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_launchable_task.hpp>
16 : #include <boost/capy/coro.hpp>
17 : #include <boost/capy/ex/executor_ref.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 52 : void set(T v)
56 : {
57 52 : value_ = std::move(v);
58 52 : }
59 :
60 45 : T get() &&
61 : {
62 45 : 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<coro, 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 2 : 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 : coro continuation_;
109 : executor_ref caller_ex_;
110 :
111 28 : when_all_state()
112 28 : : remaining_count_(task_count)
113 : {
114 28 : }
115 :
116 28 : ~when_all_state()
117 : {
118 96 : for(auto h : runner_handles_)
119 68 : if(h)
120 68 : h.destroy();
121 28 : }
122 :
123 : /** Capture an exception (first one wins).
124 : */
125 11 : void capture_exception(std::exception_ptr ep)
126 : {
127 11 : bool expected = false;
128 11 : if(has_exception_.compare_exchange_strong(
129 : expected, true, std::memory_order_relaxed))
130 8 : first_exception_ = ep;
131 11 : }
132 :
133 : /** Signal that a task has completed.
134 :
135 : The last child to complete triggers resumption of the parent.
136 : Dispatch handles thread affinity: resumes inline if on same
137 : thread, otherwise posts to the caller's executor.
138 : */
139 68 : coro signal_completion()
140 : {
141 68 : auto remaining = remaining_count_.fetch_sub(1, std::memory_order_acq_rel);
142 68 : if(remaining == 1)
143 28 : caller_ex_.dispatch(continuation_);
144 68 : return std::noop_coroutine();
145 : }
146 :
147 : };
148 :
149 : /** Wrapper coroutine that intercepts task completion.
150 :
151 : This runner awaits its assigned task and stores the result in
152 : the shared state, or captures the exception and requests stop.
153 : */
154 : template<typename T, typename... Ts>
155 : struct when_all_runner
156 : {
157 : struct promise_type // : frame_allocating_base // DISABLED FOR TESTING
158 : {
159 : when_all_state<Ts...>* state_ = nullptr;
160 : executor_ref ex_;
161 : std::stop_token stop_token_;
162 :
163 68 : when_all_runner get_return_object()
164 : {
165 68 : return when_all_runner(std::coroutine_handle<promise_type>::from_promise(*this));
166 : }
167 :
168 68 : std::suspend_always initial_suspend() noexcept
169 : {
170 68 : return {};
171 : }
172 :
173 68 : auto final_suspend() noexcept
174 : {
175 : struct awaiter
176 : {
177 : promise_type* p_;
178 :
179 68 : bool await_ready() const noexcept
180 : {
181 68 : return false;
182 : }
183 :
184 68 : coro await_suspend(coro) noexcept
185 : {
186 : // Signal completion; last task resumes parent
187 68 : return p_->state_->signal_completion();
188 : }
189 :
190 0 : void await_resume() const noexcept
191 : {
192 0 : }
193 : };
194 68 : return awaiter{this};
195 : }
196 :
197 57 : void return_void()
198 : {
199 57 : }
200 :
201 11 : void unhandled_exception()
202 : {
203 11 : state_->capture_exception(std::current_exception());
204 : // Request stop for sibling tasks
205 11 : state_->stop_source_.request_stop();
206 11 : }
207 :
208 : template<class Awaitable>
209 : struct transform_awaiter
210 : {
211 : std::decay_t<Awaitable> a_;
212 : promise_type* p_;
213 :
214 68 : bool await_ready()
215 : {
216 68 : return a_.await_ready();
217 : }
218 :
219 68 : decltype(auto) await_resume()
220 : {
221 68 : return a_.await_resume();
222 : }
223 :
224 : template<class Promise>
225 68 : auto await_suspend(std::coroutine_handle<Promise> h)
226 : {
227 68 : return a_.await_suspend(h, p_->ex_, p_->stop_token_);
228 : }
229 : };
230 :
231 : template<class Awaitable>
232 68 : auto await_transform(Awaitable&& a)
233 : {
234 : using A = std::decay_t<Awaitable>;
235 : if constexpr (IoAwaitable<A>)
236 : {
237 : return transform_awaiter<Awaitable>{
238 136 : std::forward<Awaitable>(a), this};
239 : }
240 : else
241 : {
242 : static_assert(sizeof(A) == 0, "requires IoAwaitable");
243 : }
244 68 : }
245 : };
246 :
247 : std::coroutine_handle<promise_type> h_;
248 :
249 68 : explicit when_all_runner(std::coroutine_handle<promise_type> h)
250 68 : : h_(h)
251 : {
252 68 : }
253 :
254 : // Enable move for all clang versions - some versions need it
255 : when_all_runner(when_all_runner&& other) noexcept : h_(std::exchange(other.h_, nullptr)) {}
256 :
257 : // Non-copyable
258 : when_all_runner(when_all_runner const&) = delete;
259 : when_all_runner& operator=(when_all_runner const&) = delete;
260 : when_all_runner& operator=(when_all_runner&&) = delete;
261 :
262 68 : auto release() noexcept
263 : {
264 68 : return std::exchange(h_, nullptr);
265 : }
266 : };
267 :
268 : /** Create a runner coroutine for a single task.
269 :
270 : Task is passed directly to ensure proper coroutine frame storage.
271 : */
272 : template<std::size_t Index, typename T, typename... Ts>
273 : when_all_runner<T, Ts...>
274 68 : make_when_all_runner(task<T> inner, when_all_state<Ts...>* state)
275 : {
276 : if constexpr (std::is_void_v<T>)
277 : {
278 : co_await std::move(inner);
279 : }
280 : else
281 : {
282 : std::get<Index>(state->results_).set(co_await std::move(inner));
283 : }
284 136 : }
285 :
286 : /** Internal awaitable that launches all runner coroutines and waits.
287 :
288 : This awaitable is used inside the when_all coroutine to handle
289 : the concurrent execution of child tasks.
290 : */
291 : template<typename... Ts>
292 : class when_all_launcher
293 : {
294 : std::tuple<task<Ts>...>* tasks_;
295 : when_all_state<Ts...>* state_;
296 :
297 : public:
298 28 : when_all_launcher(
299 : std::tuple<task<Ts>...>* tasks,
300 : when_all_state<Ts...>* state)
301 28 : : tasks_(tasks)
302 28 : , state_(state)
303 : {
304 28 : }
305 :
306 28 : bool await_ready() const noexcept
307 : {
308 28 : return sizeof...(Ts) == 0;
309 : }
310 :
311 28 : coro await_suspend(coro continuation, executor_ref caller_ex, std::stop_token parent_token = {})
312 : {
313 28 : state_->continuation_ = continuation;
314 28 : state_->caller_ex_ = caller_ex;
315 :
316 : // Forward parent's stop requests to children
317 28 : if(parent_token.stop_possible())
318 : {
319 12 : state_->parent_stop_callback_.emplace(
320 : parent_token,
321 6 : typename when_all_state<Ts...>::stop_callback_fn{&state_->stop_source_});
322 :
323 6 : if(parent_token.stop_requested())
324 2 : state_->stop_source_.request_stop();
325 : }
326 :
327 : // CRITICAL: If the last task finishes synchronously then the parent
328 : // coroutine resumes, destroying its frame, and destroying this object
329 : // prior to the completion of await_suspend. Therefore, await_suspend
330 : // must ensure `this` cannot be referenced after calling `launch_one`
331 : // for the last time.
332 28 : auto token = state_->stop_source_.get_token();
333 48 : [&]<std::size_t... Is>(std::index_sequence<Is...>) {
334 28 : (..., launch_one<Is>(caller_ex, token));
335 28 : }(std::index_sequence_for<Ts...>{});
336 :
337 : // Let signal_completion() handle resumption
338 56 : return std::noop_coroutine();
339 28 : }
340 :
341 28 : void await_resume() const noexcept
342 : {
343 : // Results are extracted by the when_all coroutine from state
344 28 : }
345 :
346 : private:
347 : template<std::size_t I>
348 68 : void launch_one(executor_ref caller_ex, std::stop_token token)
349 : {
350 68 : auto runner = make_when_all_runner<I>(
351 68 : std::move(std::get<I>(*tasks_)), state_);
352 :
353 68 : auto h = runner.release();
354 68 : h.promise().state_ = state_;
355 68 : h.promise().ex_ = caller_ex;
356 68 : h.promise().stop_token_ = token;
357 :
358 68 : coro ch{h};
359 68 : state_->runner_handles_[I] = ch;
360 68 : state_->caller_ex_.dispatch(ch);
361 68 : }
362 : };
363 :
364 : /** Compute the result type for when_all.
365 :
366 : Returns void when all tasks are void (P2300 aligned),
367 : otherwise returns a tuple with void types filtered out.
368 : */
369 : template<typename... Ts>
370 : using when_all_result_t = std::conditional_t<
371 : std::is_same_v<filter_void_tuple_t<Ts...>, std::tuple<>>,
372 : void,
373 : filter_void_tuple_t<Ts...>>;
374 :
375 : /** Helper to extract a single result, returning empty tuple for void.
376 : This is a separate function to work around a GCC-11 ICE that occurs
377 : when using nested immediately-invoked lambdas with pack expansion.
378 : */
379 : template<std::size_t I, typename... Ts>
380 47 : auto extract_single_result(when_all_state<Ts...>& state)
381 : {
382 : using T = std::tuple_element_t<I, std::tuple<Ts...>>;
383 : if constexpr (std::is_void_v<T>)
384 2 : return std::tuple<>();
385 : else
386 45 : return std::make_tuple(std::move(std::get<I>(state.results_)).get());
387 : }
388 :
389 : /** Extract results from state, filtering void types.
390 : */
391 : template<typename... Ts>
392 19 : auto extract_results(when_all_state<Ts...>& state)
393 : {
394 34 : return [&]<std::size_t... Is>(std::index_sequence<Is...>) {
395 19 : return std::tuple_cat(extract_single_result<Is>(state)...);
396 38 : }(std::index_sequence_for<Ts...>{});
397 : }
398 :
399 : } // namespace detail
400 :
401 : /** Execute multiple tasks concurrently and collect their results.
402 :
403 : Launches all tasks simultaneously and waits for all to complete
404 : before returning. Results are collected in input order. If any
405 : task throws, cancellation is requested for siblings and the first
406 : exception is rethrown after all tasks complete.
407 :
408 : @li All child tasks run concurrently on the caller's executor
409 : @li Results are returned as a tuple in input order
410 : @li Void-returning tasks do not contribute to the result tuple
411 : @li If all tasks return void, `when_all` returns `task<void>`
412 : @li First exception wins; subsequent exceptions are discarded
413 : @li Stop is requested for siblings on first error
414 : @li Completes only after all children have finished
415 :
416 : @par Thread Safety
417 : The returned task must be awaited from a single execution context.
418 : Child tasks execute concurrently but complete through the caller's
419 : executor.
420 :
421 : @param tasks The tasks to execute concurrently. Each task is
422 : consumed (moved-from) when `when_all` is awaited.
423 :
424 : @return A task yielding a tuple of non-void results. Returns
425 : `task<void>` when all input tasks return void.
426 :
427 : @par Example
428 :
429 : @code
430 : task<> example()
431 : {
432 : // Concurrent fetch, results collected in order
433 : auto [user, posts] = co_await when_all(
434 : fetch_user( id ), // task<User>
435 : fetch_posts( id ) // task<std::vector<Post>>
436 : );
437 :
438 : // Void tasks don't contribute to result
439 : co_await when_all(
440 : log_event( "start" ), // task<void>
441 : notify_user( id ) // task<void>
442 : );
443 : // Returns task<void>, no result tuple
444 : }
445 : @endcode
446 :
447 : @see task
448 : */
449 : template<typename... Ts>
450 : [[nodiscard]] task<detail::when_all_result_t<Ts...>>
451 28 : when_all(task<Ts>... tasks)
452 : {
453 : using result_type = detail::when_all_result_t<Ts...>;
454 :
455 : // State is stored in the coroutine frame, using the frame allocator
456 : detail::when_all_state<Ts...> state;
457 :
458 : // Store tasks in the frame
459 : std::tuple<task<Ts>...> task_tuple(std::move(tasks)...);
460 :
461 : // Launch all tasks and wait for completion
462 : co_await detail::when_all_launcher<Ts...>(&task_tuple, &state);
463 :
464 : // Propagate first exception if any.
465 : // Safe without explicit acquire: capture_exception() is sequenced-before
466 : // signal_completion()'s acq_rel fetch_sub, which synchronizes-with the
467 : // last task's decrement that resumes this coroutine.
468 : if(state.first_exception_)
469 : std::rethrow_exception(state.first_exception_);
470 :
471 : // Extract and return results
472 : if constexpr (std::is_void_v<result_type>)
473 : co_return;
474 : else
475 : co_return detail::extract_results(state);
476 56 : }
477 :
478 : /// Compute the result type of `when_all` for the given task types.
479 : template<typename... Ts>
480 : using when_all_result_type = detail::when_all_result_t<Ts...>;
481 :
482 : } // namespace capy
483 : } // namespace boost
484 :
485 : #endif
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