Boost (Beast) websocket:同步写入挂起

Question

我遇到了一个关于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，该问题会间歇性地发生(特别是由于远程端点与网络的物理连接断开)。

发生此问题时，同步stream.write()调用最终会挂起很长一段时间(分钟)，直到套接字最终关闭。我理解这种行为很可能是因为我的程序在没有远程acks的情况下继续写入流，直到发送缓冲区满为止。我想知道是否有一种方法可以将超时应用于写调用，或者是否有一个更类似于stream.try_write()的接口，在那里我可以将错误处理提升到用户级。

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)

Answer 1

Beast websockets不支持非阻塞模式。如果在与websocket流一起使用的套接字上设置此模式，则在某些情况下，websocket::stream的实现将产生未定义的行为。缺少超时是同步代码的一个普遍问题。除了使用异步操作之外，您确实别无选择。您说过要直接从堆栈发送缓冲区，这在异步上下文中通过使用协程很容易实现(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，包括在低期望延迟的情况下。

下面是来自Boost.Asio作者关于实现超低延迟的直接建议：https://groups.google.com/a/isocpp.org/d/msg/sg14/FoLFHXqZSck/i4rdO-O3BQAJ

Answer 2

我能够通过将底层流套接字设置为非阻塞模式来解决这个问题：

socket.non_blocking(true);

在此模式下，一旦发送缓冲区变满，write()调用将返回boost::system::error_code::try_again (也称为posix EAGAIN)。