Line data Source code
1 : //
2 : // Copyright (c) 2026 Steve Gerbino
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/capy
8 : //
9 :
10 : #ifndef BOOST_CAPY_WHEN_ALL_HPP
11 : #define BOOST_CAPY_WHEN_ALL_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 <coroutine>
17 : #include <boost/capy/ex/io_env.hpp>
18 : #include <boost/capy/ex/frame_allocator.hpp>
19 : #include <boost/capy/task.hpp>
20 :
21 : #include <array>
22 : #include <atomic>
23 : #include <exception>
24 : #include <optional>
25 : #include <stop_token>
26 : #include <tuple>
27 : #include <type_traits>
28 : #include <utility>
29 :
30 : namespace boost {
31 : namespace capy {
32 :
33 : namespace detail {
34 :
35 : /** Type trait to filter void types from a tuple.
36 :
37 : Void-returning tasks do not contribute a value to the result tuple.
38 : This trait computes the filtered result type.
39 :
40 : Example: filter_void_tuple_t<int, void, string> = tuple<int, string>
41 : */
42 : template<typename T>
43 : using wrap_non_void_t = std::conditional_t<std::is_void_v<T>, std::tuple<>, std::tuple<T>>;
44 :
45 : template<typename... Ts>
46 : using filter_void_tuple_t = decltype(std::tuple_cat(std::declval<wrap_non_void_t<Ts>>()...));
47 :
48 : /** Holds the result of a single task within when_all.
49 : */
50 : template<typename T>
51 : struct result_holder
52 : {
53 : std::optional<T> value_;
54 :
55 60 : void set(T v)
56 : {
57 60 : value_ = std::move(v);
58 60 : }
59 :
60 53 : T get() &&
61 : {
62 53 : return std::move(*value_);
63 : }
64 : };
65 :
66 : /** Specialization for void tasks - no value storage needed.
67 : */
68 : template<>
69 : struct result_holder<void>
70 : {
71 : };
72 :
73 : /** Shared state for when_all operation.
74 :
75 : @tparam Ts The result types of the tasks.
76 : */
77 : template<typename... Ts>
78 : struct when_all_state
79 : {
80 : static constexpr std::size_t task_count = sizeof...(Ts);
81 :
82 : // Completion tracking - when_all waits for all children
83 : std::atomic<std::size_t> remaining_count_;
84 :
85 : // Result storage in input order
86 : std::tuple<result_holder<Ts>...> results_;
87 :
88 : // Runner handles - destroyed in await_resume while allocator is valid
89 : std::array<std::coroutine_handle<>, task_count> runner_handles_{};
90 :
91 : // Exception storage - first error wins, others discarded
92 : std::atomic<bool> has_exception_{false};
93 : std::exception_ptr first_exception_;
94 :
95 : // Stop propagation - on error, request stop for siblings
96 : std::stop_source stop_source_;
97 :
98 : // Connects parent's stop_token to our stop_source
99 : struct stop_callback_fn
100 : {
101 : std::stop_source* source_;
102 4 : void operator()() const { source_->request_stop(); }
103 : };
104 : using stop_callback_t = std::stop_callback<stop_callback_fn>;
105 : std::optional<stop_callback_t> parent_stop_callback_;
106 :
107 : // Parent resumption
108 : std::coroutine_handle<> continuation_;
109 : io_env const* caller_env_ = nullptr;
110 :
111 34 : when_all_state()
112 34 : : remaining_count_(task_count)
113 : {
114 34 : }
115 :
116 : // Runners self-destruct in final_suspend. No destruction needed here.
117 :
118 : /** Capture an exception (first one wins).
119 : */
120 11 : void capture_exception(std::exception_ptr ep)
121 : {
122 11 : bool expected = false;
123 11 : if(has_exception_.compare_exchange_strong(
124 : expected, true, std::memory_order_relaxed))
125 8 : first_exception_ = ep;
126 11 : }
127 :
128 : };
129 :
130 : /** Wrapper coroutine that intercepts task completion.
131 :
132 : This runner awaits its assigned task and stores the result in
133 : the shared state, or captures the exception and requests stop.
134 : */
135 : template<typename T, typename... Ts>
136 : struct when_all_runner
137 : {
138 : struct promise_type // : frame_allocating_base // DISABLED FOR TESTING
139 : {
140 : when_all_state<Ts...>* state_ = nullptr;
141 : io_env env_;
142 :
143 80 : when_all_runner get_return_object()
144 : {
145 80 : return when_all_runner(std::coroutine_handle<promise_type>::from_promise(*this));
146 : }
147 :
148 80 : std::suspend_always initial_suspend() noexcept
149 : {
150 80 : return {};
151 : }
152 :
153 80 : auto final_suspend() noexcept
154 : {
155 : struct awaiter
156 : {
157 : promise_type* p_;
158 :
159 80 : bool await_ready() const noexcept
160 : {
161 80 : return false;
162 : }
163 :
164 80 : std::coroutine_handle<> await_suspend(std::coroutine_handle<> h) noexcept
165 : {
166 : // Extract everything needed before self-destruction.
167 80 : auto* state = p_->state_;
168 80 : auto* counter = &state->remaining_count_;
169 80 : auto* caller_env = state->caller_env_;
170 80 : auto cont = state->continuation_;
171 :
172 80 : h.destroy();
173 :
174 : // If last runner, dispatch parent for symmetric transfer.
175 80 : auto remaining = counter->fetch_sub(1, std::memory_order_acq_rel);
176 80 : if(remaining == 1)
177 34 : return caller_env->executor.dispatch(cont);
178 46 : return std::noop_coroutine();
179 : }
180 :
181 0 : void await_resume() const noexcept
182 : {
183 0 : }
184 : };
185 80 : return awaiter{this};
186 : }
187 :
188 69 : void return_void()
189 : {
190 69 : }
191 :
192 11 : void unhandled_exception()
193 : {
194 11 : state_->capture_exception(std::current_exception());
195 : // Request stop for sibling tasks
196 11 : state_->stop_source_.request_stop();
197 11 : }
198 :
199 : template<class Awaitable>
200 : struct transform_awaiter
201 : {
202 : std::decay_t<Awaitable> a_;
203 : promise_type* p_;
204 :
205 80 : bool await_ready()
206 : {
207 80 : return a_.await_ready();
208 : }
209 :
210 80 : decltype(auto) await_resume()
211 : {
212 80 : return a_.await_resume();
213 : }
214 :
215 : template<class Promise>
216 79 : auto await_suspend(std::coroutine_handle<Promise> h)
217 : {
218 79 : return a_.await_suspend(h, &p_->env_);
219 : }
220 : };
221 :
222 : template<class Awaitable>
223 80 : auto await_transform(Awaitable&& a)
224 : {
225 : using A = std::decay_t<Awaitable>;
226 : if constexpr (IoAwaitable<A>)
227 : {
228 : return transform_awaiter<Awaitable>{
229 160 : std::forward<Awaitable>(a), this};
230 : }
231 : else
232 : {
233 : static_assert(sizeof(A) == 0, "requires IoAwaitable");
234 : }
235 80 : }
236 : };
237 :
238 : std::coroutine_handle<promise_type> h_;
239 :
240 80 : explicit when_all_runner(std::coroutine_handle<promise_type> h)
241 80 : : h_(h)
242 : {
243 80 : }
244 :
245 : // Enable move for all clang versions - some versions need it
246 : when_all_runner(when_all_runner&& other) noexcept : h_(std::exchange(other.h_, nullptr)) {}
247 :
248 : // Non-copyable
249 : when_all_runner(when_all_runner const&) = delete;
250 : when_all_runner& operator=(when_all_runner const&) = delete;
251 : when_all_runner& operator=(when_all_runner&&) = delete;
252 :
253 80 : auto release() noexcept
254 : {
255 80 : return std::exchange(h_, nullptr);
256 : }
257 : };
258 :
259 : /** Create a runner coroutine for a single awaitable.
260 :
261 : Awaitable is passed directly to ensure proper coroutine frame storage.
262 : */
263 : template<std::size_t Index, IoAwaitable Awaitable, typename... Ts>
264 : when_all_runner<awaitable_result_t<Awaitable>, Ts...>
265 80 : make_when_all_runner(Awaitable inner, when_all_state<Ts...>* state)
266 : {
267 : using T = awaitable_result_t<Awaitable>;
268 : if constexpr (std::is_void_v<T>)
269 : {
270 : co_await std::move(inner);
271 : }
272 : else
273 : {
274 : std::get<Index>(state->results_).set(co_await std::move(inner));
275 : }
276 160 : }
277 :
278 : /** Internal awaitable that launches all runner coroutines and waits.
279 :
280 : This awaitable is used inside the when_all coroutine to handle
281 : the concurrent execution of child awaitables.
282 : */
283 : template<IoAwaitable... Awaitables>
284 : class when_all_launcher
285 : {
286 : using state_type = when_all_state<awaitable_result_t<Awaitables>...>;
287 :
288 : std::tuple<Awaitables...>* awaitables_;
289 : state_type* state_;
290 :
291 : public:
292 34 : when_all_launcher(
293 : std::tuple<Awaitables...>* awaitables,
294 : state_type* state)
295 34 : : awaitables_(awaitables)
296 34 : , state_(state)
297 : {
298 34 : }
299 :
300 34 : bool await_ready() const noexcept
301 : {
302 34 : return sizeof...(Awaitables) == 0;
303 : }
304 :
305 34 : std::coroutine_handle<> await_suspend(std::coroutine_handle<> continuation, io_env const* caller_env)
306 : {
307 34 : state_->continuation_ = continuation;
308 34 : state_->caller_env_ = caller_env;
309 :
310 : // Forward parent's stop requests to children
311 34 : if(caller_env->stop_token.stop_possible())
312 : {
313 16 : state_->parent_stop_callback_.emplace(
314 8 : caller_env->stop_token,
315 8 : typename state_type::stop_callback_fn{&state_->stop_source_});
316 :
317 8 : if(caller_env->stop_token.stop_requested())
318 4 : state_->stop_source_.request_stop();
319 : }
320 :
321 : // CRITICAL: If the last task finishes synchronously then the parent
322 : // coroutine resumes, destroying its frame, and destroying this object
323 : // prior to the completion of await_suspend. Therefore, await_suspend
324 : // must ensure `this` cannot be referenced after calling `launch_one`
325 : // for the last time.
326 34 : auto token = state_->stop_source_.get_token();
327 58 : [&]<std::size_t... Is>(std::index_sequence<Is...>) {
328 34 : (..., launch_one<Is>(caller_env->executor, token));
329 34 : }(std::index_sequence_for<Awaitables...>{});
330 :
331 : // Let signal_completion() handle resumption
332 68 : return std::noop_coroutine();
333 34 : }
334 :
335 34 : void await_resume() const noexcept
336 : {
337 : // Results are extracted by the when_all coroutine from state
338 34 : }
339 :
340 : private:
341 : template<std::size_t I>
342 80 : void launch_one(executor_ref caller_ex, std::stop_token token)
343 : {
344 80 : auto runner = make_when_all_runner<I>(
345 80 : std::move(std::get<I>(*awaitables_)), state_);
346 :
347 80 : auto h = runner.release();
348 80 : h.promise().state_ = state_;
349 80 : h.promise().env_ = io_env{caller_ex, token, state_->caller_env_->allocator};
350 :
351 80 : std::coroutine_handle<> ch{h};
352 80 : state_->runner_handles_[I] = ch;
353 80 : state_->caller_env_->executor.post(ch);
354 160 : }
355 : };
356 :
357 : /** Compute the result type for when_all.
358 :
359 : Returns void when all tasks are void (P2300 aligned),
360 : otherwise returns a tuple with void types filtered out.
361 : */
362 : template<typename... Ts>
363 : using when_all_result_t = std::conditional_t<
364 : std::is_same_v<filter_void_tuple_t<Ts...>, std::tuple<>>,
365 : void,
366 : filter_void_tuple_t<Ts...>>;
367 :
368 : /** Helper to extract a single result, returning empty tuple for void.
369 : This is a separate function to work around a GCC-11 ICE that occurs
370 : when using nested immediately-invoked lambdas with pack expansion.
371 : */
372 : template<std::size_t I, typename... Ts>
373 57 : auto extract_single_result(when_all_state<Ts...>& state)
374 : {
375 : using T = std::tuple_element_t<I, std::tuple<Ts...>>;
376 : if constexpr (std::is_void_v<T>)
377 4 : return std::tuple<>();
378 : else
379 53 : return std::make_tuple(std::move(std::get<I>(state.results_)).get());
380 : }
381 :
382 : /** Extract results from state, filtering void types.
383 : */
384 : template<typename... Ts>
385 24 : auto extract_results(when_all_state<Ts...>& state)
386 : {
387 43 : return [&]<std::size_t... Is>(std::index_sequence<Is...>) {
388 25 : return std::tuple_cat(extract_single_result<Is>(state)...);
389 48 : }(std::index_sequence_for<Ts...>{});
390 : }
391 :
392 : } // namespace detail
393 :
394 : /** Execute multiple awaitables concurrently and collect their results.
395 :
396 : Launches all awaitables simultaneously and waits for all to complete
397 : before returning. Results are collected in input order. If any
398 : awaitable throws, cancellation is requested for siblings and the first
399 : exception is rethrown after all awaitables complete.
400 :
401 : @li All child awaitables run concurrently on the caller's executor
402 : @li Results are returned as a tuple in input order
403 : @li Void-returning awaitables do not contribute to the result tuple
404 : @li If all awaitables return void, `when_all` returns `task<void>`
405 : @li First exception wins; subsequent exceptions are discarded
406 : @li Stop is requested for siblings on first error
407 : @li Completes only after all children have finished
408 :
409 : @par Thread Safety
410 : The returned task must be awaited from a single execution context.
411 : Child awaitables execute concurrently but complete through the caller's
412 : executor.
413 :
414 : @param awaitables The awaitables to execute concurrently. Each must
415 : satisfy @ref IoAwaitable and is consumed (moved-from) when
416 : `when_all` is awaited.
417 :
418 : @return A task yielding a tuple of non-void results. Returns
419 : `task<void>` when all input awaitables return void.
420 :
421 : @par Example
422 :
423 : @code
424 : task<> example()
425 : {
426 : // Concurrent fetch, results collected in order
427 : auto [user, posts] = co_await when_all(
428 : fetch_user( id ), // task<User>
429 : fetch_posts( id ) // task<std::vector<Post>>
430 : );
431 :
432 : // Void awaitables don't contribute to result
433 : co_await when_all(
434 : log_event( "start" ), // task<void>
435 : notify_user( id ) // task<void>
436 : );
437 : // Returns task<void>, no result tuple
438 : }
439 : @endcode
440 :
441 : @see IoAwaitable, task
442 : */
443 : template<IoAwaitable... As>
444 34 : [[nodiscard]] auto when_all(As... awaitables)
445 : -> task<detail::when_all_result_t<detail::awaitable_result_t<As>...>>
446 : {
447 : using result_type = detail::when_all_result_t<detail::awaitable_result_t<As>...>;
448 :
449 : // State is stored in the coroutine frame, using the frame allocator
450 : detail::when_all_state<detail::awaitable_result_t<As>...> state;
451 :
452 : // Store awaitables in the frame
453 : std::tuple<As...> awaitable_tuple(std::move(awaitables)...);
454 :
455 : // Launch all awaitables and wait for completion
456 : co_await detail::when_all_launcher<As...>(&awaitable_tuple, &state);
457 :
458 : // Propagate first exception if any.
459 : // Safe without explicit acquire: capture_exception() is sequenced-before
460 : // signal_completion()'s acq_rel fetch_sub, which synchronizes-with the
461 : // last task's decrement that resumes this coroutine.
462 : if(state.first_exception_)
463 : std::rethrow_exception(state.first_exception_);
464 :
465 : // Extract and return results
466 : if constexpr (std::is_void_v<result_type>)
467 : co_return;
468 : else
469 : co_return detail::extract_results(state);
470 68 : }
471 :
472 : /// Compute the result type of `when_all` for the given task types.
473 : template<typename... Ts>
474 : using when_all_result_type = detail::when_all_result_t<Ts...>;
475 :
476 : } // namespace capy
477 : } // namespace boost
478 :
479 : #endif
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