I think you should probably have a call to socket.shutdown(boost::asio::ip::tcp::socket::shutdown_both, ec) in there before the call to socket.close().
The boost::asio documentation for basic_stream_socket::close states:
For portable behaviour with respect to graceful closure of a connected socket, call shutdown() before closing the socket.
This should ensure that any pending operations on the socket are properly cancelled and any buffers are flushed prior to the call to socket.close.