TLA Line data Source code
1 : //
2 : // Copyright (c) 2025 Vinnie Falco (vinnie.falco@gmail.com)
3 : // Copyright (c) 2026 Michael Vandeberg
4 : //
5 : // Distributed under the Boost Software License, Version 1.0. (See accompanying
6 : // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
7 : //
8 : // Official repository: https://github.com/cppalliance/capy
9 : //
10 :
11 : #ifndef BOOST_CAPY_ASYNC_EVENT_HPP
12 : #define BOOST_CAPY_ASYNC_EVENT_HPP
13 :
14 : #include <boost/capy/detail/config.hpp>
15 : #include <boost/capy/detail/intrusive.hpp>
16 : #include <boost/capy/continuation.hpp>
17 : #include <boost/capy/concept/executor.hpp>
18 : #include <boost/capy/error.hpp>
19 : #include <boost/capy/ex/io_env.hpp>
20 : #include <boost/capy/io_result.hpp>
21 :
22 : #include <stop_token>
23 :
24 : #include <atomic>
25 : #include <coroutine>
26 : #include <new>
27 : #include <utility>
28 :
29 : /* async_event implementation notes
30 : =================================
31 :
32 : Same cancellation pattern as async_mutex (see that file for the
33 : full discussion on claimed_, stop_cb lifetime, member ordering,
34 : and threading assumptions).
35 :
36 : Key difference: set() wakes ALL waiters (broadcast), not one.
37 : It pops every waiter from the list and posts the ones it
38 : claims. Waiters already claimed by a stop callback are skipped.
39 :
40 : Because set() pops all waiters, a canceled waiter may have been
41 : removed from the list by set() before its await_resume runs.
42 : This requires a separate in_list_ flag (unlike async_mutex where
43 : active_ served double duty). await_resume only calls remove()
44 : when in_list_ is true.
45 : */
46 :
47 : namespace boost {
48 : namespace capy {
49 :
50 : /** Queues coroutines in `wait()` and resumes all of them when `set()` is called.
51 :
52 : This event provides a way to notify multiple coroutines that some
53 : condition has occurred. When a coroutine awaits an unset event, it
54 : suspends and is added to a wait queue. When the event is set, all
55 : waiting coroutines are resumed.
56 :
57 : @par Cancellation
58 :
59 : When a coroutine is suspended waiting for the event and its stop
60 : token is triggered, the waiter completes with `error::canceled`
61 : instead of waiting for `set()`.
62 :
63 : Cancellation only applies while the coroutine is suspended in the
64 : wait queue. If the event is already set when `wait()` is called,
65 : the wait completes immediately even if the stop token is already
66 : signaled.
67 :
68 : @par Zero Allocation
69 :
70 : No heap allocation occurs for wait operations.
71 :
72 : @par Thread Safety
73 :
74 : Distinct objects: Safe.@n
75 : Shared objects: Unsafe.
76 :
77 : The event operations are designed for single-threaded use on one
78 : executor. The stop callback may fire from any thread.
79 :
80 : This type is non-copyable and non-movable because suspended
81 : waiters hold intrusive pointers into the event's internal list.
82 :
83 : @par Example
84 : @par !example example
85 :
86 : */
87 : class async_event
88 : {
89 : public:
90 : class wait_awaiter;
91 :
92 : private:
93 : bool set_ = false;
94 : detail::intrusive_list<wait_awaiter> waiters_;
95 :
96 : public:
97 : /** Suspends the caller until `set()` runs, or resumes it with `error::canceled` on a stop request.
98 : */
99 : class wait_awaiter
100 : : public detail::intrusive_list<wait_awaiter>::node
101 : {
102 : friend class async_event;
103 :
104 : async_event* e_;
105 : continuation cont_;
106 : executor_ref ex_;
107 :
108 : // Declared before stop_cb_buf_: the callback
109 : // accesses these members, so they must still be
110 : // alive if the stop_cb_ destructor blocks.
111 : std::atomic<bool> claimed_{false};
112 : bool canceled_ = false;
113 : bool active_ = false;
114 : bool in_list_ = false;
115 :
116 : struct cancel_fn
117 : {
118 : wait_awaiter* self_;
119 :
120 HIT 4 : void operator()() const noexcept
121 : {
122 4 : if(!self_->claimed_.exchange(
123 : true, std::memory_order_acq_rel))
124 : {
125 3 : self_->canceled_ = true;
126 3 : self_->ex_.post(self_->cont_);
127 : }
128 4 : }
129 : };
130 :
131 : using stop_cb_t =
132 : std::stop_callback<cancel_fn>;
133 :
134 : // Aligned storage for stop_cb_t. Declared last:
135 : // its destructor may block while the callback
136 : // accesses the members above.
137 : BOOST_CAPY_MSVC_WARNING_PUSH
138 : BOOST_CAPY_MSVC_WARNING_DISABLE(4324) // padded due to alignas
139 : alignas(stop_cb_t)
140 : unsigned char stop_cb_buf_[sizeof(stop_cb_t)];
141 : BOOST_CAPY_MSVC_WARNING_POP
142 :
143 20 : stop_cb_t& stop_cb_() noexcept
144 : {
145 : return *reinterpret_cast<stop_cb_t*>(
146 20 : stop_cb_buf_);
147 : }
148 :
149 : public:
150 : /** Destroy the awaiter, leaving the event unable to reach it.
151 :
152 : Destroys the stop callback if one is registered, and unlinks
153 : the awaiter from the event's wait queue if it is still linked.
154 : Both are necessary when the coroutine frame is torn down while
155 : suspended, so that neither `set()` nor the stop callback can
156 : reach a destroyed awaiter.
157 : */
158 52 : ~wait_awaiter()
159 : {
160 52 : if(active_)
161 1 : stop_cb_().~stop_cb_t();
162 52 : if(in_list_)
163 1 : e_->waiters_.remove(this);
164 52 : }
165 :
166 : /** Construct an awaiter for the given event.
167 :
168 : @param e The event to wait on. It must outlive the awaiter.
169 : */
170 25 : explicit wait_awaiter(async_event* e) noexcept
171 25 : : e_(e)
172 : {
173 25 : }
174 :
175 : /** Construct by moving.
176 :
177 : The moved-from awaiter is left inert: its destructor no longer
178 : destroys the stop callback and no longer unlinks from the
179 : event's wait queue.
180 :
181 : @param o The awaiter to move from.
182 : */
183 27 : wait_awaiter(wait_awaiter&& o) noexcept
184 54 : : e_(o.e_)
185 27 : , cont_(o.cont_)
186 27 : , ex_(o.ex_)
187 27 : , claimed_(o.claimed_.load(
188 : std::memory_order_relaxed))
189 27 : , canceled_(o.canceled_)
190 27 : , active_(std::exchange(o.active_, false))
191 54 : , in_list_(std::exchange(o.in_list_, false))
192 : {
193 27 : }
194 :
195 : /** Copy construction is disabled; a waiter is linked into the
196 : event's wait queue by address.
197 :
198 : @param other The awaiter that would be copied.
199 : */
200 : wait_awaiter(wait_awaiter const& other) = delete;
201 :
202 : /** Copy assignment is disabled; a waiter is linked into the
203 : event's wait queue by address.
204 :
205 : @param other The awaiter that would be assigned from.
206 :
207 : @return A reference to `*this`.
208 : */
209 : wait_awaiter& operator=(wait_awaiter const& other) = delete;
210 :
211 : /** Move assignment is disabled; a waiter is linked into the
212 : event's wait queue by address.
213 :
214 : @param other The awaiter that would be moved from.
215 :
216 : @return A reference to `*this`.
217 : */
218 : wait_awaiter& operator=(wait_awaiter&& other) = delete;
219 :
220 : /** Report whether the event is already set.
221 :
222 : @return `true` if the event is set, in which case the awaiting
223 : coroutine does not suspend; otherwise `false`.
224 : */
225 25 : bool await_ready() const noexcept
226 : {
227 25 : return e_->set_;
228 : }
229 :
230 : /** Enqueue the awaiting coroutine until the event is set.
231 :
232 : This is the @ref IoAwaitable overload of `await_suspend`.
233 :
234 : If a stop request is already pending on `env->stop_token`, the
235 : awaiter records the cancellation and does not enqueue.
236 :
237 : Otherwise it stores `h` and `env->executor`, links itself into
238 : the event's wait queue, and registers a stop callback on
239 : `env->stop_token`. Exactly one of `set()` and that callback posts
240 : `h` through the stored executor, whichever claims the waiter
241 : first. Only the post is subject to that race. A losing stop
242 : callback does nothing at all, but `set()` unlinks every waiter it
243 : pops whether it claims it or not. That is why @ref await_resume
244 : unlinks a canceled waiter only when it is still linked.
245 :
246 : @param h The awaiting coroutine, resumed when the event is set
247 : or the wait is canceled.
248 :
249 : @param env The execution environment. Its executor posts the
250 : resumption and its stop token is watched for the duration of
251 : the wait. It must outlive the wait.
252 :
253 : @return `h` if a stop request was already pending, which
254 : resumes the awaiting coroutine immediately without enqueuing
255 : it. Otherwise `std::noop_coroutine()`, which leaves the
256 : coroutine suspended and returns control to the resumer.
257 : */
258 : std::coroutine_handle<>
259 21 : await_suspend(
260 : std::coroutine_handle<> h,
261 : io_env const* env) noexcept
262 : {
263 21 : if(env->stop_token.stop_requested())
264 : {
265 1 : canceled_ = true;
266 1 : return h;
267 : }
268 20 : cont_.h = h;
269 20 : ex_ = env->executor;
270 20 : e_->waiters_.push_back(this);
271 20 : in_list_ = true;
272 60 : ::new(stop_cb_buf_) stop_cb_t(
273 20 : env->stop_token, cancel_fn{this});
274 20 : active_ = true;
275 20 : return std::noop_coroutine();
276 : }
277 :
278 : /** Complete the wait and report the outcome.
279 :
280 : Destroys the stop callback if one is registered. If the wait
281 : was canceled while still linked into the event's wait queue,
282 : unlinks it. `set()` pops every waiter, so a canceled waiter may
283 : or may not still be linked when it resumes.
284 :
285 : @return An empty `io_result<>` if the event was set, or one
286 : holding `error::canceled` if the stop token fired first.
287 : */
288 22 : [[nodiscard]] io_result<> await_resume() noexcept
289 : {
290 22 : if(active_)
291 : {
292 19 : stop_cb_().~stop_cb_t();
293 19 : active_ = false;
294 : }
295 22 : if(canceled_)
296 : {
297 4 : if(in_list_)
298 : {
299 3 : e_->waiters_.remove(this);
300 3 : in_list_ = false;
301 : }
302 8 : return {make_error_code(
303 4 : error::canceled)};
304 : }
305 18 : return {{}};
306 : }
307 : };
308 :
309 : /// Construct an unset event.
310 : async_event() = default;
311 :
312 : /** Copy construction is disabled; suspended waiters point into the
313 : event's wait queue.
314 :
315 : @param other The event that would be copied.
316 : */
317 : async_event(async_event const& other) = delete;
318 :
319 : /** Copy assignment is disabled; suspended waiters point into the
320 : event's wait queue.
321 :
322 : @param other The event that would be assigned from.
323 :
324 : @return A reference to `*this`.
325 : */
326 : async_event& operator=(async_event const& other) = delete;
327 :
328 : /** Move construction is disabled; suspended waiters point into the
329 : event's wait queue.
330 :
331 : @param other The event that would be moved from.
332 : */
333 : async_event(async_event&& other) = delete;
334 :
335 : /** Move assignment is disabled; suspended waiters point into the
336 : event's wait queue.
337 :
338 : @param other The event that would be moved from.
339 :
340 : @return A reference to `*this`.
341 : */
342 : async_event& operator=(async_event&& other) = delete;
343 :
344 : /** Returns an awaiter that waits until the event is set.
345 :
346 : If the event is already set, completes immediately.
347 :
348 : @return An awaitable that await-returns `(error_code)`.
349 : */
350 25 : wait_awaiter wait() noexcept
351 : {
352 25 : return wait_awaiter{this};
353 : }
354 :
355 : /** Resumes every waiting coroutine and marks the event set for later `wait()` calls.
356 :
357 : All waiting coroutines are resumed. Canceled waiters
358 : are skipped. Subsequent calls to wait() complete
359 : immediately until clear() is called.
360 : */
361 17 : void set()
362 : {
363 17 : set_ = true;
364 : for(;;)
365 : {
366 33 : auto* w = waiters_.pop_front();
367 33 : if(!w)
368 17 : break;
369 16 : w->in_list_ = false;
370 16 : if(!w->claimed_.exchange(
371 : true, std::memory_order_acq_rel))
372 : {
373 16 : w->ex_.post(w->cont_);
374 : }
375 16 : }
376 17 : }
377 :
378 : /** Clears the event.
379 :
380 : Subsequent calls to wait() suspend until
381 : set() is called again.
382 : */
383 2 : void clear() noexcept
384 : {
385 2 : set_ = false;
386 2 : }
387 :
388 : /** Returns true if the event is currently set.
389 :
390 : @return `true` if the event is set; otherwise `false`.
391 : */
392 9 : bool is_set() const noexcept
393 : {
394 9 : return set_;
395 : }
396 : };
397 :
398 : } // namespace capy
399 : } // namespace boost
400 :
401 : #endif
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