ioctx_on_thread_pool.hpp

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#pragma once

#include <thread>

#include <restinio/asio_include.hpp>

#include <restinio/exception.hpp>

namespace restinio
{

namespace impl
{

/*!
 * \brief A class for holding actual instance of Asio's io_context.
 *
 * \note
 * This class is intended to be used as template argument for
 * ioctx_on_thread_pool_t template.
 *
 * \since
 * v.0.4.2
 */
class own_io_context_for_thread_pool_t
{
    asio_ns::io_context m_ioctx;

public:
    own_io_context_for_thread_pool_t() = default;

    //! Get access to io_context object.
    auto & io_context() noexcept { return m_ioctx; }
};

/*!
 * \brief A class for holding a reference to external Asio's io_context.
 *
 * \note
 * This class is intended to be used as template argument for
 * ioctx_on_thread_pool_t template.
 *
 * \since
 * v.0.4.2
 */
class external_io_context_for_thread_pool_t
{
    asio_ns::io_context & m_ioctx;

public:
    //! Initializing constructor.
    external_io_context_for_thread_pool_t(
        //! External io_context to be used.
        asio_ns::io_context & ioctx )
        :   m_ioctx{ ioctx }
    {}

    //! Get access to io_context object.
    auto & io_context() noexcept { return m_ioctx; }
};

namespace pool_size_checking
{

[[nodiscard]]
inline std::size_t
ensure_pool_size_non_zero( std::size_t pool_size )
{
    if( !pool_size )
        throw exception_t{ "pool_size can't be 0" };

    return pool_size;
}

} /* namespace pool_size_checking */

/*!
 * Helper class for creating io_context and running it
 * (via `io_context::run()`) on a thread pool.
 *
 * \note class is not thread-safe (except `io_context()` method).
 * Expected usage scenario is to start and stop it on the same thread.
 *
 * \tparam Io_Context_Holder A type which actually holds io_context object
 * or a reference to an external io_context object.
 */
template< typename Io_Context_Holder >
class ioctx_on_thread_pool_t
{
    public:
        ioctx_on_thread_pool_t( const ioctx_on_thread_pool_t & ) = delete;
        ioctx_on_thread_pool_t( ioctx_on_thread_pool_t && ) = delete;

        template< typename... Io_Context_Holder_Ctor_Args >
        ioctx_on_thread_pool_t(
            // Pool size.
            std::size_t pool_size,
            // Optional arguments for Io_Context_Holder instance.
            Io_Context_Holder_Ctor_Args && ...ioctx_holder_args )
            :   m_ioctx_holder{
                    std::forward<Io_Context_Holder_Ctor_Args>(ioctx_holder_args)... }
            ,   m_pool( pool_size_checking::ensure_pool_size_non_zero( pool_size ) )
            ,   m_status( status_t::stopped )
        {}

        // Makes sure the pool is stopped.
        ~ioctx_on_thread_pool_t()
        {
            if( started() )
            {
                stop();
                wait();
            }
        }

        void
        start()
        {
            if( started() )
            {
                throw exception_t{
                    "io_context_with_thread_pool is already started" };
            }

            try
            {
                std::generate(
                    begin( m_pool ),
                    end( m_pool ),
                    [this]{
                        return
                            std::thread{ [this] {
                                auto work{ asio_ns::make_work_guard(
                                            m_ioctx_holder.io_context() ) };

                                m_ioctx_holder.io_context().run();
                            } };
                    } );

                // When all thread started successfully
                // status can be changed.
                m_status = status_t::started;
            }
            catch( const std::exception & )
            {
                io_context().stop();
                for( auto & t : m_pool )
                    if( t.joinable() )
                        t.join();

                throw;
            }
        }

        // NOTE: this method is marked as noexcept in v.0.6.7.
        // It's because this method can be called from destructors and
        // there is no way to recover from an exception thrown from
        // this method.
        void
        stop() noexcept
        {
            if( started() )
            {
                io_context().stop();
            }
        }

        // NOTE: this method is marked as noexcept in v.0.6.7.
        // It's because this method can be called from destructors and
        // there is no way to recover from an exception thrown from
        // this method.
        void
        wait() noexcept
        {
            if( started() )
            {
                for( auto & t : m_pool )
                    t.join();

                // When all threads are stopped status can be changed.
                m_status = status_t::stopped;
            }
        }

        bool started() const noexcept { return status_t::started == m_status; }

        asio_ns::io_context &
        io_context() noexcept
        {
            return m_ioctx_holder.io_context();
        }

    private:
        enum class status_t : std::uint8_t { stopped, started };

        Io_Context_Holder m_ioctx_holder;
        std::vector< std::thread > m_pool;
        status_t m_status;
};

} /* namespace impl */

} /* namespace restinio */