. /include/boost/capy /task.hpp
100.00% Lines (95/95) 100.00% Functions (30/30)
[ ] prev/next file n p prev/next uncovered line
TLA Baseline Branch
Line Hits Code Line Hits Code
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   889 void return_value(T value) 40   889 void return_value(T value)
41   { 41   {
HITCBC 42   889 result_ = std::move(value); 42   889 result_ = std::move(value);
HITCBC 43   889 } 43   889 }
44   44  
HITCBC 45   279 T&& result() noexcept 45   279 T&& result() noexcept
46   { 46   {
HITCBC 47   279 return std::move(*result_); 47   279 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   3648 void return_void() 54   3634 void return_void()
55   { 55   {
HITCBC 56   3648 } 56   3634 }
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 ContextRunnable, ContextAwaitable, run, run_async 114   @see ContextRunnable, ContextAwaitable, 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 `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 context_awaitable_promise_base, ContextRunnable 131   @see context_awaitable_promise_base, 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   112692 promise_type() noexcept 144   112700 promise_type() noexcept
HITCBC 145   112692 : has_ep_(false) 145   112700 : has_ep_(false)
146   { 146   {
HITCBC 147   112692 } 147   112700 }
148   148  
149   /// Destroy the promise, releasing any stored exception. 149   /// Destroy the promise, releasing any stored exception.
HITCBC 150   112692 ~promise_type() 150   112700 ~promise_type()
151   { 151   {
HITCBC 152   112692 if(has_ep_) 152   112700 if(has_ep_)
HITCBC 153   490 ep_.~exception_ptr(); 153   490 ep_.~exception_ptr();
HITCBC 154   112692 } 154   112700 }
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   4526 std::exception_ptr exception() const noexcept 161   4512 std::exception_ptr exception() const noexcept
162   { 162   {
HITCBC 163   4526 if(has_ep_) 163   4512 if(has_ep_)
HITCBC 164   732 return ep_; 164   732 return ep_;
HITCBC 165   3794 return {}; 165   3780 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   112692 task get_return_object() 175   112700 task get_return_object()
176   { 176   {
HITCBC 177   112692 return task{std::coroutine_handle<promise_type>::from_promise(*this)}; 177   112700 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   112692 auto initial_suspend() noexcept 189   112700 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   112692 bool await_ready() const noexcept 195   112700 bool await_ready() const noexcept
196   { 196   {
HITCBC 197   112692 return false; 197   112700 return false;
198   } 198   }
199   199  
HITCBC 200   112692 void await_suspend(std::coroutine_handle<>) const noexcept 200   112700 void await_suspend(std::coroutine_handle<>) const noexcept
201   { 201   {
HITCBC 202   112692 } 202   112700 }
203   203  
HITCBC 204   107643 void await_resume() const noexcept 204   107651 void await_resume() const noexcept
205   { 205   {
206   // Restore TLS when body starts executing 206   // Restore TLS when body starts executing
HITCBC 207   107643 set_current_frame_allocator(p_->environment()->frame_allocator); 207   107651 set_current_frame_allocator(p_->environment()->frame_allocator);
HITCBC 208   107643 } 208   107651 }
209   }; 209   };
HITCBC 210   112692 return awaiter{this}; 210   112700 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   5027 auto final_suspend() noexcept 223   5013 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   5027 bool await_ready() const noexcept 229   5013 bool await_ready() const noexcept
230   { 230   {
HITCBC 231   5027 return false; 231   5013 return false;
232   } 232   }
233   233  
HITCBC 234   5027 std::coroutine_handle<> await_suspend(std::coroutine_handle<>) const noexcept 234   5013 std::coroutine_handle<> await_suspend(std::coroutine_handle<>) const noexcept
235   { 235   {
HITCBC 236   5027 return p_->continuation(); 236   5013 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   5027 return awaiter{this}; 241   5013 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   @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   107714 bool await_ready() noexcept 278   107722 bool await_ready() noexcept
279   { 279   {
HITCBC 280   107714 return a_.await_ready(); 280   107722 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   5098 decltype(auto) await_resume() 294   5084 decltype(auto) await_resume()
295   { 295   {
296   // Restore TLS before body resumes 296   // Restore TLS before body resumes
HITCBC 297   5098 set_current_frame_allocator(p_->environment()->frame_allocator); 297   5084 set_current_frame_allocator(p_->environment()->frame_allocator);
HITCBC 298   5098 return a_.await_resume(); 298   5084 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 ContextAwaitable overload, supplying the promise's stored 306   @ref ContextAwaitable 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   107085 auto await_suspend(std::coroutine_handle<Promise> h) noexcept 323   107102 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   106054 return detail::symmetric_transfer(a_.await_suspend(h, p_->environment())); 327   106071 return detail::symmetric_transfer(a_.await_suspend(h, p_->environment()));
328   else 328   else
HITCBC 329   1031 return a_.await_suspend(h, p_->environment()); 329   1031 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 ContextAwaitable in a @ref transform_awaiter so the 335   Wraps an @ref ContextAwaitable 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 ContextAwaitable. 337   is emitted if the awaitable does not satisfy @ref ContextAwaitable.
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   107714 auto transform_awaitable(Awaitable&& a) 344   107722 auto transform_awaitable(Awaitable&& a)
345   { 345   {
346   using A = std::decay_t<Awaitable>; 346   using A = std::decay_t<Awaitable>;
347   if constexpr (ContextAwaitable<A>) 347   if constexpr (ContextAwaitable<A>)
348   { 348   {
349   return transform_awaiter<Awaitable>{ 349   return transform_awaiter<Awaitable>{
HITCBC 350   212917 std::forward<Awaitable>(a), this}; 350   212933 std::forward<Awaitable>(a), this};
351   } 351   }
352   else 352   else
353   { 353   {
354   static_assert(ContextAwaitable<A>, "requires ContextAwaitable"); 354   static_assert(ContextAwaitable<A>, "requires ContextAwaitable");
355   } 355   }
HITCBC 356   105203 } 356   105211 }
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.
368   368  
369   @par Preconditions 369   @par Preconditions
370   The frame is not lent: no parent is suspended awaiting it, 370   The frame is not lent: no parent is suspended awaiting it,
371   unless its chain is being unwound after an @ref abandon. 371   unless its chain is being unwound after an @ref abandon.
372   Destroying a lent frame otherwise calls `std::terminate`; end 372   Destroying a lent frame otherwise calls `std::terminate`; end
373   a running chain by requesting stop, or by destroying its 373   a running chain by requesting stop, or by destroying its
374   execution context. 374   execution context.
375   */ 375   */
HITCBC 376   116762 ~task() 376   116770 ~task()
377   { 377   {
HITCBC 378   116762 destroy_frame(); 378   116770 destroy_frame();
HITCBC 379   116762 } 379   116770 }
380   380  
381   /** Report whether the awaited task is already complete. 381   /** Report whether the awaited task is already complete.
382   382  
383   Always returns `false`; a task is lazy and has not started when 383   Always returns `false`; a task is lazy and has not started when
384   it is awaited, so the awaiting coroutine always suspends. 384   it is awaited, so the awaiting coroutine always suspends.
385   385  
386   @return `false`. 386   @return `false`.
387   */ 387   */
HITCBC 388   2104 bool await_ready() const noexcept 388   2104 bool await_ready() const noexcept
389   { 389   {
HITCBC 390   2104 return false; 390   2104 return false;
391   } 391   }
392   392  
393   /** Return the task's result, rethrowing any captured exception. 393   /** Return the task's result, rethrowing any captured exception.
394   394  
395   If the coroutine body exited via an unhandled exception, that 395   If the coroutine body exited via an unhandled exception, that
396   exception is rethrown here. Otherwise the result is returned by 396   exception is rethrown here. Otherwise the result is returned by
397   move (for `task<T>`) or nothing is returned (for `task<void>`). 397   move (for `task<T>`) or nothing is returned (for `task<void>`).
398   398  
399   @return The result value for non-void `T`; otherwise `void`. 399   @return The result value for non-void `T`; otherwise `void`.
400   400  
401   @throws The exception captured by the coroutine body, if any. 401   @throws The exception captured by the coroutine body, if any.
402   402  
403   @note Discarding a `results` silently drops the error 403   @note Discarding a `results` silently drops the error
404   code, so that overload is marked `[[nodiscard]]`. 404   code, so that overload is marked `[[nodiscard]]`.
405   */ 405   */
HITCBC 406   562 [[nodiscard]] auto await_resume() 406   562 [[nodiscard]] auto await_resume()
407   requires detail::is_results_v<T> 407   requires detail::is_results_v<T>
408   { 408   {
HITCBC 409   562 lent_ = false; 409   562 lent_ = false;
HITCBC 410   562 if(h_.promise().has_ep_) 410   562 if(h_.promise().has_ep_)
HITCBC 411   105 std::rethrow_exception(h_.promise().ep_); 411   105 std::rethrow_exception(h_.promise().ep_);
HITCBC 412   457 return std::move(*h_.promise().result_); 412   457 return std::move(*h_.promise().result_);
413   } 413   }
414   414  
HITCBC 415   242 auto await_resume() 415   242 auto await_resume()
416   requires (! detail::is_results_v<T>) 416   requires (! detail::is_results_v<T>)
417   { 417   {
HITCBC 418   242 lent_ = false; 418   242 lent_ = false;
HITCBC 419   242 if(h_.promise().has_ep_) 419   242 if(h_.promise().has_ep_)
HITCBC 420   18 std::rethrow_exception(h_.promise().ep_); 420   18 std::rethrow_exception(h_.promise().ep_);
421   if constexpr (! std::is_void_v<T>) 421   if constexpr (! std::is_void_v<T>)
HITCBC 422   151 return std::move(*h_.promise().result_); 422   151 return std::move(*h_.promise().result_);
423   else 423   else
HITCBC 424   73 return; 424   73 return;
425   } 425   }
426   426  
427   /** Start the task with the awaiting coroutine's context. 427   /** Start the task with the awaiting coroutine's context.
428   428  
429   Stores `cont` as the continuation to resume on completion. 429   Stores `cont` as the continuation to resume on completion.
430   Stores `env` as the execution environment propagated to nested 430   Stores `env` as the execution environment propagated to nested
431   `co_await` expressions. Then transfers control into the task's 431   `co_await` expressions. Then transfers control into the task's
432   coroutine body via the returned handle. 432   coroutine body via the returned handle.
433   433  
434   @param cont The awaiting coroutine to resume when the task 434   @param cont The awaiting coroutine to resume when the task
435   completes. 435   completes.
436   436  
437   @param env The execution environment (executor, stop token, and 437   @param env The execution environment (executor, stop token, and
438   frame allocator). It must outlive the task. 438   frame allocator). It must outlive the task.
439   439  
440   @return The task's coroutine handle, for symmetric transfer. 440   @return The task's coroutine handle, for symmetric transfer.
441   */ 441   */
HITCBC 442   2001 std::coroutine_handle<> await_suspend(std::coroutine_handle<> cont, await_context const* env) 442   2001 std::coroutine_handle<> await_suspend(std::coroutine_handle<> cont, await_context const* env)
443   { 443   {
HITCBC 444   2001 h_.promise().set_continuation(cont); 444   2001 h_.promise().set_continuation(cont);
HITCBC 445   2001 h_.promise().set_environment(env); 445   2001 h_.promise().set_environment(env);
HITCBC 446   2001 lent_ = true; 446   2001 lent_ = true;
HITCBC 447   2001 return h_; 447   2001 return h_;
448   } 448   }
449   449  
450   /** Return the coroutine handle. 450   /** Return the coroutine handle.
451   451  
452   @note Do not call `destroy()` on the returned handle while the 452   @note Do not call `destroy()` on the returned handle while the
453   task is being awaited: the awaiting frame still holds it and 453   task is being awaited: the awaiting frame still holds it and
454   destroys it again, which has undefined behavior. A holder that 454   destroys it again, which has undefined behavior. A holder that
455   gives the coroutine up passes its continuation to @ref abandon 455   gives the coroutine up passes its continuation to @ref abandon
456   instead. (Destroying or 456   instead. (Destroying or
457   move-assigning over the task object itself while its frame is 457   move-assigning over the task object itself while its frame is
458   lent is a different operation, and calls `std::terminate`.) 458   lent is a different operation, and calls `std::terminate`.)
459   The task's lifetime is normally managed by `run_async`, `run`, 459   The task's lifetime is normally managed by `run_async`, `run`,
460   or the awaiting parent. For cooperative cancellation, use 460   or the awaiting parent. For cooperative cancellation, use
461   `std::stop_token`. 461   `std::stop_token`.
462   462  
463   @note The operation, queue or primitive holding a suspended 463   @note The operation, queue or primitive holding a suspended
464   coroutine gives it up with @ref abandon, through the 464   coroutine gives it up with @ref abandon, through the
465   continuation it keeps, never by destroying a handle. 465   continuation it keeps, never by destroying a handle.
466   `handle()` belongs to the owning task object. 466   `handle()` belongs to the owning task object.
467   467  
468   @return The coroutine handle. 468   @return The coroutine handle.
469   */ 469   */
HITCBC 470   110720 std::coroutine_handle<promise_type> handle() const noexcept 470   110728 std::coroutine_handle<promise_type> handle() const noexcept
471   { 471   {
HITCBC 472   110720 return h_; 472   110728 return h_;
473   } 473   }
474   474  
475   /** Release ownership of the coroutine frame. 475   /** Release ownership of the coroutine frame.
476   476  
477   After calling this, destroying the task does not destroy the 477   After calling this, destroying the task does not destroy the
478   coroutine frame. The caller becomes responsible for the frame's 478   coroutine frame. The caller becomes responsible for the frame's
479   lifetime. 479   lifetime.
480   480  
481   @note The caller may call `destroy()` on the released handle 481   @note The caller may call `destroy()` on the released handle
482   only when the task has not started or has fully completed. 482   only when the task has not started or has fully completed.
483   Destroying the released handle of a task that is being 483   Destroying the released handle of a task that is being
484   awaited is not supported and has undefined behavior. 484   awaited is not supported and has undefined behavior.
485   485  
486   @par Postconditions 486   @par Postconditions
487   `handle()` returns a null handle. Callers needing the 487   `handle()` returns a null handle. Callers needing the
488   original handle must save it, via @ref handle, before 488   original handle must save it, via @ref handle, before
489   calling this. 489   calling this.
490   */ 490   */
HITCBC 491   110577 void release() noexcept 491   110585 void release() noexcept
492   { 492   {
HITCBC 493   110577 h_ = nullptr; 493   110585 h_ = nullptr;
HITCBC 494   110577 } 494   110585 }
495   495  
496   /** Copy construction is disabled; a task uniquely owns its frame. 496   /** Copy construction is disabled; a task uniquely owns its frame.
497   497  
498   @param other The task that would be copied. 498   @param other The task that would be copied.
499   */ 499   */
500   task(task const& other) = delete; 500   task(task const& other) = delete;
501   501  
502   /** Copy assignment is disabled; a task uniquely owns its frame. 502   /** Copy assignment is disabled; a task uniquely owns its frame.
503   503  
504   @param other The task that would be assigned from. 504   @param other The task that would be assigned from.
505   505  
506   @return A reference to `*this`. 506   @return A reference to `*this`.
507   */ 507   */
508   task& operator=(task const& other) = delete; 508   task& operator=(task const& other) = delete;
509   509  
510   /** Construct by moving, transferring ownership of the frame. 510   /** Construct by moving, transferring ownership of the frame.
511   511  
512   @par Postconditions 512   @par Postconditions
513   `other` is empty and must not be awaited. 513   `other` is empty and must not be awaited.
514   514  
515   @param other The task to move from. 515   @param other The task to move from.
516   */ 516   */
HITCBC 517   4070 task(task&& other) noexcept 517   4070 task(task&& other) noexcept
HITCBC 518   4070 : h_(std::exchange(other.h_, nullptr)) 518   4070 : h_(std::exchange(other.h_, nullptr))
519   { 519   {
HITCBC 520   4070 } 520   4070 }
521   521  
522   /** Assign by moving, transferring ownership of the frame. 522   /** Assign by moving, transferring ownership of the frame.
523   523  
524   If this task already owns a coroutine frame, that frame is 524   If this task already owns a coroutine frame, that frame is
525   destroyed first. Self-assignment is a no-op. 525   destroyed first. Self-assignment is a no-op.
526   526  
527   @par Preconditions 527   @par Preconditions
528   The frame being replaced is not lent: no parent is suspended 528   The frame being replaced is not lent: no parent is suspended
529   awaiting it. Replacing a lent frame calls `std::terminate`. 529   awaiting it. Replacing a lent frame calls `std::terminate`.
530   530  
531   @par Postconditions 531   @par Postconditions
532   `other` is empty and must not be awaited. 532   `other` is empty and must not be awaited.
533   533  
534   @param other The task to move from. 534   @param other The task to move from.
535   535  
536   @return A reference to `*this`. 536   @return A reference to `*this`.
537   */ 537   */
HITCBC 538   1 task& operator=(task&& other) noexcept 538   1 task& operator=(task&& other) noexcept
539   { 539   {
HITCBC 540   1 if(this != &other) 540   1 if(this != &other)
541   { 541   {
HITCBC 542   1 destroy_frame(); 542   1 destroy_frame();
HITCBC 543   1 h_ = std::exchange(other.h_, nullptr); 543   1 h_ = std::exchange(other.h_, nullptr);
544   } 544   }
HITCBC 545   1 return *this; 545   1 return *this;
546   } 546   }
547   547  
548   private: 548   private:
549   // Set while a parent is suspended awaiting this task. 549   // Set while a parent is suspended awaiting this task.
550   bool lent_ = false; 550   bool lent_ = false;
551   551  
HITCBC 552   112692 explicit task(std::coroutine_handle<promise_type> h) 552   112700 explicit task(std::coroutine_handle<promise_type> h)
HITCBC 553   112692 : h_(h) 553   112700 : h_(h)
554   { 554   {
HITCBC 555   112692 } 555   112700 }
556   556  
HITCBC 557   116763 void destroy_frame() noexcept 557   116771 void destroy_frame() noexcept
558   { 558   {
HITCBC 559   116763 if(!h_) 559   116771 if(!h_)
HITCBC 560   114648 return; 560   114656 return;
HITCBC 561   2115 if(lent_ && !h_.done()) 561   2115 if(lent_ && !h_.done())
562   { 562   {
563   // Only the unwind of an abandoned chain destroys a frame 563   // Only the unwind of an abandoned chain destroys a frame
564   // that is still awaited. 564   // that is still awaited.
HITCBC 565   1278 auto* owner = h_.promise().environment()->owner; 565   1278 auto* owner = h_.promise().environment()->owner;
HITCBC 566   1278 if(!owner || !owner->abandoned()) 566   1278 if(!owner || !owner->abandoned())
567   std::terminate(); // LCOV_EXCL_LINE reached only in a death test's child process 567   std::terminate(); // LCOV_EXCL_LINE reached only in a death test's child process
568   } 568   }
HITCBC 569   2115 std::exchange(h_, nullptr).destroy(); 569   2115 std::exchange(h_, nullptr).destroy();
HITCBC 570   2115 lent_ = false; 570   2115 lent_ = false;
571   } 571   }
572   }; 572   };
573   573  
574   } // namespace capy 574   } // namespace capy
575   } // namespace boost 575   } // namespace boost
576   576  
577   #endif 577   #endif