Boost (Beast） websocket:同步写入挂起

Boost (Beast) websocket: synchronous write hangs

本文关键字：同步 挂起 websocket Beast Boost      更新时间：2023-10-16

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

发生此问题时，同步 stream.write(( 调用最终挂起很长时间 (分钟( 直到套接字最终关闭。我知道这种行为可能是因为我的程序在没有来自远程的确认的情况下继续写入流，直到发送缓冲区已满。我想知道是否有办法将超时应用于写入调用，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界面。但是，我担心这会通过将套接字写入操作推迟到下一次io_context迭代来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 '00', 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)

Beast websockets 不支持非阻塞模式。在某些情况下，如果在与 websocket 流一起使用的套接字上设置此模式，则websocket::stream的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。您说要直接从堆栈发送缓冲区，这在使用协程在异步上下文中很容易完成(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，包括低所需延迟的情况。

以下是Boost.Asio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

我能够通过将底层流套接字置于非阻塞模式来解决此问题：

socket.non_blocking(true);

在此模式下，一旦发送缓冲区已满，write()调用将立即返回boost::system::error_code::try_again(也称为 posixEAGAIN(。

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