include/boost/capy/ex/async_event.hpp

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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		21x	void operator()() const noexcept
126			{
127	2/2 ✓ Branch 1 taken 20 times. ✓ Branch 2 taken 1 time.	21x	if(!self_->claimed_.exchange(
128			true, std::memory_order_acq_rel))
129			{
130		20x	self_->canceled_ = true;
131		20x	self_->ex_.post(self_->h_);
132			}
133		21x	}
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		37x	stop_cb_t& stop_cb_() noexcept
153			{
154			return reinterpret_cast<stop_cb_t>(
155		37x	stop_cb_buf_);
156			}
157
158			public:
159		251x	~wait_awaiter()
160			{
161	2/2 ✓ Branch 0 taken 1 time. ✓ Branch 1 taken 250 times.	251x	if(active_)
162		1x	stop_cb_().~stop_cb_t();
163	2/2 ✓ Branch 0 taken 1 time. ✓ Branch 1 taken 250 times.	251x	if(in_list_)
164		1x	e_->waiters_.remove(this);
165		251x	}
166
167		57x	explicit wait_awaiter(async_event* e) noexcept
168		57x	: e_(e)
169			{
170		57x	}
171
172		194x	wait_awaiter(wait_awaiter&& o) noexcept
173		194x	: e_(o.e_)
174		194x	, h_(o.h_)
175		194x	, ex_(o.ex_)
176		194x	, claimed_(o.claimed_.load(
177			std::memory_order_relaxed))
178		194x	, canceled_(o.canceled_)
179		194x	, active_(std::exchange(o.active_, false))
180		194x	, in_list_(std::exchange(o.in_list_, false))
181			{
182		194x	}
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		57x	bool await_ready() const noexcept
189			{
190		57x	return e_->set_;
191			}
192
193			/** IoAwaitable protocol overload. */
194			std::coroutine_handle<>
195		47x	await_suspend(
196			std::coroutine_handle<> h,
197			io_env const* env) noexcept
198			{
199	2/2 ✓ Branch 1 taken 10 times. ✓ Branch 2 taken 37 times.	47x	if(env->stop_token.stop_requested())
200			{
201		10x	canceled_ = true;
202		10x	return h;
203			}
204		37x	h_ = h;
205		37x	ex_ = env->executor;
206		37x	e_->waiters_.push_back(this);
207		37x	in_list_ = true;
208		111x	::new(stop_cb_buf_) stop_cb_t(
209		37x	env->stop_token, cancel_fn{this});
210		37x	active_ = true;
211		37x	return std::noop_coroutine();
212			}
213
214		54x	io_result<> await_resume() noexcept
215			{
216	2/2 ✓ Branch 0 taken 36 times. ✓ Branch 1 taken 18 times.	54x	if(active_)
217			{
218		36x	stop_cb_().~stop_cb_t();
219		36x	active_ = false;
220			}
221	2/2 ✓ Branch 0 taken 30 times. ✓ Branch 1 taken 24 times.	54x	if(canceled_)
222			{
223	2/2 ✓ Branch 0 taken 20 times. ✓ Branch 1 taken 10 times.	30x	if(in_list_)
224			{
225		20x	e_->waiters_.remove(this);
226		20x	in_list_ = false;
227			}
228		30x	return {make_error_code(
229		30x	error::canceled)};
230			}
231		24x	return {{}};
232			}
233			};
234
235		20x	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		57x	wait_awaiter wait() noexcept
248			{
249		57x	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		23x	void set()
259			{
260		23x	set_ = true;
261			for(;;)
262			{
263		39x	auto* w = waiters_.pop_front();
264	2/2 ✓ Branch 0 taken 23 times. ✓ Branch 1 taken 16 times.	39x	if(!w)
265		23x	break;
266		16x	w->in_list_ = false;
267	1/2 ✓ Branch 1 taken 16 times. ✗ Branch 2 not taken.	16x	if(!w->claimed_.exchange(
268			true, std::memory_order_acq_rel))
269			{
270		16x	w->ex_.post(w->h_);
271			}
272		16x	}
273		23x	}
274
275			/** Clears the event.
276
277			Subsequent calls to wait() will suspend until
278			set() is called again.
279			*/
280		2x	void clear() noexcept
281			{
282		2x	set_ = false;
283		2x	}
284
285			/** Returns true if the event is currently set.
286			*/
287		9x	bool is_set() const noexcept
288			{
289		9x	return set_;
290			}
291			};
292
293			} // namespace capy
294			} // namespace boost
295
296			#endif
297