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_support.hpp>
16   
#include <boost/capy/ex/io_awaitable_support.hpp>
17   
#include <boost/capy/ex/executor_ref.hpp>
17   
#include <boost/capy/ex/executor_ref.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 IoLaunchableTask.
58   
/** Lazy coroutine task satisfying @ref IoLaunchableTask.
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 IoLaunchableTask, IoAwaitableTask, run, run_async
94   
    @see IoLaunchableTask, IoAwaitableTask, 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_support<promise_type>
101   
        : io_awaitable_support<promise_type>
102   
        , detail::task_return_base<T>
102   
        , detail::task_return_base<T>
103   
    {
103   
    {
104 - 
        std::exception_ptr ep_;
104 + 
    private:
 
105 + 
        friend task;
 
106 + 
        union { std::exception_ptr ep_; };
 
107 + 
        bool has_ep_;
 
108 +
 
109 + 
    public:
 
110 + 
        promise_type() noexcept
 
111 + 
            : has_ep_(false)
 
112 + 
        {
 
113 + 
        }
 
114 +
 
115 + 
        ~promise_type()
 
116 + 
        {
 
117 + 
            if(has_ep_)
 
118 + 
                ep_.~exception_ptr();
 
119 + 
        }
105  
120  
106   
        std::exception_ptr exception() const noexcept
121   
        std::exception_ptr exception() const noexcept
107   
        {
122   
        {
108 - 
            return ep_;
123 + 
            if(has_ep_)
 
124 + 
                return ep_;
 
125 + 
            return {};
109   
        }
126   
        }
110  
127  
111   
        task get_return_object()
128   
        task get_return_object()
112   
        {
129   
        {
113   
            return task{std::coroutine_handle<promise_type>::from_promise(*this)};
130   
            return task{std::coroutine_handle<promise_type>::from_promise(*this)};
114   
        }
131   
        }
115  
132  
116   
        auto initial_suspend() noexcept
133   
        auto initial_suspend() noexcept
117   
        {
134   
        {
118   
            struct awaiter
135   
            struct awaiter
119   
            {
136   
            {
120   
                promise_type* p_;
137   
                promise_type* p_;
121  
138  
122   
                bool await_ready() const noexcept
139   
                bool await_ready() const noexcept
123   
                {
140   
                {
124   
                    return false;
141   
                    return false;
125   
                }
142   
                }
126  
143  
127   
                void await_suspend(coro) const noexcept
144   
                void await_suspend(coro) const noexcept
128   
                {
145   
                {
129   
                    // Capture TLS allocator while it's still valid
146   
                    // Capture TLS allocator while it's still valid
130   
                    p_->set_frame_allocator(current_frame_allocator());
147   
                    p_->set_frame_allocator(current_frame_allocator());
131   
                }
148   
                }
132  
149  
133   
                void await_resume() const noexcept
150   
                void await_resume() const noexcept
134   
                {
151   
                {
135   
                    // Restore TLS when body starts executing
152   
                    // Restore TLS when body starts executing
136 - 
                    if(p_->frame_allocator())
153 + 
                    auto* fa = p_->frame_allocator();
137 - 
                        current_frame_allocator() = p_->frame_allocator();
154 + 
                    if(fa && fa != current_frame_allocator())
 
155 + 
                        current_frame_allocator() = fa;
138   
                }
156   
                }
139   
            };
157   
            };
140   
            return awaiter{this};
158   
            return awaiter{this};
141   
        }
159   
        }
142  
160  
143   
        auto final_suspend() noexcept
161   
        auto final_suspend() noexcept
144   
        {
162   
        {
145   
            struct awaiter
163   
            struct awaiter
146   
            {
164   
            {
147   
                promise_type* p_;
165   
                promise_type* p_;
148  
166  
149   
                bool await_ready() const noexcept
167   
                bool await_ready() const noexcept
150   
                {
168   
                {
151   
                    return false;
169   
                    return false;
152   
                }
170   
                }
153  
171  
154   
                coro await_suspend(coro) const noexcept
172   
                coro await_suspend(coro) const noexcept
155   
                {
173   
                {
156   
                    return p_->complete();
174   
                    return p_->complete();
157   
                }
175   
                }
158  
176  
159   
                void await_resume() const noexcept
177   
                void await_resume() const noexcept
160   
                {
178   
                {
161   
                }
179   
                }
162   
            };
180   
            };
163   
            return awaiter{this};
181   
            return awaiter{this};
164   
        }
182   
        }
165  
183  
166   
        void unhandled_exception()
184   
        void unhandled_exception()
167   
        {
185   
        {
168 - 
            ep_ = std::current_exception();
186 + 
            new (&ep_) std::exception_ptr(std::current_exception());
 
187 + 
            has_ep_ = true;
169   
        }
188   
        }
170  
189  
171   
        template<class Awaitable>
190   
        template<class Awaitable>
172   
        struct transform_awaiter
191   
        struct transform_awaiter
173   
        {
192   
        {
174   
            std::decay_t<Awaitable> a_;
193   
            std::decay_t<Awaitable> a_;
175   
            promise_type* p_;
194   
            promise_type* p_;
176  
195  
177 - 
            bool await_ready()
196 + 
            bool await_ready() noexcept
178   
            {
197   
            {
179   
                return a_.await_ready();
198   
                return a_.await_ready();
180   
            }
199   
            }
181  
200  
182   
            decltype(auto) await_resume()
201   
            decltype(auto) await_resume()
183   
            {
202   
            {
184   
                // Restore TLS before body resumes
203   
                // Restore TLS before body resumes
185 - 
                if(p_->frame_allocator())
204 + 
                auto* fa = p_->frame_allocator();
186 - 
                    current_frame_allocator() = p_->frame_allocator();
205 + 
                if(fa && fa != current_frame_allocator())
 
206 + 
                    current_frame_allocator() = fa;
187   
                return a_.await_resume();
207   
                return a_.await_resume();
188   
            }
208   
            }
189  
209  
190   
            template<class Promise>
210   
            template<class Promise>
191 - 
            auto await_suspend(std::coroutine_handle<Promise> h)
211 + 
            auto await_suspend(std::coroutine_handle<Promise> h) noexcept
192   
            {
212   
            {
193   
                return a_.await_suspend(h, p_->executor(), p_->stop_token());
213   
                return a_.await_suspend(h, p_->executor(), p_->stop_token());
194   
            }
214   
            }
195   
        };
215   
        };
196  
216  
197   
        template<class Awaitable>
217   
        template<class Awaitable>
198   
        auto transform_awaitable(Awaitable&& a)
218   
        auto transform_awaitable(Awaitable&& a)
199   
        {
219   
        {
200   
            using A = std::decay_t<Awaitable>;
220   
            using A = std::decay_t<Awaitable>;
201   
            if constexpr (IoAwaitable<A>)
221   
            if constexpr (IoAwaitable<A>)
202   
            {
222   
            {
203   
                return transform_awaiter<Awaitable>{
223   
                return transform_awaiter<Awaitable>{
204   
                    std::forward<Awaitable>(a), this};
224   
                    std::forward<Awaitable>(a), this};
205   
            }
225   
            }
206   
            else
226   
            else
207   
            {
227   
            {
208   
                static_assert(sizeof(A) == 0, "requires IoAwaitable");
228   
                static_assert(sizeof(A) == 0, "requires IoAwaitable");
209   
            }
229   
            }
210   
        }
230   
        }
211   
    };
231   
    };
212  
232  
213   
    std::coroutine_handle<promise_type> h_;
233   
    std::coroutine_handle<promise_type> h_;
214  
234  
215   
    /// Destroy the task and its coroutine frame if owned.
235   
    /// Destroy the task and its coroutine frame if owned.
216   
    ~task()
236   
    ~task()
217   
    {
237   
    {
218   
        if(h_)
238   
        if(h_)
219   
            h_.destroy();
239   
            h_.destroy();
220   
    }
240   
    }
221  
241  
222   
    /// Return false; tasks are never immediately ready.
242   
    /// Return false; tasks are never immediately ready.
223   
    bool await_ready() const noexcept
243   
    bool await_ready() const noexcept
224   
    {
244   
    {
225   
        return false;
245   
        return false;
226   
    }
246   
    }
227  
247  
228   
    /// Return the result or rethrow any stored exception.
248   
    /// Return the result or rethrow any stored exception.
229   
    auto await_resume()
249   
    auto await_resume()
230   
    {
250   
    {
231 - 
        if(h_.promise().ep_)
251 + 
        if(h_.promise().has_ep_)
232   
            std::rethrow_exception(h_.promise().ep_);
252   
            std::rethrow_exception(h_.promise().ep_);
233   
        if constexpr (! std::is_void_v<T>)
253   
        if constexpr (! std::is_void_v<T>)
234   
            return std::move(*h_.promise().result_);
254   
            return std::move(*h_.promise().result_);
235   
        else
255   
        else
236   
            return;
256   
            return;
237   
    }
257   
    }
238  
258  
239   
    /// Start execution with the caller's context.
259   
    /// Start execution with the caller's context.
240 - 
    coro await_suspend(coro cont, executor_ref caller_ex, std::stop_token token)
260 + 
    coro await_suspend(coro cont,
 
261 + 
        executor_ref const& caller_ex, std::stop_token const& token)
241   
    {
262   
    {
242   
        h_.promise().set_continuation(cont, caller_ex);
263   
        h_.promise().set_continuation(cont, caller_ex);
243   
        h_.promise().set_executor(caller_ex);
264   
        h_.promise().set_executor(caller_ex);
244   
        h_.promise().set_stop_token(token);
265   
        h_.promise().set_stop_token(token);
245   
        return h_;
266   
        return h_;
246   
    }
267   
    }
247  
268  
248   
    /// Return the coroutine handle.
269   
    /// Return the coroutine handle.
249   
    std::coroutine_handle<promise_type> handle() const noexcept
270   
    std::coroutine_handle<promise_type> handle() const noexcept
250   
    {
271   
    {
251   
        return h_;
272   
        return h_;
252   
    }
273   
    }
253  
274  
254   
    /** Release ownership of the coroutine frame.
275   
    /** Release ownership of the coroutine frame.
255  
276  
256   
        After calling this, destroying the task does not destroy the
277   
        After calling this, destroying the task does not destroy the
257   
        coroutine frame. The caller becomes responsible for the frame's
278   
        coroutine frame. The caller becomes responsible for the frame's
258   
        lifetime.
279   
        lifetime.
259  
280  
260   
        @par Postconditions
281   
        @par Postconditions
261   
        `handle()` returns the original handle, but the task no longer
282   
        `handle()` returns the original handle, but the task no longer
262   
        owns it.
283   
        owns it.
263   
    */
284   
    */
264   
    void release() noexcept
285   
    void release() noexcept
265   
    {
286   
    {
266   
        h_ = nullptr;
287   
        h_ = nullptr;
267   
    }
288   
    }
268  
289  
269   
    task(task const&) = delete;
290   
    task(task const&) = delete;
270   
    task& operator=(task const&) = delete;
291   
    task& operator=(task const&) = delete;
271  
292  
272   
    /// Move construct, transferring ownership.
293   
    /// Move construct, transferring ownership.
273   
    task(task&& other) noexcept
294   
    task(task&& other) noexcept
274   
        : h_(std::exchange(other.h_, nullptr))
295   
        : h_(std::exchange(other.h_, nullptr))
275   
    {
296   
    {
276   
    }
297   
    }
277  
298  
278   
    /// Move assign, transferring ownership.
299   
    /// Move assign, transferring ownership.
279   
    task& operator=(task&& other) noexcept
300   
    task& operator=(task&& other) noexcept
280   
    {
301   
    {
281   
        if(this != &other)
302   
        if(this != &other)
282   
        {
303   
        {
283   
            if(h_)
304   
            if(h_)
284   
                h_.destroy();
305   
                h_.destroy();
285   
            h_ = std::exchange(other.h_, nullptr);
306   
            h_ = std::exchange(other.h_, nullptr);
286   
        }
307   
        }
287   
        return *this;
308   
        return *this;
288   
    }
309   
    }
289  
310  
290   
private:
311   
private:
291   
    explicit task(std::coroutine_handle<promise_type> h)
312   
    explicit task(std::coroutine_handle<promise_type> h)
292   
        : h_(h)
313   
        : h_(h)
293   
    {
314   
    {
294   
    }
315   
    }
295   
};
316   
};
296  
317  
297   
} // namespace capy
318   
} // namespace capy
298   
} // namespace boost
319   
} // namespace boost
299  
320  
300   
#endif
321   
#endif