boost::asio::co_spawn 不传播异常

Question

我在 boost::asio 方面涉足协程，我对异常处理感到困惑。从 examples in 文档来看，看起来任何 'fail' error_code 都变成了异常 - 所以我 希望假设 任何抛出的异常都会也被传播回 co_spawn 调用。但事实并非如此：

#define BOOST_ASIO_HAS_CO_AWAIT
#define BOOST_ASIO_HAS_STD_COROUTINE

#include <iostream>

#include <boost/asio/awaitable.hpp>
#include <boost/asio/co_spawn.hpp>
#include <boost/asio/detached.hpp>
#include <boost/asio/io_context.hpp>
#include <boost/asio/executor_work_guard.hpp>

namespace this_coro = boost::asio::this_coro;

boost::asio::awaitable<void> async_op()
{
    std::cout << "About to throw" << std::endl;
    throw std::runtime_error{"Bang!"};
}

int main()
{
    auto ctx = boost::asio::io_context{};
    auto guard = boost::asio::make_work_guard(ctx.get_executor());

    boost::asio::co_spawn(ctx, async_op, boost::asio::detached);

    ctx.run();
}

如果这是运行在调试器中，您可以看到抛出的异常，但它似乎只是挂起。暂停调试器显示 ctx.run() 正在等待新工作（由于 executor_work_guard）。所以看起来 boost::asio 里面的东西已经默默地吞下了异常。

作为实验，我将异步操作切换为使用 boost::asio 库调用：

boost::asio::awaitable<void> async_op()
{
    auto executor = co_await this_coro::executor;
    auto socket = boost::asio::ip::tcp::socket{executor};

    std::cout << "Starting resolve" << std::endl;
    auto resolver = boost::asio::ip::tcp::resolver{executor};
    const auto endpoints = co_await resolver.async_resolve("localhost",
                                                           "4444",
                                                           boost::asio::use_awaitable);



    std::cout << "Starting connect (num endpoints: " << endpoints.size() << ")" << std::endl;
    co_await boost::asio::async_connect(socket, endpoints, boost::asio::use_awaitable);
    std::cout << "Exited" << std::endl;
}

我在端口 4444 上没有服务器运行，所以这应该会立即失败 - 而且它会静静地失败。暂停调试器表明它卡在 epoll 中等待某些东西（我在 Linux）。

将 async_connect CompletionToken 换成 boost::asio::redirect_error 表明操作失败：

co_await boost::asio::async_connect(socket,
                                    endpoints, 
                                    boost::asio::redirect_error(boost::asio::use_awaitable, ec));
std::cout << "Exited: " << ec.message() << std::endl;

产量：

Starting resolve
Starting connect (num endpoints: 1)
Exited: Connection refused

那么我如何传播异常，并从 error_code 中创建它们，而不是 boost::asio 中的协程？

Answer 1

boost::asio::co_spawn 创建一个 separate thread。这意味着不会传播异常。您可以在这里阅读更多相关信息：

Will main() catch exceptions thrown from threads?
How can I propagate exceptions between threads?

但是 co_spawn 支持带有签名 void(std::exception_ptr, R) 的完成处理程序。在您的示例中，您使用了 boost::asio::detached ，这意味着忽略完成结果。简单地传播它 write a custom handler.

boost::asio::co_spawn 不传播异常

boost::asio::co_spawn does not propagate exception

c++

boost

exception

coroutine