1   
//
1   
//
2   
// Copyright (c) 2025 Vinnie Falco (vinnie.falco@gmail.com)
2   
// Copyright (c) 2025 Vinnie Falco (vinnie.falco@gmail.com)
3   
//
3   
//
4   
// Distributed under the Boost Software License, Version 1.0. (See accompanying
4   
// Distributed under the Boost Software License, Version 1.0. (See accompanying
5   
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
5   
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
6   
//
6   
//
7   
// Official repository: https://github.com/cppalliance/corosio
7   
// Official repository: https://github.com/cppalliance/corosio
8   
//
8   
//
9  
9  
10   
#ifndef BOOST_CAPY_TASK_HPP
10   
#ifndef BOOST_CAPY_TASK_HPP
11   
#define BOOST_CAPY_TASK_HPP
11   
#define BOOST_CAPY_TASK_HPP
12  
12  
13   
#include <boost/capy/detail/config.hpp>
13   
#include <boost/capy/detail/config.hpp>
14   
#include <boost/capy/concept/executor.hpp>
14   
#include <boost/capy/concept/executor.hpp>
15   
#include <boost/capy/concept/io_awaitable.hpp>
15   
#include <boost/capy/concept/io_awaitable.hpp>
16   
#include <boost/capy/ex/io_awaitable_promise_base.hpp>
16   
#include <boost/capy/ex/io_awaitable_promise_base.hpp>
17   
#include <boost/capy/ex/io_env.hpp>
17   
#include <boost/capy/ex/io_env.hpp>
18   
#include <boost/capy/ex/frame_allocator.hpp>
18   
#include <boost/capy/ex/frame_allocator.hpp>
19  
19  
20   
#include <exception>
20   
#include <exception>
21   
#include <optional>
21   
#include <optional>
22   
#include <type_traits>
22   
#include <type_traits>
23   
#include <utility>
23   
#include <utility>
24   
#include <variant>
24   
#include <variant>
25  
25  
26   
namespace boost {
26   
namespace boost {
27   
namespace capy {
27   
namespace capy {
28  
28  
29   
namespace detail {
29   
namespace detail {
30  
30  
31   
// Helper base for result storage and return_void/return_value
31   
// Helper base for result storage and return_void/return_value
32   
template<typename T>
32   
template<typename T>
33   
struct task_return_base
33   
struct task_return_base
34   
{
34   
{
35   
    std::optional<T> result_;
35   
    std::optional<T> result_;
36  
36  
37   
    void return_value(T value)
37   
    void return_value(T value)
38   
    {
38   
    {
39   
        result_ = std::move(value);
39   
        result_ = std::move(value);
40   
    }
40   
    }
41  
41  
42   
    T&& result() noexcept
42   
    T&& result() noexcept
43   
    {
43   
    {
44   
        return std::move(*result_);
44   
        return std::move(*result_);
45   
    }
45   
    }
46   
};
46   
};
47  
47  
48   
template<>
48   
template<>
49   
struct task_return_base<void>
49   
struct task_return_base<void>
50   
{
50   
{
51   
    void return_void()
51   
    void return_void()
52   
    {
52   
    {
53   
    }
53   
    }
54   
};
54   
};
55  
55  
56   
} // namespace detail
56   
} // namespace detail
57  
57  
58   
/** Lazy coroutine task satisfying @ref IoRunnable.
58   
/** Lazy coroutine task satisfying @ref IoRunnable.
59  
59  
60   
    Use `task<T>` as the return type for coroutines that perform I/O
60   
    Use `task<T>` as the return type for coroutines that perform I/O
61   
    and return a value of type `T`. The coroutine body does not start
61   
    and return a value of type `T`. The coroutine body does not start
62   
    executing until the task is awaited, enabling efficient composition
62   
    executing until the task is awaited, enabling efficient composition
63   
    without unnecessary eager execution.
63   
    without unnecessary eager execution.
64  
64  
65   
    The task participates in the I/O awaitable protocol: when awaited,
65   
    The task participates in the I/O awaitable protocol: when awaited,
66   
    it receives the caller's executor and stop token, propagating them
66   
    it receives the caller's executor and stop token, propagating them
67   
    to nested `co_await` expressions. This enables cancellation and
67   
    to nested `co_await` expressions. This enables cancellation and
68   
    proper completion dispatch across executor boundaries.
68   
    proper completion dispatch across executor boundaries.
69  
69  
70   
    @tparam T The result type. Use `task<>` for `task<void>`.
70   
    @tparam T The result type. Use `task<>` for `task<void>`.
71  
71  
72   
    @par Thread Safety
72   
    @par Thread Safety
73   
    Distinct objects: Safe.
73   
    Distinct objects: Safe.
74   
    Shared objects: Unsafe.
74   
    Shared objects: Unsafe.
75  
75  
76   
    @par Example
76   
    @par Example
77  
77  
78   
    @code
78   
    @code
79   
    task<int> compute_value()
79   
    task<int> compute_value()
80   
    {
80   
    {
81   
        auto [ec, n] = co_await stream.read_some( buf );
81   
        auto [ec, n] = co_await stream.read_some( buf );
82   
        if( ec )
82   
        if( ec )
83   
            co_return 0;
83   
            co_return 0;
84   
        co_return process( buf, n );
84   
        co_return process( buf, n );
85   
    }
85   
    }
86  
86  
87   
    task<> run_session( tcp_socket sock )
87   
    task<> run_session( tcp_socket sock )
88   
    {
88   
    {
89   
        int result = co_await compute_value();
89   
        int result = co_await compute_value();
90   
        // ...
90   
        // ...
91   
    }
91   
    }
92   
    @endcode
92   
    @endcode
93  
93  
94   
    @see IoRunnable, IoAwaitable, run, run_async
94   
    @see IoRunnable, IoAwaitable, run, run_async
95   
*/
95   
*/
96   
template<typename T = void>
96   
template<typename T = void>
97   
struct [[nodiscard]] BOOST_CAPY_CORO_AWAIT_ELIDABLE
97   
struct [[nodiscard]] BOOST_CAPY_CORO_AWAIT_ELIDABLE
98   
    task
98   
    task
99   
{
99   
{
100   
    struct promise_type
100   
    struct promise_type
101   
        : io_awaitable_promise_base<promise_type>
101   
        : io_awaitable_promise_base<promise_type>
102   
        , detail::task_return_base<T>
102   
        , detail::task_return_base<T>
103   
    {
103   
    {
104   
    private:
104   
    private:
105   
        friend task;
105   
        friend task;
106   
        union { std::exception_ptr ep_; };
106   
        union { std::exception_ptr ep_; };
107   
        bool has_ep_;
107   
        bool has_ep_;
108  
108  
109   
    public:
109   
    public:
110   
        promise_type() noexcept
110   
        promise_type() noexcept
111   
            : has_ep_(false)
111   
            : has_ep_(false)
112   
        {
112   
        {
113   
        }
113   
        }
114  
114  
115   
        ~promise_type()
115   
        ~promise_type()
116   
        {
116   
        {
117   
            if(has_ep_)
117   
            if(has_ep_)
118   
                ep_.~exception_ptr();
118   
                ep_.~exception_ptr();
119   
        }
119   
        }
120  
120  
121   
        std::exception_ptr exception() const noexcept
121   
        std::exception_ptr exception() const noexcept
122   
        {
122   
        {
123   
            if(has_ep_)
123   
            if(has_ep_)
124   
                return ep_;
124   
                return ep_;
125   
            return {};
125   
            return {};
126   
        }
126   
        }
127  
127  
128   
        task get_return_object()
128   
        task get_return_object()
129   
        {
129   
        {
130   
            return task{std::coroutine_handle<promise_type>::from_promise(*this)};
130   
            return task{std::coroutine_handle<promise_type>::from_promise(*this)};
131   
        }
131   
        }
132  
132  
133   
        auto initial_suspend() noexcept
133   
        auto initial_suspend() noexcept
134   
        {
134   
        {
135   
            struct awaiter
135   
            struct awaiter
136   
            {
136   
            {
137   
                promise_type* p_;
137   
                promise_type* p_;
138  
138  
139   
                bool await_ready() const noexcept
139   
                bool await_ready() const noexcept
140   
                {
140   
                {
141   
                    return false;
141   
                    return false;
142   
                }
142   
                }
143  
143  
144   
                void await_suspend(std::coroutine_handle<>) const noexcept
144   
                void await_suspend(std::coroutine_handle<>) const noexcept
145   
                {
145   
                {
146   
                }
146   
                }
147  
147  
148   
                void await_resume() const noexcept
148   
                void await_resume() const noexcept
149   
                {
149   
                {
150   
                    // Restore TLS when body starts executing
150   
                    // Restore TLS when body starts executing
151   
                    set_current_frame_allocator(p_->environment()->frame_allocator);
151   
                    set_current_frame_allocator(p_->environment()->frame_allocator);
152   
                }
152   
                }
153   
            };
153   
            };
154   
            return awaiter{this};
154   
            return awaiter{this};
155   
        }
155   
        }
156  
156  
157   
        auto final_suspend() noexcept
157   
        auto final_suspend() noexcept
158   
        {
158   
        {
159   
            struct awaiter
159   
            struct awaiter
160   
            {
160   
            {
161   
                promise_type* p_;
161   
                promise_type* p_;
162  
162  
163   
                bool await_ready() const noexcept
163   
                bool await_ready() const noexcept
164   
                {
164   
                {
165   
                    return false;
165   
                    return false;
166   
                }
166   
                }
167  
167  
168   
                std::coroutine_handle<> await_suspend(std::coroutine_handle<>) const noexcept
168   
                std::coroutine_handle<> await_suspend(std::coroutine_handle<>) const noexcept
169   
                {
169   
                {
170   
                    return p_->continuation();
170   
                    return p_->continuation();
171   
                }
171   
                }
172  
172  
173   
                void await_resume() const noexcept
173   
                void await_resume() const noexcept
174   
                {
174   
                {
175   
                }
175   
                }
176   
            };
176   
            };
177   
            return awaiter{this};
177   
            return awaiter{this};
178   
        }
178   
        }
179  
179  
180   
        void unhandled_exception()
180   
        void unhandled_exception()
181   
        {
181   
        {
182   
            new (&ep_) std::exception_ptr(std::current_exception());
182   
            new (&ep_) std::exception_ptr(std::current_exception());
183   
            has_ep_ = true;
183   
            has_ep_ = true;
184   
        }
184   
        }
185  
185  
186   
        template<class Awaitable>
186   
        template<class Awaitable>
187   
        struct transform_awaiter
187   
        struct transform_awaiter
188   
        {
188   
        {
189   
            std::decay_t<Awaitable> a_;
189   
            std::decay_t<Awaitable> a_;
190   
            promise_type* p_;
190   
            promise_type* p_;
191  
191  
192   
            bool await_ready() noexcept
192   
            bool await_ready() noexcept
193   
            {
193   
            {
194   
                return a_.await_ready();
194   
                return a_.await_ready();
195   
            }
195   
            }
196  
196  
197   
            decltype(auto) await_resume()
197   
            decltype(auto) await_resume()
198   
            {
198   
            {
199   
                // Restore TLS before body resumes
199   
                // Restore TLS before body resumes
200   
                set_current_frame_allocator(p_->environment()->frame_allocator);
200   
                set_current_frame_allocator(p_->environment()->frame_allocator);
201   
                return a_.await_resume();
201   
                return a_.await_resume();
202   
            }
202   
            }
203  
203  
204   
            template<class Promise>
204   
            template<class Promise>
205   
            auto await_suspend(std::coroutine_handle<Promise> h) noexcept
205   
            auto await_suspend(std::coroutine_handle<Promise> h) noexcept
206   
            {
206   
            {
207   
#ifdef _MSC_VER
207   
#ifdef _MSC_VER
208   
                // Workaround: MSVC stores the coroutine_handle<> return
208   
                // Workaround: MSVC stores the coroutine_handle<> return
209   
                // value on the coroutine frame via hidden __$ReturnUdt$.
209   
                // value on the coroutine frame via hidden __$ReturnUdt$.
210   
                // After await_suspend publishes the handle to another
210   
                // After await_suspend publishes the handle to another
211   
                // thread, that thread can resume/destroy the frame before
211   
                // thread, that thread can resume/destroy the frame before
212   
                // __resume reads the handle back for the symmetric
212   
                // __resume reads the handle back for the symmetric
213   
                // transfer tail-call, causing a use-after-free.
213   
                // transfer tail-call, causing a use-after-free.
214   
                using R = decltype(a_.await_suspend(h, p_->environment()));
214   
                using R = decltype(a_.await_suspend(h, p_->environment()));
215   
                if constexpr (std::is_same_v<R, std::coroutine_handle<>>)
215   
                if constexpr (std::is_same_v<R, std::coroutine_handle<>>)
216   
                    a_.await_suspend(h, p_->environment()).resume();
216   
                    a_.await_suspend(h, p_->environment()).resume();
217   
                else
217   
                else
218   
                    return a_.await_suspend(h, p_->environment());
218   
                    return a_.await_suspend(h, p_->environment());
219   
#else
219   
#else
220   
                return a_.await_suspend(h, p_->environment());
220   
                return a_.await_suspend(h, p_->environment());
221   
#endif
221   
#endif
222   
            }
222   
            }
223   
        };
223   
        };
224  
224  
225   
        template<class Awaitable>
225   
        template<class Awaitable>
226   
        auto transform_awaitable(Awaitable&& a)
226   
        auto transform_awaitable(Awaitable&& a)
227   
        {
227   
        {
228   
            using A = std::decay_t<Awaitable>;
228   
            using A = std::decay_t<Awaitable>;
229   
            if constexpr (IoAwaitable<A>)
229   
            if constexpr (IoAwaitable<A>)
230   
            {
230   
            {
231   
                return transform_awaiter<Awaitable>{
231   
                return transform_awaiter<Awaitable>{
232   
                    std::forward<Awaitable>(a), this};
232   
                    std::forward<Awaitable>(a), this};
233   
            }
233   
            }
234   
            else
234   
            else
235   
            {
235   
            {
236   
                static_assert(sizeof(A) == 0, "requires IoAwaitable");
236   
                static_assert(sizeof(A) == 0, "requires IoAwaitable");
237   
            }
237   
            }
238   
        }
238   
        }
239   
    };
239   
    };
240  
240  
241   
    std::coroutine_handle<promise_type> h_;
241   
    std::coroutine_handle<promise_type> h_;
242  
242  
243   
    /// Destroy the task and its coroutine frame if owned.
243   
    /// Destroy the task and its coroutine frame if owned.
244   
    ~task()
244   
    ~task()
245   
    {
245   
    {
246   
        if(h_)
246   
        if(h_)
247   
            h_.destroy();
247   
            h_.destroy();
248   
    }
248   
    }
249  
249  
250   
    /// Return false; tasks are never immediately ready.
250   
    /// Return false; tasks are never immediately ready.
251   
    bool await_ready() const noexcept
251   
    bool await_ready() const noexcept
252   
    {
252   
    {
253   
        return false;
253   
        return false;
254   
    }
254   
    }
255  
255  
256   
    /// Return the result or rethrow any stored exception.
256   
    /// Return the result or rethrow any stored exception.
257   
    auto await_resume()
257   
    auto await_resume()
258   
    {
258   
    {
259   
        if(h_.promise().has_ep_)
259   
        if(h_.promise().has_ep_)
260   
            std::rethrow_exception(h_.promise().ep_);
260   
            std::rethrow_exception(h_.promise().ep_);
261   
        if constexpr (! std::is_void_v<T>)
261   
        if constexpr (! std::is_void_v<T>)
262   
            return std::move(*h_.promise().result_);
262   
            return std::move(*h_.promise().result_);
263   
        else
263   
        else
264   
            return;
264   
            return;
265   
    }
265   
    }
266  
266  
267   
    /// Start execution with the caller's context.
267   
    /// Start execution with the caller's context.
268   
    std::coroutine_handle<> await_suspend(std::coroutine_handle<> cont, io_env const* env)
268   
    std::coroutine_handle<> await_suspend(std::coroutine_handle<> cont, io_env const* env)
269   
    {
269   
    {
270   
        h_.promise().set_continuation(cont);
270   
        h_.promise().set_continuation(cont);
271   
        h_.promise().set_environment(env);
271   
        h_.promise().set_environment(env);
272   
        return h_;
272   
        return h_;
273   
    }
273   
    }
274  
274  
275   
    /// Return the coroutine handle.
275   
    /// Return the coroutine handle.
276   
    std::coroutine_handle<promise_type> handle() const noexcept
276   
    std::coroutine_handle<promise_type> handle() const noexcept
277   
    {
277   
    {
278   
        return h_;
278   
        return h_;
279   
    }
279   
    }
280  
280  
281   
    /** Release ownership of the coroutine frame.
281   
    /** Release ownership of the coroutine frame.
282  
282  
283   
        After calling this, destroying the task does not destroy the
283   
        After calling this, destroying the task does not destroy the
284   
        coroutine frame. The caller becomes responsible for the frame's
284   
        coroutine frame. The caller becomes responsible for the frame's
285   
        lifetime.
285   
        lifetime.
286  
286  
287   
        @par Postconditions
287   
        @par Postconditions
288   
        `handle()` returns the original handle, but the task no longer
288   
        `handle()` returns the original handle, but the task no longer
289   
        owns it.
289   
        owns it.
290   
    */
290   
    */
291   
    void release() noexcept
291   
    void release() noexcept
292   
    {
292   
    {
293   
        h_ = nullptr;
293   
        h_ = nullptr;
294   
    }
294   
    }
295  
295  
296   
    task(task const&) = delete;
296   
    task(task const&) = delete;
297   
    task& operator=(task const&) = delete;
297   
    task& operator=(task const&) = delete;
298  
298  
299   
    /// Move construct, transferring ownership.
299   
    /// Move construct, transferring ownership.
300   
    task(task&& other) noexcept
300   
    task(task&& other) noexcept
301   
        : h_(std::exchange(other.h_, nullptr))
301   
        : h_(std::exchange(other.h_, nullptr))
302   
    {
302   
    {
303   
    }
303   
    }
304  
304  
305   
    /// Move assign, transferring ownership.
305   
    /// Move assign, transferring ownership.
306   
    task& operator=(task&& other) noexcept
306   
    task& operator=(task&& other) noexcept
307   
    {
307   
    {
308   
        if(this != &other)
308   
        if(this != &other)
309   
        {
309   
        {
310   
            if(h_)
310   
            if(h_)
311   
                h_.destroy();
311   
                h_.destroy();
312   
            h_ = std::exchange(other.h_, nullptr);
312   
            h_ = std::exchange(other.h_, nullptr);
313   
        }
313   
        }
314   
        return *this;
314   
        return *this;
315   
    }
315   
    }
316  
316  
317   
private:
317   
private:
318   
    explicit task(std::coroutine_handle<promise_type> h)
318   
    explicit task(std::coroutine_handle<promise_type> h)
319   
        : h_(h)
319   
        : h_(h)
320   
    {
320   
    {
321   
    }
321   
    }
322   
};
322   
};
323  
323  
324   
} // namespace capy
324   
} // namespace capy
325   
} // namespace boost
325   
} // namespace boost
326  
326  
327   
#endif
327   
#endif