include/boost/capy/detail/await_suspend_helper.hpp
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boost::capy::detail::symmetric_transfer(std::__n4861::coroutine_handle<void>)
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|---|---|---|---|
| 1 | // | ||
| 2 | // Copyright (c) 2025 Vinnie Falco (vinnie.falco@gmail.com) | ||
| 3 | // Copyright (c) 2026 Steve Gerbino | ||
| 4 | // | ||
| 5 | // Distributed under the Boost Software License, Version 1.0. (See accompanying | ||
| 6 | // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) | ||
| 7 | // | ||
| 8 | // Official repository: https://github.com/cppalliance/capy | ||
| 9 | // | ||
| 10 | |||
| 11 | #ifndef BOOST_CAPY_DETAIL_AWAIT_SUSPEND_HELPER_HPP | ||
| 12 | #define BOOST_CAPY_DETAIL_AWAIT_SUSPEND_HELPER_HPP | ||
| 13 | |||
| 14 | #include <coroutine> | ||
| 15 | #include <boost/capy/ex/io_env.hpp> | ||
| 16 | |||
| 17 | #include <type_traits> | ||
| 18 | |||
| 19 | namespace boost { | ||
| 20 | namespace capy { | ||
| 21 | namespace detail { | ||
| 22 | |||
| 23 | /** Perform symmetric transfer, working around an MSVC codegen bug. | ||
| 24 | |||
| 25 | MSVC stores the `std::coroutine_handle<>` returned from | ||
| 26 | `await_suspend` in a hidden `__$ReturnUdt$` variable located | ||
| 27 | on the coroutine frame. When another thread resumes or destroys | ||
| 28 | the frame between the store and the read-back for the | ||
| 29 | symmetric-transfer tail-call, the read hits freed memory. | ||
| 30 | |||
| 31 | This occurs in two scenarios: | ||
| 32 | |||
| 33 | @li `await_suspend` calls `h.destroy()` then returns a handle | ||
| 34 | (e.g. `when_all_runner` and `when_any_runner` final_suspend). | ||
| 35 | The return value is written to the now-destroyed frame. | ||
| 36 | |||
| 37 | @li `await_suspend` hands the continuation to another thread | ||
| 38 | via an executor handoff (e.g. `post()` or `dispatch()`), | ||
| 39 | which may resume the parent. The parent can destroy this | ||
| 40 | frame before the runtime reads `__$ReturnUdt$` (e.g. | ||
| 41 | `boundary_trampoline` final_suspend). | ||
| 42 | |||
| 43 | On MSVC this function calls `h.resume()` on the current stack | ||
| 44 | and returns `void`, causing unconditional suspension. The | ||
| 45 | trade-off is O(n) stack growth instead of O(1) tail-calls. | ||
| 46 | |||
| 47 | On other compilers the handle is returned directly for proper | ||
| 48 | symmetric transfer. | ||
| 49 | |||
| 50 | Callers must use `auto` return type on their `await_suspend` | ||
| 51 | so the return type adapts per platform. | ||
| 52 | |||
| 53 | @param h The coroutine handle to transfer to. | ||
| 54 | */ | ||
| 55 | #if BOOST_CAPY_WORKAROUND(_MSC_VER, >= 1) | ||
| 56 | inline void symmetric_transfer(std::coroutine_handle<> h) noexcept | ||
| 57 | { | ||
| 58 | // safe_resume is not needed here: the calling coroutine is | ||
| 59 | // about to suspend unconditionally. When it later resumes, | ||
| 60 | // await_resume restores TLS from the promise's environment. | ||
| 61 | h.resume(); | ||
| 62 | } | ||
| 63 | #else | ||
| 64 | inline std::coroutine_handle<> | ||
| 65 | 2971x | symmetric_transfer(std::coroutine_handle<> h) noexcept | |
| 66 | { | ||
| 67 | 2971x | return h; | |
| 68 | } | ||
| 69 | #endif | ||
| 70 | |||
| 71 | // Helper to normalize await_suspend return types to std::coroutine_handle<> | ||
| 72 | template<typename Awaitable> | ||
| 73 | 7x | std::coroutine_handle<> call_await_suspend( | |
| 74 | Awaitable* a, | ||
| 75 | std::coroutine_handle<> h, | ||
| 76 | io_env const* env) | ||
| 77 | { | ||
| 78 | using R = decltype(a->await_suspend(h, env)); | ||
| 79 | if constexpr (std::is_void_v<R>) | ||
| 80 | { | ||
| 81 | ✗ | a->await_suspend(h, env); | |
| 82 | ✗ | return std::noop_coroutine(); | |
| 83 | } | ||
| 84 | else if constexpr (std::is_same_v<R, bool>) | ||
| 85 | { | ||
| 86 | if(a->await_suspend(h, env)) | ||
| 87 | return std::noop_coroutine(); | ||
| 88 | return h; | ||
| 89 | } | ||
| 90 | else | ||
| 91 | { | ||
| 92 | 7x | return a->await_suspend(h, env); | |
| 93 | } | ||
| 94 | } | ||
| 95 | |||
| 96 | } // namespace detail | ||
| 97 | } // namespace capy | ||
| 98 | } // namespace boost | ||
| 99 | |||
| 100 | #endif | ||
| 101 |