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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   903 void return_value(T value) 40   904 void return_value(T value)
41   { 41   {
HITCBC 42   903 result_ = std::move(value); 42   904 result_ = std::move(value);
HITCBC 43   903 } 43   904 }
44   44  
HITCBC 45   282 T&& result() noexcept 45   283 T&& result() noexcept
46   { 46   {
HITCBC 47   282 return std::move(*result_); 47   283 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   4145 void return_void() 54   4097 void return_void()
55   { 55   {
HITCBC 56   4145 } 56   4097 }
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   109252 promise_type() noexcept 144   109232 promise_type() noexcept
HITCBC 145   109252 : has_ep_(false) 145   109232 : has_ep_(false)
146   { 146   {
HITCBC 147   109252 } 147   109232 }
148   148  
149   /// Destroy the promise, releasing any stored exception. 149   /// Destroy the promise, releasing any stored exception.
HITCBC 150   109252 ~promise_type() 150   109232 ~promise_type()
151   { 151   {
HITCBC 152   109252 if(has_ep_) 152   109232 if(has_ep_)
HITCBC 153   490 ep_.~exception_ptr(); 153   490 ep_.~exception_ptr();
HITCBC 154   109252 } 154   109232 }
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   5024 std::exception_ptr exception() const noexcept 161   4977 std::exception_ptr exception() const noexcept
162   { 162   {
HITCBC 163   5024 if(has_ep_) 163   4977 if(has_ep_)
HITCBC 164   732 return ep_; 164   732 return ep_;
HITCBC 165   4292 return {}; 165   4245 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   109252 task get_return_object() 175   109232 task get_return_object()
176   { 176   {
HITCBC 177   109252 return task{std::coroutine_handle<promise_type>::from_promise(*this)}; 177   109232 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   109252 auto initial_suspend() noexcept 189   109232 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   109252 bool await_ready() const noexcept 195   109232 bool await_ready() const noexcept
196   { 196   {
HITCBC 197   109252 return false; 197   109232 return false;
198   } 198   }
199   199  
HITCBC 200   109252 void await_suspend(std::coroutine_handle<>) const noexcept 200   109232 void await_suspend(std::coroutine_handle<>) const noexcept
201   { 201   {
HITCBC 202   109252 } 202   109232 }
203   203  
HITCBC 204   108169 void await_resume() const noexcept 204   108149 void await_resume() const noexcept
205   { 205   {
206   // Restore TLS when body starts executing 206   // Restore TLS when body starts executing
HITCBC 207   108169 set_current_frame_allocator(p_->environment()->frame_allocator); 207   108149 set_current_frame_allocator(p_->environment()->frame_allocator);
HITCBC 208   108169 } 208   108149 }
209   }; 209   };
HITCBC 210   109252 return awaiter{this}; 210   109232 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   5538 auto final_suspend() noexcept 223   5491 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   5538 bool await_ready() const noexcept 229   5491 bool await_ready() const noexcept
230   { 230   {
HITCBC 231   5538 return false; 231   5491 return false;
232   } 232   }
233   233  
HITCBC 234   5538 std::coroutine_handle<> await_suspend(std::coroutine_handle<>) const noexcept 234   5491 std::coroutine_handle<> await_suspend(std::coroutine_handle<>) const noexcept
235   { 235   {
HITCBC 236   5538 return p_->continuation(); 236   5491 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   5538 return awaiter{this}; 241   5491 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,
  276 + this always returns `false`, and @ref await_suspend asks
  277 + the wrapped awaitable instead. See
  278 + `detail::await_ready_then_suspend`.
  279 +
275   @return The wrapped awaitable's own `await_ready` result: 280   @return The wrapped awaitable's own `await_ready` result:
276 - `true` if no suspension is needed. 281 + `true` if no suspension is needed. Always `false` where
  282 + the workaround is on.
277   */ 283   */
HITCBC 278   108242 bool await_ready() noexcept 284   108222 bool await_ready() noexcept
279   { 285   {
  286 + #if BOOST_CAPY_SYMMETRIC_TRANSFER_WORKAROUND
  287 + return false;
  288 + #else
HITCBC 280   108242 return a_.await_ready(); 289   108222 return a_.await_ready();
  290 + #endif
281   } 291   }
282   292  
283   /** Restore the frame allocator, then resume the wrapped 293   /** Restore the frame allocator, then resume the wrapped
284   awaitable. 294   awaitable.
285   295  
286   Reinstalls the thread-local frame allocator from the stored 296   Reinstalls the thread-local frame allocator from the stored
287   environment before the body continues. This is needed 297   environment before the body continues. This is needed
288   because the resumption may arrive on a different thread 298   because the resumption may arrive on a different thread
289   than the one that suspended. 299   than the one that suspended.
290   300  
291   @return The wrapped awaitable's await-result, forwarded 301   @return The wrapped awaitable's await-result, forwarded
292   unchanged. 302   unchanged.
293   */ 303   */
HITCBC 294   5611 decltype(auto) await_resume() 304   5564 decltype(auto) await_resume()
295   { 305   {
296   // Restore TLS before body resumes 306   // Restore TLS before body resumes
HITCBC 297   5611 set_current_frame_allocator(p_->environment()->frame_allocator); 307   5564 set_current_frame_allocator(p_->environment()->frame_allocator);
HITCBC 298   5611 return a_.await_resume(); 308   5564 return a_.await_resume();
299   } 309   }
300   310  
301   /** Suspend by calling the wrapped awaitable with the 311   /** Suspend by calling the wrapped awaitable with the
302   environment. 312   environment.
303   313  
304   This is the plain `await_suspend` the compiler calls for the 314   This is the plain `await_suspend` the compiler calls for the
305   nested `co_await`. It forwards to the wrapped awaitable's 315   nested `co_await`. It forwards to the wrapped awaitable's
306   @ref ContextAwaitable overload, supplying the promise's stored 316   @ref ContextAwaitable overload, supplying the promise's stored
307   environment as the second argument. It then hands back 317   environment as the second argument. It then hands back
308   that call's result unchanged, so the wrapped awaitable's 318   that call's result unchanged, so the wrapped awaitable's
309   suspension decision, whatever form it takes, is preserved. 319   suspension decision, whatever form it takes, is preserved.
310   320  
311   @param h The coroutine performing the `co_await`. 321   @param h The coroutine performing the `co_await`.
312   322  
313   @return Whatever the wrapped awaitable's `await_suspend` 323   @return Whatever the wrapped awaitable's `await_suspend`
314   returns. When that is a `std::coroutine_handle<>`, the 324   returns. When that is a `std::coroutine_handle<>`, the
315   handle is routed through `detail::symmetric_transfer`. 325   handle is routed through `detail::symmetric_transfer`.
316 - On MSVC that helper resumes the handle on the current 326 + Where `BOOST_CAPY_SYMMETRIC_TRANSFER_WORKAROUND` is on,
317 - stack, and this function returns `void`, so the awaiting 327 + this function instead returns `bool` from
318 - coroutine suspends unconditionally. On every other 328 + `detail::await_ready_then_suspend`. It first asks the
319 - compiler the handle is returned unchanged for symmetric 329 + wrapped awaitable's `await_ready`. When that reports
320 - transfer. 330 + ready, it returns `false` to continue without
  331 + suspending. Otherwise it resumes any returned handle on
  332 + the current stack and returns `true`. Where the
  333 + workaround is off, a returned handle is passed back
  334 + unchanged for symmetric transfer.
321   */ 335   */
322   template<class Promise> 336   template<class Promise>
HITCBC 323   107603 auto await_suspend(std::coroutine_handle<Promise> h) noexcept 337   107666 auto await_suspend(std::coroutine_handle<Promise> h) noexcept
324   { 338   {
  339 + #if BOOST_CAPY_SYMMETRIC_TRANSFER_WORKAROUND
  340 + return detail::await_ready_then_suspend(a_, h, p_->environment());
  341 + #else
325   using R = decltype(a_.await_suspend(h, p_->environment())); 342   using R = decltype(a_.await_suspend(h, p_->environment()));
326   if constexpr (std::is_same_v<R, std::coroutine_handle<>>) 343   if constexpr (std::is_same_v<R, std::coroutine_handle<>>)
HITCBC 327   106562 return detail::symmetric_transfer(a_.await_suspend(h, p_->environment())); 344   106624 return detail::symmetric_transfer(a_.await_suspend(h, p_->environment()));
328   else 345   else
HITCBC 329   1041 return a_.await_suspend(h, p_->environment()); 346   1042 return a_.await_suspend(h, p_->environment());
  347 + #endif
330   } 348   }
331   }; 349   };
332   350  
333   /** Transform a nested awaitable before `co_await`. 351   /** Transform a nested awaitable before `co_await`.
334   352  
335   Wraps an @ref ContextAwaitable in a @ref transform_awaiter so the 353   Wraps an @ref ContextAwaitable in a @ref transform_awaiter so the
336   coroutine's environment is propagated into it. A diagnostic 354   coroutine's environment is propagated into it. A diagnostic
337   is emitted if the awaitable does not satisfy @ref ContextAwaitable. 355   is emitted if the awaitable does not satisfy @ref ContextAwaitable.
338   356  
339   @param a The awaitable expression from `co_await a`. 357   @param a The awaitable expression from `co_await a`.
340   358  
341   @return A @ref transform_awaiter wrapping `a`. 359   @return A @ref transform_awaiter wrapping `a`.
342   */ 360   */
343   template<class Awaitable> 361   template<class Awaitable>
HITCBC 344   108242 auto transform_awaitable(Awaitable&& a) 362   108222 auto transform_awaitable(Awaitable&& a)
345   { 363   {
346   using A = std::decay_t<Awaitable>; 364   using A = std::decay_t<Awaitable>;
347   if constexpr (ContextAwaitable<A>) 365   if constexpr (ContextAwaitable<A>)
348   { 366   {
349   return transform_awaiter<Awaitable>{ 367   return transform_awaiter<Awaitable>{
HITCBC 350   213961 std::forward<Awaitable>(a), this}; 368   213919 std::forward<Awaitable>(a), this};
351   } 369   }
352   else 370   else
353   { 371   {
354   static_assert(ContextAwaitable<A>, "requires ContextAwaitable"); 372   static_assert(ContextAwaitable<A>, "requires ContextAwaitable");
355   } 373   }
HITCBC 356   105719 } 374   105697 }
357   }; 375   };
358   376  
359   /** Handle to the owned coroutine frame. 377   /** Handle to the owned coroutine frame.
360   378  
361   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
362   @ref release). Prefer @ref handle to read this; the member is 380   @ref release). Prefer @ref handle to read this; the member is
363   public for use by the coroutine machinery. 381   public for use by the coroutine machinery.
364   */ 382   */
365   std::coroutine_handle<promise_type> h_; 383   std::coroutine_handle<promise_type> h_;
366   384  
367   /** Destroy the task and its coroutine frame if owned. 385   /** Destroy the task and its coroutine frame if owned.
368   386  
369   @par Preconditions 387   @par Preconditions
370   The frame is not lent: no parent is suspended awaiting it, 388   The frame is not lent: no parent is suspended awaiting it,
371   unless its chain is being unwound after an @ref abandon. 389   unless its chain is being unwound after an @ref abandon.
372   Destroying a lent frame otherwise calls `std::terminate`; end 390   Destroying a lent frame otherwise calls `std::terminate`; end
373   a running chain by requesting stop, or by destroying its 391   a running chain by requesting stop, or by destroying its
374   execution context. 392   execution context.
375   */ 393   */
HITCBC 376   113340 ~task() 394   113320 ~task()
377   { 395   {
HITCBC 378   113340 destroy_frame(); 396   113320 destroy_frame();
HITCBC 379   113340 } 397   113320 }
380   398  
381   /** Report whether the awaited task is already complete. 399   /** Report whether the awaited task is already complete.
382   400  
383   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
384   it is awaited, so the awaiting coroutine always suspends. 402   it is awaited, so the awaiting coroutine always suspends.
385   403  
386   @return `false`. 404   @return `false`.
387   */ 405   */
HITCBC 388   2117 bool await_ready() const noexcept 406   2117 bool await_ready() const noexcept
389   { 407   {
HITCBC 390   2117 return false; 408   2117 return false;
391   } 409   }
392   410  
393   /** Return the task's result, rethrowing any captured exception. 411   /** Return the task's result, rethrowing any captured exception.
394   412  
395   If the coroutine body exited via an unhandled exception, that 413   If the coroutine body exited via an unhandled exception, that
396   exception is rethrown here. Otherwise the result is returned by 414   exception is rethrown here. Otherwise the result is returned by
397   move (for `task<T>`) or nothing is returned (for `task<void>`). 415   move (for `task<T>`) or nothing is returned (for `task<void>`).
398   416  
399   @return The result value for non-void `T`; otherwise `void`. 417   @return The result value for non-void `T`; otherwise `void`.
400   418  
401   @throws The exception captured by the coroutine body, if any. 419   @throws The exception captured by the coroutine body, if any.
402   420  
403   @note Discarding a `results` silently drops the error 421   @note Discarding a `results` silently drops the error
404   code, so that overload is marked `[[nodiscard]]`. 422   code, so that overload is marked `[[nodiscard]]`.
405   */ 423   */
HITCBC 406   570 [[nodiscard]] auto await_resume() 424   570 [[nodiscard]] auto await_resume()
407   requires detail::is_results_v<T> 425   requires detail::is_results_v<T>
408   { 426   {
HITCBC 409   570 lent_ = false; 427   570 lent_ = false;
HITCBC 410   570 if(h_.promise().has_ep_) 428   570 if(h_.promise().has_ep_)
HITCBC 411   105 std::rethrow_exception(h_.promise().ep_); 429   105 std::rethrow_exception(h_.promise().ep_);
HITCBC 412   465 return std::move(*h_.promise().result_); 430   465 return std::move(*h_.promise().result_);
413   } 431   }
414   432  
HITCBC 415   247 auto await_resume() 433   247 auto await_resume()
416   requires (! detail::is_results_v<T>) 434   requires (! detail::is_results_v<T>)
417   { 435   {
HITCBC 418   247 lent_ = false; 436   247 lent_ = false;
HITCBC 419   247 if(h_.promise().has_ep_) 437   247 if(h_.promise().has_ep_)
HITCBC 420   18 std::rethrow_exception(h_.promise().ep_); 438   18 std::rethrow_exception(h_.promise().ep_);
421   if constexpr (! std::is_void_v<T>) 439   if constexpr (! std::is_void_v<T>)
HITCBC 422   154 return std::move(*h_.promise().result_); 440   154 return std::move(*h_.promise().result_);
423   else 441   else
HITCBC 424   75 return; 442   75 return;
425   } 443   }
426   444  
427   /** Start the task with the awaiting coroutine's context. 445   /** Start the task with the awaiting coroutine's context.
428   446  
429   Stores `cont` as the continuation to resume on completion. 447   Stores `cont` as the continuation to resume on completion.
430   Stores `env` as the execution environment propagated to nested 448   Stores `env` as the execution environment propagated to nested
431   `co_await` expressions. Then transfers control into the task's 449   `co_await` expressions. Then transfers control into the task's
432   coroutine body via the returned handle. 450   coroutine body via the returned handle.
433   451  
434   @param cont The awaiting coroutine to resume when the task 452   @param cont The awaiting coroutine to resume when the task
435   completes. 453   completes.
436   454  
437   @param env The execution environment (executor, stop token, and 455   @param env The execution environment (executor, stop token, and
438   frame allocator). It must outlive the task. 456   frame allocator). It must outlive the task.
439   457  
440   @return The task's coroutine handle, for symmetric transfer. 458   @return The task's coroutine handle, for symmetric transfer.
441   */ 459   */
HITCBC 442   2014 std::coroutine_handle<> await_suspend(std::coroutine_handle<> cont, await_context const* env) 460   2014 std::coroutine_handle<> await_suspend(std::coroutine_handle<> cont, await_context const* env)
443   { 461   {
HITCBC 444   2014 h_.promise().set_continuation(cont); 462   2014 h_.promise().set_continuation(cont);
HITCBC 445   2014 h_.promise().set_environment(env); 463   2014 h_.promise().set_environment(env);
HITCBC 446   2014 lent_ = true; 464   2014 lent_ = true;
HITCBC 447   2014 return h_; 465   2014 return h_;
448   } 466   }
449   467  
450   /** Return the coroutine handle. 468   /** Return the coroutine handle.
451   469  
452   @note Do not call `destroy()` on the returned handle while the 470   @note Do not call `destroy()` on the returned handle while the
453   task is being awaited: the awaiting frame still holds it and 471   task is being awaited: the awaiting frame still holds it and
454   destroys it again, which has undefined behavior. A holder that 472   destroys it again, which has undefined behavior. A holder that
455   gives the coroutine up passes its continuation to @ref abandon 473   gives the coroutine up passes its continuation to @ref abandon
456   instead. (Destroying or 474   instead. (Destroying or
457   move-assigning over the task object itself while its frame is 475   move-assigning over the task object itself while its frame is
458   lent is a different operation, and calls `std::terminate`.) 476   lent is a different operation, and calls `std::terminate`.)
459   The task's lifetime is normally managed by `run_async`, `run`, 477   The task's lifetime is normally managed by `run_async`, `run`,
460   or the awaiting parent. For cooperative cancellation, use 478   or the awaiting parent. For cooperative cancellation, use
461   `std::stop_token`. 479   `std::stop_token`.
462   480  
463   @note The operation, queue or primitive holding a suspended 481   @note The operation, queue or primitive holding a suspended
464   coroutine gives it up with @ref abandon, through the 482   coroutine gives it up with @ref abandon, through the
465   continuation it keeps, never by destroying a handle. 483   continuation it keeps, never by destroying a handle.
466   `handle()` belongs to the owning task object. 484   `handle()` belongs to the owning task object.
467   485  
468   @return The coroutine handle. 486   @return The coroutine handle.
469   */ 487   */
HITCBC 470   107267 std::coroutine_handle<promise_type> handle() const noexcept 488   107247 std::coroutine_handle<promise_type> handle() const noexcept
471   { 489   {
HITCBC 472   107267 return h_; 490   107247 return h_;
473   } 491   }
474   492  
475   /** Release ownership of the coroutine frame. 493   /** Release ownership of the coroutine frame.
476   494  
477   After calling this, destroying the task does not destroy the 495   After calling this, destroying the task does not destroy the
478   coroutine frame. The caller becomes responsible for the frame's 496   coroutine frame. The caller becomes responsible for the frame's
479   lifetime. 497   lifetime.
480   498  
481   @note The caller may call `destroy()` on the released handle 499   @note The caller may call `destroy()` on the released handle
482   only when the task has not started or has fully completed. 500   only when the task has not started or has fully completed.
483   Destroying the released handle of a task that is being 501   Destroying the released handle of a task that is being
484   awaited is not supported and has undefined behavior. 502   awaited is not supported and has undefined behavior.
485   503  
486   @par Postconditions 504   @par Postconditions
487   `handle()` returns a null handle. Callers needing the 505   `handle()` returns a null handle. Callers needing the
488   original handle must save it, via @ref handle, before 506   original handle must save it, via @ref handle, before
489   calling this. 507   calling this.
490   */ 508   */
HITCBC 491   107124 void release() noexcept 509   107104 void release() noexcept
492   { 510   {
HITCBC 493   107124 h_ = nullptr; 511   107104 h_ = nullptr;
HITCBC 494   107124 } 512   107104 }
495   513  
496   /** Copy construction is disabled; a task uniquely owns its frame. 514   /** Copy construction is disabled; a task uniquely owns its frame.
497   515  
498   @param other The task that would be copied. 516   @param other The task that would be copied.
499   */ 517   */
500   task(task const& other) = delete; 518   task(task const& other) = delete;
501   519  
502   /** Copy assignment is disabled; a task uniquely owns its frame. 520   /** Copy assignment is disabled; a task uniquely owns its frame.
503   521  
504   @param other The task that would be assigned from. 522   @param other The task that would be assigned from.
505   523  
506   @return A reference to `*this`. 524   @return A reference to `*this`.
507   */ 525   */
508   task& operator=(task const& other) = delete; 526   task& operator=(task const& other) = delete;
509   527  
510   /** Construct by moving, transferring ownership of the frame. 528   /** Construct by moving, transferring ownership of the frame.
511   529  
512   @par Postconditions 530   @par Postconditions
513   `other` is empty and must not be awaited. 531   `other` is empty and must not be awaited.
514   532  
515   @param other The task to move from. 533   @param other The task to move from.
516   */ 534   */
HITCBC 517   4088 task(task&& other) noexcept 535   4088 task(task&& other) noexcept
HITCBC 518   4088 : h_(std::exchange(other.h_, nullptr)) 536   4088 : h_(std::exchange(other.h_, nullptr))
519   { 537   {
HITCBC 520   4088 } 538   4088 }
521   539  
522   /** Assign by moving, transferring ownership of the frame. 540   /** Assign by moving, transferring ownership of the frame.
523   541  
524   If this task already owns a coroutine frame, that frame is 542   If this task already owns a coroutine frame, that frame is
525   destroyed first. Self-assignment is a no-op. 543   destroyed first. Self-assignment is a no-op.
526   544  
527   @par Preconditions 545   @par Preconditions
528   The frame being replaced is not lent: no parent is suspended 546   The frame being replaced is not lent: no parent is suspended
529   awaiting it. Replacing a lent frame calls `std::terminate`. 547   awaiting it. Replacing a lent frame calls `std::terminate`.
530   548  
531   @par Postconditions 549   @par Postconditions
532   `other` is empty and must not be awaited. 550   `other` is empty and must not be awaited.
533   551  
534   @param other The task to move from. 552   @param other The task to move from.
535   553  
536   @return A reference to `*this`. 554   @return A reference to `*this`.
537   */ 555   */
HITCBC 538   1 task& operator=(task&& other) noexcept 556   1 task& operator=(task&& other) noexcept
539   { 557   {
HITCBC 540   1 if(this != &other) 558   1 if(this != &other)
541   { 559   {
HITCBC 542   1 destroy_frame(); 560   1 destroy_frame();
HITCBC 543   1 h_ = std::exchange(other.h_, nullptr); 561   1 h_ = std::exchange(other.h_, nullptr);
544   } 562   }
HITCBC 545   1 return *this; 563   1 return *this;
546   } 564   }
547   565  
548   private: 566   private:
549   // Set while a parent is suspended awaiting this task. 567   // Set while a parent is suspended awaiting this task.
550   bool lent_ = false; 568   bool lent_ = false;
551   569  
HITCBC 552   109252 explicit task(std::coroutine_handle<promise_type> h) 570   109232 explicit task(std::coroutine_handle<promise_type> h)
HITCBC 553   109252 : h_(h) 571   109232 : h_(h)
554   { 572   {
HITCBC 555   109252 } 573   109232 }
556   574  
HITCBC 557   113341 void destroy_frame() noexcept 575   113321 void destroy_frame() noexcept
558   { 576   {
HITCBC 559   113341 if(!h_) 577   113321 if(!h_)
HITCBC 560   111213 return; 578   111193 return;
HITCBC 561   2128 if(lent_ && !h_.done()) 579   2128 if(lent_ && !h_.done())
562   { 580   {
563   // Only the unwind of an abandoned chain destroys a frame 581   // Only the unwind of an abandoned chain destroys a frame
564   // that is still awaited. 582   // that is still awaited.
HITCBC 565   1278 auto* owner = h_.promise().environment()->owner; 583   1278 auto* owner = h_.promise().environment()->owner;
HITCBC 566   1278 if(!owner || !owner->abandoned()) 584   1278 if(!owner || !owner->abandoned())
567   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
568   } 586   }
HITCBC 569   2128 std::exchange(h_, nullptr).destroy(); 587   2128 std::exchange(h_, nullptr).destroy();
HITCBC 570   2128 lent_ = false; 588   2128 lent_ = false;
571   } 589   }
572   }; 590   };
573   591  
574   } // namespace capy 592   } // namespace capy
575   } // namespace boost 593   } // namespace boost
576   594  
577   #endif 595   #endif