fixed_size.hpp

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/*
 * RESTinio
 */

/*!
 * @file
 * @brief Stuff related to fixed-size chain of request-headers.
 *
 * @since v.0.6.13
 */

#pragma once

#include <restinio/request_handler.hpp>

#include <array>

namespace restinio
{

namespace sync_chain
{

//
// fixed_size_chain_t
//
/*!
 * @brief A holder of fixed-size chain of synchronous handlers.
 *
 * @note
 * An instance of that type is intended to be filled with actual handlers
 * at the creation time. After that new handlers can't be added to the chain,
 * and old handlers can't be removed from the chain.
 *
 * Usage example for the case when there is no extra-data in a request object.
 * @code
 * struct my_traits : public restinio::default_traits_t {
 *  using request_handler_t = restinio::sync_chain::fixed_size_chain_t<3>;
 * };
 *
 * // The first handler in the chain.
 * restinio::request_handling_status_t headers_checker(
 *  const restinio::request_handle_t & req )
 * {
 *  ... // Checks values of HTTP-fields and rejects invalid requests.
 * }
 *
 * // The second handler in the chain.
 * restinio::request_handling_status_t authentificator(
 *  const restinio::request_handle_t & req )
 * {
 *  ... // Checks user's credentials and rejects requests from
 *      // non-authentificated users.
 * }
 *
 * // The last handler in the chain.
 * restinio::request_handling_status_t actual_handler(
 *  const restinio::request_handle_t & req )
 * {
 *  ... // Actual processing.
 * }
 *
 * restinio::run(
 *  on_thread_pool<my_traits>(16)
 *      .address(...)
 *      .port(...)
 *      .request_handler(
 *          // Just enumerate all handlers.
 *          headers_checker,
 *          authentificator,
 *          actual_handler )
 * );
 * @endcode
 *
 * An instance of `fixed_size_chain_t` can also be created manually and
 * passed to server's settings by `unique_ptr`:
 * @code
 * auto chain = std::make_unique<restinio::sync_chain::fixed_size_chain_t<3>>(
 *  headers_checker, authentificator, actual_handler);
 * ...
 * restinio::run(
 *  on_thread_pool<my_traits>(16)
 *      .address(...)
 *      .port(...)
 *      .request_handler(std::move(chain))
 * );
 * @endcode
 *
 * Usage example for the case when some extra-data is incorporated into
 * a request object.
 * @code
 * struct my_extra_data_factory {
 *  // A data formed by checker of HTTP-fields.
 *  struct request_specific_fields_t {...};
 *
 *  // A data formed by user-authentificator.
 *  struct user_info_t {...};
 *
 *  // A data to be incorporated into a request object.
 *  using data_t = std::tuple<request_specific_fields_t, user_info_t>;
 *
 *  void make_within(restinio::extra_data_buffer_t<data_t> buf) {
 *      new(buf.get()) data_t{};
 *  }
 * };
 *
 * struct my_traits : public restinio::default_traits_t {
 *  using extra_data_factory_t = my_extra_data_factory;
 *  using request_handler_t = restinio::sync_chain::fixed_size_chain_t<
 *          3,
 *          extra_data_factory>;
 * };
 *
 * using my_request_handle_t =
 *  restinio::generic_request_handle_t<my_extra_data_factory::data_t>;
 *
 * // The first handler in the chain.
 * restinio::request_handling_status_t headers_checker(
 *  const my_request_handle_t & req )
 * {
 *  ... // Checks values of HTTP-fields and rejects invalid requests.
 * }
 *
 * // The second handler in the chain.
 * restinio::request_handling_status_t authentificator(
 *  const my_request_handle_t & req )
 * {
 *  ... // Checks user's credentials and rejects requests from
 *      // non-authentificated users.
 * }
 *
 * // The last handler in the chain.
 * restinio::request_handling_status_t actual_handler(
 *  const my_request_handle_t & req )
 * {
 *  auto & field_values = std::get<my_extra_data_factory::request_specific_fields_t>(req->extra_data());
 *  auto & user_info = std::get<my_extra_data_factory::user_info_t>(req->extra_data());
 *  ... // Actual processing.
 * }
 *
 * restinio::run(
 *  on_thread_pool<my_traits>(16)
 *      .address(...)
 *      .port(...)
 *      .request_handler(
 *          // Just enumerate all handlers.
 *          headers_checker,
 *          authentificator,
 *          actual_handler )
 * );
 * @endcode
 *
 * @tparam Size The exact number of handlers in the chain.
 *
 * @tparam Extra_Data_Factory The type of extra-data-factory specified in
 * the server's traits.
 *
 * @since v.0.6.13
 */
template<
    std::size_t Size,
    typename Extra_Data_Factory = no_extra_data_factory_t >
class fixed_size_chain_t
{
    using actual_request_handle_t =
            generic_request_handle_t< typename Extra_Data_Factory::data_t >;

    using handler_holder_t = std::function<
            request_handling_status_t(const actual_request_handle_t &)
    >;

    std::array< handler_holder_t, Size > m_handlers;

    template<
        typename Head,
        typename... Tail >
    void
    store_to( std::size_t index, Head && head, Tail && ...tail )
    {
        m_handlers[ index ] =
            [handler = std::move(head)]
            ( const actual_request_handle_t & req ) -> request_handling_status_t
            {
                return handler( req );
            };

        if constexpr( 0u != sizeof...(tail) )
            store_to( index + 1u, std::forward<Tail>(tail)... );
    }

public:
    /*!
     * @attention
     * The default constructor is disabled. It's because a chain should
     * be initialized by handlers at the creation time. Because of that
     * fixed_size_chain_t isn't a DefaultConstructible type.
     */
    fixed_size_chain_t() = delete;

    /*!
     * @brief Initializing constructor.
     *
     * @note
     * The number of parameters should match the value of @a Size
     * template parameter.
     */
    template< typename... Handlers >
    fixed_size_chain_t( Handlers && ...handlers )
    {
        static_assert( Size == sizeof...(handlers),
                "Wrong number of parameters for the constructor of "
                "fixed_size_chain_t<Size>. Exact `Size` parameters expected" );

        store_to( 0u, std::forward<Handlers>(handlers)... );
    }

    [[nodiscard]]
    request_handling_status_t
    operator()( const actual_request_handle_t & req ) const
    {
        for( auto & h : m_handlers )
        {
            const request_handling_status_t result = h( req );

            switch( result )
            {
                case request_handling_status_t::accepted:
                case request_handling_status_t::rejected:
                    // There is no need to try next handler.
                    return result;

                case request_handling_status_t::not_handled:
                    // Nothing to do. The next handler should be tried.
                    break;
            }
        }

        return request_not_handled();
    }
};

} /* namespace sync_chain */

} /* namespace restinio */