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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/context_awaitable.hpp> 16   #include <boost/capy/concept/context_awaitable.hpp>
17   #include <boost/capy/ex/context_awaitable_promise_base.hpp> 17   #include <boost/capy/ex/context_awaitable_promise_base.hpp>
18   #include <boost/capy/ex/await_context.hpp> 18   #include <boost/capy/ex/await_context.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/results.hpp> 21   #include <boost/capy/results.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   904 void return_value(T value) 40   901 void return_value(T value)
41   { 41   {
HITCBC 42   904 result_ = std::move(value); 42   901 result_ = std::move(value);
HITCBC 43   904 } 43   901 }
44   44  
HITCBC 45   286 T&& result() noexcept 45   285 T&& result() noexcept
46   { 46   {
HITCBC 47   286 return std::move(*result_); 47   285 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   4172 void return_void() 54   4135 void return_void()
55   { 55   {
HITCBC 56   4172 } 56   4135 }
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 @ref ContextRunnable, @ref ContextAwaitable, @ref run, @ref run_async 114   @see @ref ContextRunnable, @ref ContextAwaitable, @ref run, @ref 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 `context_awaitable_promise_base`. 120   /** Stores `task<T>`'s result and joins the I/O awaitable protocol via `context_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 context_awaitable_promise_base. It is part of the coroutine 125   @ref context_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 @ref context_awaitable_promise_base, @ref ContextRunnable 131   @see @ref context_awaitable_promise_base, @ref ContextRunnable
132   */ 132   */
133   struct promise_type 133   struct promise_type
134   : context_awaitable_promise_base<promise_type> 134   : context_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   109265 promise_type() noexcept 144   109247 promise_type() noexcept
HITCBC 145   109265 : has_ep_(false) 145   109247 : has_ep_(false)
146   { 146   {
HITCBC 147   109265 } 147   109247 }
148   148  
149   /// Destroy the promise, releasing any stored exception. 149   /// Destroy the promise, releasing any stored exception.
HITCBC 150   109265 ~promise_type() 150   109247 ~promise_type()
151   { 151   {
HITCBC 152   109265 if(has_ep_) 152   109247 if(has_ep_)
HITCBC 153   490 ep_.~exception_ptr(); 153   490 ep_.~exception_ptr();
HITCBC 154   109265 } 154   109247 }
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   5055 std::exception_ptr exception() const noexcept 161   5017 std::exception_ptr exception() const noexcept
162   { 162   {
HITCBC 163   5055 if(has_ep_) 163   5017 if(has_ep_)
HITCBC 164   732 return ep_; 164   732 return ep_;
HITCBC 165   4323 return {}; 165   4285 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   109265 task get_return_object() 175   109247 task get_return_object()
176   { 176   {
HITCBC 177   109265 return task{std::coroutine_handle<promise_type>::from_promise(*this)}; 177   109247 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   109265 auto initial_suspend() noexcept 189   109247 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   109265 bool await_ready() const noexcept 195   109247 bool await_ready() const noexcept
196   { 196   {
HITCBC 197   109265 return false; 197   109247 return false;
198   } 198   }
199   199  
HITCBC 200   109265 void await_suspend(std::coroutine_handle<>) const noexcept 200   109247 void await_suspend(std::coroutine_handle<>) const noexcept
201   { 201   {
HITCBC 202   109265 } 202   109247 }
203   203  
HITCBC 204   108182 void await_resume() const noexcept 204   108164 void await_resume() const noexcept
205   { 205   {
206   // Restore TLS when body starts executing 206   // Restore TLS when body starts executing
HITCBC 207   108182 set_current_frame_allocator(p_->environment()->frame_allocator); 207   108164 set_current_frame_allocator(p_->environment()->frame_allocator);
HITCBC 208   108182 } 208   108164 }
209   }; 209   };
HITCBC 210   109265 return awaiter{this}; 210   109247 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   5566 auto final_suspend() noexcept 223   5526 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   5566 bool await_ready() const noexcept 229   5526 bool await_ready() const noexcept
230   { 230   {
HITCBC 231   5566 return false; 231   5526 return false;
232   } 232   }
233   233  
HITCBC 234   5566 std::coroutine_handle<> await_suspend(std::coroutine_handle<>) const noexcept 234   5526 std::coroutine_handle<> await_suspend(std::coroutine_handle<>) const noexcept
235   { 235   {
HITCBC 236   5566 return p_->continuation(); 236   5526 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   5566 return awaiter{this}; 241   5526 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   490 void unhandled_exception() noexcept 250   490 void unhandled_exception() noexcept
251   { 251   {
HITCBC 252   490 new (&ep_) std::exception_ptr(std::current_exception()); 252   490 new (&ep_) std::exception_ptr(std::current_exception());
HITCBC 253   490 has_ep_ = true; 253   490 has_ep_ = true;
HITCBC 254   490 } 254   490 }
255   255  
256   /** Awaiter wrapping a nested `co_await` of an @ref ContextAwaitable. 256   /** Awaiter wrapping a nested `co_await` of an @ref ContextAwaitable.
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   Where `BOOST_CAPY_SYMMETRIC_TRANSFER_WORKAROUND` is on, 275   Where `BOOST_CAPY_SYMMETRIC_TRANSFER_WORKAROUND` is on,
276   this always returns `false`, and @ref await_suspend asks 276   this always returns `false`, and @ref await_suspend asks
277   the wrapped awaitable instead. See 277   the wrapped awaitable instead. See
278   `detail::await_ready_then_suspend`. 278   `detail::await_ready_then_suspend`.
279   279  
280   @return The wrapped awaitable's own `await_ready` result: 280   @return The wrapped awaitable's own `await_ready` result:
281   `true` if no suspension is needed. Always `false` where 281   `true` if no suspension is needed. Always `false` where
282   the workaround is on. 282   the workaround is on.
283   */ 283   */
HITCBC 284   108251 bool await_ready() noexcept 284   108230 bool await_ready() noexcept
285   { 285   {
286   #if BOOST_CAPY_SYMMETRIC_TRANSFER_WORKAROUND 286   #if BOOST_CAPY_SYMMETRIC_TRANSFER_WORKAROUND
287   return false; 287   return false;
288   #else 288   #else
HITCBC 289   108251 return a_.await_ready(); 289   108230 return a_.await_ready();
290   #endif 290   #endif
291   } 291   }
292   292  
293   /** Restore the frame allocator, then resume the wrapped 293   /** Restore the frame allocator, then resume the wrapped
294   awaitable. 294   awaitable.
295   295  
296   Reinstalls the thread-local frame allocator from the stored 296   Reinstalls the thread-local frame allocator from the stored
297   environment before the body continues. This is needed 297   environment before the body continues. This is needed
298   because the resumption may arrive on a different thread 298   because the resumption may arrive on a different thread
299   than the one that suspended. 299   than the one that suspended.
300   300  
301   @return The wrapped awaitable's await-result, forwarded 301   @return The wrapped awaitable's await-result, forwarded
302   unchanged. 302   unchanged.
303   */ 303   */
HITCBC 304   5635 decltype(auto) await_resume() 304   5592 decltype(auto) await_resume()
305   { 305   {
306   // Restore TLS before body resumes 306   // Restore TLS before body resumes
HITCBC 307   5635 set_current_frame_allocator(p_->environment()->frame_allocator); 307   5592 set_current_frame_allocator(p_->environment()->frame_allocator);
HITCBC 308   5635 return a_.await_resume(); 308   5592 return a_.await_resume();
309   } 309   }
310   310  
311   /** Suspend by calling the wrapped awaitable with the 311   /** Suspend by calling the wrapped awaitable with the
312   environment. 312   environment.
313   313  
314   This is the plain `await_suspend` the compiler calls for the 314   This is the plain `await_suspend` the compiler calls for the
315   nested `co_await`. It forwards to the wrapped awaitable's 315   nested `co_await`. It forwards to the wrapped awaitable's
316   @ref ContextAwaitable overload, supplying the promise's stored 316   @ref ContextAwaitable overload, supplying the promise's stored
317   environment as the second argument. It then hands back 317   environment as the second argument. It then hands back
318   that call's result unchanged, so the wrapped awaitable's 318   that call's result unchanged, so the wrapped awaitable's
319   suspension decision, whatever form it takes, is preserved. 319   suspension decision, whatever form it takes, is preserved.
320   320  
321   @param h The coroutine performing the `co_await`. 321   @param h The coroutine performing the `co_await`.
322   322  
323   @return Whatever the wrapped awaitable's `await_suspend` 323   @return Whatever the wrapped awaitable's `await_suspend`
324   returns. When that is a `std::coroutine_handle<>`, the 324   returns. When that is a `std::coroutine_handle<>`, the
325   handle is routed through `detail::symmetric_transfer`. 325   handle is routed through `detail::symmetric_transfer`.
326   Where `BOOST_CAPY_SYMMETRIC_TRANSFER_WORKAROUND` is on, 326   Where `BOOST_CAPY_SYMMETRIC_TRANSFER_WORKAROUND` is on,
327   this function instead returns `bool` from 327   this function instead returns `bool` from
328   `detail::await_ready_then_suspend`. It first asks the 328   `detail::await_ready_then_suspend`. It first asks the
329   wrapped awaitable's `await_ready`. When that reports 329   wrapped awaitable's `await_ready`. When that reports
330   ready, it returns `false` to continue without 330   ready, it returns `false` to continue without
331   suspending. Otherwise it resumes any returned handle on 331   suspending. Otherwise it resumes any returned handle on
332   the current stack and returns `true`. Where the 332   the current stack and returns `true`. Where the
333   workaround is off, a returned handle is passed back 333   workaround is off, a returned handle is passed back
334   unchanged for symmetric transfer. 334   unchanged for symmetric transfer.
335   */ 335   */
336   template<class Promise> 336   template<class Promise>
HITCBC 337   107635 auto await_suspend(std::coroutine_handle<Promise> h) noexcept 337   107602 auto await_suspend(std::coroutine_handle<Promise> h) noexcept
338   { 338   {
339   #if BOOST_CAPY_SYMMETRIC_TRANSFER_WORKAROUND 339   #if BOOST_CAPY_SYMMETRIC_TRANSFER_WORKAROUND
340   return detail::await_ready_then_suspend(a_, h, p_->environment()); 340   return detail::await_ready_then_suspend(a_, h, p_->environment());
341   #else 341   #else
342   using R = decltype(a_.await_suspend(h, p_->environment())); 342   using R = decltype(a_.await_suspend(h, p_->environment()));
343   if constexpr (std::is_same_v<R, std::coroutine_handle<>>) 343   if constexpr (std::is_same_v<R, std::coroutine_handle<>>)
HITCBC 344   106595 return detail::symmetric_transfer(a_.await_suspend(h, p_->environment())); 344   106564 return detail::symmetric_transfer(a_.await_suspend(h, p_->environment()));
345   else 345   else
HITCBC 346   1040 return a_.await_suspend(h, p_->environment()); 346   1038 return a_.await_suspend(h, p_->environment());
347   #endif 347   #endif
348   } 348   }
349   }; 349   };
350   350  
351   /** Transform a nested awaitable before `co_await`. 351   /** Transform a nested awaitable before `co_await`.
352   352  
353   Wraps an @ref ContextAwaitable in a @ref transform_awaiter so the 353   Wraps an @ref ContextAwaitable in a @ref transform_awaiter so the
354   coroutine's environment is propagated into it. A diagnostic 354   coroutine's environment is propagated into it. A diagnostic
355   is emitted if the awaitable does not satisfy @ref ContextAwaitable. 355   is emitted if the awaitable does not satisfy @ref ContextAwaitable.
356   356  
357   @param a The awaitable expression from `co_await a`. 357   @param a The awaitable expression from `co_await a`.
358   358  
359   @return A @ref transform_awaiter wrapping `a`. 359   @return A @ref transform_awaiter wrapping `a`.
360   */ 360   */
361   template<class Awaitable> 361   template<class Awaitable>
HITCBC 362   108251 auto transform_awaitable(Awaitable&& a) 362   108230 auto transform_awaitable(Awaitable&& a)
363   { 363   {
364   using A = std::decay_t<Awaitable>; 364   using A = std::decay_t<Awaitable>;
365   if constexpr (ContextAwaitable<A>) 365   if constexpr (ContextAwaitable<A>)
366   { 366   {
367   return transform_awaiter<Awaitable>{ 367   return transform_awaiter<Awaitable>{
HITCBC 368   213979 std::forward<Awaitable>(a), this}; 368   213939 std::forward<Awaitable>(a), this};
369   } 369   }
370   else 370   else
371   { 371   {
372   static_assert(ContextAwaitable<A>, "requires ContextAwaitable"); 372   static_assert(ContextAwaitable<A>, "requires ContextAwaitable");
373   } 373   }
HITCBC 374   105728 } 374   105709 }
375   }; 375   };
376   376  
377   /** Handle to the owned coroutine frame. 377   /** Handle to the owned coroutine frame.
378   378  
379   Null when the task is empty (for example after a move or after 379   Null when the task is empty (for example after a move or after
380   @ref release). Prefer @ref handle to read this; the member is 380   @ref release). Prefer @ref handle to read this; the member is
381   public for use by the coroutine machinery. 381   public for use by the coroutine machinery.
382   */ 382   */
383   std::coroutine_handle<promise_type> h_; 383   std::coroutine_handle<promise_type> h_;
384   384  
385   /** Destroy the task and its coroutine frame if owned. 385   /** Destroy the task and its coroutine frame if owned.
386   386  
387   @par Preconditions 387   @par Preconditions
388   The frame is not lent: no parent is suspended awaiting it, 388   The frame is not lent: no parent is suspended awaiting it,
389   unless its chain is being unwound after an @ref abandon. 389   unless its chain is being unwound after an @ref abandon.
390   Destroying a lent frame otherwise calls `std::terminate`; end 390   Destroying a lent frame otherwise calls `std::terminate`; end
391   a running chain by requesting stop, or by destroying its 391   a running chain by requesting stop, or by destroying its
392   execution context. 392   execution context.
393   */ 393   */
HITCBC 394   113348 ~task() 394   113326 ~task()
395   { 395   {
HITCBC 396   113348 destroy_frame(); 396   113326 destroy_frame();
HITCBC 397   113348 } 397   113326 }
398   398  
399   /** Report whether the awaited task is already complete. 399   /** Report whether the awaited task is already complete.
400   400  
401   Always returns `false`; a task is lazy and has not started when 401   Always returns `false`; a task is lazy and has not started when
402   it is awaited, so the awaiting coroutine always suspends. 402   it is awaited, so the awaiting coroutine always suspends.
403   403  
404   @return `false`. 404   @return `false`.
405   */ 405   */
HITCBC 406   2114 bool await_ready() const noexcept 406   2112 bool await_ready() const noexcept
407   { 407   {
HITCBC 408   2114 return false; 408   2112 return false;
409   } 409   }
410   410  
411   /** Return the task's result, rethrowing any captured exception. 411   /** Return the task's result, rethrowing any captured exception.
412   412  
413   If the coroutine body exited via an unhandled exception, that 413   If the coroutine body exited via an unhandled exception, that
414   exception is rethrown here. Otherwise the result is returned by 414   exception is rethrown here. Otherwise the result is returned by
415   move (for `task<T>`) or nothing is returned (for `task<void>`). 415   move (for `task<T>`) or nothing is returned (for `task<void>`).
416   416  
417   @return The result value for non-void `T`; otherwise `void`. 417   @return The result value for non-void `T`; otherwise `void`.
418   418  
419   @throws The exception captured by the coroutine body, if any. 419   @throws The exception captured by the coroutine body, if any.
420   420  
421   @note Discarding a `results` silently drops the error 421   @note Discarding a `results` silently drops the error
422   code, so that overload is marked `[[nodiscard]]`. 422   code, so that overload is marked `[[nodiscard]]`.
423   */ 423   */
HITCBC 424   569 [[nodiscard]] auto await_resume() 424   567 [[nodiscard]] auto await_resume()
425   requires detail::is_results_v<T> 425   requires detail::is_results_v<T>
426   { 426   {
HITCBC 427   569 lent_ = false; 427   567 lent_ = false;
HITCBC 428   569 if(h_.promise().has_ep_) 428   567 if(h_.promise().has_ep_)
HITCBC 429   105 std::rethrow_exception(h_.promise().ep_); 429   105 std::rethrow_exception(h_.promise().ep_);
HITCBC 430   464 return std::move(*h_.promise().result_); 430   462 return std::move(*h_.promise().result_);
431   } 431   }
432   432  
HITCBC 433   245 auto await_resume() 433   245 auto await_resume()
434   requires (! detail::is_results_v<T>) 434   requires (! detail::is_results_v<T>)
435   { 435   {
HITCBC 436   245 lent_ = false; 436   245 lent_ = false;
HITCBC 437   245 if(h_.promise().has_ep_) 437   245 if(h_.promise().has_ep_)
HITCBC 438   18 std::rethrow_exception(h_.promise().ep_); 438   18 std::rethrow_exception(h_.promise().ep_);
439   if constexpr (! std::is_void_v<T>) 439   if constexpr (! std::is_void_v<T>)
HITCBC 440   152 return std::move(*h_.promise().result_); 440   152 return std::move(*h_.promise().result_);
441   else 441   else
HITCBC 442   75 return; 442   75 return;
443   } 443   }
444   444  
445   /** Start the task with the awaiting coroutine's context. 445   /** Start the task with the awaiting coroutine's context.
446   446  
447   Stores `cont` as the continuation to resume on completion. 447   Stores `cont` as the continuation to resume on completion.
448   Stores `env` as the execution environment propagated to nested 448   Stores `env` as the execution environment propagated to nested
449   `co_await` expressions. Then transfers control into the task's 449   `co_await` expressions. Then transfers control into the task's
450   coroutine body via the returned handle. 450   coroutine body via the returned handle.
451   451  
452   @param cont The awaiting coroutine to resume when the task 452   @param cont The awaiting coroutine to resume when the task
453   completes. 453   completes.
454   454  
455   @param env The execution environment (executor, stop token, and 455   @param env The execution environment (executor, stop token, and
456   frame allocator). It must outlive the task. 456   frame allocator). It must outlive the task.
457   457  
458   @return The task's coroutine handle, for symmetric transfer. 458   @return The task's coroutine handle, for symmetric transfer.
459   */ 459   */
HITCBC 460   2011 std::coroutine_handle<> await_suspend(std::coroutine_handle<> cont, await_context const* env) 460   2009 std::coroutine_handle<> await_suspend(std::coroutine_handle<> cont, await_context const* env)
461   { 461   {
HITCBC 462   2011 h_.promise().set_continuation(cont); 462   2009 h_.promise().set_continuation(cont);
HITCBC 463   2011 h_.promise().set_environment(env); 463   2009 h_.promise().set_environment(env);
HITCBC 464   2011 lent_ = true; 464   2009 lent_ = true;
HITCBC 465   2011 return h_; 465   2009 return h_;
466   } 466   }
467   467  
468   /** Return the coroutine handle. 468   /** Return the coroutine handle.
469   469  
470   @note Do not call `destroy()` on the returned handle while the 470   @note Do not call `destroy()` on the returned handle while the
471   task is being awaited: the awaiting frame still holds it and 471   task is being awaited: the awaiting frame still holds it and
472   destroys it again, which has undefined behavior. A holder that 472   destroys it again, which has undefined behavior. A holder that
473   gives the coroutine up passes its continuation to @ref abandon 473   gives the coroutine up passes its continuation to @ref abandon
474   instead. (Destroying or 474   instead. (Destroying or
475   move-assigning over the task object itself while its frame is 475   move-assigning over the task object itself while its frame is
476   lent is a different operation, and calls `std::terminate`.) 476   lent is a different operation, and calls `std::terminate`.)
477   The task's lifetime is normally managed by `run_async`, `run`, 477   The task's lifetime is normally managed by `run_async`, `run`,
478   or the awaiting parent. For cooperative cancellation, use 478   or the awaiting parent. For cooperative cancellation, use
479   `std::stop_token`. 479   `std::stop_token`.
480   480  
481   @note The operation, queue or primitive holding a suspended 481   @note The operation, queue or primitive holding a suspended
482   coroutine gives it up with @ref abandon, through the 482   coroutine gives it up with @ref abandon, through the
483   continuation it keeps, never by destroying a handle. 483   continuation it keeps, never by destroying a handle.
484   `handle()` belongs to the owning task object. 484   `handle()` belongs to the owning task object.
485   485  
486   @return The coroutine handle. 486   @return The coroutine handle.
487   */ 487   */
HITCBC 488   107283 std::coroutine_handle<promise_type> handle() const noexcept 488   107267 std::coroutine_handle<promise_type> handle() const noexcept
489   { 489   {
HITCBC 490   107283 return h_; 490   107267 return h_;
491   } 491   }
492   492  
493   /** Release ownership of the coroutine frame. 493   /** Release ownership of the coroutine frame.
494   494  
495   After calling this, destroying the task does not destroy the 495   After calling this, destroying the task does not destroy the
496   coroutine frame. The caller becomes responsible for the frame's 496   coroutine frame. The caller becomes responsible for the frame's
497   lifetime. 497   lifetime.
498   498  
499   @note The caller may call `destroy()` on the released handle 499   @note The caller may call `destroy()` on the released handle
500   only when the task has not started or has fully completed. 500   only when the task has not started or has fully completed.
501   Destroying the released handle of a task that is being 501   Destroying the released handle of a task that is being
502   awaited is not supported and has undefined behavior. 502   awaited is not supported and has undefined behavior.
503   503  
504   @par Postconditions 504   @par Postconditions
505   `handle()` returns a null handle. Callers needing the 505   `handle()` returns a null handle. Callers needing the
506   original handle must save it, via @ref handle, before 506   original handle must save it, via @ref handle, before
507   calling this. 507   calling this.
508   */ 508   */
HITCBC 509   107140 void release() noexcept 509   107124 void release() noexcept
510   { 510   {
HITCBC 511   107140 h_ = nullptr; 511   107124 h_ = nullptr;
HITCBC 512   107140 } 512   107124 }
513   513  
514   /** Copy construction is disabled; a task uniquely owns its frame. 514   /** Copy construction is disabled; a task uniquely owns its frame.
515   515  
516   @param other The task that would be copied. 516   @param other The task that would be copied.
517   */ 517   */
518   task(task const& other) = delete; 518   task(task const& other) = delete;
519   519  
520   /** Copy assignment is disabled; a task uniquely owns its frame. 520   /** Copy assignment is disabled; a task uniquely owns its frame.
521   521  
522   @param other The task that would be assigned from. 522   @param other The task that would be assigned from.
523   523  
524   @return A reference to `*this`. 524   @return A reference to `*this`.
525   */ 525   */
526   task& operator=(task const& other) = delete; 526   task& operator=(task const& other) = delete;
527   527  
528   /** Construct by moving, transferring ownership of the frame. 528   /** Construct by moving, transferring ownership of the frame.
529   529  
530   @par Postconditions 530   @par Postconditions
531   `other` is empty and must not be awaited. 531   `other` is empty and must not be awaited.
532   532  
533   @param other The task to move from. 533   @param other The task to move from.
534   */ 534   */
HITCBC 535   4083 task(task&& other) noexcept 535   4079 task(task&& other) noexcept
HITCBC 536   4083 : h_(std::exchange(other.h_, nullptr)) 536   4079 : h_(std::exchange(other.h_, nullptr))
537   { 537   {
HITCBC 538   4083 } 538   4079 }
539   539  
540   /** Assign by moving, transferring ownership of the frame. 540   /** Assign by moving, transferring ownership of the frame.
541   541  
542   If this task already owns a coroutine frame, that frame is 542   If this task already owns a coroutine frame, that frame is
543   destroyed first. Self-assignment is a no-op. 543   destroyed first. Self-assignment is a no-op.
544   544  
545   @par Preconditions 545   @par Preconditions
546   The frame being replaced is not lent: no parent is suspended 546   The frame being replaced is not lent: no parent is suspended
547   awaiting it. Replacing a lent frame calls `std::terminate`. 547   awaiting it. Replacing a lent frame calls `std::terminate`.
548   548  
549   @par Postconditions 549   @par Postconditions
550   `other` is empty and must not be awaited. 550   `other` is empty and must not be awaited.
551   551  
552   @param other The task to move from. 552   @param other The task to move from.
553   553  
554   @return A reference to `*this`. 554   @return A reference to `*this`.
555   */ 555   */
HITCBC 556   1 task& operator=(task&& other) noexcept 556   1 task& operator=(task&& other) noexcept
557   { 557   {
HITCBC 558   1 if(this != &other) 558   1 if(this != &other)
559   { 559   {
HITCBC 560   1 destroy_frame(); 560   1 destroy_frame();
HITCBC 561   1 h_ = std::exchange(other.h_, nullptr); 561   1 h_ = std::exchange(other.h_, nullptr);
562   } 562   }
HITCBC 563   1 return *this; 563   1 return *this;
564   } 564   }
565   565  
566   private: 566   private:
567   // Set while a parent is suspended awaiting this task. 567   // Set while a parent is suspended awaiting this task.
568   bool lent_ = false; 568   bool lent_ = false;
569   569  
HITCBC 570   109265 explicit task(std::coroutine_handle<promise_type> h) 570   109247 explicit task(std::coroutine_handle<promise_type> h)
HITCBC 571   109265 : h_(h) 571   109247 : h_(h)
572   { 572   {
HITCBC 573   109265 } 573   109247 }
574   574  
HITCBC 575   113349 void destroy_frame() noexcept 575   113327 void destroy_frame() noexcept
576   { 576   {
HITCBC 577   113349 if(!h_) 577   113327 if(!h_)
HITCBC 578   111224 return; 578   111204 return;
HITCBC 579   2125 if(lent_ && !h_.done()) 579   2123 if(lent_ && !h_.done())
580   { 580   {
581   // Only the unwind of an abandoned chain destroys a frame 581   // Only the unwind of an abandoned chain destroys a frame
582   // that is still awaited. 582   // that is still awaited.
HITCBC 583   1278 auto* owner = h_.promise().environment()->owner; 583   1278 auto* owner = h_.promise().environment()->owner;
HITCBC 584   1278 if(!owner || !owner->abandoned()) 584   1278 if(!owner || !owner->abandoned())
585   std::terminate(); // LCOV_EXCL_LINE reached only in a death test's child process 585   std::terminate(); // LCOV_EXCL_LINE reached only in a death test's child process
586   } 586   }
HITCBC 587   2125 std::exchange(h_, nullptr).destroy(); 587   2123 std::exchange(h_, nullptr).destroy();
HITCBC 588   2125 lent_ = false; 588   2123 lent_ = false;
589   } 589   }
590   }; 590   };
591   591  
592   } // namespace capy 592   } // namespace capy
593   } // namespace boost 593   } // namespace boost
594   594  
595   #endif 595   #endif