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1   // 1   //
2   // Copyright (c) 2025 Vinnie Falco (vinnie.falco@gmail.com) 2   // Copyright (c) 2025 Vinnie Falco (vinnie.falco@gmail.com)
3   // Copyright (c) 2026 Michael Vandeberg 3   // Copyright (c) 2026 Michael Vandeberg
4   // 4   //
5   // Distributed under the Boost Software License, Version 1.0. (See accompanying 5   // Distributed under the Boost Software License, Version 1.0. (See accompanying
6   // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) 6   // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
7   // 7   //
8   // Official repository: https://github.com/cppalliance/capy 8   // Official repository: https://github.com/cppalliance/capy
9   // 9   //
10   10  
11   #ifndef BOOST_CAPY_TASK_HPP 11   #ifndef BOOST_CAPY_TASK_HPP
12   #define BOOST_CAPY_TASK_HPP 12   #define BOOST_CAPY_TASK_HPP
13   13  
14   #include <boost/capy/detail/config.hpp> 14   #include <boost/capy/detail/config.hpp>
15   #include <boost/capy/concept/executor.hpp> 15   #include <boost/capy/concept/executor.hpp>
16   #include <boost/capy/concept/io_awaitable.hpp> 16   #include <boost/capy/concept/io_awaitable.hpp>
17   #include <boost/capy/ex/io_awaitable_promise_base.hpp> 17   #include <boost/capy/ex/io_awaitable_promise_base.hpp>
18   #include <boost/capy/ex/io_env.hpp> 18   #include <boost/capy/ex/io_env.hpp>
19   #include <boost/capy/ex/frame_allocator.hpp> 19   #include <boost/capy/ex/frame_allocator.hpp>
20   #include <boost/capy/detail/await_suspend_helper.hpp> 20   #include <boost/capy/detail/await_suspend_helper.hpp>
21   #include <boost/capy/io_result.hpp> 21   #include <boost/capy/io_result.hpp>
22   22  
23   #include <exception> 23   #include <exception>
24   #include <optional> 24   #include <optional>
25   #include <type_traits> 25   #include <type_traits>
26   #include <utility> 26   #include <utility>
27   #include <variant> 27   #include <variant>
28   28  
29   namespace boost { 29   namespace boost {
30   namespace capy { 30   namespace capy {
31   31  
32   namespace detail { 32   namespace detail {
33   33  
34   // Helper base for result storage and return_void/return_value 34   // Helper base for result storage and return_void/return_value
35   template<typename T> 35   template<typename T>
36   struct task_return_base 36   struct task_return_base
37   { 37   {
38   std::optional<T> result_; 38   std::optional<T> result_;
39   39  
HITCBC 40   870 void return_value(T value) 40   870 void return_value(T value)
41   { 41   {
HITCBC 42   870 result_ = std::move(value); 42   870 result_ = std::move(value);
HITCBC 43   870 } 43   870 }
44   44  
HITCBC 45   273 T&& result() noexcept 45   273 T&& result() noexcept
46   { 46   {
HITCBC 47   273 return std::move(*result_); 47   273 return std::move(*result_);
48   } 48   }
49   }; 49   };
50   50  
51   template<> 51   template<>
52   struct task_return_base<void> 52   struct task_return_base<void>
53   { 53   {
HITCBC 54   1262 void return_void() 54   1247 void return_void()
55   { 55   {
HITCBC 56   1262 } 56   1247 }
57   }; 57   };
58   58  
59   } // namespace detail 59   } // namespace detail
60   60  
61   /** Defers a coroutine body until awaited, then runs it inline on the caller's thread. 61   /** Defers a coroutine body until awaited, then runs it inline on the caller's thread.
62   62  
63   Use `task<T>` as the return type for coroutines that perform I/O 63   Use `task<T>` as the return type for coroutines that perform I/O
64   and return a value of type `T`. The coroutine body does not start 64   and return a value of type `T`. The coroutine body does not start
65   executing until the task is awaited, enabling efficient composition 65   executing until the task is awaited, enabling efficient composition
66   without unnecessary eager execution. 66   without unnecessary eager execution.
67   67  
68   The task participates in the I/O awaitable protocol: when awaited, 68   The task participates in the I/O awaitable protocol: when awaited,
69   it receives the caller's executor and stop token, propagating them 69   it receives the caller's executor and stop token, propagating them
70   to nested `co_await` expressions. This enables cancellation and 70   to nested `co_await` expressions. This enables cancellation and
71   proper completion dispatch across executor boundaries. 71   proper completion dispatch across executor boundaries.
72   72  
73   @par Await-effects 73   @par Await-effects
74   74  
75   Let `t` be a `task<T>`. `co_await t` always suspends the awaiting 75   Let `t` be a `task<T>`. `co_await t` always suspends the awaiting
76   coroutine, then transfers control directly into the task's coroutine 76   coroutine, then transfers control directly into the task's coroutine
77   body on the current thread; no executor operation is posted. The task 77   body on the current thread; no executor operation is posted. The task
78   records the caller's environment (executor, stop token, and frame 78   records the caller's environment (executor, stop token, and frame
79   allocator) by pointer rather than copying it. It propagates that 79   allocator) by pointer rather than copying it. It propagates that
80   environment to every `co_await` inside the body. 80   environment to every `co_await` inside the body.
81   81  
82   The body runs until it returns or exits via an exception. Control 82   The body runs until it returns or exits via an exception. Control
83   then transfers directly back to the awaiting coroutine, again 83   then transfers directly back to the awaiting coroutine, again
84   without an executor operation. 84   without an executor operation.
85   85  
86   `task` never inspects the stop token; it only propagates it. A task 86   `task` never inspects the stop token; it only propagates it. A task
87   body observes a stop request through the results of the operations it 87   body observes a stop request through the results of the operations it
88   awaits, or by reading the token itself. See @ref quitter for a task 88   awaits, or by reading the token itself. See @ref quitter for a task
89   that stops its own body. 89   that stops its own body.
90   90  
91   @par Await-returns 91   @par Await-returns
92   The value the body passed to `co_return`, moved out of the task, or 92   The value the body passed to `co_return`, moved out of the task, or
93   nothing when `T` is `void`. 93   nothing when `T` is `void`.
94   94  
95   If the body exits via an unhandled exception, that exception is 95   If the body exits via an unhandled exception, that exception is
96   rethrown instead. 96   rethrown instead.
97   97  
98   @par Await-postcondition 98   @par Await-postcondition
99   The task's coroutine has run to completion and is suspended at its 99   The task's coroutine has run to completion and is suspended at its
100   final suspend point. The task still owns the frame, but not the 100   final suspend point. The task still owns the frame, but not the
101   result: the await moves it out, so a task must not be awaited twice. 101   result: the await moves it out, so a task must not be awaited twice.
102   102  
103   @par Thread Safety 103   @par Thread Safety
104   Distinct objects: Safe. 104   Distinct objects: Safe.
105   Shared objects: Unsafe. 105   Shared objects: Unsafe.
106   106  
107   @par Example 107   @par Example
108   108  
109   @par !example example 109   @par !example example
110   110  
111   111  
112   @tparam T The result type. Use `task<>` for `task<void>`. 112   @tparam T The result type. Use `task<>` for `task<void>`.
113   113  
114   @see IoRunnable, IoAwaitable, run, run_async 114   @see IoRunnable, IoAwaitable, run, run_async
115   */ 115   */
116   template<typename T = void> 116   template<typename T = void>
117   struct [[nodiscard]] BOOST_CAPY_CORO_AWAIT_ELIDABLE 117   struct [[nodiscard]] BOOST_CAPY_CORO_AWAIT_ELIDABLE
118   task 118   task
119   { 119   {
120   /** Stores `task<T>`'s result and joins the I/O awaitable protocol via `io_awaitable_promise_base`. 120   /** Stores `task<T>`'s result and joins the I/O awaitable protocol via `io_awaitable_promise_base`.
121   121  
122   This is the promise object the compiler associates with a 122   This is the promise object the compiler associates with a
123   `task<T>` coroutine. It satisfies the coroutine promise 123   `task<T>` coroutine. It satisfies the coroutine promise
124   requirements and participates in the I/O awaitable protocol via 124   requirements and participates in the I/O awaitable protocol via
125   @ref io_awaitable_promise_base. It is part of the coroutine 125   @ref io_awaitable_promise_base. It is part of the coroutine
126   machinery and is not intended to be used directly by callers. 126   machinery and is not intended to be used directly by callers.
127   127  
128   Result storage and `return_value`/`return_void` are provided by 128   Result storage and `return_value`/`return_void` are provided by
129   `detail::task_return_base<T>`. 129   `detail::task_return_base<T>`.
130   130  
131   @see io_awaitable_promise_base, IoRunnable 131   @see io_awaitable_promise_base, IoRunnable
132   */ 132   */
133   struct promise_type 133   struct promise_type
134   : io_awaitable_promise_base<promise_type> 134   : io_awaitable_promise_base<promise_type>
135   , detail::task_return_base<T> 135   , detail::task_return_base<T>
136   { 136   {
137   private: 137   private:
138   friend task; 138   friend task;
139   union { std::exception_ptr ep_; }; 139   union { std::exception_ptr ep_; };
140   bool has_ep_; 140   bool has_ep_;
141   141  
142   public: 142   public:
143   /// Construct the promise with no stored exception. 143   /// Construct the promise with no stored exception.
HITCBC 144   2766 promise_type() noexcept 144   2773 promise_type() noexcept
HITCBC 145   2766 : has_ep_(false) 145   2773 : has_ep_(false)
146   { 146   {
HITCBC 147   2766 } 147   2773 }
148   148  
149   /// Destroy the promise, releasing any stored exception. 149   /// Destroy the promise, releasing any stored exception.
HITCBC 150   2766 ~promise_type() 150   2773 ~promise_type()
151   { 151   {
HITCBC 152   2766 if(has_ep_) 152   2773 if(has_ep_)
HITCBC 153   489 ep_.~exception_ptr(); 153   489 ep_.~exception_ptr();
HITCBC 154   2766 } 154   2773 }
155   155  
156   /** Return the exception captured by the coroutine body, if any. 156   /** Return the exception captured by the coroutine body, if any.
157   157  
158   @return The stored exception, or a null `std::exception_ptr` 158   @return The stored exception, or a null `std::exception_ptr`
159   if the coroutine did not exit via an unhandled exception. 159   if the coroutine did not exit via an unhandled exception.
160   */ 160   */
HITCBC 161   2162 std::exception_ptr exception() const noexcept 161   2147 std::exception_ptr exception() const noexcept
162   { 162   {
HITCBC 163   2162 if(has_ep_) 163   2147 if(has_ep_)
HITCBC 164   730 return ep_; 164   730 return ep_;
HITCBC 165   1432 return {}; 165   1417 return {};
166   } 166   }
167   167  
168   /** Return the owning `task` for this coroutine. 168   /** Return the owning `task` for this coroutine.
169   169  
170   Called by the compiler to produce the object returned to the 170   Called by the compiler to produce the object returned to the
171   caller when the coroutine is created. 171   caller when the coroutine is created.
172   172  
173   @return A `task` owning the coroutine frame. 173   @return A `task` owning the coroutine frame.
174   */ 174   */
HITCBC 175   2766 task get_return_object() 175   2773 task get_return_object()
176   { 176   {
HITCBC 177   2766 return task{std::coroutine_handle<promise_type>::from_promise(*this)}; 177   2773 return task{std::coroutine_handle<promise_type>::from_promise(*this)};
178   } 178   }
179   179  
180   /** Return the initial-suspend awaiter. 180   /** Return the initial-suspend awaiter.
181   181  
182   The coroutine always suspends at the initial suspend point, 182   The coroutine always suspends at the initial suspend point,
183   so the body does not start until the task is awaited. When the 183   so the body does not start until the task is awaited. When the
184   body is resumed, the awaiter restores the thread-local frame 184   body is resumed, the awaiter restores the thread-local frame
185   allocator from the stored environment. 185   allocator from the stored environment.
186   186  
187   @return An awaiter that suspends unconditionally. 187   @return An awaiter that suspends unconditionally.
188   */ 188   */
HITCBC 189   2766 auto initial_suspend() noexcept 189   2773 auto initial_suspend() noexcept
190   { 190   {
191   struct awaiter 191   struct awaiter
192   { 192   {
193   promise_type* p_; 193   promise_type* p_;
194   194  
HITCBC 195   2766 bool await_ready() const noexcept 195   2773 bool await_ready() const noexcept
196   { 196   {
HITCBC 197   2766 return false; 197   2773 return false;
198   } 198   }
199   199  
HITCBC 200   2766 void await_suspend(std::coroutine_handle<>) const noexcept 200   2773 void await_suspend(std::coroutine_handle<>) const noexcept
201   { 201   {
HITCBC 202   2766 } 202   2773 }
203   203  
HITCBC 204   2762 void await_resume() const noexcept 204   2769 void await_resume() const noexcept
205   { 205   {
206   // Restore TLS when body starts executing 206   // Restore TLS when body starts executing
HITCBC 207   2762 set_current_frame_allocator(p_->environment()->frame_allocator); 207   2769 set_current_frame_allocator(p_->environment()->frame_allocator);
HITCBC 208   2762 } 208   2769 }
209   }; 209   };
HITCBC 210   2766 return awaiter{this}; 210   2773 return awaiter{this};
211   } 211   }
212   212  
213   /** Return the final-suspend awaiter. 213   /** Return the final-suspend awaiter.
214   214  
215   The coroutine always suspends at the final suspend point. The 215   The coroutine always suspends at the final suspend point. The
216   awaiter's `await_suspend` performs symmetric transfer to the 216   awaiter's `await_suspend` performs symmetric transfer to the
217   stored continuation (consuming it), resuming the awaiting 217   stored continuation (consuming it), resuming the awaiting
218   coroutine. 218   coroutine.
219   219  
220   @return An awaiter that suspends and transfers to the 220   @return An awaiter that suspends and transfers to the
221   continuation. 221   continuation.
222   */ 222   */
HITCBC 223   2621 auto final_suspend() noexcept 223   2606 auto final_suspend() noexcept
224   { 224   {
225   struct awaiter 225   struct awaiter
226   { 226   {
227   promise_type* p_; 227   promise_type* p_;
228   228  
HITCBC 229   2621 bool await_ready() const noexcept 229   2606 bool await_ready() const noexcept
230   { 230   {
HITCBC 231   2621 return false; 231   2606 return false;
232   } 232   }
233   233  
HITCBC 234   2621 std::coroutine_handle<> await_suspend(std::coroutine_handle<>) const noexcept 234   2606 std::coroutine_handle<> await_suspend(std::coroutine_handle<>) const noexcept
235   { 235   {
HITCBC 236   2621 return p_->continuation(); 236   2606 return p_->continuation();
237   } 237   }
238   238  
239   void await_resume() const noexcept {} // LCOV_EXCL_LINE final_suspend awaiter, never resumed 239   void await_resume() const noexcept {} // LCOV_EXCL_LINE final_suspend awaiter, never resumed
240   }; 240   };
HITCBC 241   2621 return awaiter{this}; 241   2606 return awaiter{this};
242   } 242   }
243   243  
244   /** Capture the in-flight exception from the coroutine body. 244   /** Capture the in-flight exception from the coroutine body.
245   245  
246   Called by the compiler when the coroutine body exits via an 246   Called by the compiler when the coroutine body exits via an
247   unhandled exception. The captured exception is rethrown when 247   unhandled exception. The captured exception is rethrown when
248   the task is awaited. 248   the task is awaited.
249   */ 249   */
HITCBC 250   489 void unhandled_exception() noexcept 250   489 void unhandled_exception() noexcept
251   { 251   {
HITCBC 252   489 new (&ep_) std::exception_ptr(std::current_exception()); 252   489 new (&ep_) std::exception_ptr(std::current_exception());
HITCBC 253   489 has_ep_ = true; 253   489 has_ep_ = true;
HITCBC 254   489 } 254   489 }
255   255  
256   /** Awaiter wrapping a nested `co_await` of an @ref IoAwaitable. 256   /** Awaiter wrapping a nested `co_await` of an @ref IoAwaitable.
257   257  
258   Forwards the environment to the inner awaitable's 258   Forwards the environment to the inner awaitable's
259   environment-taking `await_suspend` and restores the 259   environment-taking `await_suspend` and restores the
260   thread-local frame allocator before the body resumes. 260   thread-local frame allocator before the body resumes.
261   261  
262   @tparam Awaitable The awaitable being transformed. 262   @tparam Awaitable The awaitable being transformed.
263   */ 263   */
264   template<class Awaitable> 264   template<class Awaitable>
265   struct transform_awaiter 265   struct transform_awaiter
266   { 266   {
267   /// The wrapped awaitable, decayed and stored by value. 267   /// The wrapped awaitable, decayed and stored by value.
268   std::decay_t<Awaitable> a_; 268   std::decay_t<Awaitable> a_;
269   269  
270   /// The promise of the coroutine performing the `co_await`. 270   /// The promise of the coroutine performing the `co_await`.
271   promise_type* p_; 271   promise_type* p_;
272   272  
273   /** Report whether the wrapped awaitable is already complete. 273   /** Report whether the wrapped awaitable is already complete.
274   274  
275   @return The wrapped awaitable's own `await_ready` result: 275   @return The wrapped awaitable's own `await_ready` result:
276   `true` if no suspension is needed. 276   `true` if no suspension is needed.
277   */ 277   */
HITCBC 278   2877 bool await_ready() noexcept 278   2884 bool await_ready() noexcept
279   { 279   {
HITCBC 280   2877 return a_.await_ready(); 280   2884 return a_.await_ready();
281   } 281   }
282   282  
283   /** Restore the frame allocator, then resume the wrapped 283   /** Restore the frame allocator, then resume the wrapped
284   awaitable. 284   awaitable.
285   285  
286   Reinstalls the thread-local frame allocator from the stored 286   Reinstalls the thread-local frame allocator from the stored
287   environment before the body continues. This is needed 287   environment before the body continues. This is needed
288   because the resumption may arrive on a different thread 288   because the resumption may arrive on a different thread
289   than the one that suspended. 289   than the one that suspended.
290   290  
291   @return The wrapped awaitable's await-result, forwarded 291   @return The wrapped awaitable's await-result, forwarded
292   unchanged. 292   unchanged.
293   */ 293   */
HITCBC 294   2736 decltype(auto) await_resume() 294   2721 decltype(auto) await_resume()
295   { 295   {
296   // Restore TLS before body resumes 296   // Restore TLS before body resumes
HITCBC 297   2736 set_current_frame_allocator(p_->environment()->frame_allocator); 297   2721 set_current_frame_allocator(p_->environment()->frame_allocator);
HITCBC 298   2736 return a_.await_resume(); 298   2721 return a_.await_resume();
299   } 299   }
300   300  
301   /** Suspend by calling the wrapped awaitable with the 301   /** Suspend by calling the wrapped awaitable with the
302   environment. 302   environment.
303   303  
304   This is the plain `await_suspend` the compiler calls for the 304   This is the plain `await_suspend` the compiler calls for the
305   nested `co_await`. It forwards to the wrapped awaitable's 305   nested `co_await`. It forwards to the wrapped awaitable's
306   @ref IoAwaitable overload, supplying the promise's stored 306   @ref IoAwaitable overload, supplying the promise's stored
307   environment as the second argument. It then hands back 307   environment as the second argument. It then hands back
308   that call's result unchanged, so the wrapped awaitable's 308   that call's result unchanged, so the wrapped awaitable's
309   suspension decision, whatever form it takes, is preserved. 309   suspension decision, whatever form it takes, is preserved.
310   310  
311   @param h The coroutine performing the `co_await`. 311   @param h The coroutine performing the `co_await`.
312   312  
313   @return Whatever the wrapped awaitable's `await_suspend` 313   @return Whatever the wrapped awaitable's `await_suspend`
314   returns. When that is a `std::coroutine_handle<>`, the 314   returns. When that is a `std::coroutine_handle<>`, the
315   handle is routed through `detail::symmetric_transfer`. 315   handle is routed through `detail::symmetric_transfer`.
316   On MSVC that helper resumes the handle on the current 316   On MSVC that helper resumes the handle on the current
317   stack, and this function returns `void`, so the awaiting 317   stack, and this function returns `void`, so the awaiting
318   coroutine suspends unconditionally. On every other 318   coroutine suspends unconditionally. On every other
319   compiler the handle is returned unchanged for symmetric 319   compiler the handle is returned unchanged for symmetric
320   transfer. 320   transfer.
321   */ 321   */
322   template<class Promise> 322   template<class Promise>
HITCBC 323   2266 auto await_suspend(std::coroutine_handle<Promise> h) noexcept 323   2260 auto await_suspend(std::coroutine_handle<Promise> h) noexcept
324   { 324   {
325   using R = decltype(a_.await_suspend(h, p_->environment())); 325   using R = decltype(a_.await_suspend(h, p_->environment()));
326   if constexpr (std::is_same_v<R, std::coroutine_handle<>>) 326   if constexpr (std::is_same_v<R, std::coroutine_handle<>>)
HITCBC 327   1266 return detail::symmetric_transfer(a_.await_suspend(h, p_->environment())); 327   1260 return detail::symmetric_transfer(a_.await_suspend(h, p_->environment()));
328   else 328   else
HITCBC 329   1000 return a_.await_suspend(h, p_->environment()); 329   1000 return a_.await_suspend(h, p_->environment());
330   } 330   }
331   }; 331   };
332   332  
333   /** Transform a nested awaitable before `co_await`. 333   /** Transform a nested awaitable before `co_await`.
334   334  
335   Wraps an @ref IoAwaitable in a @ref transform_awaiter so the 335   Wraps an @ref IoAwaitable in a @ref transform_awaiter so the
336   coroutine's environment is propagated into it. A diagnostic 336   coroutine's environment is propagated into it. A diagnostic
337   is emitted if the awaitable does not satisfy @ref IoAwaitable. 337   is emitted if the awaitable does not satisfy @ref IoAwaitable.
338   338  
339   @param a The awaitable expression from `co_await a`. 339   @param a The awaitable expression from `co_await a`.
340   340  
341   @return A @ref transform_awaiter wrapping `a`. 341   @return A @ref transform_awaiter wrapping `a`.
342   */ 342   */
343   template<class Awaitable> 343   template<class Awaitable>
HITCBC 344   2877 auto transform_awaitable(Awaitable&& a) 344   2884 auto transform_awaitable(Awaitable&& a)
345   { 345   {
346   using A = std::decay_t<Awaitable>; 346   using A = std::decay_t<Awaitable>;
347   if constexpr (IoAwaitable<A>) 347   if constexpr (IoAwaitable<A>)
348   { 348   {
349   return transform_awaiter<Awaitable>{ 349   return transform_awaiter<Awaitable>{
HITCBC 350   4408 std::forward<Awaitable>(a), this}; 350   4422 std::forward<Awaitable>(a), this};
351   } 351   }
352   else 352   else
353   { 353   {
354   static_assert(IoAwaitable<A>, "requires IoAwaitable"); 354   static_assert(IoAwaitable<A>, "requires IoAwaitable");
355   } 355   }
HITCBC 356   1531 } 356   1538 }
357   }; 357   };
358   358  
359   /** Handle to the owned coroutine frame. 359   /** Handle to the owned coroutine frame.
360   360  
361   Null when the task is empty (for example after a move or after 361   Null when the task is empty (for example after a move or after
362   @ref release). Prefer @ref handle to read this; the member is 362   @ref release). Prefer @ref handle to read this; the member is
363   public for use by the coroutine machinery. 363   public for use by the coroutine machinery.
364   */ 364   */
365   std::coroutine_handle<promise_type> h_; 365   std::coroutine_handle<promise_type> h_;
366   366  
367   /// Destroy the task and its coroutine frame if owned. 367   /// Destroy the task and its coroutine frame if owned.
HITCBC 368   5856 ~task() 368   5863 ~task()
369   { 369   {
HITCBC 370   5856 if(h_) 370   5863 if(h_)
HITCBC 371   767 h_.destroy(); 371   767 h_.destroy();
HITCBC 372   5856 } 372   5863 }
373   373  
374   /** Report whether the awaited task is already complete. 374   /** Report whether the awaited task is already complete.
375   375  
376   Always returns `false`; a task is lazy and has not started when 376   Always returns `false`; a task is lazy and has not started when
377   it is awaited, so the awaiting coroutine always suspends. 377   it is awaited, so the awaiting coroutine always suspends.
378   378  
379   @return `false`. 379   @return `false`.
380   */ 380   */
HITCBC 381   764 bool await_ready() const noexcept 381   764 bool await_ready() const noexcept
382   { 382   {
HITCBC 383   764 return false; 383   764 return false;
384   } 384   }
385   385  
386   /** Return the task's result, rethrowing any captured exception. 386   /** Return the task's result, rethrowing any captured exception.
387   387  
388   If the coroutine body exited via an unhandled exception, that 388   If the coroutine body exited via an unhandled exception, that
389   exception is rethrown here. Otherwise the result is returned by 389   exception is rethrown here. Otherwise the result is returned by
390   move (for `task<T>`) or nothing is returned (for `task<void>`). 390   move (for `task<T>`) or nothing is returned (for `task<void>`).
391   391  
392   @return The result value for non-void `T`; otherwise `void`. 392   @return The result value for non-void `T`; otherwise `void`.
393   393  
394   @throws The exception captured by the coroutine body, if any. 394   @throws The exception captured by the coroutine body, if any.
395   395  
396   @note Discarding an `io_result` silently drops the error 396   @note Discarding an `io_result` silently drops the error
397   code, so that overload is marked `[[nodiscard]]`. 397   code, so that overload is marked `[[nodiscard]]`.
398   */ 398   */
HITCBC 399   552 [[nodiscard]] auto await_resume() 399   552 [[nodiscard]] auto await_resume()
400   requires detail::is_io_result_v<T> 400   requires detail::is_io_result_v<T>
401   { 401   {
HITCBC 402   552 if(h_.promise().has_ep_) 402   552 if(h_.promise().has_ep_)
HITCBC 403   105 std::rethrow_exception(h_.promise().ep_); 403   105 std::rethrow_exception(h_.promise().ep_);
HITCBC 404   447 return std::move(*h_.promise().result_); 404   447 return std::move(*h_.promise().result_);
405   } 405   }
406   406  
HITCBC 407   211 auto await_resume() 407   211 auto await_resume()
408   requires (! detail::is_io_result_v<T>) 408   requires (! detail::is_io_result_v<T>)
409   { 409   {
HITCBC 410   211 if(h_.promise().has_ep_) 410   211 if(h_.promise().has_ep_)
HITCBC 411   18 std::rethrow_exception(h_.promise().ep_); 411   18 std::rethrow_exception(h_.promise().ep_);
412   if constexpr (! std::is_void_v<T>) 412   if constexpr (! std::is_void_v<T>)
HITCBC 413   148 return std::move(*h_.promise().result_); 413   148 return std::move(*h_.promise().result_);
414   else 414   else
HITCBC 415   45 return; 415   45 return;
416   } 416   }
417   417  
418   /** Start the task with the awaiting coroutine's context. 418   /** Start the task with the awaiting coroutine's context.
419   419  
420   Stores `cont` as the continuation to resume on completion. 420   Stores `cont` as the continuation to resume on completion.
421   Stores `env` as the execution environment propagated to nested 421   Stores `env` as the execution environment propagated to nested
422   `co_await` expressions. Then transfers control into the task's 422   `co_await` expressions. Then transfers control into the task's
423   coroutine body via the returned handle. 423   coroutine body via the returned handle.
424   424  
425   @param cont The awaiting coroutine to resume when the task 425   @param cont The awaiting coroutine to resume when the task
426   completes. 426   completes.
427   427  
428   @param env The execution environment (executor, stop token, and 428   @param env The execution environment (executor, stop token, and
429   frame allocator). It must outlive the task. 429   frame allocator). It must outlive the task.
430   430  
431   @return The task's coroutine handle, for symmetric transfer. 431   @return The task's coroutine handle, for symmetric transfer.
432   */ 432   */
HITCBC 433   683 std::coroutine_handle<> await_suspend(std::coroutine_handle<> cont, io_env const* env) 433   683 std::coroutine_handle<> await_suspend(std::coroutine_handle<> cont, io_env const* env)
434   { 434   {
HITCBC 435   683 h_.promise().set_continuation(cont); 435   683 h_.promise().set_continuation(cont);
HITCBC 436   683 h_.promise().set_environment(env); 436   683 h_.promise().set_environment(env);
HITCBC 437   683 return h_; 437   683 return h_;
438   } 438   }
439   439  
440   /** Return the coroutine handle. 440   /** Return the coroutine handle.
441   441  
442   @note Do not call `destroy()` on the returned handle while the 442   @note Do not call `destroy()` on the returned handle while the
443   task is being awaited. The task's lifetime is normally managed 443   task is being awaited. The task's lifetime is normally managed
444   by `run_async`, `run`, or the awaiting parent. Manually 444   by `run_async`, `run`, or the awaiting parent. Manually
445   destroying a suspended task that another coroutine is awaiting 445   destroying a suspended task that another coroutine is awaiting
446   produces undefined behavior. For cooperative cancellation, use 446   produces undefined behavior. For cooperative cancellation, use
447   `std::stop_token`. 447   `std::stop_token`.
448   448  
449   @return The coroutine handle. 449   @return The coroutine handle.
450   */ 450   */
HITCBC 451   2082 std::coroutine_handle<promise_type> handle() const noexcept 451   2089 std::coroutine_handle<promise_type> handle() const noexcept
452   { 452   {
HITCBC 453   2082 return h_; 453   2089 return h_;
454   } 454   }
455   455  
456   /** Release ownership of the coroutine frame. 456   /** Release ownership of the coroutine frame.
457   457  
458   After calling this, destroying the task does not destroy the 458   After calling this, destroying the task does not destroy the
459   coroutine frame. The caller becomes responsible for the frame's 459   coroutine frame. The caller becomes responsible for the frame's
460   lifetime. 460   lifetime.
461   461  
462   @note The caller may call `destroy()` on the released handle 462   @note The caller may call `destroy()` on the released handle
463   only when the task has not started or has fully completed. 463   only when the task has not started or has fully completed.
464   Destroying a suspended task that is being awaited produces 464   Destroying a suspended task that is being awaited produces
465   undefined behavior. 465   undefined behavior.
466   466  
467   @par Postconditions 467   @par Postconditions
468   `handle()` returns a null handle. Callers needing the 468   `handle()` returns a null handle. Callers needing the
469   original handle must save it, via @ref handle, before 469   original handle must save it, via @ref handle, before
470   calling this. 470   calling this.
471   */ 471   */
HITCBC 472   1999 void release() noexcept 472   2006 void release() noexcept
473   { 473   {
HITCBC 474   1999 h_ = nullptr; 474   2006 h_ = nullptr;
HITCBC 475   1999 } 475   2006 }
476   476  
477   /** Copy construction is disabled; a task uniquely owns its frame. 477   /** Copy construction is disabled; a task uniquely owns its frame.
478   478  
479   @param other The task that would be copied. 479   @param other The task that would be copied.
480   */ 480   */
481   task(task const& other) = delete; 481   task(task const& other) = delete;
482   482  
483   /** Copy assignment is disabled; a task uniquely owns its frame. 483   /** Copy assignment is disabled; a task uniquely owns its frame.
484   484  
485   @param other The task that would be assigned from. 485   @param other The task that would be assigned from.
486   486  
487   @return A reference to `*this`. 487   @return A reference to `*this`.
488   */ 488   */
489   task& operator=(task const& other) = delete; 489   task& operator=(task const& other) = delete;
490   490  
491   /** Construct by moving, transferring ownership of the frame. 491   /** Construct by moving, transferring ownership of the frame.
492   492  
493   @par Postconditions 493   @par Postconditions
494   `other` is empty and must not be awaited. 494   `other` is empty and must not be awaited.
495   495  
496   @param other The task to move from. 496   @param other The task to move from.
497   */ 497   */
HITCBC 498   3090 task(task&& other) noexcept 498   3090 task(task&& other) noexcept
HITCBC 499   3090 : h_(std::exchange(other.h_, nullptr)) 499   3090 : h_(std::exchange(other.h_, nullptr))
500   { 500   {
HITCBC 501   3090 } 501   3090 }
502   502  
503   /** Assign by moving, transferring ownership of the frame. 503   /** Assign by moving, transferring ownership of the frame.
504   504  
505   If this task already owns a coroutine frame, that frame is 505   If this task already owns a coroutine frame, that frame is
506   destroyed first. Self-assignment is a no-op. 506   destroyed first. Self-assignment is a no-op.
507   507  
508   @par Postconditions 508   @par Postconditions
509   `other` is empty and must not be awaited. 509   `other` is empty and must not be awaited.
510   510  
511   @param other The task to move from. 511   @param other The task to move from.
512   512  
513   @return A reference to `*this`. 513   @return A reference to `*this`.
514   */ 514   */
515   task& operator=(task&& other) noexcept 515   task& operator=(task&& other) noexcept
516   { 516   {
517   if(this != &other) 517   if(this != &other)
518   { 518   {
519   if(h_) 519   if(h_)
520   h_.destroy(); 520   h_.destroy();
521   h_ = std::exchange(other.h_, nullptr); 521   h_ = std::exchange(other.h_, nullptr);
522   } 522   }
523   return *this; 523   return *this;
524   } 524   }
525   525  
526   private: 526   private:
HITCBC 527   2766 explicit task(std::coroutine_handle<promise_type> h) 527   2773 explicit task(std::coroutine_handle<promise_type> h)
HITCBC 528   2766 : h_(h) 528   2773 : h_(h)
529   { 529   {
HITCBC 530   2766 } 530   2773 }
531   }; 531   };
532   532  
533   } // namespace capy 533   } // namespace capy
534   } // namespace boost 534   } // namespace boost
535   535  
536   #endif 536   #endif