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include/boost/capy/detail/await_suspend_helper.hpp

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await_suspend_helper.hpp
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boost::capy::detail::symmetric_transfer(std::__n4861::coroutine_handle<void>) :118 107386x 100.0% 100.0% std::__n4861::coroutine_handle<void> boost::capy::detail::call_await_suspend<boost::capy::test::write_stream::write_some<boost::capy::detail::slice_of<boost::capy::const_buffer> >(boost::capy::detail::slice_of<boost::capy::const_buffer>)::awaitable>(boost::capy::test::write_stream::write_some<boost::capy::detail::slice_of<boost::capy::const_buffer> >(boost::capy::detail::slice_of<boost::capy::const_buffer>)::awaitable*, std::__n4861::coroutine_handle<void>, boost::capy::await_context const*) :190 5x 75.0% 57.0%
Line TLA Hits Source Code
1 //
2 // Copyright (c) 2025 Vinnie Falco ([email protected])
3 // Copyright (c) 2026 Steve Gerbino
4 // Copyright (c) 2026 Michael Vandeberg
5 //
6 // Distributed under the Boost Software License, Version 1.0. (See accompanying
7 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
8 //
9 // Official repository: https://github.com/cppalliance/capy
10 //
11
12 #ifndef BOOST_CAPY_DETAIL_AWAIT_SUSPEND_HELPER_HPP
13 #define BOOST_CAPY_DETAIL_AWAIT_SUSPEND_HELPER_HPP
14
15 #include <coroutine>
16 #include <boost/capy/ex/await_context.hpp>
17 #include <boost/capy/detail/config.hpp>
18
19 #include <type_traits>
20
21 namespace boost {
22 namespace capy {
23 namespace detail {
24
25 /** Perform symmetric transfer, working around MSVC codegen bugs.
26
27 MSVC stores the `std::coroutine_handle<>` returned from
28 `await_suspend` in a hidden `__$ReturnUdt$` variable located
29 on the coroutine frame. When another thread resumes or destroys
30 the frame between the store and the read-back for the
31 symmetric-transfer tail-call, the read hits freed memory.
32
33 This occurs in two scenarios:
34
35 @li `await_suspend` calls `h.destroy()` then returns a handle
36 (e.g. `when_all_runner` and `when_any_runner` final_suspend).
37 The return value is written to the now-destroyed frame.
38
39 @li `await_suspend` hands the continuation to another thread
40 via an executor handoff (e.g. `post()` or `dispatch()`),
41 which may resume the parent. The parent can destroy this
42 frame before the runtime reads `__$ReturnUdt$` (e.g.
43 `boundary_trampoline` final_suspend).
44
45 On affected compilers this function calls `h.resume()` on the
46 current stack and returns `void`, causing unconditional
47 suspension. The trade-off is O(n) stack growth instead of
48 O(1) tail-calls.
49
50 This defect affects MSVC 19.34 through 19.44 on every target
51 and is fixed in MSVC 19.50 (VS 2026 / 18.0). Measured on
52 19.44 the caller builds the hidden return slot at
53 `__coro_frame_ptr$ + 0xC0`, on the coroutine frame; on 19.51
54 it is an `rsp`-relative stack temporary, so destroying the
55 frame no longer invalidates it.
56
57 Do not widen this gate on the basis of Developer Community
58 ticket 10251975, tagged "Fixed in VS 2022 17.9 Preview 2";
59 19.39 reproduces the fault identically to 19.34.
60
61 On ARM64 the workaround does not retire at 19.50, because
62 that target has a second, unrelated defect. With a real
63 symmetric transfer, MSVC ARM64 release loses the handler for
64 a `try` region that spans the suspend point: a `throw`
65 inside that region is not caught by the `catch` beside it and
66 escapes to the promise's `unhandled_exception`. A `catch(...)`
67 misses it too, so the region is not found at all rather than
68 the handler failing to match. Any `await_suspend` that returns
69 a handle triggers it, whether the coroutine resumes at once
70 or later from another call stack, and whether the throw comes
71 from `await_resume` or from the body after it. A standalone
72 program without capy reproduces it on 19.44 and 19.51; the
73 first clause of the gate already covers 19.44. Debug builds
74 are unaffected, as is x64 at the same toolset.
75 `testCatchAfterDeferredResume` in test/unit/task.cpp covers
76 this.
77
78 The void-returning transfer alone does not cover every case on
79 19.51. See `await_ready_then_suspend` below for the second
80 safeguard that `task` applies where this one is on.
81
82 `_M_ARM64EC` is included conservatively. It generates ARM64
83 code and has not been tested here; keeping the workaround on
84 is the safe direction, since it costs stack depth rather than
85 correctness.
86
87 The gate deliberately excludes Clang by testing
88 `BOOST_CAPY_MSVC` rather than `_MSC_VER`. Both `clang-cl` and
89 `clang++` targeting Windows define `_MSC_VER` for ABI
90 compatibility, but generate a correct tail-call.
91
92 Note that a probe which merely poisons the destroyed frame
93 cannot validate this gate. Routing the return through this
94 function moves the frame write to after `destroy()`, which
95 repairs the poison pattern and hides the defect. The
96 regression test in
97 test/unit/detail/await_suspend_helper.cpp unmaps the frame
98 instead, so any post-destroy access faults.
99
100 On unaffected compilers the handle is returned directly for
101 proper symmetric transfer.
102
103 Callers must use `auto` return type on their `await_suspend`
104 so the return type adapts per platform.
105
106 @param h The coroutine handle to transfer to.
107 */
108 #if BOOST_CAPY_SYMMETRIC_TRANSFER_WORKAROUND
109 inline void symmetric_transfer(std::coroutine_handle<> h) noexcept
110 {
111 // safe_resume is not needed here: the calling coroutine is
112 // about to suspend unconditionally. When it later resumes,
113 // await_resume restores TLS from the promise's environment.
114 h.resume();
115 }
116 #else
117 inline std::coroutine_handle<>
118 107386x symmetric_transfer(std::coroutine_handle<> h) noexcept
119 {
120 107386x return h;
121 }
122 #endif
123
124 #if BOOST_CAPY_SYMMETRIC_TRANSFER_WORKAROUND
125 /** Ask whether an awaitable is ready, and suspend on it if not.
126
127 Used by the `transform_awaiter` of `task` where the
128 symmetric-transfer workaround is on. There the awaiter's own
129 `await_ready` returns `false`, and its `await_suspend` calls this
130 instead, so the wrapped awaitable's `await_ready` runs inside
131 `await_suspend` rather than at the start of the `co_await`.
132
133 This avoids a second MSVC ARM64 release defect. When the wrapped
134 `await_ready` is evaluated at the start of the `co_await`, and
135 performs an atomic read-modify-write (as
136 `async_waker::wait_awaiter` does to consume a latched wakeup), a
137 `try` region spanning the `co_await` can lose its handler if a
138 call precedes the `try`. This happens even with the workaround
139 above in place. Observed on 19.51; not on 19.44. A plain or
140 atomic load in `await_ready` was not affected. The regression
141 tests are `testCatchAfterRmwAwaitReady` and
142 `testCatchAroundWakerWait` in test/unit/task.cpp.
143
144 The awaiting coroutine continues without suspending when the
145 wrapped awaitable is ready, or when its `await_suspend` returns
146 `false`. When the wrapped `await_suspend` returns a handle, that
147 handle is resumed through `symmetric_transfer` and the awaiting
148 coroutine stays suspended. Returning `false` instead of resuming
149 inline keeps immediate completions from growing the stack.
150
151 @param a The wrapped awaitable.
152
153 @param h The coroutine performing the `co_await`.
154
155 @param env The execution environment to pass to the wrapped
156 awaitable's `await_suspend`.
157
158 @return `false` to continue without suspending, otherwise `true`.
159 */
160 template<class Awaitable>
161 bool
162 await_ready_then_suspend(
163 Awaitable& a,
164 std::coroutine_handle<> h,
165 await_context const* env) noexcept
166 {
167 // Called exactly once: await_ready may consume state.
168 if(a.await_ready())
169 return false;
170 using R = decltype(a.await_suspend(h, env));
171 if constexpr (std::is_same_v<R, bool>)
172 {
173 return a.await_suspend(h, env);
174 }
175 else if constexpr (std::is_void_v<R>)
176 {
177 a.await_suspend(h, env);
178 return true;
179 }
180 else
181 {
182 symmetric_transfer(a.await_suspend(h, env));
183 return true;
184 }
185 }
186 #endif
187
188 // Helper to normalize await_suspend return types to std::coroutine_handle<>
189 template<typename Awaitable>
190 256x std::coroutine_handle<> call_await_suspend(
191 Awaitable* a,
192 std::coroutine_handle<> h,
193 await_context const* env)
194 {
195 using R = decltype(a->await_suspend(h, env));
196 if constexpr (std::is_void_v<R>)
197 {
198 1x a->await_suspend(h, env);
199 1x return std::noop_coroutine();
200 }
201 else if constexpr (std::is_same_v<R, bool>)
202 {
203 232x if(a->await_suspend(h, env))
204 1x return std::noop_coroutine();
205 231x return h;
206 }
207 else
208 {
209 23x return a->await_suspend(h, env);
210 }
211 }
212
213 } // namespace detail
214 } // namespace capy
215 } // namespace boost
216
217 #endif
218