include/boost/corosio/detail/timeout_awaitable.hpp

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timeout_awaitable.hpp

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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/corosio
8		//
9
10		#ifndef BOOST_COROSIO_DETAIL_TIMEOUT_AWAITABLE_HPP
11		#define BOOST_COROSIO_DETAIL_TIMEOUT_AWAITABLE_HPP
12
13		#include <boost/corosio/io_context.hpp>
14		#include <boost/corosio/detail/timeout_coro.hpp>
15		#include <boost/corosio/detail/timer.hpp>
16		#include <boost/corosio/detail/except.hpp>
17		#include <boost/capy/cond.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 <chrono>
23		#include <coroutine>
24		#include <new>
25		#include <optional>
26		#include <stdexcept>
27		#include <stop_token>
28		#include <type_traits>
29		#include <utility>
30
31		/* Races an inner IoAwaitable against a timer via a shared
32		stop_source. await_suspend arms the timer by launching a
33		fire-and-forget timeout_coro, then starts the inner op with
34		an interposed stop_token. Whichever completes first signals
35		the stop_source, cancelling the other.
36
37		Parent cancellation is forwarded through a stop_callback
38		stored in a placement-new buffer (stop_callback is not
39		movable, but the awaitable must be movable for
40		transform_awaiter). The buffer is inert during moves
41		(before await_suspend) and constructed in-place once the
42		awaitable is pinned on the coroutine frame.
43
44		The timeout_coro can outlive this awaitable — it owns its
45		env and self-destroys via suspend_never. The timer lives in
46		std::optional and is constructed lazily in await_suspend,
47		once the awaiting coroutine's executor context is known. */
48
49		namespace boost::corosio::detail {
50
51		// Local stand-in for capy::detail's io_result trait: corosio must not
52		// reach into capy::detail, but the result-mapping switch in
53		// await_resume needs to distinguish io_result from other return types.
54		template<typename T>
55		struct is_io_result : std::false_type
56		{
57		};
58
59		template<typename... Ts>
60		struct is_io_result<capy::io_result<Ts...>> : std::true_type
61		{
62		};
63
64		template<typename T>
65		inline constexpr bool is_io_result_v = is_io_result<T>::value;
66
67		/** Awaitable adapter that cancels an inner operation after a deadline.
68
69		Races the inner awaitable against a timer. A shared stop_source
70		ties them together: whichever completes first cancels the other.
71		Parent cancellation is forwarded via stop_callback.
72
73		The timer is constructed internally in `await_suspend` from the
74		execution context in `io_env`.
75
76		@tparam A The inner IoAwaitable type (decayed).
77		*/
78		template<typename A>
79		struct timeout_awaitable
80		{
81		struct stop_forwarder
82		{
83		std::stop_source* src_;
84	3005x	void operator()() const noexcept
85		{
86	3005x	src_->request_stop();
87	3005x	}
88		};
89
90		using time_point = std::chrono::steady_clock::time_point;
91		using stop_cb_type = std::stop_callback<stop_forwarder>;
92
93		A inner_;
94		std::optional<timer> timer_;
95		time_point deadline_;
96		std::chrono::nanoseconds dur_{};
97		bool has_deadline_ = true;
98		std::stop_source stop_src_;
99		std::stop_token parent_token_;
100		capy::io_env inner_env_;
101		alignas(stop_cb_type) unsigned char cb_buf_[sizeof(stop_cb_type)];
102		bool cb_active_ = false;
103
104		/// Construct without a timer, deadline given as an absolute time.
105	6x	timeout_awaitable(A&& inner, time_point deadline)
106	6x	: inner_(std::move(inner))
107	6x	, deadline_(deadline)
108		{
109	6x	}
110
111		/// Construct without a timer, deadline measured from suspension.
112	3057x	timeout_awaitable(A&& inner, std::chrono::nanoseconds dur)
113	3057x	: inner_(std::move(inner))
114	3057x	, dur_(dur)
115	3057x	, has_deadline_(false)
116		{
117	3057x	}
118
119	6126x	~timeout_awaitable()
120		{
121	6126x	destroy_parent_cb();
122	6126x	}
123
124		// Only moved before await_suspend, when cb_active_ is false
125	3063x	timeout_awaitable(timeout_awaitable&& o) noexcept(
126		std::is_nothrow_move_constructible_v<A>)
127	3063x	: inner_(std::move(o.inner_))
128	3063x	, timer_(std::move(o.timer_))
129	3063x	, deadline_(o.deadline_)
130	3063x	, dur_(o.dur_)
131	3063x	, has_deadline_(o.has_deadline_)
132	3063x	, stop_src_(std::move(o.stop_src_))
133		{
134	3063x	}
135
136		timeout_awaitable(timeout_awaitable const&) = delete;
137		timeout_awaitable& operator=(timeout_awaitable const&) = delete;
138		timeout_awaitable& operator=(timeout_awaitable&&) = delete;
139
140	3057x	bool await_ready() const noexcept
141		{
142	3057x	return false;
143		}
144
145	3063x	auto await_suspend(std::coroutine_handle<> h, capy::io_env const* env)
146		{
147	3063x	parent_token_ = env->stop_token;
148
149		// The deadline timer is built here from the awaiting
150		// coroutine's executor context, the first point at which it
151		// is known. await_suspend is driven through a noexcept
152		// wrapper, so a failure cannot be surfaced as a catchable
153		// exception. An executor whose context is not an io_context
154		// cannot supply a timer service; silently running the
155		// operation with no deadline would be a worse failure than
156		// aborting, so translate the service-lookup error into a
157		// clear precondition diagnostic. This terminates by design
158		// (a usage error) rather than dropping the requested timeout.
159		// The detached timeout coroutine must own its executor by
160		// value (see timeout_coro::set_env_owned); io_env carries
161		// only a non-owning executor_ref. Recover the concrete
162		// executor from the context rather than the executor_ref:
163		// wrapped executors (a strand over the io_context) satisfy
164		// the documented precondition but do not expose the io
165		// executor as their target. The timer construction below
166		// validates the context is an io_context, and the detached
167		// coroutine shares only the thread-safe stop_source with
168		// the caller, so resuming it on the raw io executor instead
169		// of the caller's wrapper is safe.
170		try
171		{
172	3063x	timer_.emplace(env->executor.context());
173		}
174	6x	catch (std::logic_error const&)
175		{
176	3x	throw_logic_error(
177		"timeout requires an io_context-backed executor");
178		}
179		auto ex = static_cast<io_context&>(
180	3060x	env->executor.context()).get_executor();
181
182	3060x	if (has_deadline_)
183	6x	timer_->expires_at(deadline_);
184		else
185	3054x	timer_->expires_after(dur_);
186
187		// Launch fire-and-forget timeout (starts suspended)
188	3060x	auto timeout = make_timeout(*timer_, stop_src_);
189	6120x	timeout.h_.promise().set_env_owned(
190	3060x	ex, stop_src_.get_token(), env->frame_allocator);
191		// Runs synchronously until timer.wait() suspends
192	3060x	timeout.h_.resume();
193		// timeout goes out of scope; destructor is a no-op,
194		// the coroutine self-destroys via suspend_never
195
196		// Forward parent cancellation
197	3060x	new (cb_buf_) stop_cb_type(env->stop_token, stop_forwarder{&stop_src_});
198	3060x	cb_active_ = true;
199
200		// Start the inner op with our interposed stop_token
201	3060x	inner_env_ = {
202	3060x	env->executor, stop_src_.get_token(), env->frame_allocator};
203	6120x	return inner_.await_suspend(h, &inner_env_);
204	3060x	}
205
206	3054x	decltype(auto) await_resume()
207		{
208		// Read before request_stop: afterwards stop_requested()
209		// can no longer distinguish who fired first. This must also
210		// happen before inner_.await_resume() rather than after: when
211		// the inner awaitable is itself a timeout_awaitable (nested
212		// timeout()), our own request_stop() below is visible through
213		// its parent_token_ (aliasing our stop_src_), and would
214		// otherwise make its read of "parent" look like a
215		// cancellation that never happened.
216	3054x	bool const parent = parent_token_.stop_requested();
217	3054x	bool const fired = stop_src_.stop_requested();
218
219		// If inner_.await_resume() throws below, request_stop() is
220		// skipped; the still-armed timeout coroutine is then drained
221		// by timer_'s destructor rather than by us.
222	3054x	auto r = inner_.await_resume();
223
224		// Cancel whichever is still pending (idempotent)
225	3051x	stop_src_.request_stop();
226	3051x	destroy_parent_cb();
227
228		// Deadline won: stop_src_ is assumed to be the only
229		// cancellation source, whose only writers are the timer
230		// coroutine and the parent forwarder, so fired && !parent
231		// identifies a timeout. A third-party cancellation of the
232		// inner op (e.g. a socket cancel issued from elsewhere)
233		// landing in the same window as the deadline firing is
234		// reported as a timeout.
235	3081x	if (fired && !parent &&
236	3081x	r.ec == capy::cond::canceled)
237		{
238	30x	std::remove_cvref_t<decltype(r)> t{};
239	30x	t.ec = make_error_code(capy::error::timeout);
240	30x	return t;
241		}
242	3021x	return r;
243		}
244
245	9177x	void destroy_parent_cb() noexcept
246		{
247	9177x	if (cb_active_)
248		{
249	3060x	std::launder(reinterpret_cast<stop_cb_type*>(cb_buf_))
250	3060x	->~stop_cb_type();
251	3060x	cb_active_ = false;
252		}
253	9177x	}
254		};
255
256		} // namespace boost::corosio::detail
257
258		#endif
259