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		2004x	void operator()() const noexcept
85			{
86		2004x	src_->request_stop();
87		2004x	}
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		4x	std::chrono::nanoseconds dur_{};
97		4x	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		4117x	bool cb_active_ = false;
103
104			/// Construct without a timer, deadline given as an absolute time.
105	3/6 ✓ Branch 0 taken 4 times. ✗ Branch 1 not taken. ✓ Branch 2 taken 4 times. ✗ Branch 3 not taken. ✓ Branch 4 taken 4 times. ✗ Branch 5 not taken.	16x	timeout_awaitable(A&& inner, time_point deadline)
106		4x	: inner_(std::move(inner))
107		4x	, deadline_(deadline)
108		4x	{
109		8x	}
110
111			/// Construct without a timer, deadline measured from suspension.
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113		2055x	: inner_(std::move(inner))
114		2054x	, dur_(dur)
115		2054x	, has_deadline_(false)
116		2055x	{
117		4110x	}
118
119		8235x	~timeout_awaitable()
120		4118x	{
121		4117x	destroy_parent_cb();
122		8235x	}
123
124			// Only moved before await_suspend, when cb_active_ is false
125		6180x	timeout_awaitable(timeout_awaitable&& o) noexcept(
126			std::is_nothrow_move_constructible_v<A>)
127		2060x	: inner_(std::move(o.inner_))
128		2060x	, timer_(std::move(o.timer_))
129		2060x	, deadline_(o.deadline_)
130		2060x	, dur_(o.dur_)
131		2060x	, has_deadline_(o.has_deadline_)
132		2060x	, stop_src_(std::move(o.stop_src_))
133		2060x	{
134		4120x	}
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		2055x	bool await_ready()
146			{
147		2055x	return inner_.await_ready();
148			}
149
150		2054x	auto await_suspend(std::coroutine_handle<> h, capy::io_env const* env)
151			{
152		2054x	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			{
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178		2056x	}
179			catch (std::logic_error const&)
180			{
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182			"timeout requires an io_context-backed executor");
183		2x	}
184			auto ex = static_cast<io_context&>(
185		2052x	env->executor.context()).get_executor();
186
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188		4x	timer_->expires_at(deadline_);
189			else
190		2048x	timer_->expires_after(dur_);
191
192			// Launch fire-and-forget timeout (starts suspended)
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195		2052x	ex, stop_src_.get_token(), env->frame_allocator);
196			// Runs synchronously until timer.wait() suspends
197		2052x	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		2052x	new (cb_buf_) stop_cb_type(env->stop_token, stop_forwarder{&stop_src_});
203		2052x	cb_active_ = true;
204
205			// Start the inner op with our interposed stop_token
206		8208x	inner_env_ = {
207		6156x	env->executor, stop_src_.get_token(), env->frame_allocator};
208		2052x	return inner_.await_suspend(h, &inner_env_);
209		4x	}
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		2050x	stop_src_.request_stop();
231		2050x	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.
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241		24x	std::get<0>(r) == capy::cond::canceled)
242			{
243		24x	std::remove_cvref_t<decltype(r)> t{};
244		24x	std::get<0>(t) = make_error_code(capy::error::timeout);
245		24x	return t;
246			}
247		2026x	return r;
248		2050x	}
249
250		6161x	void destroy_parent_cb() noexcept
251			{
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253			{
254		2051x	std::launder(reinterpret_cast<stop_cb_type*>(cb_buf_))
255		2051x	->~stop_cb_type();
256		2051x	cb_active_ = false;
257		2051x	}
258		6161x	}
259			};
260
261			} // namespace boost::corosio::detail
262
263			#endif
264