Detect fatal errors

[anarthal]

When issuing operations on connections, two kind of errors may arise:

• Recoverable errors (e.g. a SQL query failed). Operations following a recoverable error can succeed without reconnecting the connection.
• Fatal errors (e.g. packet mismatch). After such an error happens, the connection is left in an unknown state, and should be reconnected.

We should detect such errors and provide a way for the user to check them. For instance:

bool is_fatal_error(boost::mysql::error_code ec) noexcept
{
    // If there is no failure, it's not fatal
    if (!ec)
        return false;

    // Retrieve the error category
    const auto& cat = ec.category();

    if (cat == boost::mysql::get_common_server_category())
    {
        // Server errors may or may not be fatal. MySQL defines a ton of different errors.
        // After some research, these are the ones I'd recommend to consider fatal
        auto code = static_cast<boost::mysql::common_server_errc>(ec.value());
        switch (code)
        {
        // Diferent flavors of communication errors. These usually indicate that the connection
        // has been left in an unspecified state, and the safest is to reconnect it.
        case boost::mysql::common_server_errc::er_unknown_com_error:
        case boost::mysql::common_server_errc::er_aborting_connection:
        case boost::mysql::common_server_errc::er_net_packet_too_large:
        case boost::mysql::common_server_errc::er_net_read_error_from_pipe:
        case boost::mysql::common_server_errc::er_net_fcntl_error:
        case boost::mysql::common_server_errc::er_net_packets_out_of_order:
        case boost::mysql::common_server_errc::er_net_uncompress_error:
        case boost::mysql::common_server_errc::er_net_read_error:
        case boost::mysql::common_server_errc::er_net_read_interrupted:
        case boost::mysql::common_server_errc::er_net_error_on_write:
        case boost::mysql::common_server_errc::er_net_write_interrupted:
        case boost::mysql::common_server_errc::er_malformed_packet:
        // This one indicates that you are preparing statements dynamically and
        // never calling statement::close or connection::reset_connection.
        // Restarting the connection will clean up any leaked statements.
        // It's recommended that you design your code so this never happens.
        // But better safe than sorry.
        case boost::mysql::common_server_errc::er_max_prepared_stmt_count_reached: return true;
        default: return false;
        }
    }
    else if (cat == boost::mysql::get_mysql_server_category() || cat == boost::mysql::get_mariadb_server_category())
    {
        // This is a MySQL-specific or a MariaDB specific error. They are all
        // considered non-fatal. If you're working with a specific DB system,
        // you can comment the one you're not using
        return false;
    }
    else if (ec == boost::mysql::client_errc::wrong_num_params)
    {
        // Errors in the boost::mysql::get_client_category() are all fatal
        // except for this one.
        return false;
    }
    else
    {
        // Errors in any all category are considered fatal. They will usually
        // be asio or SSL related errors. The only option here is to reconnect.
        return true;
    }
}