Line data Source code
1 : //
2 : // Copyright (c) 2025 Vinnie Falco (vinnie.falco@gmail.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/capy
8 : //
9 :
10 : #ifndef BOOST_CAPY_ASYNC_EVENT_HPP
11 : #define BOOST_CAPY_ASYNC_EVENT_HPP
12 :
13 : #include <boost/capy/detail/config.hpp>
14 : #include <boost/capy/detail/intrusive.hpp>
15 : #include <boost/capy/concept/executor.hpp>
16 : #include <boost/capy/error.hpp>
17 : #include <boost/capy/ex/io_env.hpp>
18 : #include <boost/capy/io_result.hpp>
19 :
20 : #include <stop_token>
21 :
22 : #include <atomic>
23 : #include <coroutine>
24 : #include <new>
25 : #include <utility>
26 :
27 : /* async_event implementation notes
28 : =================================
29 :
30 : Same cancellation pattern as async_mutex (see that file for the
31 : full discussion on claimed_, stop_cb lifetime, member ordering,
32 : and threading assumptions).
33 :
34 : Key difference: set() wakes ALL waiters (broadcast), not one.
35 : It pops every waiter from the list and posts the ones it
36 : claims. Waiters already claimed by a stop callback are skipped.
37 :
38 : Because set() pops all waiters, a canceled waiter may have been
39 : removed from the list by set() before its await_resume runs.
40 : This requires a separate in_list_ flag (unlike async_mutex where
41 : active_ served double duty). await_resume only calls remove()
42 : when in_list_ is true.
43 : */
44 :
45 : namespace boost {
46 : namespace capy {
47 :
48 : /** An asynchronous event for coroutines.
49 :
50 : This event provides a way to notify multiple coroutines that some
51 : condition has occurred. When a coroutine awaits an unset event, it
52 : suspends and is added to a wait queue. When the event is set, all
53 : waiting coroutines are resumed.
54 :
55 : @par Cancellation
56 :
57 : When a coroutine is suspended waiting for the event and its stop
58 : token is triggered, the waiter completes with `error::canceled`
59 : instead of waiting for `set()`.
60 :
61 : Cancellation only applies while the coroutine is suspended in the
62 : wait queue. If the event is already set when `wait()` is called,
63 : the wait completes immediately even if the stop token is already
64 : signaled.
65 :
66 : @par Zero Allocation
67 :
68 : No heap allocation occurs for wait operations.
69 :
70 : @par Thread Safety
71 :
72 : The event operations are designed for single-threaded use on one
73 : executor. The stop callback may fire from any thread.
74 :
75 : @par Example
76 : @code
77 : async_event event;
78 :
79 : task<> waiter() {
80 : auto [ec] = co_await event.wait();
81 : if(ec)
82 : co_return;
83 : // ... event was set ...
84 : }
85 :
86 : task<> notifier() {
87 : // ... do some work ...
88 : event.set(); // Wake all waiters
89 : }
90 : @endcode
91 : */
92 : class async_event
93 : {
94 : public:
95 : class wait_awaiter;
96 :
97 : private:
98 : bool set_ = false;
99 : detail::intrusive_list<wait_awaiter> waiters_;
100 :
101 : public:
102 : /** Awaiter returned by wait().
103 : */
104 : class wait_awaiter
105 : : public detail::intrusive_list<wait_awaiter>::node
106 : {
107 : friend class async_event;
108 :
109 : async_event* e_;
110 : std::coroutine_handle<> h_;
111 : executor_ref ex_;
112 :
113 : // Declared before stop_cb_buf_: the callback
114 : // accesses these members, so they must still be
115 : // alive if the stop_cb_ destructor blocks.
116 : std::atomic<bool> claimed_{false};
117 : bool canceled_ = false;
118 : bool active_ = false;
119 : bool in_list_ = false;
120 :
121 : struct cancel_fn
122 : {
123 : wait_awaiter* self_;
124 :
125 21 : void operator()() const noexcept
126 : {
127 21 : if(!self_->claimed_.exchange(
128 : true, std::memory_order_acq_rel))
129 : {
130 20 : self_->canceled_ = true;
131 20 : self_->ex_.post(self_->h_);
132 : }
133 21 : }
134 : };
135 :
136 : using stop_cb_t =
137 : std::stop_callback<cancel_fn>;
138 :
139 : // Aligned storage for stop_cb_t. Declared last:
140 : // its destructor may block while the callback
141 : // accesses the members above.
142 : #ifdef _MSC_VER
143 : # pragma warning(push)
144 : # pragma warning(disable: 4324) // padded due to alignas
145 : #endif
146 : alignas(stop_cb_t)
147 : unsigned char stop_cb_buf_[sizeof(stop_cb_t)];
148 : #ifdef _MSC_VER
149 : # pragma warning(pop)
150 : #endif
151 :
152 37 : stop_cb_t& stop_cb_() noexcept
153 : {
154 : return *reinterpret_cast<stop_cb_t*>(
155 37 : stop_cb_buf_);
156 : }
157 :
158 : public:
159 251 : ~wait_awaiter()
160 : {
161 251 : if(active_)
162 1 : stop_cb_().~stop_cb_t();
163 251 : if(in_list_)
164 1 : e_->waiters_.remove(this);
165 251 : }
166 :
167 57 : explicit wait_awaiter(async_event* e) noexcept
168 57 : : e_(e)
169 : {
170 57 : }
171 :
172 194 : wait_awaiter(wait_awaiter&& o) noexcept
173 194 : : e_(o.e_)
174 194 : , h_(o.h_)
175 194 : , ex_(o.ex_)
176 194 : , claimed_(o.claimed_.load(
177 : std::memory_order_relaxed))
178 194 : , canceled_(o.canceled_)
179 194 : , active_(std::exchange(o.active_, false))
180 194 : , in_list_(std::exchange(o.in_list_, false))
181 : {
182 194 : }
183 :
184 : wait_awaiter(wait_awaiter const&) = delete;
185 : wait_awaiter& operator=(wait_awaiter const&) = delete;
186 : wait_awaiter& operator=(wait_awaiter&&) = delete;
187 :
188 57 : bool await_ready() const noexcept
189 : {
190 57 : return e_->set_;
191 : }
192 :
193 : /** IoAwaitable protocol overload. */
194 : std::coroutine_handle<>
195 47 : await_suspend(
196 : std::coroutine_handle<> h,
197 : io_env const* env) noexcept
198 : {
199 47 : if(env->stop_token.stop_requested())
200 : {
201 10 : canceled_ = true;
202 10 : return h;
203 : }
204 37 : h_ = h;
205 37 : ex_ = env->executor;
206 37 : e_->waiters_.push_back(this);
207 37 : in_list_ = true;
208 111 : ::new(stop_cb_buf_) stop_cb_t(
209 37 : env->stop_token, cancel_fn{this});
210 37 : active_ = true;
211 37 : return std::noop_coroutine();
212 : }
213 :
214 54 : io_result<> await_resume() noexcept
215 : {
216 54 : if(active_)
217 : {
218 36 : stop_cb_().~stop_cb_t();
219 36 : active_ = false;
220 : }
221 54 : if(canceled_)
222 : {
223 30 : if(in_list_)
224 : {
225 20 : e_->waiters_.remove(this);
226 20 : in_list_ = false;
227 : }
228 30 : return {make_error_code(
229 30 : error::canceled)};
230 : }
231 24 : return {{}};
232 : }
233 : };
234 :
235 20 : async_event() = default;
236 :
237 : // Non-copyable, non-movable
238 : async_event(async_event const&) = delete;
239 : async_event& operator=(async_event const&) = delete;
240 :
241 : /** Returns an awaiter that waits until the event is set.
242 :
243 : If the event is already set, completes immediately.
244 :
245 : @return An awaitable yielding `(error_code)`.
246 : */
247 57 : wait_awaiter wait() noexcept
248 : {
249 57 : return wait_awaiter{this};
250 : }
251 :
252 : /** Sets the event.
253 :
254 : All waiting coroutines are resumed. Canceled waiters
255 : are skipped. Subsequent calls to wait() complete
256 : immediately until clear() is called.
257 : */
258 23 : void set()
259 : {
260 23 : set_ = true;
261 : for(;;)
262 : {
263 39 : auto* w = waiters_.pop_front();
264 39 : if(!w)
265 23 : break;
266 16 : w->in_list_ = false;
267 16 : if(!w->claimed_.exchange(
268 : true, std::memory_order_acq_rel))
269 : {
270 16 : w->ex_.post(w->h_);
271 : }
272 16 : }
273 23 : }
274 :
275 : /** Clears the event.
276 :
277 : Subsequent calls to wait() will suspend until
278 : set() is called again.
279 : */
280 2 : void clear() noexcept
281 : {
282 2 : set_ = false;
283 2 : }
284 :
285 : /** Returns true if the event is currently set.
286 : */
287 9 : bool is_set() const noexcept
288 : {
289 9 : return set_;
290 : }
291 : };
292 :
293 : } // namespace capy
294 : } // namespace boost
295 :
296 : #endif
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