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 <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 59 : void set(T v)
56 : {
57 59 : value_ = std::move(v);
58 59 : }
59 :
60 52 : T get() &&
61 : {
62 52 : 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 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 : coro continuation_;
109 : executor_ref caller_ex_;
110 :
111 33 : when_all_state()
112 33 : : remaining_count_(task_count)
113 : {
114 33 : }
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 : executor_ref ex_;
142 : std::stop_token stop_token_;
143 :
144 78 : when_all_runner get_return_object()
145 : {
146 78 : return when_all_runner(std::coroutine_handle<promise_type>::from_promise(*this));
147 : }
148 :
149 78 : std::suspend_always initial_suspend() noexcept
150 : {
151 78 : return {};
152 : }
153 :
154 78 : auto final_suspend() noexcept
155 : {
156 : struct awaiter
157 : {
158 : promise_type* p_;
159 :
160 78 : bool await_ready() const noexcept
161 : {
162 78 : return false;
163 : }
164 :
165 78 : void await_suspend(coro h) noexcept
166 : {
167 : // Extract everything needed for signaling before
168 : // self-destruction. Inline dispatch may destroy
169 : // when_all_state, so we can't access members after.
170 78 : auto* state = p_->state_;
171 78 : auto* counter = &state->remaining_count_;
172 78 : auto caller_ex = state->caller_ex_;
173 78 : auto cont = state->continuation_;
174 :
175 : // Self-destruct first - state no longer destroys runners
176 78 : h.destroy();
177 :
178 : // Signal completion. If last, dispatch parent.
179 : // Uses only local copies - safe even if state
180 : // is destroyed during inline dispatch.
181 78 : auto remaining = counter->fetch_sub(1, std::memory_order_acq_rel);
182 78 : if(remaining == 1)
183 33 : caller_ex.dispatch(cont);
184 78 : }
185 :
186 0 : void await_resume() const noexcept
187 : {
188 0 : }
189 : };
190 78 : return awaiter{this};
191 : }
192 :
193 67 : void return_void()
194 : {
195 67 : }
196 :
197 11 : void unhandled_exception()
198 : {
199 11 : state_->capture_exception(std::current_exception());
200 : // Request stop for sibling tasks
201 11 : state_->stop_source_.request_stop();
202 11 : }
203 :
204 : template<class Awaitable>
205 : struct transform_awaiter
206 : {
207 : std::decay_t<Awaitable> a_;
208 : promise_type* p_;
209 :
210 78 : bool await_ready()
211 : {
212 78 : return a_.await_ready();
213 : }
214 :
215 78 : decltype(auto) await_resume()
216 : {
217 78 : return a_.await_resume();
218 : }
219 :
220 : template<class Promise>
221 77 : auto await_suspend(std::coroutine_handle<Promise> h)
222 : {
223 77 : return a_.await_suspend(h, p_->ex_, p_->stop_token_);
224 : }
225 : };
226 :
227 : template<class Awaitable>
228 78 : auto await_transform(Awaitable&& a)
229 : {
230 : using A = std::decay_t<Awaitable>;
231 : if constexpr (IoAwaitable<A>)
232 : {
233 : return transform_awaiter<Awaitable>{
234 156 : std::forward<Awaitable>(a), this};
235 : }
236 : else
237 : {
238 : static_assert(sizeof(A) == 0, "requires IoAwaitable");
239 : }
240 78 : }
241 : };
242 :
243 : std::coroutine_handle<promise_type> h_;
244 :
245 78 : explicit when_all_runner(std::coroutine_handle<promise_type> h)
246 78 : : h_(h)
247 : {
248 78 : }
249 :
250 : // Enable move for all clang versions - some versions need it
251 : when_all_runner(when_all_runner&& other) noexcept : h_(std::exchange(other.h_, nullptr)) {}
252 :
253 : // Non-copyable
254 : when_all_runner(when_all_runner const&) = delete;
255 : when_all_runner& operator=(when_all_runner const&) = delete;
256 : when_all_runner& operator=(when_all_runner&&) = delete;
257 :
258 78 : auto release() noexcept
259 : {
260 78 : return std::exchange(h_, nullptr);
261 : }
262 : };
263 :
264 : /** Create a runner coroutine for a single awaitable.
265 :
266 : Awaitable is passed directly to ensure proper coroutine frame storage.
267 : */
268 : template<std::size_t Index, IoAwaitable Awaitable, typename... Ts>
269 : when_all_runner<awaitable_result_t<Awaitable>, Ts...>
270 78 : make_when_all_runner(Awaitable inner, when_all_state<Ts...>* state)
271 : {
272 : using T = awaitable_result_t<Awaitable>;
273 : if constexpr (std::is_void_v<T>)
274 : {
275 : co_await std::move(inner);
276 : }
277 : else
278 : {
279 : std::get<Index>(state->results_).set(co_await std::move(inner));
280 : }
281 156 : }
282 :
283 : /** Internal awaitable that launches all runner coroutines and waits.
284 :
285 : This awaitable is used inside the when_all coroutine to handle
286 : the concurrent execution of child awaitables.
287 : */
288 : template<IoAwaitable... Awaitables>
289 : class when_all_launcher
290 : {
291 : using state_type = when_all_state<awaitable_result_t<Awaitables>...>;
292 :
293 : std::tuple<Awaitables...>* awaitables_;
294 : state_type* state_;
295 :
296 : public:
297 33 : when_all_launcher(
298 : std::tuple<Awaitables...>* awaitables,
299 : state_type* state)
300 33 : : awaitables_(awaitables)
301 33 : , state_(state)
302 : {
303 33 : }
304 :
305 33 : bool await_ready() const noexcept
306 : {
307 33 : return sizeof...(Awaitables) == 0;
308 : }
309 :
310 33 : coro await_suspend(coro continuation, executor_ref const& caller_ex, std::stop_token const& parent_token = {})
311 : {
312 33 : state_->continuation_ = continuation;
313 33 : state_->caller_ex_ = caller_ex;
314 :
315 : // Forward parent's stop requests to children
316 33 : if(parent_token.stop_possible())
317 : {
318 16 : state_->parent_stop_callback_.emplace(
319 : parent_token,
320 8 : typename state_type::stop_callback_fn{&state_->stop_source_});
321 :
322 8 : if(parent_token.stop_requested())
323 4 : state_->stop_source_.request_stop();
324 : }
325 :
326 : // CRITICAL: If the last task finishes synchronously then the parent
327 : // coroutine resumes, destroying its frame, and destroying this object
328 : // prior to the completion of await_suspend. Therefore, await_suspend
329 : // must ensure `this` cannot be referenced after calling `launch_one`
330 : // for the last time.
331 33 : auto token = state_->stop_source_.get_token();
332 58 : [&]<std::size_t... Is>(std::index_sequence<Is...>) {
333 33 : (..., launch_one<Is>(caller_ex, token));
334 33 : }(std::index_sequence_for<Awaitables...>{});
335 :
336 : // Let signal_completion() handle resumption
337 66 : return std::noop_coroutine();
338 33 : }
339 :
340 33 : void await_resume() const noexcept
341 : {
342 : // Results are extracted by the when_all coroutine from state
343 33 : }
344 :
345 : private:
346 : template<std::size_t I>
347 78 : void launch_one(executor_ref caller_ex, std::stop_token token)
348 : {
349 78 : auto runner = make_when_all_runner<I>(
350 78 : std::move(std::get<I>(*awaitables_)), state_);
351 :
352 78 : auto h = runner.release();
353 78 : h.promise().state_ = state_;
354 78 : h.promise().ex_ = caller_ex;
355 78 : h.promise().stop_token_ = token;
356 :
357 78 : coro ch{h};
358 78 : state_->runner_handles_[I] = ch;
359 78 : state_->caller_ex_.dispatch(ch);
360 78 : }
361 : };
362 :
363 : /** Compute the result type for when_all.
364 :
365 : Returns void when all tasks are void (P2300 aligned),
366 : otherwise returns a tuple with void types filtered out.
367 : */
368 : template<typename... Ts>
369 : using when_all_result_t = std::conditional_t<
370 : std::is_same_v<filter_void_tuple_t<Ts...>, std::tuple<>>,
371 : void,
372 : filter_void_tuple_t<Ts...>>;
373 :
374 : /** Helper to extract a single result, returning empty tuple for void.
375 : This is a separate function to work around a GCC-11 ICE that occurs
376 : when using nested immediately-invoked lambdas with pack expansion.
377 : */
378 : template<std::size_t I, typename... Ts>
379 55 : auto extract_single_result(when_all_state<Ts...>& state)
380 : {
381 : using T = std::tuple_element_t<I, std::tuple<Ts...>>;
382 : if constexpr (std::is_void_v<T>)
383 3 : return std::tuple<>();
384 : else
385 52 : return std::make_tuple(std::move(std::get<I>(state.results_)).get());
386 : }
387 :
388 : /** Extract results from state, filtering void types.
389 : */
390 : template<typename... Ts>
391 23 : auto extract_results(when_all_state<Ts...>& state)
392 : {
393 42 : return [&]<std::size_t... Is>(std::index_sequence<Is...>) {
394 24 : return std::tuple_cat(extract_single_result<Is>(state)...);
395 46 : }(std::index_sequence_for<Ts...>{});
396 : }
397 :
398 : } // namespace detail
399 :
400 : /** Execute multiple awaitables concurrently and collect their results.
401 :
402 : Launches all awaitables simultaneously and waits for all to complete
403 : before returning. Results are collected in input order. If any
404 : awaitable throws, cancellation is requested for siblings and the first
405 : exception is rethrown after all awaitables complete.
406 :
407 : @li All child awaitables run concurrently on the caller's executor
408 : @li Results are returned as a tuple in input order
409 : @li Void-returning awaitables do not contribute to the result tuple
410 : @li If all awaitables return void, `when_all` returns `task<void>`
411 : @li First exception wins; subsequent exceptions are discarded
412 : @li Stop is requested for siblings on first error
413 : @li Completes only after all children have finished
414 :
415 : @par Thread Safety
416 : The returned task must be awaited from a single execution context.
417 : Child awaitables execute concurrently but complete through the caller's
418 : executor.
419 :
420 : @param awaitables The awaitables to execute concurrently. Each must
421 : satisfy @ref IoAwaitable and is consumed (moved-from) when
422 : `when_all` is awaited.
423 :
424 : @return A task yielding a tuple of non-void results. Returns
425 : `task<void>` when all input awaitables 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 awaitables 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 IoAwaitable, task
448 : */
449 : template<IoAwaitable... As>
450 33 : [[nodiscard]] auto when_all(As... awaitables)
451 : -> task<detail::when_all_result_t<detail::awaitable_result_t<As>...>>
452 : {
453 : using result_type = detail::when_all_result_t<detail::awaitable_result_t<As>...>;
454 :
455 : // State is stored in the coroutine frame, using the frame allocator
456 : detail::when_all_state<detail::awaitable_result_t<As>...> state;
457 :
458 : // Store awaitables in the frame
459 : std::tuple<As...> awaitable_tuple(std::move(awaitables)...);
460 :
461 : // Launch all awaitables and wait for completion
462 : co_await detail::when_all_launcher<As...>(&awaitable_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 66 : }
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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