include/sdbusplus/async/stdexec/task.hpp

/*
 * Copyright (c) 2021-2022 NVIDIA Corporation
 *
 * Licensed under the Apache License Version 2.0 with LLVM Exceptions
 * (the "License"); you may not use this file except in compliance with
 * the License. You may obtain a copy of the License at
 *
 *   https://llvm.org/LICENSE.txt
 *
 * 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.
 */
#pragma once

#include "../stdexec/__detail/__meta.hpp"
#include "../stdexec/coroutine.hpp"
#include "../stdexec/execution.hpp"
#include "any_sender_of.hpp"
#include "at_coroutine_exit.hpp"
#include "inline_scheduler.hpp"
#include "scope.hpp"

#include <any>
#include <cassert>
#include <exception>
#include <utility>
#include <variant>

STDEXEC_PRAGMA_PUSH()
STDEXEC_PRAGMA_IGNORE_GNU("-Wundefined-inline")

namespace exec
{
namespace __task
{
using namespace stdexec;

using __any_scheduler =                        //
    any_receiver_ref<                          //
        completion_signatures<set_error_t(std::exception_ptr),
                              set_stopped_t()> //
        >::any_sender<>::any_scheduler<>;
static_assert(scheduler<__any_scheduler>);

template <class _Ty>
concept __stop_token_provider = //
    requires(const _Ty& t) {    //
        get_stop_token(t);
    };

template <class _Ty>
concept __indirect_stop_token_provider = //
    requires(const _Ty& t) {
        {
            get_env(t)
        } -> __stop_token_provider;
    };

template <class _Ty>
concept __indirect_scheduler_provider = //
    requires(const _Ty& t) {
        {
            get_env(t)
        } -> __scheduler_provider;
    };

template <class _ParentPromise>
void __check_parent_promise_has_scheduler() noexcept
{
    static_assert(__indirect_scheduler_provider<_ParentPromise>,
                  "exec::task<T> cannot be co_await-ed in a coroutine that "
                  "does not have an associated scheduler.");
}

struct __forward_stop_request
{
    in_place_stop_source& __stop_source_;

    void operator()() noexcept
    {
        __stop_source_.request_stop();
    }
};

template <class _ParentPromise>
struct __default_awaiter_context;

////////////////////////////////////////////////////////////////////////////////
// This is the context that is associated with basic_task's promise type
// by default. It handles forwarding of stop requests from parent to child.
enum class __scheduler_affinity
{
    __none,
    __sticky
};

template <__scheduler_affinity _SchedulerAffinity =
              __scheduler_affinity::__sticky>
class __default_task_context_impl
{
    template <class _ParentPromise>
    friend struct __default_awaiter_context;

    static constexpr bool __with_scheduler = _SchedulerAffinity ==
                                             __scheduler_affinity::__sticky;

    STDEXEC_ATTRIBUTE((no_unique_address))
    __if_c<__with_scheduler, __any_scheduler, __ignore> //
        __scheduler_{exec::inline_scheduler{}};
    in_place_stop_token __stop_token_;

    friend const __any_scheduler&
        tag_invoke(get_scheduler_t,
                   const __default_task_context_impl& __self) noexcept
        requires(__with_scheduler)
    {
        return __self.__scheduler_;
    }

    friend auto tag_invoke(get_stop_token_t,
                           const __default_task_context_impl& __self) noexcept
        -> in_place_stop_token
    {
        return __self.__stop_token_;
    }

  public:
    __default_task_context_impl() = default;

    template <scheduler _Scheduler>
    explicit __default_task_context_impl(_Scheduler&& __scheduler) :
        __scheduler_{(_Scheduler&&)__scheduler}
    {}

    bool stop_requested() const noexcept
    {
        return __stop_token_.stop_requested();
    }

    template <scheduler _Scheduler>
    void set_scheduler(_Scheduler&& __sched)
        requires(__with_scheduler)
    {
        __scheduler_ = (_Scheduler&&)__sched;
    }

    template <class _ThisPromise>
    using promise_context_t = __default_task_context_impl;

    template <class _ThisPromise, class _ParentPromise = void>
    using awaiter_context_t = __default_awaiter_context<_ParentPromise>;
};

template <class _Ty>
using default_task_context =
    __default_task_context_impl<__scheduler_affinity::__sticky>;

template <class _Ty>
using __raw_task_context =
    __default_task_context_impl<__scheduler_affinity::__none>;

// This is the context associated with basic_task's awaiter. By default
// it does nothing.
template <class _ParentPromise>
struct __default_awaiter_context
{
    template <__scheduler_affinity _Affinity>
    explicit __default_awaiter_context(
        __default_task_context_impl<_Affinity>& __self,
        _ParentPromise& __parent) noexcept
    {
        if constexpr (_Affinity == __scheduler_affinity::__sticky)
        {
            __check_parent_promise_has_scheduler<_ParentPromise>();
            __self.__scheduler_ = get_scheduler(get_env(__parent));
        }
    }
};

////////////////////////////////////////////////////////////////////////////////
// This is the context to be associated with basic_task's awaiter when
// the parent coroutine's promise type is known, is a __stop_token_provider,
// and its stop token type is neither in_place_stop_token nor unstoppable.
template <__indirect_stop_token_provider _ParentPromise>
struct __default_awaiter_context<_ParentPromise>
{
    using __stop_token_t = stop_token_of_t<env_of_t<_ParentPromise>>;
    using __stop_callback_t =
        typename __stop_token_t::template callback_type<__forward_stop_request>;

    template <__scheduler_affinity _Affinity>
    explicit __default_awaiter_context(
        __default_task_context_impl<_Affinity>& __self,
        _ParentPromise& __parent) noexcept
        // Register a callback that will request stop on this basic_task's
        // stop_source when stop is requested on the parent coroutine's stop
        // token.
        :
        __stop_callback_{get_stop_token(get_env(__parent)),
                         __forward_stop_request{__stop_source_}}
    {
        if constexpr (_Affinity == __scheduler_affinity::__sticky)
        {
            __check_parent_promise_has_scheduler<_ParentPromise>();
            __self.__scheduler_ = get_scheduler(get_env(__parent));
        }
        static_assert(
            std::is_nothrow_constructible_v<__stop_callback_t, __stop_token_t,
                                            __forward_stop_request>);
        __self.__stop_token_ = __stop_source_.get_token();
    }

    in_place_stop_source __stop_source_{};
    __stop_callback_t __stop_callback_;
};

// If the parent coroutine's type has a stop token of type in_place_stop_token,
// we don't need to register a stop callback.
template <__indirect_stop_token_provider _ParentPromise>
    requires std::same_as<in_place_stop_token,
                          stop_token_of_t<env_of_t<_ParentPromise>>>
struct __default_awaiter_context<_ParentPromise>
{
    template <__scheduler_affinity _Affinity>
    explicit __default_awaiter_context(
        __default_task_context_impl<_Affinity>& __self,
        _ParentPromise& __parent) noexcept
    {
        if constexpr (_Affinity == __scheduler_affinity::__sticky)
        {
            __check_parent_promise_has_scheduler<_ParentPromise>();
            __self.__scheduler_ = get_scheduler(get_env(__parent));
        }
        __self.__stop_token_ = get_stop_token(get_env(__parent));
    }
};

// If the parent coroutine's stop token is unstoppable, there's no point
// forwarding stop tokens or stop requests at all.
template <__indirect_stop_token_provider _ParentPromise>
    requires unstoppable_token<stop_token_of_t<env_of_t<_ParentPromise>>>
struct __default_awaiter_context<_ParentPromise>
{
    template <__scheduler_affinity _Affinity>
    explicit __default_awaiter_context(
        __default_task_context_impl<_Affinity>& __self,
        _ParentPromise& __parent) noexcept
    {
        if constexpr (_Affinity == __scheduler_affinity::__sticky)
        {
            __check_parent_promise_has_scheduler<_ParentPromise>();
            __self.__scheduler_ = get_scheduler(get_env(__parent));
        }
    }
};

// Finally, if we don't know the parent coroutine's promise type, assume the
// worst and save a type-erased stop callback.
template <>
struct __default_awaiter_context<void>
{
    template <__scheduler_affinity _Affinity, class _ParentPromise>
    explicit __default_awaiter_context(
        __default_task_context_impl<_Affinity>& __self,
        _ParentPromise& __parent) noexcept
    {
        if constexpr (_Affinity == __scheduler_affinity::__sticky)
        {
            __check_parent_promise_has_scheduler<_ParentPromise>();
            __self.__scheduler_ = get_scheduler(get_env(__parent));
        }
    }

    template <__scheduler_affinity _Affinity,
              __indirect_stop_token_provider _ParentPromise>
    explicit __default_awaiter_context(
        __default_task_context_impl<_Affinity>& __self,
        _ParentPromise& __parent)
    {
        if constexpr (_Affinity == __scheduler_affinity::__sticky)
        {
            __check_parent_promise_has_scheduler<_ParentPromise>();
            __self.__scheduler_ = get_scheduler(get_env(__parent));
        }
        // Register a callback that will request stop on this basic_task's
        // stop_source when stop is requested on the parent coroutine's stop
        // token.
        using __stop_token_t = stop_token_of_t<env_of_t<_ParentPromise>>;
        using __stop_callback_t =
            typename __stop_token_t::template callback_type<
                __forward_stop_request>;

        if constexpr (std::same_as<__stop_token_t, in_place_stop_token>)
        {
            __self.__stop_token_ = get_stop_token(get_env(__parent));
        }
        else if (auto __token = get_stop_token(get_env(__parent));
                 __token.stop_possible())
        {
            __stop_callback_.emplace<__stop_callback_t>(
                std::move(__token), __forward_stop_request{__stop_source_});
            __self.__stop_token_ = __stop_source_.get_token();
        }
    }

    in_place_stop_source __stop_source_{};
    std::any __stop_callback_{};
};

template <class _Promise, class _ParentPromise = void>
using awaiter_context_t =                  //
    typename __decay_t<env_of_t<_Promise>> //
    ::template awaiter_context_t<_Promise, _ParentPromise>;

////////////////////////////////////////////////////////////////////////////////
// In a base class so it can be specialized when _Ty is void:
template <class _Ty>
struct __promise_base
{
    void return_value(_Ty value) noexcept
    {
        __data_.template emplace<1>(std::move(value));
    }

    std::variant<std::monostate, _Ty, std::exception_ptr> __data_{};
};

template <>
struct __promise_base<void>
{
    struct __void
    {};

    void return_void() noexcept
    {
        __data_.template emplace<1>(__void{});
    }

    std::variant<std::monostate, __void, std::exception_ptr> __data_{};
};

enum class disposition : unsigned
{
    stopped,
    succeeded,
    failed,
};

struct __reschedule_coroutine_on
{
    template <class _Scheduler>
    struct __wrap
    {
        _Scheduler __sched_;
    };

    template <scheduler _Scheduler>
    __wrap<_Scheduler> operator()(_Scheduler __sched) const noexcept
    {
        return {(_Scheduler&&)__sched};
    }
};

////////////////////////////////////////////////////////////////////////////////
// basic_task
template <class _Ty, class _Context = default_task_context<_Ty>>
class basic_task
{
    struct __promise;

  public:
    using __t = basic_task;
    using __id = basic_task;
    using promise_type = __promise;

    basic_task(basic_task&& __that) noexcept :
        __coro_(std::exchange(__that.__coro_, {}))
    {}

    ~basic_task()
    {
        if (__coro_)
            __coro_.destroy();
    }

  private:
    struct __final_awaitable
    {
        static constexpr bool await_ready() noexcept
        {
            return false;
        }

        static __coro::coroutine_handle<>
            await_suspend(__coro::coroutine_handle<__promise> __h) noexcept
        {
            return __h.promise().continuation().handle();
        }

        static void await_resume() noexcept {}
    };

    struct __promise : __promise_base<_Ty>, with_awaitable_senders<__promise>
    {
        basic_task get_return_object() noexcept
        {
            return basic_task(
                __coro::coroutine_handle<__promise>::from_promise(*this));
        }

        __coro::suspend_always initial_suspend() noexcept
        {
            return {};
        }

        __final_awaitable final_suspend() noexcept
        {
            return {};
        }

        __task::disposition disposition() const noexcept
        {
            return static_cast<__task::disposition>(this->__data_.index());
        }

        void unhandled_exception() noexcept
        {
            this->__data_.template emplace<2>(std::current_exception());
        }

        template <sender _Awaitable>
            requires __scheduler_provider<_Context>
        decltype(auto) await_transform(_Awaitable&& __awaitable) noexcept
        {
            // TODO: If we have a complete-where-it-starts query then we can
            // optimize this to avoid the reschedule
            return as_awaitable(
                transfer((_Awaitable&&)__awaitable, get_scheduler(__context_)),
                *this);
        }

        template <class _Scheduler>
            requires __scheduler_provider<_Context>
        decltype(auto) await_transform(
            __reschedule_coroutine_on::__wrap<_Scheduler> __box) noexcept
        {
            if (!std::exchange(__rescheduled_, true))
            {
                // Create a cleanup action that transitions back onto the
                // current scheduler:
                auto __sched = get_scheduler(__context_);
                auto __cleanup_task = at_coroutine_exit(schedule,
                                                        std::move(__sched));
                // Insert the cleanup action into the head of the continuation
                // chain by making direct calls to the cleanup task's awaiter
                // member functions. See type _cleanup_task in
                // at_coroutine_exit.hpp:
                __cleanup_task.await_suspend(
                    __coro::coroutine_handle<__promise>::from_promise(*this));
                (void)__cleanup_task.await_resume();
            }
            __context_.set_scheduler(__box.__sched_);
            return as_awaitable(schedule(__box.__sched_), *this);
        }

        template <class _Awaitable>
        decltype(auto) await_transform(_Awaitable&& __awaitable) noexcept
        {
            return with_awaitable_senders<__promise>::await_transform(
                (_Awaitable&&)__awaitable);
        }

        using __context_t =
            typename _Context::template promise_context_t<__promise>;

        friend const __context_t& tag_invoke(get_env_t,
                                             const __promise& __self) noexcept
        {
            return __self.__context_;
        }

        __context_t __context_;
        bool __rescheduled_{false};
    };

    template <class _ParentPromise = void>
    struct __task_awaitable
    {
        __coro::coroutine_handle<__promise> __coro_;
        std::optional<awaiter_context_t<__promise, _ParentPromise>>
            __context_{};

        ~__task_awaitable()
        {
            if (__coro_)
                __coro_.destroy();
        }

        static constexpr bool await_ready() noexcept
        {
            return false;
        }

        template <class _ParentPromise2>
        __coro::coroutine_handle<> await_suspend(
            __coro::coroutine_handle<_ParentPromise2> __parent) noexcept
        {
            static_assert(__one_of<_ParentPromise, _ParentPromise2, void>);
            __context_.emplace(__coro_.promise().__context_,
                               __parent.promise());
            __coro_.promise().set_continuation(__parent);
            if constexpr (requires {
                              __coro_.promise().stop_requested() ? 0 : 1;
                          })
            {
                if (__coro_.promise().stop_requested())
                    return __parent.promise().unhandled_stopped();
            }
            return __coro_;
        }

        _Ty await_resume()
        {
            __context_.reset();
            scope_guard __on_exit{
                [this]() noexcept { std::exchange(__coro_, {}).destroy(); }};
            if (__coro_.promise().__data_.index() == 2)
                std::rethrow_exception(
                    std::get<2>(std::move(__coro_.promise().__data_)));
            if constexpr (!std::is_void_v<_Ty>)
                return std::get<1>(std::move(__coro_.promise().__data_));
        }
    };

    // Make this task awaitable within a particular context:
    template <class _ParentPromise>
        requires constructible_from<
            awaiter_context_t<__promise, _ParentPromise>, __promise&,
            _ParentPromise&>
    friend __task_awaitable<_ParentPromise> tag_invoke(as_awaitable_t,
                                                       basic_task&& __self,
                                                       _ParentPromise&) noexcept
    {
        return __task_awaitable<_ParentPromise>{
            std::exchange(__self.__coro_, {})};
    }

    // Make this task generally awaitable:
    friend __task_awaitable<> operator co_await(basic_task && __self) noexcept
        requires __mvalid<awaiter_context_t, __promise>
    {
        return __task_awaitable<>{std::exchange(__self.__coro_, {})};
    }

    // From the list of types [_Ty], remove any types that are void, and send
    //   the resulting list to __qf<set_value_t>, which uses the list of types
    //   as arguments of a function type. In other words, set_value_t() if _Ty
    //   is void, and set_value_t(_Ty) otherwise.
    using __set_value_sig_t = __minvoke<__remove<void, __qf<set_value_t>>, _Ty>;

    // Specify basic_task's completion signatures
    //   This is only necessary when basic_task is not generally awaitable
    //   owing to constraints imposed by its _Context parameter.
    using __task_traits_t = //
        completion_signatures<__set_value_sig_t,
                              set_error_t(std::exception_ptr), set_stopped_t()>;

    friend auto tag_invoke(get_completion_signatures_t, const basic_task&, auto)
        -> __task_traits_t
    {
        return {};
    }

    explicit basic_task(__coro::coroutine_handle<promise_type> __coro) noexcept
        :
        __coro_(__coro)
    {}

    __coro::coroutine_handle<promise_type> __coro_;
};
} // namespace __task

using task_disposition = __task::disposition;

template <class _Ty>
using default_task_context = __task::default_task_context<_Ty>;

template <class _Promise, class _ParentPromise = void>
using awaiter_context_t = __task::awaiter_context_t<_Promise, _ParentPromise>;

template <class _Ty, class _Context = default_task_context<_Ty>>
using basic_task = __task::basic_task<_Ty, _Context>;

template <class _Ty>
using task = basic_task<_Ty, default_task_context<_Ty>>;

inline constexpr __task::__reschedule_coroutine_on reschedule_coroutine_on{};
} // namespace exec

STDEXEC_PRAGMA_POP()