mysql/example/default_completion_tokens.cpp

//
// Copyright (c) 2019-2022 Ruben Perez Hidalgo (rubenperez038 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)
//

//[example_default_completion_tokens]
#include <boost/mysql.hpp>
#include <boost/asio/basic_stream_socket.hpp>
#include <boost/asio/ip/tcp.hpp>
#include <boost/asio/ssl/context.hpp>
#include <boost/asio/io_context.hpp>
#include <boost/system/system_error.hpp>
#include <boost/asio/co_spawn.hpp>
#include <boost/asio/use_awaitable.hpp>
#include <boost/asio/awaitable.hpp>
#include <boost/asio/detached.hpp>
#include <boost/asio/use_future.hpp>
#include <iostream>

using boost::mysql::error_code;


#ifdef BOOST_ASIO_HAS_CO_AWAIT

void print_employee(const boost::mysql::row& employee)
{
    std::cout << "Employee '"
              << employee.values()[0] << " "                   // first_name (type boost::string_view)
              << employee.values()[1] << "' earns "            // last_name  (type boost::string_view)
              << employee.values()[2] << " dollars yearly\n";  // salary     (type double)
}

/**
 * Default completion tokens are associated to executors.
 * boost::mysql::connection objects use the same executor
 * as the underlying stream (socket). boost::mysql::tcp_ssl_connection
 * objects use boost::asio::ssl::stream<boost::asio::ip::tcp::socket>,
 * which use the polymorphic boost::asio::executor as executor type, 
 * which does not have a default completion token associated.
 *
 * We will use the io_context's executor as base executor. We will then
 * use use_awaitable_t::executor_with_default on this type, which creates
 * a new executor type acting the same as the base executor, but having
 * use_awaitable_t as default completion token type.
 *
 * We will then obtain the connection type to use by rebinding
 * the usual tcp_ssl_connection to our new executor type, coro_executor_type.
 *
 * The reward for this hard work is not having to pass the completion
 * token (boost::asio::use_awaitable) to any of the asynchronous operations
 * initiated by this connection or any of the I/O objects (e.g. resultsets)
 * associated to them.
 */
using base_executor_type = boost::asio::io_context::executor_type;
using coro_executor_type = boost::asio::use_awaitable_t<
    base_executor_type>::executor_with_default<base_executor_type>;
using stream_type = boost::asio::ssl::stream<
    boost::asio::basic_stream_socket<boost::asio::ip::tcp, coro_executor_type>
>;
using connection_type = boost::mysql::connection<
    stream_type
>;
using resolver_type = boost::asio::ip::basic_resolver<
    boost::asio::ip::tcp, coro_executor_type
>;

// Our coroutine
boost::asio::awaitable<void, base_executor_type> start_query(
    connection_type& conn,
    resolver_type& resolver,
    const char* hostname,
    const boost::mysql::connection_params& params
)
{
    try
    {
        // Resolve hostname. Note: we didn't have to pass boost::asio::use_awaitable.
        auto endpoints = co_await resolver.async_resolve(
            hostname,
            boost::mysql::default_port_string
        );

        // Connect to server
        co_await conn.async_connect(*endpoints.begin(), params);

        /**
        * Issue the query to the server. Note that async_query returns a
        * boost::asio::awaitable<boost::mysql::resultset<stream_type>, base_executor_type>,
        * where stream_type is the type alias we defined previously.
        * Calling co_await on this expression will yield a boost::mysql::resultset<stream_type>.
        * Note that this is not the same type as a boost::mysql::tcp_ssl_resultset because we
        * used a custom stream type.
        */
        const char* sql = "SELECT first_name, last_name, salary FROM employee WHERE company_id = 'HGS'";
        auto result = co_await conn.async_query(sql);

        /**
        * Get all rows in the resultset. We will employ resultset::async_read_one(),
        * which reads a single row at every call. The row is read in-place, preventing
        * unnecessary copies. resultset::async_read_one() returns true if a row has been
        * read, false if no more rows are available or an error occurred.
        */
        boost::mysql::row row;
        while (co_await result.async_read_one(row))
        {
            print_employee(row);
        }

        // Notify the MySQL server we want to quit, then close the underlying connection.
        co_await conn.async_close();
    }
    catch (const boost::system::system_error& err)
    {
        std::cerr << "Error: " << err.what() << ", error code: " << err.code() << std::endl;
    }
    catch (const std::exception& err)
    {
        std::cerr << "Error: " << err.what() << std::endl;
    }
}

void main_impl(int argc, char** argv)
{
    if (argc != 4)
    {
        std::cerr << "Usage: " << argv[0] << " <username> <password> <server-hostname>\n";
        exit(1);
    }

    const char* hostname = argv[3];

    // I/O context and connection. We use SSL because MySQL 8+ default settings require it.
    boost::asio::io_context ctx;
    boost::asio::ssl::context ssl_ctx (boost::asio::ssl::context::tls_client);
    connection_type conn (ctx.get_executor(), ssl_ctx);

    // Resolver, for hostname resolution
    resolver_type resolver (ctx.get_executor());

    // Connection parameters
    boost::mysql::connection_params params (
        argv[1],               // username
        argv[2],               // password
        "boost_mysql_examples" // database to use; leave empty or omit the parameter for no database
    );

    /**
     * The entry point. We pass in a function returning
     * a boost::asio::awaitable<void>, as required.
     */
    boost::asio::co_spawn(ctx.get_executor(), [&conn, &resolver, hostname, params] {
        return start_query(conn, resolver, hostname, params);
    }, boost::asio::detached);

    // Calling run will actually start the requested operations.
    ctx.run();
}


#else

void main_impl(int, char**)
{
    std::cout << "Sorry, your compiler does not support C++20 coroutines" << std::endl;
}

#endif


int main(int argc, char** argv)
{
    try
    {
        main_impl(argc, argv);
    }
    catch (const std::exception& err)
    {
        std::cerr << "Error: " << err.what() << std::endl;
        return 1;
    }
}

//]