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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 1081 void return_value(T value)
38 {
39 1081 result_ = std::move(value);
40 1081 }
41
42 108 T&& result() noexcept
43 {
44 108 return std::move(*result_);
45 }
46 };
47
48 template<>
49 struct task_return_base<void>
50 {
51 972 void return_void()
52 {
53 972 }
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 2283 std::exception_ptr exception() const noexcept
107 {
108 2283 return ep_;
109 }
110
111 3170 task get_return_object()
112 {
113 3170 return task{std::coroutine_handle<promise_type>::from_promise(*this)};
114 }
115
116 3170 auto initial_suspend() noexcept
117 {
118 struct awaiter
119 {
120 promise_type* p_;
121
122 144 bool await_ready() const noexcept
123 {
124 144 return false;
125 }
126
127 144 void await_suspend(coro) const noexcept
128 {
129 // Capture TLS allocator while it's still valid
130 144 p_->set_frame_allocator(current_frame_allocator());
131 144 }
132
133 144 void await_resume() const noexcept
134 {
135 // Restore TLS when body starts executing
136
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 144 if(p_->frame_allocator())
137 140 current_frame_allocator() = p_->frame_allocator();
138 144 }
139 };
140 3170 return awaiter{this};
141 }
142
143 3167 auto final_suspend() noexcept
144 {
145 struct awaiter
146 {
147 promise_type* p_;
148
149 144 bool await_ready() const noexcept
150 {
151 144 return false;
152 }
153
154 144 coro await_suspend(coro) const noexcept
155 {
156 144 return p_->complete();
157 }
158
159 ✗ void await_resume() const noexcept
160 {
161 ✗ }
162 };
163 3167 return awaiter{this};
164 }
165
166 1114 void unhandled_exception()
167 {
168 1114 ep_ = std::current_exception();
169 1114 }
170
171 template<class Awaitable>
172 struct transform_awaiter
173 {
174 std::decay_t<Awaitable> a_;
175 promise_type* p_;
176
177 6973 bool await_ready()
178 {
179 6973 return a_.await_ready();
180 }
181
182 6972 decltype(auto) await_resume()
183 {
184 // Restore TLS before body resumes
185
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 6972 if(p_->frame_allocator())
186 6908 current_frame_allocator() = p_->frame_allocator();
187 6972 return a_.await_resume();
188 }
189
190 template<class Promise>
191 1941 auto await_suspend(std::coroutine_handle<Promise> h)
192 {
193
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 1941 return a_.await_suspend(h, p_->executor(), p_->stop_token());
194 }
195 };
196
197 template<class Awaitable>
198 6973 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 8238 std::forward<Awaitable>(a), this};
205 }
206 else
207 {
208 static_assert(sizeof(A) == 0, "requires IoAwaitable");
209 }
210 1265 }
211 };
212
213 std::coroutine_handle<promise_type> h_;
214
215 /// Destroy the task and its coroutine frame if owned.
216 7068 ~task()
217 {
218
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 7068 if(h_)
219 1468 h_.destroy();
220 7068 }
221
222 /// Return false; tasks are never immediately ready.
223 1340 bool await_ready() const noexcept
224 {
225 1340 return false;
226 }
227
228 /// Return the result or rethrow any stored exception.
229 1465 auto await_resume()
230 {
231
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 1465 if(h_.promise().ep_)
232 486 std::rethrow_exception(h_.promise().ep_);
233 if constexpr (! std::is_void_v<T>)
234 956 return std::move(*h_.promise().result_);
235 else
236 23 return;
237 }
238
239 /// Start execution with the caller's context.
240 1453 coro await_suspend(coro cont, executor_ref caller_ex, std::stop_token token)
241 {
242 1453 h_.promise().set_continuation(cont, caller_ex);
243 1453 h_.promise().set_executor(caller_ex);
244 1453 h_.promise().set_stop_token(token);
245 1453 return h_;
246 }
247
248 /// Return the coroutine handle.
249 1718 std::coroutine_handle<promise_type> handle() const noexcept
250 {
251 1718 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 1702 void release() noexcept
265 {
266 1702 h_ = nullptr;
267 1702 }
268
269 task(task const&) = delete;
270 task& operator=(task const&) = delete;
271
272 /// Move construct, transferring ownership.
273 3898 task(task&& other) noexcept
274 3898 : h_(std::exchange(other.h_, nullptr))
275 {
276 3898 }
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 3170 explicit task(std::coroutine_handle<promise_type> h)
292 3170 : h_(h)
293 {
294 3170 }
295 };
296
297 } // namespace capy
298 } // namespace boost
299
300 #endif
301