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libs/capy/include/boost/capy/task.hpp

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libs/capy/include/boost/capy/task.hpp
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1 //
2 // Copyright (c) 2025 Vinnie Falco (vinnie dot falco at gmail dot com)
3 //
4 // Distributed under the Boost Software License, Version 1.0. (See accompanying
5 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
6 //
7 // Official repository: https://github.com/cppalliance/corosio
8 //
9
10 #ifndef BOOST_CAPY_TASK_HPP
11 #define BOOST_CAPY_TASK_HPP
12
13 #include <boost/capy/detail/config.hpp>
14 #include <boost/capy/concept/executor.hpp>
15 #include <boost/capy/concept/io_awaitable.hpp>
16 #include <boost/capy/ex/io_awaitable_support.hpp>
17 #include <boost/capy/ex/executor_ref.hpp>
18 #include <boost/capy/ex/frame_allocator.hpp>
19
20 #include <exception>
21 #include <optional>
22 #include <type_traits>
23 #include <utility>
24 #include <variant>
25
26 namespace boost {
27 namespace capy {
28
29 namespace detail {
30
31 // Helper base for result storage and return_void/return_value
32 template<typename T>
33 struct task_return_base
34 {
35 std::optional<T> result_;
36
37 835 void return_value(T value)
38 {
39 835 result_ = std::move(value);
40 835 }
41
42 62 T&& result() noexcept
43 {
44 62 return std::move(*result_);
45 }
46 };
47
48 template<>
49 struct task_return_base<void>
50 {
51 957 void return_void()
52 {
53 957 }
54 };
55
56 } // namespace detail
57
58 /** Lazy coroutine task satisfying @ref IoLaunchableTask.
59
60 Use `task<T>` as the return type for coroutines that perform I/O
61 and return a value of type `T`. The coroutine body does not start
62 executing until the task is awaited, enabling efficient composition
63 without unnecessary eager execution.
64
65 The task participates in the I/O awaitable protocol: when awaited,
66 it receives the caller's executor and stop token, propagating them
67 to nested `co_await` expressions. This enables cancellation and
68 proper completion dispatch across executor boundaries.
69
70 @tparam T The result type. Use `task<>` for `task<void>`.
71
72 @par Thread Safety
73 Distinct objects: Safe.
74 Shared objects: Unsafe.
75
76 @par Example
77
78 @code
79 task<int> compute_value()
80 {
81 auto [ec, n] = co_await stream.read_some( buf );
82 if( ec.failed() )
83 co_return 0;
84 co_return process( buf, n );
85 }
86
87 task<> run_session( tcp_socket sock )
88 {
89 int result = co_await compute_value();
90 // ...
91 }
92 @endcode
93
94 @see IoLaunchableTask, IoAwaitableTask, run, run_async
95 */
96 template<typename T = void>
97 struct [[nodiscard]] BOOST_CAPY_CORO_AWAIT_ELIDABLE
98 task
99 {
100 struct promise_type
101 : io_awaitable_support<promise_type>
102 , detail::task_return_base<T>
103 {
104 std::exception_ptr ep_;
105
106 2204 std::exception_ptr exception() const noexcept
107 {
108 2204 return ep_;
109 }
110
111 2869 task get_return_object()
112 {
113 2869 return task{std::coroutine_handle<promise_type>::from_promise(*this)};
114 }
115
116 2869 auto initial_suspend() noexcept
117 {
118 struct awaiter
119 {
120 promise_type* p_;
121
122 279 bool await_ready() const noexcept
123 {
124 279 return false;
125 }
126
127 279 void await_suspend(coro) const noexcept
128 {
129 // Capture TLS allocator while it's still valid
130 279 p_->set_frame_allocator(current_frame_allocator());
131 279 }
132
133 279 void await_resume() const noexcept
134 {
135 // Restore TLS when body starts executing
136
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 279 if(p_->frame_allocator())
137 255 current_frame_allocator() = p_->frame_allocator();
138 279 }
139 };
140 2869 return awaiter{this};
141 }
142
143 2866 auto final_suspend() noexcept
144 {
145 struct awaiter
146 {
147 promise_type* p_;
148
149 279 bool await_ready() const noexcept
150 {
151 279 return false;
152 }
153
154 279 coro await_suspend(coro) const noexcept
155 {
156 279 return p_->complete();
157 }
158
159 ✗ void await_resume() const noexcept
160 {
161 ✗ }
162 };
163 2866 return awaiter{this};
164 }
165
166 1074 void unhandled_exception()
167 {
168 1074 ep_ = std::current_exception();
169 1074 }
170
171 template<class Awaitable>
172 struct transform_awaiter
173 {
174 std::decay_t<Awaitable> a_;
175 promise_type* p_;
176
177 6845 bool await_ready()
178 {
179 6845 return a_.await_ready();
180 }
181
182 6844 decltype(auto) await_resume()
183 {
184 // Restore TLS before body resumes
185
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 6844 if(p_->frame_allocator())
186 6780 current_frame_allocator() = p_->frame_allocator();
187 6844 return a_.await_resume();
188 }
189
190 template<class Promise>
191 1813 auto await_suspend(std::coroutine_handle<Promise> h)
192 {
193
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 1813 return a_.await_suspend(h, p_->executor(), p_->stop_token());
194 }
195 };
196
197 template<class Awaitable>
198 6845 auto transform_awaitable(Awaitable&& a)
199 {
200 using A = std::decay_t<Awaitable>;
201 if constexpr (IoAwaitable<A>)
202 {
203 return transform_awaiter<Awaitable>{
204 8085 std::forward<Awaitable>(a), this};
205 }
206 else
207 {
208 static_assert(sizeof(A) == 0, "requires IoAwaitable");
209 }
210 1240 }
211 };
212
213 std::coroutine_handle<promise_type> h_;
214
215 /// Destroy the task and its coroutine frame if owned.
216 5675 ~task()
217 {
218
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 5675 if(h_)
219 1230 h_.destroy();
220 5675 }
221
222 /// Return false; tasks are never immediately ready.
223 1103 bool await_ready() const noexcept
224 {
225 1103 return false;
226 }
227
228 /// Return the result or rethrow any stored exception.
229 1228 auto await_resume()
230 {
231
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 1228 if(h_.promise().ep_)
232 462 std::rethrow_exception(h_.promise().ep_);
233 if constexpr (! std::is_void_v<T>)
234 756 return std::move(*h_.promise().result_);
235 else
236 10 return;
237 }
238
239 /// Start execution with the caller's context.
240 1216 coro await_suspend(coro cont, executor_ref caller_ex, std::stop_token token)
241 {
242 1216 h_.promise().set_continuation(cont, caller_ex);
243 1216 h_.promise().set_executor(caller_ex);
244 1216 h_.promise().set_stop_token(token);
245 1216 return h_;
246 }
247
248 /// Return the coroutine handle.
249 1654 std::coroutine_handle<promise_type> handle() const noexcept
250 {
251 1654 return h_;
252 }
253
254 /** Release ownership of the coroutine frame.
255
256 After calling this, destroying the task does not destroy the
257 coroutine frame. The caller becomes responsible for the frame's
258 lifetime.
259
260 @par Postconditions
261 `handle()` returns the original handle, but the task no longer
262 owns it.
263 */
264 1639 void release() noexcept
265 {
266 1639 h_ = nullptr;
267 1639 }
268
269 task(task const&) = delete;
270 task& operator=(task const&) = delete;
271
272 /// Move construct, transferring ownership.
273 2806 task(task&& other) noexcept
274 2806 : h_(std::exchange(other.h_, nullptr))
275 {
276 2806 }
277
278 /// Move assign, transferring ownership.
279 task& operator=(task&& other) noexcept
280 {
281 if(this != &other)
282 {
283 if(h_)
284 h_.destroy();
285 h_ = std::exchange(other.h_, nullptr);
286 }
287 return *this;
288 }
289
290 private:
291 2869 explicit task(std::coroutine_handle<promise_type> h)
292 2869 : h_(h)
293 {
294 2869 }
295 };
296
297 } // namespace capy
298 } // namespace boost
299
300 #endif
301