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	1937x	void operator()() const noexcept
85		{
86	1937x	src_->request_stop();
87	1937x	}
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	4x	timeout_awaitable(A&& inner, time_point deadline)
106	4x	: inner_(std::move(inner))
107	4x	, deadline_(deadline)
108		{
109	4x	}
110
111		/// Construct without a timer, deadline measured from suspension.
112	2052x	timeout_awaitable(A&& inner, std::chrono::nanoseconds dur)
113	2052x	: inner_(std::move(inner))
114	2052x	, dur_(dur)
115	2052x	, has_deadline_(false)
116		{
117	2052x	}
118
119	4114x	~timeout_awaitable()
120		{
121	4114x	destroy_parent_cb();
122	4114x	}
123
124		// Only moved before await_suspend, when cb_active_ is false
125	2058x	timeout_awaitable(timeout_awaitable&& o) noexcept(
126		std::is_nothrow_move_constructible_v<A>)
127	2058x	: inner_(std::move(o.inner_))
128	2058x	, timer_(std::move(o.timer_))
129	2058x	, deadline_(o.deadline_)
130	2058x	, dur_(o.dur_)
131	2058x	, has_deadline_(o.has_deadline_)
132	2058x	, stop_src_(std::move(o.stop_src_))
133		{
134	2058x	}
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		// Forwarding here is load-bearing, not an optimization: awaitables
141		// may perform setup in await_ready (type-erased stream wrappers
142		// construct their cached inner op there), so the full awaiter
143		// protocol must reach inner_ before await_suspend is driven. An
144		// already-ready inner op also skips arming the timer entirely.
145	2054x	bool await_ready()
146		{
147	2054x	return inner_.await_ready();
148		}
149
150	2052x	auto await_suspend(std::coroutine_handle<> h, capy::io_env const* env)
151		{
152	2052x	parent_token_ = env->stop_token;
153
154		// The deadline timer is built here from the awaiting
155		// coroutine's executor context, the first point at which it
156		// is known. await_suspend is driven through a noexcept
157		// wrapper, so a failure cannot be surfaced as a catchable
158		// exception. An executor whose context is not an io_context
159		// cannot supply a timer service; silently running the
160		// operation with no deadline would be a worse failure than
161		// aborting, so translate the service-lookup error into a
162		// clear precondition diagnostic. This terminates by design
163		// (a usage error) rather than dropping the requested timeout.
164		// The detached timeout coroutine must own its executor by
165		// value (see timeout_coro::set_env_owned); io_env carries
166		// only a non-owning executor_ref. Recover the concrete
167		// executor from the context rather than the executor_ref:
168		// wrapped executors (a strand over the io_context) satisfy
169		// the documented precondition but do not expose the io
170		// executor as their target. The timer construction below
171		// validates the context is an io_context, and the detached
172		// coroutine shares only the thread-safe stop_source with
173		// the caller, so resuming it on the raw io executor instead
174		// of the caller's wrapper is safe.
175		try
176		{
177	2052x	timer_.emplace(env->executor.context());
178		}
179	4x	catch (std::logic_error const&)
180		{
181	2x	throw_logic_error(
182		"timeout requires an io_context-backed executor");
183		}
184		auto ex = static_cast<io_context&>(
185	2050x	env->executor.context()).get_executor();
186
187	2050x	if (has_deadline_)
188	4x	timer_->expires_at(deadline_);
189		else
190	2046x	timer_->expires_after(dur_);
191
192		// Launch fire-and-forget timeout (starts suspended)
193	2050x	auto timeout = make_timeout(*timer_, stop_src_);
194	4100x	timeout.h_.promise().set_env_owned(
195	2050x	ex, stop_src_.get_token(), env->frame_allocator);
196		// Runs synchronously until timer.wait() suspends
197	2050x	timeout.h_.resume();
198		// timeout goes out of scope; destructor is a no-op,
199		// the coroutine self-destroys via suspend_never
200
201		// Forward parent cancellation
202	2050x	new (cb_buf_) stop_cb_type(env->stop_token, stop_forwarder{&stop_src_});
203	2050x	cb_active_ = true;
204
205		// Start the inner op with our interposed stop_token
206	2050x	inner_env_ = {
207	2050x	env->executor, stop_src_.get_token(), env->frame_allocator};
208	4100x	return inner_.await_suspend(h, &inner_env_);
209	2050x	}
210
211	2050x	decltype(auto) await_resume()
212		{
213		// Read before request_stop: afterwards stop_requested()
214		// can no longer distinguish who fired first. This must also
215		// happen before inner_.await_resume() rather than after: when
216		// the inner awaitable is itself a timeout_awaitable (nested
217		// timeout()), our own request_stop() below is visible through
218		// its parent_token_ (aliasing our stop_src_), and would
219		// otherwise make its read of "parent" look like a
220		// cancellation that never happened.
221	2050x	bool const parent = parent_token_.stop_requested();
222	2050x	bool const fired = stop_src_.stop_requested();
223
224		// If inner_.await_resume() throws below, request_stop() is
225		// skipped; the still-armed timeout coroutine is then drained
226		// by timer_'s destructor rather than by us.
227	2050x	auto r = inner_.await_resume();
228
229		// Cancel whichever is still pending (idempotent)
230	2048x	stop_src_.request_stop();
231	2048x	destroy_parent_cb();
232
233		// Deadline won: stop_src_ is assumed to be the only
234		// cancellation source, whose only writers are the timer
235		// coroutine and the parent forwarder, so fired && !parent
236		// identifies a timeout. A third-party cancellation of the
237		// inner op (e.g. a socket cancel issued from elsewhere)
238		// landing in the same window as the deadline firing is
239		// reported as a timeout.
240	2070x	if (fired && !parent &&
241	2070x	r.ec == capy::cond::canceled)
242		{
243	22x	std::remove_cvref_t<decltype(r)> t{};
244	22x	t.ec = make_error_code(capy::error::timeout);
245	22x	return t;
246		}
247	2026x	return r;
248		}
249
250	6162x	void destroy_parent_cb() noexcept
251		{
252	6162x	if (cb_active_)
253		{
254	2050x	std::launder(reinterpret_cast<stop_cb_type*>(cb_buf_))
255	2050x	->~stop_cb_type();
256	2050x	cb_active_ = false;
257		}
258	6162x	}
259		};
260
261		} // namespace boost::corosio::detail
262
263		#endif
264