boost_beast/example/common/write_msg.hpp

//
// Copyright (c) 2016-2017 Vinnie Falco (vinnie dot falco at gmail dot com)
//
// Distributed under the Boost Software License, Version 1.0. (See accompanying
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
//
// Official repository: https://github.com/boostorg/beast
//

#ifndef BOOST_BEAST_EXAMPLE_COMMON_WRITE_MSG_HPP
#define BOOST_BEAST_EXAMPLE_COMMON_WRITE_MSG_HPP

#include <boost/beast/core/async_result.hpp>
#include <boost/beast/core/handler_ptr.hpp>
#include <boost/beast/core/type_traits.hpp>
#include <boost/beast/http/message.hpp>
#include <boost/beast/http/write.hpp>
#include <boost/beast/http/type_traits.hpp>
#include <boost/asio/handler_alloc_hook.hpp>
#include <boost/asio/handler_continuation_hook.hpp>
#include <boost/asio/handler_invoke_hook.hpp>

namespace detail {

/** Composed operation to send an HTTP message

    This implements the composed operation needed for the
    @ref async_write_msg function.
*/
template<
    class AsyncWriteStream,
    class Handler,
    bool isRequest, class Body, class Fields>
class write_msg_op
{
    // This composed operation has a state which is not trivial
    // to copy (msg) so we need to store the state in an allocated
    // object.
    //
    struct data
    {
        // The stream we are writing to
        AsyncWriteStream& stream;

        // The message we are sending. Note that this composed
        // operation takes ownership of the message and destroys
        // it when it is done.
        //
        boost::beast::http::message<isRequest, Body, Fields> msg;

        // Serializer for the message
        boost::beast::http::serializer<isRequest, Body, Fields> sr;

        data(
            Handler& handler,
            AsyncWriteStream& stream_,
            boost::beast::http::message<isRequest, Body, Fields>&& msg_)
            : stream(stream_)
            , msg(std::move(msg_))
            , sr(msg)
        {
            boost::ignore_unused(handler);
        }
    };

    // `handler_ptr` is a utility which helps to manage a composed
    // operation's state. It is similar to a shared pointer, but
    // it uses handler allocation hooks to allocate and free memory,
    // and it also helps to meet Asio's deallocate-before-invocation
    // guarantee.
    //
    boost::beast::handler_ptr<data, Handler> d_;

public:
    // Asio can move and copy the handler, we support both
    write_msg_op(write_msg_op&&) = default;
    write_msg_op(write_msg_op const&) = default;

    // Constructor
    //
    // We take the handler as a template type to
    // support both const and rvalue references.
    //
    template<
        class DeducedHandler,
        class... Args>
    write_msg_op(
        DeducedHandler&& h,
        AsyncWriteStream& s,
        Args&&... args)
        : d_(std::forward<DeducedHandler>(h),
            s, std::forward<Args>(args)...)
    {
    }

    // Entry point
    //
    // The initiation function calls this to start the operation
    //
    void
    operator()()
    {
        auto& d = *d_;
        boost::beast::http::async_write(
            d.stream, d.sr, std::move(*this));
    }

    // Completion handler
    //
    // This gets called when boost::beast::http::async_write completes
    //
    void
    operator()(boost::beast::error_code ec)
    {
        d_.invoke(ec);
    }

    //
    // These hooks are necessary for Asio
    //
    // The meaning is explained in the Beast documentation
    //

    friend
    void* asio_handler_allocate(
        std::size_t size, write_msg_op* op)
    {
        using boost::asio::asio_handler_allocate;
        return asio_handler_allocate(
            size, std::addressof(op->d_.handler()));
    }

    friend
    void asio_handler_deallocate(
        void* p, std::size_t size, write_msg_op* op)
    {
        using boost::asio::asio_handler_deallocate;
        asio_handler_deallocate(
            p, size, std::addressof(op->d_.handler()));
    }

    friend
    bool asio_handler_is_continuation(write_msg_op* op)
    {
        using boost::asio::asio_handler_is_continuation;
        return asio_handler_is_continuation(std::addressof(op->d_.handler()));
    }

    template<class Function>
    friend
    void asio_handler_invoke(Function&& f, write_msg_op* op)
    {
        using boost::asio::asio_handler_invoke;
        asio_handler_invoke(
            f, std::addressof(op->d_.handler()));
    }
};

} // detail

/** Write an HTTP message to a stream asynchronously

    This function is used to write a complete message to a stream asynchronously
    using HTTP/1. The function call always returns immediately. The asynchronous
    operation will continue until one of the following conditions is true:

    @li The entire message is written.

    @li An error occurs.

    This operation is implemented in terms of zero or more calls to the stream's
    `async_write_some` function, and is known as a <em>composed operation</em>.
    The program must ensure that the stream performs no other write operations
    until this operation completes. The algorithm will use a temporary
    @ref serializer to produce buffers. If the semantics of the message
    indicate that the connection should be closed after the message is sent,
    the error delivered by this function
    will be @ref error::end_of_stream

    @param stream The stream to which the data is to be written.
    The type must support the @b AsyncWriteStream concept.

    @param msg The message to write. The function will take ownership
    of the object as if by move constrction.

    @param handler The handler to be called when the operation
    completes. Copies will be made of the handler as required.
    The equivalent function signature of the handler must be:
    @code void handler(
        error_code const& error // result of operation
    ); @endcode
    Regardless of whether the asynchronous operation completes
    immediately or not, the handler will not be invoked from within
    this function. Invocation of the handler will be performed in a
    manner equivalent to using `boost::asio::io_service::post`.
*/
template<
    class AsyncWriteStream,
    bool isRequest, class Body, class Fields,
    class WriteHandler>
boost::beast::async_return_type<WriteHandler, void(boost::beast::error_code)>
async_write_msg(
    AsyncWriteStream& stream,
    boost::beast::http::message<isRequest, Body, Fields>&& msg,
    WriteHandler&& handler)
{
    static_assert(
        boost::beast::is_async_write_stream<AsyncWriteStream>::value,
        "AsyncWriteStream requirements not met");

    static_assert(boost::beast::http::is_body<Body>::value,
        "Body requirements not met");

    static_assert(boost::beast::http::is_body_reader<Body>::value,
        "BodyReader requirements not met");

    boost::beast::async_completion<WriteHandler, void(boost::beast::error_code)> init{handler};

    ::detail::write_msg_op<
        AsyncWriteStream,
        boost::beast::handler_type<WriteHandler, void(boost::beast::error_code)>,
        isRequest, Body, Fields>{
            init.completion_handler,
            stream,
            std::move(msg)}();

    return init.result.get();
}

#endif