Index/include / boost / corosio / detail / cancel_at_awaitable.hpp

include/boost/corosio/detail/cancel_at_awaitable.hpp

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include/boost/corosio/detail/cancel_at_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_CANCEL_AT_AWAITABLE_HPP
11 #define BOOST_COROSIO_DETAIL_CANCEL_AT_AWAITABLE_HPP
12
13 #include <boost/corosio/detail/timeout_coro.hpp>
14 #include <boost/capy/ex/io_env.hpp>
15
16 #include <chrono>
17 #include <coroutine>
18 #include <new>
19 #include <optional>
20 #include <stop_token>
21 #include <type_traits>
22 #include <utility>
23
24 /* Races an inner IoAwaitable against a timer via a shared
25 stop_source. await_suspend arms the timer by launching a
26 fire-and-forget timeout_coro, then starts the inner op with
27 an interposed stop_token. Whichever completes first signals
28 the stop_source, cancelling the other.
29
30 Parent cancellation is forwarded through a stop_callback
31 stored in a placement-new buffer (stop_callback is not
32 movable, but the awaitable must be movable for
33 transform_awaiter). The buffer is inert during moves
34 (before await_suspend) and constructed in-place once the
35 awaitable is pinned on the coroutine frame.
36
37 The timeout_coro can outlive this awaitable — it owns its
38 env and self-destroys via suspend_never. When Owning is
39 false the caller-supplied timer must outlive both; when
40 Owning is true the timer lives in std::optional and is
41 constructed lazily in await_suspend. */
42
43 namespace boost::corosio::detail {
44
45 /** Awaitable adapter that cancels an inner operation after a deadline.
46
47 Races the inner awaitable against a timer. A shared stop_source
48 ties them together: whichever completes first cancels the other.
49 Parent cancellation is forwarded via stop_callback.
50
51 When @p Owning is `false` (default), the caller supplies a timer
52 reference that must outlive the awaitable. When @p Owning is
53 `true`, the timer is constructed internally in `await_suspend`
54 from the execution context in `io_env`.
55
56 @tparam A The inner IoAwaitable type (decayed).
57 @tparam Timer The timer type (`timer` or `native_timer<B>`).
58 @tparam Owning When `true`, the awaitable owns its timer.
59 */
60 template<typename A, typename Timer, bool Owning = false>
61 struct cancel_at_awaitable
62 {
63 struct stop_forwarder
64 {
65 std::stop_source* src_;
66 2 void operator()() const noexcept
67 {
68 2 src_->request_stop();
69 2 }
70 };
71
72 using time_point = std::chrono::steady_clock::time_point;
73 using stop_cb_type = std::stop_callback<stop_forwarder>;
74 using timer_storage = std::conditional_t<
75 Owning, std::optional<Timer>, Timer*>;
76
77 A inner_;
78 timer_storage timer_;
79 time_point deadline_;
80 std::stop_source stop_src_;
81 capy::io_env inner_env_;
82 alignas(stop_cb_type) unsigned char cb_buf_[sizeof(stop_cb_type)];
83 bool cb_active_ = false;
84
85 /// Construct with a caller-supplied timer reference.
86 18 cancel_at_awaitable(
87 A&& inner,
88 Timer& timer,
89 time_point deadline)
90 requires (!Owning)
91 18 : inner_(std::move(inner))
92 18 , timer_(&timer)
93 18 , deadline_(deadline)
94 {
95 18 }
96
97 /// Construct without a timer (created in `await_suspend`).
98 6 cancel_at_awaitable(
99 A&& inner,
100 time_point deadline)
101 requires Owning
102 6 : inner_(std::move(inner))
103 6 , deadline_(deadline)
104 {
105 6 }
106
107 48 ~cancel_at_awaitable()
108 {
109 48 destroy_parent_cb();
110 48 }
111
112 // Only moved before await_suspend, when cb_active_ is false
113 24 cancel_at_awaitable(cancel_at_awaitable&& o) noexcept(
114 std::is_nothrow_move_constructible_v<A>)
115 24 : inner_(std::move(o.inner_))
116 24 , timer_(std::move(o.timer_))
117 24 , deadline_(o.deadline_)
118 24 , stop_src_(std::move(o.stop_src_))
119 {
120 24 }
121
122 cancel_at_awaitable(cancel_at_awaitable const&) = delete;
123 cancel_at_awaitable& operator=(cancel_at_awaitable const&) = delete;
124 cancel_at_awaitable& operator=(cancel_at_awaitable&&) = delete;
125
126 24 bool await_ready() const noexcept { return false; }
127
128 24 auto await_suspend(
129 std::coroutine_handle<> h,
130 capy::io_env const* env)
131 {
132 if constexpr (Owning)
133 6 timer_.emplace(env->executor.context());
134
135 24 timer_->expires_at(deadline_);
136
137 // Launch fire-and-forget timeout (starts suspended)
138 24 auto timeout = make_timeout(*timer_, stop_src_);
139 48 timeout.h_.promise().set_env_owned({
140 env->executor,
141 stop_src_.get_token(),
142 24 env->frame_allocator});
143 // Runs synchronously until timer.wait() suspends
144 24 timeout.h_.resume();
145 // timeout goes out of scope; destructor is a no-op,
146 // the coroutine self-destroys via suspend_never
147
148 // Forward parent cancellation
149 24 new (cb_buf_) stop_cb_type(
150 24 env->stop_token, stop_forwarder{&stop_src_});
151 24 cb_active_ = true;
152
153 // Start the inner op with our interposed stop_token
154 24 inner_env_ = {
155 env->executor,
156 stop_src_.get_token(),
157 24 env->frame_allocator};
158 48 return inner_.await_suspend(h, &inner_env_);
159 48 }
160
161 24 decltype(auto) await_resume()
162 {
163 // Cancel whichever is still pending (idempotent)
164 24 stop_src_.request_stop();
165 24 destroy_parent_cb();
166 24 return inner_.await_resume();
167 }
168
169 72 void destroy_parent_cb() noexcept
170 {
171 72 if (cb_active_)
172 {
173 std::launder(reinterpret_cast<stop_cb_type*>(
174 24 cb_buf_))->~stop_cb_type();
175 24 cb_active_ = false;
176 }
177 72 }
178 };
179
180 } // namespace boost::corosio::detail
181
182 #endif
183