对称传输不会阻止 C++20 协程的堆栈溢出

分类：编程问答

如何解决对称传输不会阻止 C++20 协程的堆栈溢出

根据博文 C++ Coroutines: Understanding Symmetric Transfer 对称传输，您可以暂停一个协程并恢复另一个协程，而不会消耗任何额外的堆栈空间。这可以防止堆栈溢出，当协程包含循环和 co_await 任务可能会在该循环的主体内同步完成时，可能会发生这种情况。

即使下面的代码示例使用对称传输，它也会由于堆栈溢出而崩溃。请注意，下面的代码是重现堆栈溢出的最小示例：例如，如果我在头文件中包含类型为 Type 的析构函数的定义，那么我不会得到堆栈溢出。 

// type.h
#pragma once

struct Type {
  ~Type();
};

// type.cc
#include "type.h"

Type::~Type() {}

// main.cc
#include <cstdint>
#include <exception>
#include <type_traits>
#include <utility>

#include "type.h"

#if __has_include(<coroutine>)  // when using g++
#include <coroutine>
namespace coro {
using std::coroutine_handle;
using std::noop_coroutine;
using std::suspend_always;
}  // namespace coro
#elif __has_include(<experimental/coroutine>)  // when using clang++
#include <experimental/coroutine>
namespace coro {
using std::experimental::coroutine_handle;
using std::experimental::noop_coroutine;
using std::experimental::suspend_always;
}  // namespace coro
#endif

template <typename T = void>
class Task {
 public:
  struct PromiseBase {
    friend struct final_awaitable;

    struct final_awaitable {
      bool await_ready() const noexcept { return false; }

      template <typename PROMISE>
      coro::coroutine_handle<> await_suspend(
          coro::coroutine_handle<PROMISE> coro) noexcept {
        if (coro.promise().m_continuation) {
          return coro.promise().m_continuation;
        } else {
          // The top-level task started from within main() does not have a
          // continuation. This will give control back to the main function.
          return coro::noop_coroutine();
        }
      }

      void await_resume() noexcept {}
    };

    coro::suspend_always initial_suspend() noexcept { return {}; }

    auto final_suspend() noexcept { return final_awaitable{}; }

    void unhandled_exception() noexcept { std::terminate(); }

    void set_continuation(coro::coroutine_handle<> continuation) noexcept {
      m_continuation = continuation;
    }

   private:
    coro::coroutine_handle<> m_continuation;
  };

  struct PromiseVoid : public PromiseBase {
    auto get_return_object() { return coroutine_handle_t::from_promise(*this); }

    void return_void() noexcept {}

    void result() {}
  };

  struct PromiseT : public PromiseBase {
    auto get_return_object() { return coroutine_handle_t::from_promise(*this); }

    void return_value(T&& v) { value = std::move(v); }

    T&& result() && { return std::move(value); }

    T value;
  };

  using promise_type =
      std::conditional_t<std::is_same_v<T,void>,PromiseVoid,PromiseT>;

  using coroutine_handle_t = coro::coroutine_handle<promise_type>;

  Task(coroutine_handle_t coroutine) : m_coroutine(coroutine) {}

  ~Task() {
    if (m_coroutine) {
      m_coroutine.destroy();
    }
  }

  void start() noexcept { m_coroutine.resume(); }

  auto operator co_await() const noexcept { return awaitable{m_coroutine}; }

 private:
  struct awaitable {
    coroutine_handle_t m_coroutine;

    awaitable(coroutine_handle_t coroutine) noexcept : m_coroutine(coroutine) {}

    bool await_ready() const noexcept { return false; }

    coro::coroutine_handle<> await_suspend(
        coro::coroutine_handle<> awaitingCoroutine) noexcept {
      m_coroutine.promise().set_continuation(awaitingCoroutine);
      return m_coroutine;
    }

    auto await_resume() { return std::move(m_coroutine.promise()).result(); }
  };
  coroutine_handle_t m_coroutine;
};

Task<Type> coro2() { co_return Type{}; }

Task<> coro1() { auto s = co_await coro2(); }

Task<> test() {
  for (std::uint64_t i = 0; i != 50000000; ++i) {
    co_await coro1();
  }
}

int main() {
  auto task = test();
  task.start();
}

我使用 clang++ version 12.0.1 和 g++ version 11.1.0 编译代码：

clang++-12 main.cc type.cc -std=c++20 -stdlib=libc++ -O3 -fsanitize=address

g++-11 main.cc type.cc -std=c++20 -O3 -fsanitize=address

这是 clang++ 的截断输出：

$ ./a.out 

AddressSanitizer:DEADLYSIGNAL
=================================================================
==20846==ERROR: AddressSanitizer: stack-overflow on address 0x7ffc76b1aff8 (pc 0x0000004cb7ab bp 0x7ffc76b1b050 sp 0x7ffc76b1afa0 T0)
    #0 0x4cb7ab in coro1() (.resume) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x4cb7ab)
    #1 0x4cbe4a in test() (.resume) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x4cbe4a)
    #2 0x4cbe4a in test() (.resume) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x4cbe4a)
    #3 0x4cbe4a in test() (.resume) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x4cbe4a)
    #4 0x4cbe4a in test() (.resume) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x4cbe4a)
    #5 0x4cbe4a in test() (.resume) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x4cbe4a)
    #6 0x4cbe4a in test() (.resume) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x4cbe4a)
    #7 0x4cbe4a in test() (.resume) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x4cbe4a)
    #8 0x4cbe4a in test() (.resume) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x4cbe4a)
    #9 0x4cbe4a in test() (.resume) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x4cbe4a)
    #10 0x4cbe4a in test() (.resume) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x4cbe4a)
    #11 0x4cbe4a in test() (.resume) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x4cbe4a)
    #12 0x4cbe4a in test() (.resume) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x4cbe4a)
    #13 0x4cbe4a in test() (.resume) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x4cbe4a)
    #14 0x4cbe4a in test() (.resume) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x4cbe4a)
    #15 0x4cbe4a in test() (.resume) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x4cbe4a)
    #16 0x4cbe4a in test() (.resume) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x4cbe4a)
    #17 0x4cbe4a in test() (.resume) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x4cbe4a)
    #18 0x4cbe4a in test() (.resume) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x4cbe4a)
    #19 0x4cbe4a in test() (.resume) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x4cbe4a)
    #20 0x4cbe4a in test() (.resume) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x4cbe4a)
    #21 0x4cbe4a in test() (.resume) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x4cbe4a)
    #22 0x4cbe4a in test() (.resume) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x4cbe4a)
    #23 0x4cbe4a in test() (.resume) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x4cbe4a)
    #24 0x4cbe4a in test() (.resume) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x4cbe4a)
    #25 0x4cbe4a in test() (.resume) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x4cbe4a)
...

这是 g++ 的截断输出：

$ ./a.out

AddressSanitizer:DEADLYSIGNAL
=================================================================
==21434==ERROR: AddressSanitizer: stack-overflow on address 0x7fff2904dff8 (pc 0x7fd5f7825180 bp 0x7fff2904e880 sp 0x7fff2904dff0 T0)
    #0 0x7fd5f7825180 in __sanitizer::BufferedStackTrace::UnwindImpl(unsigned long,unsigned long,void*,bool,unsigned int) ../../../../src/libsanitizer/asan/asan_stack.cpp:57
    #1 0x7fd5f781b0eb in __sanitizer::BufferedStackTrace::Unwind(unsigned long,unsigned int) ../../../../src/libsanitizer/sanitizer_common/sanitizer_stacktrace.h:122
    #2 0x7fd5f781b0eb in operator delete(void*) ../../../../src/libsanitizer/asan/asan_new_delete.cpp:160
    #3 0x5643118400b7 in _Z5coro2v.destroy(coro2()::_Z5coro2v.frame*) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x20b7)
    #4 0x564311840e36 in _Z5coro1v.actor(coro1()::_Z5coro1v.frame*) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x2e36)
    #5 0x56431183fe20 in _Z5coro2v.actor(coro2()::_Z5coro2v.frame*) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x1e20)
    #6 0x564311840f15 in _Z5coro1v.actor(coro1()::_Z5coro1v.frame*) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x2f15)
    #7 0x564311841741 in _Z4testv.actor(test()::_Z4testv.frame*) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x3741)
    #8 0x564311840f15 in _Z5coro1v.actor(coro1()::_Z5coro1v.frame*) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x2f15)
    #9 0x56431183fe20 in _Z5coro2v.actor(coro2()::_Z5coro2v.frame*) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x1e20)
    #10 0x564311840f15 in _Z5coro1v.actor(coro1()::_Z5coro1v.frame*) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x2f15)
    #11 0x564311841741 in _Z4testv.actor(test()::_Z4testv.frame*) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x3741)
    #12 0x564311840f15 in _Z5coro1v.actor(coro1()::_Z5coro1v.frame*) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x2f15)
    #13 0x56431183fe20 in _Z5coro2v.actor(coro2()::_Z5coro2v.frame*) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x1e20)
    #14 0x564311840f15 in _Z5coro1v.actor(coro1()::_Z5coro1v.frame*) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x2f15)
    #15 0x564311841741 in _Z4testv.actor(test()::_Z4testv.frame*) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x3741)
    #16 0x564311840f15 in _Z5coro1v.actor(coro1()::_Z5coro1v.frame*) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x2f15)
    #17 0x56431183fe20 in _Z5coro2v.actor(coro2()::_Z5coro2v.frame*) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x1e20)
    #18 0x564311840f15 in _Z5coro1v.actor(coro1()::_Z5coro1v.frame*) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x2f15)
    #19 0x564311841741 in _Z4testv.actor(test()::_Z4testv.frame*) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x3741)
    #20 0x564311840f15 in _Z5coro1v.actor(coro1()::_Z5coro1v.frame*) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x2f15)
    #21 0x56431183fe20 in _Z5coro2v.actor(coro2()::_Z5coro2v.frame*) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x1e20)
    #22 0x564311840f15 in _Z5coro1v.actor(coro1()::_Z5coro1v.frame*) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x2f15)
    #23 0x564311841741 in _Z4testv.actor(test()::_Z4testv.frame*) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x3741)
    #24 0x564311840f15 in _Z5coro1v.actor(coro1()::_Z5coro1v.frame*) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x2f15)
    #25 0x56431183fe20 in _Z5coro2v.actor(coro2()::_Z5coro2v.frame*) (/home/leonard/Desktop/hiwi/async_io_uring/stack_overflow/a.out+0x1e20)

有趣的是，如果我用 clang++-12 main.cc type.cc -std=c++20 -stdlib=libc++ -O0 -fsanitize=address 编译，程序不会触发堆栈溢出，并且没有任何错误地退出。此外，如果我省略 -fsanitize=address，那么在使用 -O3 时会出现分段错误，而在使用 -O0 时则完全没有错误。

谁能告诉我我做错了什么？

解决方法

我在玩协程时遇到了类似的问题。我不是 100% 确定原因 堆栈建立起来，但我认为这可能会发生。

首先，我不认为对称传输是给定的，它取决于编译器优化，在某些情况下，编译器可能难以进行这种尾调用转换。一个原因可能是因为位于另一个 Type 编译单元中的非平凡析构函数（这只是一个猜测）。

阅读您提到的博客文章，它说：“但是，与对称传输形式相比，返回布尔值的版本在某些情况下在可优化性方面可能略胜一筹。”，所以这可能是因为编译器支持尚未完全成熟 (?)，尝试使用 bool-returning 形式可能是一个不错的选择。

我也很想对这个问题有一个好的答案，只是想根据我目前的发现发表我的意见，所以请不要把这个答案当作绝对真理。

编辑：

这是防止堆栈溢出的解决方法。它使用 bool 函数的 await_suspend() 返回版本。不幸的是，该解决方法引入了其他问题。例如，Task 类型不再是线程安全的。有关更多信息，请查看博文 C++ Coroutines: Understanding Symmetric Transfer 的“协程 TS 解决方案”部分。

// in main.cc
struct PromiseBase {
// ...
  struct final_awaitable {
  // ...
    template <typename PROMISE>
    void await_suspend(coro::coroutine_handle<PROMISE> coro) noexcept {
      if (coro.promise().m_continuation &&
          std::exchange(coro.promise().ready,true)) {
        // coro did not complete synchronously,therefore we need to resume
        // the continuation
        coro.promise().m_continuation.resume();
      }
    }
  // ...
  };

  bool ready{false};
// ...
};

// in main.cc
struct awaitable {
// ...
    // The bool returning version of await_suspend resumes awaitingCoroutine
    // without consuming any additional stack-space if the value false is
    // returned. Otherwise,it returns control to the caller/resumer of
    // awaitingCoroutine.
    bool await_suspend(coro::coroutine_handle<> awaitingCoroutine) noexcept {
      m_coroutine.promise().set_continuation(awaitingCoroutine);
      m_coroutine.resume();
      // resume awaitingCoroutine if m_coroutine completed synchronously
      return !std::exchange(m_coroutine.promise().ready,true);
    }
// ...
};

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