nothing for
            // moved out values, so we omit calling it. This is critical for the code quality produced for this
            // function. Without the out of line call, gcc can figure out that both sides of the if produce
            // identical code and merges them.if
            // We don't make any assumptions about other c++ libraries.
            // There is request with gcc to define it inline: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90295
#ifndef __GLIBCXX__
            ex.~exception_ptr();
#endif
            st = state::invalid;
            return ret;
        }
        any(any&& x) {
            if (x.st < state::exception_min) {
                st = x.st;
                x.st = state::invalid;
            } else {
                new (&ex) std::exception_ptr(x.take_exception());
            }
        }
        bool has_result() const {
            return st == state::result || st == state::result_unavailable;
        }
        state st;
        std::exception_ptr ex;
    } _u;

    future_state_base() noexcept { }
    future_state_base(state st) noexcept : _u(st) { }
    future_state_base(std::exception_ptr&& ex) noexcept : _u(std::move(ex)) { }
    future_state_base(future_state_base&& x) noexcept : _u(std::move(x._u)) { }

    // We never need to destruct this polymorphicly, so we can make it
    // protected instead of virtual.
protected:
    ~future_state_base() noexcept {
        if (failed()) {
            report_failed_future(_u.take_exception());
        }
    }

public:

    bool valid() const noexcept { return _u.st != state::invalid; }
    bool available() const noexcept { return _u.st == state::result || _u.st >= state::exception_min; }
    bool failed() const noexcept { return __builtin_expect(_u.st >= state::exception_min, false); }

    void set_to_broken_promise() noexcept;

    void ignore() noexcept;

    void set_exception(std::exception_ptr&& ex) noexcept {
        assert(_u.st == state::future);
        _u.set_exception(std::move(ex));
    }
    future_state_base& operator=(future_state_base&& x) noexcept {
        this->~future_state_base();
        new (this) future_state_base(std::move(x));
        return *this;
    }
    void set_exception(future_state_base&& state) noexcept {
        assert(_u.st == state::future);
        *this = std::move(state);
    }
    std::exception_ptr get_exception() && noexcept {
        assert(_u.st >= state::exception_min);
        // Move ex out so future::~future() knows we've handled it
        return _u.take_exception();
    }
    const std::exception_ptr& get_exception() const& noexcept {
        assert(_u.st >= state::exception_min);
        return _u.ex;
    }

    static future_state_base current_exception();

    template <typename... U>
    friend future<U...> internal::current_exception_as_future() noexcept;
};

struct ready_future_marker {};
struct exception_future_marker {};
struct future_for_get_promise_marker {};

template <typename... T>
struct future_state :  public future_state_base, private internal::uninitialized_wrapper<std::tuple<T...>> {
    static constexpr bool copy_noexcept = std::is_nothrow_copy_constructible<std::tuple<T...>>::value;
    static constexpr bool has_trivial_move_and_destroy =
        internal::all_true<internal::is_trivially_move_constructible_and_destructible<T>::value...>::value;
    static_assert(std::is_nothrow_move_constructible<std::tuple<T...>>::value,
                  "Types must be no-throw move constructible");
    static_assert(std::is_nothrow_destructible<std::tuple<T...>>::value,
                  "Types must be no-throw destructible");
    future_state() noexcept {}
    [[gnu::always_inline]]
    future_state(future_state&& x) noexcept : future_state_base(std::move(x)) {
        if (has_trivial_move_and_destroy) {
#pragma GCC diagnostic push
// Unfortunately gcc 8 warns about the memcpy of uninitialized
// memory. We can drop this when we drop support for gcc 8.
#pragma GCC diagnostic ignored "-Wuninitialized"
            memcpy(reinterpret_cast<char*>(&this->uninitialized_get()),
                   &x.uninitialized_get(),
                   internal::used_size<std::tuple<T...>>::value);
#pragma GCC diagnostic pop
        } else if (_u.has_result()) {
            this->uninitialized_set(std::move(x.uninitialized_get()));
            x.uninitialized_get().~tuple();
        }
    }
    __attribute__((always_inline))
    ~future_state() noexcept {
        if (_u.has_result()) {
            this->uninitialized_get().~tuple();
        }
    }
    future_state& operator=(future_state&& x) noexcept {
        this->~future_state();
        new (this) future_state(std::move(x));
        return *this;
    }
    template <typename... A>
    future_state(ready_future_marker, A&&... a) : future_state_base(state::result) {
        this->uninitialized_set(std::forward<A>(a)...);
    }
    template <typename... A>
    void set(A&&... a) {
        assert(_u.st == state::future);
        new (this) future_state(ready_future_marker(), std::forward<A>(a)...);
    }
    future_state(exception_future_marker m, std::exception_ptr&& ex) : future_state_base(std::move(ex)) { }
    future_state(exception_future_marker m, future_state_base&& state) : future_state_base(std::move(state)) { }
    std::tuple<T...>&& get_value() && noexcept {
        assert(_u.st == state::result);
        return std::move(this->uninitialized_get());
    }
    std::tuple<T...>&& take_value() && noexcept {
        assert(_u.st == state::result);
        _u.st = state::result_unavailable;
        return std::move(this->uninitialized_get());
    }
    template<typename U = std::tuple<T...>>
    const std::enable_if_t<std::is_copy_constructible<U>::value, U>& get_value() const& noexcept(copy_noexcept) {
        assert(_u.st == state::result);
        return this->uninitialized_get();
    }
    std::tuple<T...>&& take() && {
        assert(available());
        if (_u.st >= state::exception_min) {
            // Move ex out so future::~future() knows we've handled it
            std::rethrow_exception(std::move(*this).get_exception());
        }
        _u.st = state::result_unavailable;
        return std::move(this->uninitialized_get());
    }
    std::tuple<T...>&& get() && {
        assert(available());
        if (_u.st >= state::exception_min) {
            // Move ex out so future::~future() knows we've handled it
            std::rethrow_exception(std::move(*this).get_exception());
        }
        return std::move(this->uninitialized_get());
    }
    const std::tuple<T...>& get() const& {
        assert(available());
        if (_u.st >= state::exception_min) {
            std::rethrow_exception(_u.ex);
        }
        return this->uninitialized_get();
    }
    using get0_return_type = typename internal::get0_return_type<T...>::type;
    static get0_return_type get0(std::tuple<T...>&& x) {
        return internal::get0_return_type<T...>::get0(std::move(x));
    }
};

static_assert(sizeof(future_state<>) <= 8, "future_state<> is too large");
static_assert(sizeof(future_state<long>) <= 16, "future_state<long> is too large");

template <typename... T>
class continuation_base : public task {
protected:
    future_state<T...> _state;
    using future_type = future<T...>;
    using promise_type = promise<T...>;
public:
    continuation_base() = default;
    explicit continuation_base(future_state<T...>&& state) : _state(std::move(state)) {}
    void set_state(future_state<T...>&& state) {
        _state = std::move(state);
    }
    friend class internal::promise_base_with_type<T...>;
    friend class promise<T...>;
    friend class future<T...>;
};

template <typename Promise, typename... T>
class continuation_base_with_promise : public continuation_base<T...> {
protected:
    continuation_base_with_promise(Promise&& pr, future_state<T...>&& state)
        : continuation_base<T...>(std::move(state)), _pr(std::move(pr)) {}
    continuation_base_with_promise(Promise&& pr) : _pr(std::move(pr)) {}
    Promise _pr;
};

template <typename Promise, typename Func, typename... T>
struct continuation final : continuation_base_with_promise<Promise, T...> {
    continuation(Promise&& pr, Func&& func, future_state<T...>&& state) : continuation_base_with_promise<Promise, T...>(std::move(pr), std::move(state)), _func(std::move(func)) {}
    continuation(Promise&& pr, Func&& func) : continuation_base_with_promise<Promise, T...>(std::move(pr)), _func(std::move(func)) {}
    virtual void run_and_dispose() noexcept override {
        _func(this->_pr, std::move(this->_state));
        delete this;
    }
    Func _func;
};

namespace internal {

template <typename... T>
future<T...> make_exception_future(future_state_base&& state) noexcept;

template <typename... T, typename U>
void set_callback(future<T...>& fut, U* callback) noexcept;

class future_base;

class promise_base {
protected:
    enum class urgent { no, yes };
    future_base* _future = nullptr;

    // This points to the future_state that is currently being
    // used. See comment above the future_state struct definition for
    // details.
    future_state_base* _state;

    task* _task = nullptr;

    promise_base(const promise_base&) = delete;
    promise_base(future_state_base* state) noexcept : _state(state) {}
    promise_base(future_base* future, future_state_base* state) noexcept;
    promise_base(promise_base&& x) noexcept;

    // We never need to destruct this polymorphicly, so we can make it
    // protected instead of virtual
    ~promise_base() noexcept;

    void operator=(const promise_base&) = delete;
    promise_base& operator=(promise_base&& x) = delete;

    template<urgent Urgent>
    void make_ready() noexcept;

    template<typename T>
    void set_exception_impl(T&& val) noexcept {
        if (_state) {
            _state->set_exception(std::move(val));
            make_ready<urgent::no>();
        } else {
            // We get here if promise::get_future is called and the
            // returned future is destroyed without creating a
            // continuation.
            // In older versions of seastar we would store a local
            // copy of ex and warn in the promise destructor.
            // Since there isn't any way for the user to clear
            // the exception, we issue the warning from here.
            report_failed_future(val);
        }
    }

    void set_exception(future_state_base&& state) noexcept {
        set_exception_impl(std::move(state));
    }

    void set_exception(std::exception_ptr&& ex) noexcept {
        set_exception_impl(std::move(ex));
    }

    void set_exception(const std::exception_ptr& ex) noexcept {
        set_exception(std::exception_ptr(ex));
    }

    template<typename Exception>
    std::enable_if_t<!std::is_same<std::remove_reference_t<Exception>, std::exception_ptr>::value, void> set_exception(Exception&& e) noexcept {
        set_exception(make_exception_ptr(std::forward<Exception>(e)));
    }

    friend class future_base;
    template <typename... U> friend class seastar::future;
};

template <typename... T>
class promise_base_with_type : protected internal::promise_base {
protected:
    future_state<T...>* get_state() {
        return static_cast<future_state<T...>*>(_state);
    }
    static constexpr bool copy_noexcept = future_state<T...>::copy_noexcept;
public:
    promise_base_with_type(future_state_base* state) noexcept : promise_base(state) { }
    promise_base_with_type(future<T...>* future) noexcept : promise_base(future, &future->_state) { }
    promise_base_with_type(promise_base_with_type&& x) noexcept : promise_base(std::move(x)) { }
    promise_base_with_type(const promise_base_with_type&) = delete;
    promise_base_with_type& operator=(promise_base_with_type&& x) noexcept {
        this->~promise_base_with_type();
        new (this) promise_base_with_type(std::move(x));
        return *this;
    }
    void operator=(const promise_base_with_type&) = delete;

    void set_urgent_state(future_state<T...>&& state) noexcept {
        auto* ptr = get_state();
        // The state can be null if the corresponding future has been
        // destroyed without producing a continuation.
        if (ptr) {
            // FIXME: This is a fairly expensive assert. It would be a
            // good candidate for being disabled in release builds if
            // we had such an assert.
            assert(ptr->_u.st == future_state_base::state::future);
            new (ptr) future_state<T...>(std::move(state));
            make_ready<urgent::yes>();
        }
    }

    template <typename... A>
    void set_value(A&&... a) {
        if (auto *s = get_state()) {
            s->set(std::forward<A>(a)...);
            make_ready<urgent::no>();
        }
    }

#if SEASTAR_COROUTINES_TS
    void set_coroutine(future_state<T...>& state, task& coroutine) noexcept {
        _state = &state;
        _task = &coroutine;
    }
#endif
private:
    template <typename Pr, typename Func>
    void schedule(Pr&& pr, Func&& func) noexcept {
        auto tws = new continuation<Pr, Func, T...>(std::move(pr), std::move(func));
        _state = &tws->_state;
        _task = tws;
    }
    void schedule(continuation_base<T...>* callback) noexcept {
        _state = &callback->_state;
        _task = callback;
    }

    template <typename... U>
    friend class seastar::future;

    friend struct seastar::future_state<T...>;
};
}

template <typename... T>
class promise : private internal::promise_base_with_type<T...> {
    future_state<T...> _local_state;

public:
    promise() noexcept : internal::promise_base_with_type<T...>(&_local_state) {}

    promise(promise&& x) noexcept;
    promise(const promise&) = delete;
    promise& operator=(promise&& x) noexcept {
        this->~promise();
        new (this) promise(std::move(x));
        return *this;
    }
    void operator=(const promise&) = delete;

    future<T...> get_future() noexcept;

    template <typename... A>
    void set_value(A&&... a) {
        internal::promise_base_with_type<T...>::set_value(std::forward<A>(a)...);
    }

    void set_exception(std::exception_ptr&& ex) noexcept {
        internal::promise_base::set_exception(std::move(ex));
    }

    void set_exception(const std::exception_ptr& ex) noexcept {
        internal::promise_base::set_exception(ex);
    }

    template<typename Exception>
    std::enable_if_t<!std::is_same<std::remove_reference_t<Exception>, std::exception_ptr>::value, void> set_exception(Exception&& e) noexcept {
        internal::promise_base::set_exception(std::forward<Exception>(e));
    }

    using internal::promise_base_with_type<T...>::set_urgent_state;

    template <typename... U>
    friend class future;
};

template<>
class promise<void> : public promise<> {};




template <typename... T> struct is_future : std::false_type {};

template <typename... T> struct is_future<future<T...>> : std::true_type {};



template <typename T>
struct futurize;

template <typename T>
struct futurize {
    using type = future<T>;
    using promise_type = promise<T>;
    using value_type = std::tuple<T>;

    template<typename Func, typename... FuncArgs>
    static inline type apply(Func&& func, std::tuple<FuncArgs...>&& args) noexcept;

    template<typename Func, typename... FuncArgs>
    static inline type invoke(Func&& func, FuncArgs&&... args) noexcept;

    template<typename Func, typename... FuncArgs>
    [[deprecated("Use invoke for varargs")]]
    static inline type apply(Func&& func, FuncArgs&&... args) noexcept;

    static inline type convert(T&& value) { return make_ready_future<T>(std::move(value)); }
    static inline type convert(type&& value) { return std::move(value); }

    static type from_tuple(value_type&& value);
    static type from_tuple(const value_type& value);

    template <typename Arg>
    static type make_exception_future(Arg&& arg) noexcept;
};

template <>
struct futurize<void> {
    using type = future<>;
    using promise_type = promise<>;
    using value_type = std::tuple<>;

    template<typename Func, typename... FuncArgs>
    static inline type apply(Func&& func, std::tuple<FuncArgs...>&& args) noexcept;

    template<typename Func, typename... FuncArgs>
    static inline type invoke(Func&& func, FuncArgs&&... args) noexcept;

    template<typename Func, typename... FuncArgs>
    [[deprecated("Use invoke for varargs")]]
    static inline type apply(Func&& func, FuncArgs&&... args) noexcept;

    static inline type from_tuple(value_type&& value);
    static inline type from_tuple(const value_type& value);

    template <typename Arg>
    static type make_exception_future(Arg&& arg) noexcept;
};

template <typename... Args>
struct futurize<future<Args...>> {
    using type = future<Args...>;
    using promise_type = promise<Args...>;
    using value_type = std::tuple<Args...>;

    template<typename Func, typename... FuncArgs>
    static inline type apply(Func&& func, std::tuple<FuncArgs...>&& args) noexcept;

    template<typename Func, typename... FuncArgs>
    static inline type invoke(Func&& func, FuncArgs&&... args) noexcept;

    template<typename Func, typename... FuncArgs>
    [[deprecated("Use invoke for varargs")]]
    static inline type apply(Func&& func, FuncArgs&&... args) noexcept;

    static inline type from_tuple(value_type&& value);
    static inline type from_tuple(const value_type& value);

    static inline type convert(Args&&... values) { return make_ready_future<Args...>(std::move(values)...); }
    static inline type convert(type&& value) { return std::move(value); }

    template <typename Arg>
    static type make_exception_future(Arg&& arg) noexcept;
};

// Converts a type to a future type, if it isn't already.
template <typename T>
using futurize_t = typename futurize<T>::type;


template<typename Func, typename... Args>
auto futurize_invoke(Func&& func, Args&&... args);

template<typename Func, typename... Args>
auto futurize_apply(Func&& func, std::tuple<Args...>&& args);

GCC6_CONCEPT(

template <typename T>
concept bool Future = is_future<T>::value;

template <typename Func, typename... T>
concept bool CanApply = requires (Func f, T... args) {
    f(std::forward<T>(args)...);
};

template <typename Func, typename Return, typename... T>
concept bool ApplyReturns = requires (Func f, T... args) {
    { f(std::forward<T>(args)...) } -> Return;
};

template <typename Func, typename... T>
concept bool ApplyReturnsAnyFuture = requires (Func f, T... args) {
    requires is_future<decltype(f(std::forward<T>(args)...))>::value;
};

)

namespace internal {
class future_base {
protected:
    promise_base* _promise;
    future_base() noexcept : _promise(nullptr) {}
    future_base(promise_base* promise, future_state_base* state) noexcept : _promise(promise) {
        _promise->_future = this;
        _promise->_state = state;
    }

    future_base(future_base&& x, future_state_base* state) noexcept : _promise(x._promise) {
        if (auto* p = _promise) {
            x.detach_promise();
            p->_future = this;
            p->_state = state;
        }
    }
    ~future_base() noexcept {
        if (_promise) {
            detach_promise();
        }
    }

    promise_base* detach_promise() noexcept {
        _promise->_state = nullptr;
        _promise->_future = nullptr;
        return std::exchange(_promise, nullptr);
    }

    friend class promise_base;
};

template <bool IsVariadic>
struct warn_variadic_future {
    // Non-varidic case, do nothing
    void check_deprecation() {}
};


// Note: placing the deprecated attribute on the class specialization has no effect.
template <>
struct warn_variadic_future<true> {
    // Variadic case, has deprecation attribute
    [[deprecated("Variadic future<> with more than one template parmeter is deprecated, replace with future<std::tuple<...>>")]]
    void check_deprecation() {}
};

}

template <typename... T>
class SEASTAR_NODISCARD future : private internal::future_base, internal::warn_variadic_future<(sizeof...(T) > 1)> {
    future_state<T...> _state;
    static constexpr bool copy_noexcept = future_state<T...>::copy_noexcept;
private:
    // This constructor creates a future that is not ready but has no
    // associated promise yet. The use case is to have a less flexible
    // but more efficient future/promise pair where we know that
    // promise::set_value cannot possibly be called without a matching
    // future and so that promise doesn't need to store a
    // future_state.
    future(future_for_get_promise_marker m) { }

    future(promise<T...>* pr) noexcept : future_base(pr, &_state), _state(std::move(pr->_local_state)) { }
    template <typename... A>
    future(ready_future_marker m, A&&... a) : _state(m, std::forward<A>(a)...) { }
    future(exception_future_marker m, std::exception_ptr&& ex) noexcept : _state(m, std::move(ex)) { }
    future(exception_future_marker m, future_state_base&& state) noexcept : _state(m, std::move(state)) { }
    [[gnu::always_inline]]
    explicit future(future_state<T...>&& state) noexcept
            : _state(std::move(state)) {
        this->check_deprecation();
    }
    internal::promise_base_with_type<T...> get_promise() noexcept {
        assert(!_promise);
        return internal::promise_base_with_type<T...>(this);
    }
    internal::promise_base_with_type<T...>* detach_promise() {
        return static_cast<internal::promise_base_with_type<T...>*>(future_base::detach_promise());
    }
    template <typename Pr, typename Func>
    void schedule(Pr&& pr, Func&& func) noexcept {
        if (_state.available() || !_promise) {
            if (__builtin_expect(!_state.available() && !_promise, false)) {
                _state.set_to_broken_promise();
            }
            ::seastar::schedule(new continuation<Pr, Func, T...>(std::move(pr), std::move(func), std::move(_state)));
        } else {
            assert(_promise);
            detach_promise()->schedule(std::move(pr), std::move(func));
            _state._u.st = future_state_base::state::invalid;
        }
    }

    [[gnu::always_inline]]
    future_state<T...>&& get_available_state_ref() noexcept {
        if (_promise) {
            detach_promise();
        }
        return std::move(_state);
    }

    [[gnu::noinline]]
    future<T...> rethrow_with_nested() {
        if (!failed()) {
            return internal::current_exception_as_future<T...>();
        } else {
            //
            // Encapsulate the current exception into the
            // std::nested_exception because the current libstdc++
            // implementation has a bug requiring the value of a
            // std::throw_with_nested() parameter to be of a polymorphic
            // type.
            //
            std::nested_exception f_ex;
            try {
                get();
            } catch (...) {
                std::throw_with_nested(f_ex);
            }
            __builtin_unreachable();
        }
    }

    template<typename... U>
    friend class shared_future;
public:
    using value_type = std::tuple<T...>;
    using promise_type = promise<T...>;
    [[gnu::always_inline]]
    future(future&& x) noexcept : future_base(std::move(x), &_state), _state(std::move(x._state)) { }
    future(const future&) = delete;
    future& operator=(future&& x) noexcept {
        this->~future();
        new (this) future(std::move(x));
        return *this;
    }
    void operator=(const future&) = delete;
    [[gnu::always_inline]]
    std::tuple<T...>&& get() {
        if (!_state.available()) {
            do_wait();
        }
        return get_available_state_ref().take();
    }

    [[gnu::always_inline]]
     std::exception_ptr get_exception() {
        return get_available_state_ref().get_exception();
    }

    typename future_state<T...>::get0_return_type get0() {
        return future_state<T...>::get0(get());
    }

    void wait() noexcept {
        if (!_state.available()) {
            do_wait();
        }
    }
private:
    class thread_wake_task final : public continuation_base<T...> {
        thread_context* _thread;
        future* _waiting_for;
    public:
        thread_wake_task(thread_context* thread, future* waiting_for)
                : _thread(thread), _waiting_for(waiting_for) {
        }
        virtual void run_and_dispose() noexcept override {
            _waiting_for->_state = std::move(this->_state);
            thread_impl::switch_in(_thread);
            // no need to delete, since this is always allocated on
            // _thread's stack.
        }
    };
    void do_wait() noexcept {
        if (__builtin_expect(!_promise, false)) {
            _state.set_to_broken_promise();
            return;
        }
        auto thread = thread_impl::get();
        assert(thread);
        thread_wake_task wake_task{thread, this};
        detach_promise()->schedule(static_cast<continuation_base<T...>*>(&wake_task));
        thread_impl::switch_out(thread);
    }

public:
    [[gnu::always_inline]]
    bool available() const noexcept {
        return _state.available();
    }

    [[gnu::always_inline]]
    bool failed() const noexcept {
        return _state.failed();
    }

    template <typename Func, typename Result = futurize_t<std::result_of_t<Func(T&&...)>>>
    GCC6_CONCEPT( requires ::seastar::CanApply<Func, T...> )
    Result
    then(Func&& func) noexcept {
#ifndef SEASTAR_TYPE_ERASE_MORE
        return then_impl(std::move(func));
#else
        return then_impl(noncopyable_function<Result (T&&...)>([func = std::forward<Func>(func)] (T&&... args) mutable {
            return futurize_apply(func, std::forward_as_tuple(std::move(args)...));
        }));
#endif
    }

private:

    // Keep this simple so that Named Return Value Optimization is used.
    template <typename Func, typename Result>
    Result then_impl_nrvo(Func&& func) noexcept {
        using futurator = futurize<std::result_of_t<Func(T&&...)>>;
        typename futurator::type fut(future_for_get_promise_marker{});
        // If there is a std::bad_alloc in schedule() there is nothing that can be done about it, we cannot break future
        // chain by returning ready future while 'this' future is not ready. The noexcept will call std::terminate if
        // that happens.
        [&] () noexcept {
            using pr_type = decltype(fut.get_promise());
            memory::disable_failure_guard dfg;
            schedule(fut.get_promise(), [func = std::forward<Func>(func)] (pr_type& pr, future_state<T...>&& state) mutable {
                if (state.failed()) {
                    pr.set_exception(static_cast<future_state_base&&>(std::move(state)));
                } else {
                    futurator::apply(std::forward<Func>(func), std::move(state).get_value()).forward_to(std::move(pr));
                }
            });
        } ();
        return fut;
    }

    template <typename Func, typename Result = futurize_t<std::result_of_t<Func(T&&...)>>>
    Result
    then_impl(Func&& func) noexcept {
        using futurator = futurize<std::result_of_t<Func(T&&...)>>;
        if (available() && !need_preempt()) {
            if (failed()) {
                return futurator::make_exception_future(static_cast<future_state_base&&>(get_available_state_ref()));
            } else {
                return futurator::apply(std::forward<Func>(func), get_available_state_ref().take_value());
            }
        }
        return then_impl_nrvo<Func, Result>(std::forward<Func>(func));
    }

public:
    template <typename Func, typename FuncResult = std::result_of_t<Func(future)>>
    GCC6_CONCEPT( requires ::seastar::CanApply<Func, future> )
    futurize_t<FuncResult>
    then_wrapped(Func&& func) & noexcept {
        return then_wrapped_maybe_erase<false, FuncResult>(std::forward<Func>(func));
    }

    template <typename Func, typename FuncResult = std::result_of_t<Func(future&&)>>
    GCC6_CONCEPT( requires ::seastar::CanApply<Func, future&&> )
    futurize_t<FuncResult>
    then_wrapped(Func&& func) && noexcept {
        return then_wrapped_maybe_erase<true, FuncResult>(std::forward<Func>(func));
    }

private:

    template <bool AsSelf, typename FuncResult, typename Func>
    futurize_t<FuncResult>
    then_wrapped_maybe_erase(Func&& func) noexcept {
#ifndef SEASTAR_TYPE_ERASE_MORE
        return then_wrapped_common<AsSelf, FuncResult>(std::forward<Func>(func));
#else
        using futurator = futurize<FuncResult>;
        using WrapFuncResult = typename futurator::type;
        return then_wrapped_common<AsSelf, WrapFuncResult>(noncopyable_function<WrapFuncResult (future&&)>([func = std::forward<Func>(func)] (future&& f) mutable {
            return futurator::invoke(std::forward<Func>(func), std::move(f));
        }));
#endif
    }

    // Keep this simple so that Named Return Value Optimization is used.
    template <typename FuncResult, typename Func>
    futurize_t<FuncResult>
    then_wrapped_nrvo(Func&& func) noexcept {
        using futurator = futurize<FuncResult>;
        typename futurator::type fut(future_for_get_promise_marker{});
        // If there is a std::bad_alloc in schedule() there is nothing that can be done about it, we cannot break future
        // chain by returning ready future while 'this' future is not ready. The noexcept will call std::terminate if
        // that happens.
        [&] () noexcept {
            using pr_type = decltype(fut.get_promise());
            memory::disable_failure_guard dfg;
            schedule(fut.get_promise(), [func = std::forward<Func>(func)] (pr_type& pr, future_state<T...>&& state) mutable {
                futurator::invoke(std::forward<Func>(func), future(std::move(state))).forward_to(std::move(pr));
            });
        } ();
        return fut;
    }


    template <bool AsSelf, typename FuncResult, typename Func>
    futurize_t<FuncResult>
    then_wrapped_common(Func&& func) noexcept {
        using futurator = futurize<FuncResult>;
        if (available() && !need_preempt()) {
            // TODO: after dropping C++14 support use `if constexpr ()` instead.
            if (AsSelf) {
                if (_promise) {
                    detach_promise();
                }
                return futurator::invoke(std::forward<Func>(func), std::move(*this));
            } else {
                return futurator::invoke(std::forward<Func>(func), future(get_available_state_ref()));
            }
        }
        return then_wrapped_nrvo<FuncResult, Func>(std::forward<Func>(func));
    }

    void forward_to(internal::promise_base_with_type<T...>&& pr) noexcept {
        if (_state.available()) {
            pr.set_urgent_state(std::move(_state));
        } else {
            *detach_promise() = std::move(pr);
        }
    }

public:
    void forward_to(promise<T...>&& pr) noexcept {
        if (_state.available()) {
            pr.set_urgent_state(std::move(_state));
        } else if (&pr._local_state != pr._state) {
            // The only case when _state points to _local_state is
            // when get_future was never called. Given that pr will
            // soon be destroyed, we know get_future will never be
            // called and we can just ignore this request.
            *detach_promise() = std::move(pr);
        }
    }



    template <typename Func>
    GCC6_CONCEPT( requires ::seastar::CanApply<Func> )
    future<T...> finally(Func&& func) noexcept {
        return then_wrapped(finally_body<Func, is_future<std::result_of_t<Func()>>::value>(std::forward<Func>(func)));
    }


    template <typename Func, bool FuncReturnsFuture>
    struct finally_body;

    template <typename Func>
    struct finally_body<Func, true> {
        Func _func;

        finally_body(Func&& func) : _func(std::forward<Func>(func))
        { }

        future<T...> operator()(future<T...>&& result) {
            return futurize_invoke(_func).then_wrapped([result = std::move(result)](auto f_res) mutable {
                if (!f_res.failed()) {
                    return std::move(result);
                } else {
                    try {
                        f_res.get();
                    } catch (...) {
                        return result.rethrow_with_nested();
                    }
                    __builtin_unreachable();
                }
            });
        }
    };

    template <typename Func>
    struct finally_body<Func, false> {
        Func _func;

        finally_body(Func&& func) : _func(std::forward<Func>(func))
        { }

        future<T...> operator()(future<T...>&& result) {
            try {
                _func();
                return std::move(result);
            } catch (...) {
                return result.rethrow_with_nested();
            }
        };
    };

    future<> or_terminate() noexcept {
        return then_wrapped([] (auto&& f) {
            try {
                f.get();
            } catch (...) {
                engine_exit(std::current_exception());
            }
        });
    }

    future<> discard_result() noexcept {
        return then([] (T&&...) {});
    }

    template <typename Func>
    /* Broken?
    GCC6_CONCEPT( requires ::seastar::ApplyReturns<Func, future<T...>, std::exception_ptr>
                    || (sizeof...(T) == 0 && ::seastar::ApplyReturns<Func, void, std::exception_ptr>)
                    || (sizeof...(T) == 1 && ::seastar::ApplyReturns<Func, T..., std::exception_ptr>)
    ) */
    future<T...> handle_exception(Func&& func) noexcept {
        return then_wrapped([func = std::forward<Func>(func)]
                             (auto&& fut) mutable -> future<T...> {
            if (!fut.failed()) {
                return make_ready_future<T...>(fut.get());
            } else {
                return futurize_invoke(func, fut.get_exception());
            }
        });
    }

    template <typename Func>
    future<T...> handle_exception_type(Func&& func) noexcept {
        using trait = function_traits<Func>;
        static_assert(trait::arity == 1, "func can take only one parameter");
        using ex_type = typename trait::template arg<0>::type;
        return then_wrapped([func = std::forward<Func>(func)]
                             (auto&& fut) mutable -> future<T...> {
            try {
                return make_ready_future<T...>(fut.get());
            } catch(ex_type& ex) {
                return futurize_invoke(func, ex);
            }
        });
    }

    void ignore_ready_future() noexcept {
        _state.ignore();
    }

#if SEASTAR_COROUTINES_TS
    void set_coroutine(task& coroutine) noexcept {
        assert(!_state.available());
        assert(_promise);
        detach_promise()->set_coroutine(_state, coroutine);
    }
#endif
private:
    void set_callback(continuation_base<T...>* callback) noexcept {
        if (_state.available()) {
            callback->set_state(get_available_state_ref());
            ::seastar::schedule(callback);
        } else {
            assert(_promise);
            detach_promise()->schedule(callback);
        }

    }

    template <typename... U>
    friend class future;
    template <typename... U>
    friend class promise;
    template <typename... U>
    friend class internal::promise_base_with_type;
    template <typename... U, typename... A>
    friend future<U...> make_ready_future(A&&... value);
    template <typename... U>
    friend future<U...> make_exception_future(std::exception_ptr&& ex) noexcept;
    template <typename... U, typename Exception>
    friend future<U...> make_exception_future(Exception&& ex) noexcept;
    template <typename... U>
    friend future<U...> internal::make_exception_future(future_state_base&& state) noexcept;
    template <typename... U, typename V>
    friend void internal::set_callback(future<U...>&, V*) noexcept;
};

inline internal::promise_base::promise_base(future_base* future, future_state_base* state) noexcept
    : _future(future), _state(state) {
    _future->_promise = this;
}

template <typename... T>
inline
future<T...>
promise<T...>::get_future() noexcept {
    assert(!this->_future && this->_state && !this->_task);
    return future<T...>(this);
}

template <typename... T>
inline
promise<T...>::promise(promise&& x) noexcept : internal::promise_base_with_type<T...>(std::move(x)) {
    if (this->_state == &x._local_state) {
        this->_state = &_local_state;
        _local_state = std::move(x._local_state);
    }
}

template <typename... T, typename... A>
inline
future<T...> make_ready_future(A&&... value) {
    return future<T...>(ready_future_marker(), std::forward<A>(value)...);
}

template <typename... T>
inline
future<T...> make_exception_future(std::exception_ptr&& ex) noexcept {
    return future<T...>(exception_future_marker(), std::move(ex));
}

template <typename... T>
inline
future<T...> internal::make_exception_future(future_state_base&& state) noexcept {
    return future<T...>(exception_future_marker(), std::move(state));
}

template <typename... T>
future<T...> internal::current_exception_as_future() noexcept {
    return internal::make_exception_future<T...>(future_state_base::current_exception());
}

void log_exception_trace() noexcept;

template <typename... T, typename Exception>
inline
future<T...> make_exception_future(Exception&& ex) noexcept {
    log_exception_trace();
    return make_exception_future<T...>(std::make_exception_ptr(std::forward<Exception>(ex)));
}



template<typename T>
template<typename Func, typename... FuncArgs>
typename futurize<T>::type futurize<T>::apply(Func&& func, std::tuple<FuncArgs...>&& args) noexcept {
    try {
        return convert(::seastar::apply(std::forward<Func>(func), std::move(args)));
    } catch (...) {
        return internal::current_exception_as_future<T>();
    }
}

template<typename T>
template<typename Func, typename... FuncArgs>
typename futurize<T>::type futurize<T>::invoke(Func&& func, FuncArgs&&... args) noexcept {
    try {
        return convert(func(std::forward<FuncArgs>(args)...));
    } catch (...) {
        return internal::current_exception_as_future<T>();
    }
}

template<typename T>
template<typename Func, typename... FuncArgs>
typename futurize<T>::type futurize<T>::apply(Func&& func, FuncArgs&&... args) noexcept {
    return invoke(std::forward<Func>(func), std::forward<FuncArgs>(args)...);
}

template<typename Func, typename... FuncArgs>
typename futurize<void>::type futurize<void>::apply(Func&& func, std::tuple<FuncArgs...>&& args) noexcept {
    try {
        ::seastar::apply(std::forward<Func>(func), std::move(args));
        return make_ready_future<>();
    } catch (...) {
        return internal::current_exception_as_future<>();
    }
}

template<typename Func, typename... FuncArgs>
typename futurize<void>::type futurize<void>::invoke(Func&& func, FuncArgs&&... args) noexcept {
    try {
        func(std::forward<FuncArgs>(args)...);
        return make_ready_future<>();
    } catch (...) {
        return internal::current_exception_as_future<>();
    }
}

template<typename Func, typename... FuncArgs>
typename futurize<void>::type futurize<void>::apply(Func&& func, FuncArgs&&... args) noexcept {
    return invoke(std::forward<Func>(func), std::forward<FuncArgs>(args)...);
}

template<typename... Args>
template<typename Func, typename... FuncArgs>
typename futurize<future<Args...>>::type futurize<future<Args...>>::apply(Func&& func, std::tuple<FuncArgs...>&& args) noexcept {
    try {
        return ::seastar::apply(std::forward<Func>(func), std::move(args));
    } catch (...) {
        return internal::current_exception_as_future<Args...>();
    }
}

template<typename... Args>
template<typename Func, typename... FuncArgs>
typename futurize<future<Args...>>::type futurize<future<Args...>>::invoke(Func&& func, FuncArgs&&... args) noexcept {
    try {
        return func(std::forward<FuncArgs>(args)...);
    } catch (...) {
        return internal::current_exception_as_future<Args...>();
    }
}

template<typename... Args>
template<typename Func, typename... FuncArgs>
typename futurize<future<Args...>>::type futurize<future<Args...>>::apply(Func&& func, FuncArgs&&... args) noexcept {
    return invoke(std::forward<Func>(func), std::forward<FuncArgs>(args)...);
}

template <typename T>
template <typename Arg>
inline
future<T>
futurize<T>::make_exception_future(Arg&& arg) noexcept {
    using ::seastar::make_exception_future;
    using ::seastar::internal::make_exception_future;
    return make_exception_future<T>(std::forward<Arg>(arg));
}

template <typename... T>
template <typename Arg>
inline
future<T...>
futurize<future<T...>>::make_exception_future(Arg&& arg) noexcept {
    using ::seastar::make_exception_future;
    using ::seastar::internal::make_exception_future;
    return make_exception_future<T...>(std::forward<Arg>(arg));
}

template <typename Arg>
inline
future<>
futurize<void>::make_exception_future(Arg&& arg) noexcept {
    using ::seastar::make_exception_future;
    using ::seastar::internal::make_exception_future;
    return make_exception_future<>(std::forward<Arg>(arg));
}

template <typename T>
inline
future<T>
futurize<T>::from_tuple(std::tuple<T>&& value) {
    return make_ready_future<T>(std::move(value));
}

template <typename T>
inline
future<T>
futurize<T>::from_tuple(const std::tuple<T>& value) {
    return make_ready_future<T>(value);
}

inline
future<>
futurize<void>::from_tuple(std::tuple<>&& value) {
    return make_ready_future<>();
}

inline
future<>
futurize<void>::from_tuple(const std::tuple<>& value) {
    return make_ready_future<>();
}

template <typename... Args>
inline
future<Args...>
futurize<future<Args...>>::from_tuple(std::tuple<Args...>&& value) {
    return make_ready_future<Args...>(std::move(value));
}

template <typename... Args>
inline
future<Args...>
futurize<future<Args...>>::from_tuple(const std::tuple<Args...>& value) {
    return make_ready_future<Args...>(value);
}

template<typename Func, typename... Args>
auto futurize_invoke(Func&& func, Args&&... args) {
    using futurator = futurize<std::result_of_t<Func(Args&&...)>>;
    return futurator::invoke(std::forward<Func>(func), std::forward<Args>(args)...);
}

template<typename Func, typename... Args>
[[deprecated("Use futurize_invoke for varargs")]]
auto futurize_apply(Func&& func, Args&&... args) {
    return futurize_invoke(std::forward<Func>(func), std::forward<Args>(args)...);
}

template<typename Func, typename... Args>
auto futurize_apply(Func&& func, std::tuple<Args...>&& args) {
    using futurator = futurize<std::result_of_t<Func(Args&&...)>>;
    return futurator::apply(std::forward<Func>(func), std::move(args));
}

namespace internal {

template <typename... T, typename U>
inline
void set_callback(future<T...>& fut, U* callback) noexcept {
    // It would be better to use continuation_base<T...> for U, but
    // then a derived class of continuation_base<T...> won't be matched
    return fut.set_callback(callback);
}

}



}