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我遇到了 boost beast websocket 流的问题。当我尝试写入远程端点已停止响应的流时（特别是由于远程与网络物理断开连接），此问题会间歇性发生。

发生此问题时，同步 stream.write() 调用最终会挂起很长时间（几分钟），直到套接字最终关闭。我知道这种行为很可能是因为我的程序继续写入流而没有来自远程的确认，直到发送缓冲区已满。我想知道是否有一种方法可以对 write 调用应用超时，或者是否有一个更类似于 stream.try_write() 的接口，我可以在其中将错误处理提升到用户级别。

我确实意识到一种选择是使用 async_write 接口。但是，我担心这会将套接字写入操作推迟到 io_context 的下一次迭代，从而对我的发送性能产生负面影响。

下面是调用挂起时线程的堆栈跟踪。

#0  0x00007f468cf33624 in poll () from /lib64/libc.so.6
#1  0x000000000043e5a7 in boost::asio::detail::socket_ops::poll_write (ec=..., msec=-1, state=0 '\000', s=16)
    at /usr/include/boost/asio/detail/impl/socket_ops.ipp:1898
#2  boost::asio::detail::socket_ops::sync_send (ec=..., all_empty=<optimized out>, flags=0, count=<optimized out>, bufs=0x7fff43c17e20, 
    state=<optimized out>, s=<optimized out>) at /usr/include/boost/asio/detail/impl/socket_ops.ipp:1224
#3  boost::asio::detail::reactive_socket_service_base::send<boost::asio::detail::prepared_buffers<boost::asio::const_buffer, 64ul> > (impl=..., 
    buffers=..., ec=..., this=<optimized out>, flags=0) at /usr/include/boost/asio/detail/reactive_socket_service_base.hpp:245
#4  0x0000000000481c71 in boost::asio::basic_stream_socket<boost::asio::ip::tcp>::write_some<boost::asio::detail::prepared_buffers<boost::asio::const_buffer, 64ul> > (ec=..., buffers=..., this=0x108ad50) at /usr/include/boost/asio/buffer.hpp:941
#5  boost::asio::detail::write_buffer_sequence<boost::asio::basic_stream_socket<boost::asio::ip::tcp>, boost::beast::buffers_cat_view<boost::asio::mutable_buffer, boost::beast::buffers_prefix_view<boost::beast::buffers_suffix<boost::beast::basic_multi_buffer<std::allocator<char> >::const_buffers_type> > >, boost::beast::buffers_cat_view<boost::asio::mutable_buffer, boost::beast::buffers_prefix_view<boost::beast::buffers_suffix<boost::beast::basic_multi_buffer<std::allocator<char> >::const_buffers_type> > >::const_iterator, boost::asio::detail::transfer_all_t> (completion_condition=..., ec=..., buffers=..., s=...)
    at /usr/include/boost/asio/impl/write.hpp:53
#6  boost::asio::write<boost::asio::basic_stream_socket<boost::asio::ip::tcp>, boost::beast::buffers_cat_view<boost::asio::mutable_buffer, boost::beast::buffers_prefix_view<boost::beast::buffers_suffix<boost::beast::basic_multi_buffer<std::allocator<char> >::const_buffers_type> > >, boost::asio::detail::transfer_all_t> (ec=..., buffers=..., s=..., completion_condition=...) at /usr/include/boost/asio/impl/write.hpp:69
#7  boost::asio::write<boost::asio::basic_stream_socket<boost::asio::ip::tcp>, boost::beast::buffers_cat_view<boost::asio::mutable_buffer, boost::beast::buffers_prefix_view<boost::beast::buffers_suffix<boost::beast::basic_multi_buffer<std::allocator<char> >::const_buffers_type> > > > (ec=..., buffers=..., s=...)
    at /usr/include/boost/asio/impl/write.hpp:92
#8  boost::beast::websocket::stream<boost::asio::basic_stream_socket<boost::asio::ip::tcp> >::write_some<boost::beast::basic_multi_buffer<std::allocator<char> >::const_buffers_type> (this=this@entry=0x108ad50, fin=fin@entry=true, buffers=..., ec=...) at /usr/include/boost/beast/websocket/impl/write.ipp:625
#9  0x000000000042c5e1 in boost::beast::websocket::stream<boost::asio::basic_stream_socket<boost::asio::ip::tcp> >::write<boost::beast::basic_multi_buffer<std::allocator<char> >::const_buffers_type> (ec=..., buffers=..., this=0x108ad50)
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2 回答 2

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Beast websockets 不支持非阻塞模式。websocket::stream如果您在与 websocket 流一起使用的套接字上设置此模式，则在某些情况下，实现将产生未定义的行为。缺少超时是同步代码的普遍问题。除了使用异步操作之外，您真的别无选择。您说您想直接从堆栈发送缓冲区，这可以通过使用协程在异步上下文中轻松完成（请参阅boost::asio::spawn和boost::asio::yield_context）。

有许多技术允许异步 I/O 执行与同步 I/O 相同或更好的性能，包括在所需延迟较低的情况下。

以下是 Boost.Asio 的作者直接提供的关于实现超低延迟的建议：https ://groups.google.com/a/isocpp.org/d/msg/sg14/FoLFHXqZSck/i4rdO-O3BQAJ

于 2018-09-26T03:23:36.303 回答
-1

我能够通过将底层流套接字置于非阻塞模式来解决这个问题：

socket.non_blocking(true);

在这种模式下，一旦发送缓冲区变满，write()调用将立即返回boost::system::error_code::try_again（又名 posix EAGAIN）。

于 2018-09-24T13:07:57.497 回答