pub.hpp

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/*!
 * \file
 * \brief Implementation of Asio's Thread Pool dispatcher.
 *
 * \since
 * v.1.0.2
 */

#pragma once

#include <so_5_extra/error_ranges.hpp>

#include <so_5/disp_binder.hpp>
#include <so_5/send_functions.hpp>

#include <so_5/disp/reuse/actual_work_thread_factory_to_use.hpp>
#include <so_5/disp/reuse/work_thread_activity_tracking.hpp>
#include <so_5/disp/reuse/data_source_prefix_helpers.hpp>
#include <so_5/disp/reuse/work_thread_factory_params.hpp>

#include <so_5/stats/repository.hpp>
#include <so_5/stats/messages.hpp>
#include <so_5/stats/std_names.hpp>
#include <so_5/stats/impl/activity_tracking.hpp>

#include <so_5/details/invoke_noexcept_code.hpp>
#include <so_5/details/rollback_on_exception.hpp>
#include <so_5/details/abort_on_fatal_error.hpp>

#include <so_5/outliving.hpp>

#include <asio/io_context.hpp>
#include <asio/io_context_strand.hpp>
#include <asio/post.hpp>

namespace so_5 {

namespace extra {

namespace disp {

namespace asio_thread_pool {

namespace errors {

//! Asio IoService is not set for asio_thread_pool dispatcher.
const int rc_io_context_is_not_set =
        so_5::extra::errors::asio_thread_pool_errors;

} /* namespace errors */

//
// disp_params_t
//
/*!
 * \brief Parameters for %asio_thread_pool dispatcher.
 *
 * \since
 * v.1.0.2
 */
class disp_params_t
    :   public ::so_5::disp::reuse::work_thread_activity_tracking_flag_mixin_t< disp_params_t >
    ,   public ::so_5::disp::reuse::work_thread_factory_mixin_t< disp_params_t >
    {
        using activity_tracking_mixin_t = ::so_5::disp::reuse::
                work_thread_activity_tracking_flag_mixin_t< disp_params_t >;
        using thread_factory_mixin_t = ::so_5::disp::reuse::
                work_thread_factory_mixin_t< disp_params_t >;

    public :
        //! Default constructor.
        disp_params_t() = default;

        friend inline void
        swap(
            disp_params_t & a, disp_params_t & b ) noexcept
            {
                using std::swap;

                swap(
                        static_cast< activity_tracking_mixin_t & >(a),
                        static_cast< activity_tracking_mixin_t & >(b) );

                swap(
                        static_cast< thread_factory_mixin_t & >(a),
                        static_cast< thread_factory_mixin_t & >(b) );

                swap( a.m_thread_count, b.m_thread_count );
                swap( a.m_io_context, b.m_io_context );
            }

        //! Setter for thread count.
        disp_params_t &
        thread_count( std::size_t count )
            {
                m_thread_count = count;
                return *this;
            }

        //! Getter for thread count.
        std::size_t
        thread_count() const
            {
                return m_thread_count;
            }

        //! Use external Asio io_context object with dispatcher.
        /*!
         * Usage example:
         * \code
         * int main() {
         *  asio::io_context svc;
         *  so_5::launch( [&](so_5::environment_t & env) {
         *      namespace asio_tp = so_5::extra::disp::asio_thread_pool;
         *      auto disp = asio_tp::create_private_disp(
         *              env, "asio_tp",
         *              asio_tp::disp_params_t{}.use_external_io_context(
         *                  so_5::outliving_mutable(svc) ) );
         *      ...
         *  } );
         * }
         * \endcode
         */
        disp_params_t &
        use_external_io_context(
            ::asio::io_context & service )
            {
                m_io_context = std::shared_ptr< ::asio::io_context >(
                        std::addressof( service ),
                        // Empty deleter.
                        [](::asio::io_context *) {} );
                return *this;
            }

        //! Use external Asio io_context object with dispatcher.
        /*!
         * \note
         * Ownership of this io_context object must be shared with
         * others.
         */
        disp_params_t &
        use_external_io_context(
            std::shared_ptr< ::asio::io_context > service )
            {
                m_io_context = std::move(service);
                return *this;
            }

        //! Use own Asio io_context object.
        /*!
         * Note this object will be dynamically created at the start
         * of the dispatcher. And will be destroyed with the dispatcher object.
         *
         * A created io_context can be accessed later via io_context() method.
         */
        disp_params_t &
        use_own_io_context()
            {
                m_io_context = std::make_shared< ::asio::io_context >();
                return *this;
            }

        //! Get the io_context.
        std::shared_ptr< ::asio::io_context >
        io_context() const noexcept
            {
                return m_io_context;
            }

    private :
        //! Count of working threads.
        /*!
         * Value 0 means that actual thread will be detected automatically.
         */
        std::size_t m_thread_count = { 0 };

        //! Asio's io_context which must be used with this dispatcher.
        std::shared_ptr< ::asio::io_context > m_io_context;
    };

namespace impl {

class actual_dispatcher_iface_t;

//
// basic_dispatcher_iface_t
//
/*!
 * \brief The very basic interface of %asio_thread_pool dispatcher.
 *
 * This class contains a minimum that is necessary for implementation
 * of dispatcher_handle class.
 *
 * \since
 * v.1.3.0
 */
class basic_dispatcher_iface_t
    :   public std::enable_shared_from_this<actual_dispatcher_iface_t>
    {
    public :
        virtual ~basic_dispatcher_iface_t() noexcept = default;

        //! Create a binder for that dispatcher.
        /*!
         * The binder will use an external strand object.
         */
        [[nodiscard]]
        virtual disp_binder_shptr_t
        binder_with_external_strand( ::asio::io_context::strand & ) = 0;

        //! Create a binder for that dispatcher.
        /*!
         * The binder will use an internal (automatically created)
         * strand object.
         */
        [[nodiscard]]
        virtual disp_binder_shptr_t
        binder_with_own_strand() = 0;

        //! Get reference to io_context from that dispatcher.
        virtual ::asio::io_context &
        io_context() const noexcept = 0;
    };

using basic_dispatcher_iface_shptr_t =
        std::shared_ptr< basic_dispatcher_iface_t >;

class dispatcher_handle_maker_t;

} /* namespace impl */

//
// dispatcher_handle_t
//

/*!
 * \brief A handle for %asio_thread_pool dispatcher.
 *
 * \since
 * v.1.3.0
 */
class [[nodiscard]] dispatcher_handle_t
    {
        friend class impl::dispatcher_handle_maker_t;

        //! A reference to actual implementation of a dispatcher.
        impl::basic_dispatcher_iface_shptr_t m_dispatcher;

        dispatcher_handle_t(
            impl::basic_dispatcher_iface_shptr_t dispatcher ) noexcept
            :   m_dispatcher{ std::move(dispatcher) }
            {}

        //! Is this handle empty?
        [[nodiscard]]
        bool
        empty() const noexcept { return !m_dispatcher; }

    public :
        dispatcher_handle_t() noexcept = default;

        //! Get a binder for that dispatcher.
        /*!
         * This method requires a reference to manually created strand
         * object for protection of agents bound via binder returned.
         * A user should create this strand object and ensure the right
         * lifetime of it.
         *
         * Usage example:
         * \code
         * using namespace so_5::extra::disp::asio_thread_pool;
         *
         * asio::io_context io_ctx;
         * asio::io_context::strand agents_strand{ io_ctx };
         *
         * so_5::environment_t & env = ...;
         * auto disp = make_dispatcher( env, "my_disp", io_ctx );
         *
         * env.introduce_coop( [&]( so_5::coop_t & coop ) {
         *  coop.make_agent_with_binder< some_agent_type >(
         *      disp.binder( agents_strand ),
         *      ... );
         *
         *  coop.make_agent_with_binder< another_agent_type >(
         *      disp.binder( agents_strand ),
         *      ... );
         *
         *  ...
         * } );
         * \endcode
         *
         * \attention
         * An attempt to call this method on empty handle is UB.
         */
        [[nodiscard]]
        disp_binder_shptr_t
        binder(
            ::asio::io_context::strand & strand ) const
            {
                return m_dispatcher->binder_with_external_strand( strand );
            }

        //! Get a binder for that dispatcher.
        /*!
         * This method requires creates an internal strand object by itself.
         *
         * Usage example:
         * \code
         * using namespace so_5::extra::disp::asio_thread_pool;
         *
         * asio::io_context io_ctx;
         *
         * so_5::environment_t & env = ...;
         * auto disp = make_dispatcher( env, "my_disp", io_ctx );
         *
         * env.introduce_coop( [&]( so_5::coop_t & coop ) {
         *  // This agent will use its own strand object.
         *  coop.make_agent_with_binder< some_agent_type >(
         *      disp.binder(),
         *      ... );
         *
         *  // This agent will use its own strand object.
         *  coop.make_agent_with_binder< another_agent_type >(
         *      disp.binder(),
         *      ... );
         *  ...
         * } );
         * \endcode
         *
         * \attention
         * An attempt to call this method on empty handle is UB.
         *
         * \since
         * v.1.3.0
         */
        [[nodiscard]]
        disp_binder_shptr_t
        binder() const
            {
                return m_dispatcher->binder_with_own_strand();
            }

        //! Get reference to io_context from that dispatcher.
        /*!
         * \attention
         * An attempt to call this method on empty handle is UB.
         */
        [[nodiscard]]
        ::asio::io_context &
        io_context() noexcept
            {
                return m_dispatcher->io_context();
            }

        //! Is this handle empty?
        [[nodiscard]]
        operator bool() const noexcept { return empty(); }

        //! Does this handle contain a reference to dispatcher?
        [[nodiscard]]
        bool
        operator!() const noexcept { return !empty(); }

        //! Drop the content of handle.
        void
        reset() noexcept { m_dispatcher.reset(); }
    };

namespace impl {

//
// demands_counter_t
//
/*!
 * \brief Type of atomic counter for counting waiting demands.
 *
 * \since
 * v.1.0.2
 */
using demands_counter_t = std::atomic< std::size_t >;

//
// actual_dispatcher_iface_t
//
/*!
 * \brief An actual interface of thread pool dispatcher.
 *
 * \since
 * v.1.3.0
 */
class actual_dispatcher_iface_t : public basic_dispatcher_iface_t
    {
    public :
        //! Notification about binding of yet another agent.
        virtual void
        agent_bound() noexcept = 0;

        //! Notification about unbinding of an agent.
        virtual void
        agent_unbound() noexcept = 0;

        //! Get a reference for counter of pending demands.
        virtual demands_counter_t &
        demands_counter() noexcept = 0;
    };

//
// actual_dispatcher_shptr_t
//
using actual_dispatcher_shptr_t =
        std::shared_ptr< actual_dispatcher_iface_t >;

//
// thread_local_ptr_holder_t
//
/*!
 * \brief A helper for declaration of static and thread_local pointer
 * in a header file.
 *
 * If non-template class will define a static member in a header file
 * then there is a possibility to get a link-time error about multiple
 * definition of that member. But if a static member is defined for
 * template class then there won't be such problem.
 *
 * A typical usage intended to be:
 * \code
 * class some_useful_class_t : public thread_local_ptr_holder_t<some_useful_class_t> {
 *  ...
 * };
 * \endcode
 *
 * \since
 * v.1.0.2
 */
template< class T >
class thread_local_ptr_holder_t
    {
    private :
        //! Value of the pointer which need to be stored.
        static thread_local T * m_ptr;

    protected :
        //! Access to the current value of the pointer.
        static T *
        ptr() noexcept { return m_ptr; }

        //! Setter for the pointer.
        static void
        set_ptr( T * p ) noexcept { m_ptr = p; }
    };

template< class T >
thread_local T * thread_local_ptr_holder_t<T>::m_ptr = nullptr;

//
// work_thread_t
//
/*!
 * \brief Base type for implementations of work thread wrappers.
 *
 * Work thread wrapper creates an instance of some type on the stack
 * of the new thread. Then the pointer of this instance is stored in
 * thread_local variable (as a pointer to work_thread_t). This pointer
 * then can be retrieved later by demand handlers to get access to
 * some dispatcher-specific data.
 *
 * It is assumed that there will be two derived classes:
 * 1. One for the case when thread activity should not be tracked.
 * 2. Another for the case when thread activity must be tracked.
 *
 * These derived classes will reuse some functionality from
 * work_thread_t. And should implement on_demand() method for
 * actual demands processing.
 *
 * \since
 * v.1.0.2
 */
class work_thread_t : private thread_local_ptr_holder_t< work_thread_t >
    {
    private :
        //! ID of the work thread.
        /*!
         * Gets its value in the constructor and doesn't changed later.
         */
        current_thread_id_t m_thread_id;

    protected :
        // Constructor and destructor are accessible for derived classes only.
        work_thread_t()
            : m_thread_id( query_current_thread_id() )
            {}

        // Just to make compilers happy.
        virtual ~work_thread_t() = default;

        //! Actual processing of the demand.
        /*!
         * Must be implemented in derived classes.
         */
        virtual void
        on_demand( execution_demand_t demand ) noexcept = 0;

        //! ID of the work thread.
        current_thread_id_t
        thread_id() const noexcept
            {
                return m_thread_id;
            }

    public :
        //! Lauch processing of demand on the context of current thread.
        /*!
         * Creates an instance of Derived class, stores pointer to it into
         * a thread_local static variable, then calls io_svc.run() method.
         *
         * \attention
         * Terminates the whole application if an exception will be thrown.
         *
         * \tparam Derived Type of an object to be created on the stack.
         * \tparam Args Types of arguments for Derived's constructor.
         */
        template< typename Derived, typename... Args >
        static void
        run(
            //! SObjectizer Environment for which work thread was created.
            environment_t & env,
            //! Asio IoService to be run on the context of that thread.
            ::asio::io_context & io_svc,
            //! Arguments to Derived's constructor.
            Args &&... args )
            {
                // We don't expect any errors here.
                // But if something happens then there is no way to
                // recover and the whole application should be aborted.
                try
                    {
                        Derived actual_handler{ std::forward<Args>(args)... };
                        // actual_handler must be accessible via thread_local variable.
                        set_ptr( &actual_handler );

                        // Prevent return from io_context::run() if there is no
                        // more Asio's events.
                        auto work = ::asio::make_work_guard( io_svc );
                        io_svc.run();
                    }
                catch( const std::exception & x )
                    {
                        ::so_5::details::abort_on_fatal_error( [&] {
                            SO_5_LOG_ERROR( env, log_stream ) {
                                log_stream << "An exception caught in work thread "
                                    "of so_5::extra::disp::asio_thread_pool dispatcher."
                                    " Exception: "
                                    << x.what() << std::endl;
                            }
                        } );
                    }
                catch( ... )
                    {
                        ::so_5::details::abort_on_fatal_error( [&] {
                            SO_5_LOG_ERROR( env, log_stream ) {
                                log_stream << "An unknown exception caught in work thread "
                                    "of so_5::extra::disp::asio_thread_pool dispatcher."
                                    << std::endl;
                            }
                        } );
                    }
            }

        //! An interface method for passing a demand to processing.
        static void
        handle_demand( execution_demand_t demand )
            {
                ptr()->on_demand( std::move(demand) );
            }
    };

//
// work_thread_without_activity_tracking_t
//
/*!
 * \brief An implementation of work thread stuff for the case when
 * thread activity tracking is not needed.
 *
 * \since
 * v.1.0.2
 */
class work_thread_without_activity_tracking_t final : public work_thread_t
    {
    public :
        work_thread_without_activity_tracking_t() = default;
        ~work_thread_without_activity_tracking_t() override = default;

    protected :
        virtual void
        on_demand( execution_demand_t demand ) noexcept override
            {
                demand.call_handler( thread_id() );
            }
    };

//
// work_thread_activity_collector_t
//
/*!
 * \brief Type of collector of work thread activity data.
 *
 * Objects of this class store also an ID of work thread. This ID is
 * necessary for so_5::stats::messages::work_thread_activity message.
 * Because of that a work thread must call setup_thread_id() method
 * before use of activity collector.
 *
 * \since
 * v.1.0.2
 */
class work_thread_activity_collector_t
    {
    private :
        //! ID of thread for which activity stats is collected.
        current_thread_id_t m_thread_id;

        //! Collected activity stats.
        ::so_5::stats::activity_tracking_stuff::stats_collector_t<
                    ::so_5::stats::activity_tracking_stuff::internal_lock >
                m_work_activity;

    public :
        /*!
         * \brief Setup ID of the current work thread.
         *
         * \attention
         * Must be called as soon as possible after the start of the work thread.
         */
        void
        setup_thread_id( current_thread_id_t tid )
            {
                m_thread_id = std::move(tid);
            }

        /*!
         * \brief Get the ID of the thread.
         *
         * \attention
         * Returns actual value only after call to setup_thread_id.
         */
        current_thread_id_t
        thread_id() const noexcept { return m_thread_id; }

        /*!
         * \brief Mark start point of new activity.
         */
        void
        activity_started() noexcept
            {
                m_work_activity.start();
            }

        /*!
         * \brief Mark completion of the current activity.
         */
        void
        activity_finished() noexcept
            {
                m_work_activity.stop();
            }

        /*!
         * \brief Get the current stats.
         */
        ::so_5::stats::work_thread_activity_stats_t
        take_activity_stats() noexcept
        {
            ::so_5::stats::work_thread_activity_stats_t result;
            result.m_working_stats = m_work_activity.take_stats();

            return result;
        }
    };

//
// work_thread_with_activity_tracking_t
//
/*!
 * \brief An implementation of work thread stuff for the case when
 * thread activity tracking must be used.
 *
 * \since
 * v.1.0.2
 */
class work_thread_with_activity_tracking_t final : public work_thread_t
    {
    private :
        //! Activity statistics.
        outliving_reference_t< work_thread_activity_collector_t > m_activity_stats;

    public :
        work_thread_with_activity_tracking_t(
            outliving_reference_t< work_thread_activity_collector_t > activity_stats )
            :   m_activity_stats(activity_stats)
            {
                // Collector must receive ID of this thread.
                m_activity_stats.get().setup_thread_id( thread_id() );
            }

        ~work_thread_with_activity_tracking_t() override = default;

    protected :
        virtual void
        on_demand( execution_demand_t demand ) noexcept override
            {
                m_activity_stats.get().activity_started();

                demand.call_handler( thread_id() );

                m_activity_stats.get().activity_finished();
            }
    };

//
// class basic_binder_impl_t
//
/*!
 * \brief Basic part of implementation of a binder for %asio_thread_pool
 * dispatcher.
 *
 * \since
 * v.1.3.0
 */
class basic_binder_impl_t
    :   public disp_binder_t
    ,   public event_queue_t
    {
    public :
        //! Initializing constructor.
        basic_binder_impl_t(
            //! The actual dispatcher to be used with that binder.
            actual_dispatcher_shptr_t dispatcher )
            :   m_dispatcher{ std::move(dispatcher) }
            {}

        void
        preallocate_resources(
            agent_t & /*agent*/ ) override
            {
                // There is no need to do something.
            }

        void
        undo_preallocation(
            agent_t & /*agent*/ ) noexcept override
            {
                // There is no need to do something.
            }

        void
        bind(
            agent_t & agent ) noexcept override
            {
                // Dispatcher should know about yet another agent bound.
                m_dispatcher->agent_bound();
                // Agent should receive its event_queue.
                agent.so_bind_to_dispatcher( *this );
            }

        void
        unbind(
            agent_t & /*agent*/ ) noexcept override
            {
                // Dispatcher should know about yet another agent unbound.
                m_dispatcher->agent_bound();
            }

    protected :
        //! The actual dispatcher.
        actual_dispatcher_shptr_t m_dispatcher;
    };

//
// binder_template_t
//
/*!
 * \brief An implementation of a binder for %asio_thread_pool dispatcher.
 *
 * This binder is also an event_queue for the agents bound via that binder.
 *
 * There is no such thing as event_queue for %asio_thread_pool dispacher.
 * All execution demands will be stored inside Asio IoServce and dispatched
 * for execution via asio::post mechanism. But SObjectizer requires
 * an implementation of event_queue which must be used for agents bound
 * to %asio_thread_pool dispatcher. This class implements this event_queue
 * concepts.
 *
 * This templates implements CRTP and should be parametrized by
 * derived type. The derived type should provide method:
 * \code
 * ::asio::io_context::strand & strand() noexcept;
 * \endcode
 *
 * \since
 * v.1.3.0
 */
template< typename Derived >
class binder_template_t
    :   public basic_binder_impl_t
    {
        auto &
        self_reference() noexcept
            {
                return static_cast< Derived & >( *this );
            }

    public :
        using basic_binder_impl_t::basic_binder_impl_t;

        void
        push( execution_demand_t demand ) override
            {
                demands_counter_t & counter = m_dispatcher->demands_counter();

                // Another demand will wait for processing.
                ++counter;

                asio::post( self_reference().strand(),
                    [d = std::move(demand), &counter]() mutable {
                        // Another demand will be processed.
                        --counter;

                        // Delegate processing of the demand to actual
                        // work thread.
                        work_thread_t::handle_demand( std::move(d) );
                    } );
            }
    };

//
// binder_with_external_strand_t
//
/*!
 * \brief An implementation of binder that uses an external strand object.
 *
 * \since
 * v.1.3.0
 */
class binder_with_external_strand_t final
    : public binder_template_t< binder_with_external_strand_t >
    {
        using base_type = binder_template_t< binder_with_external_strand_t >;

    public :
        binder_with_external_strand_t(
            actual_dispatcher_shptr_t dispatcher,
            outliving_reference_t< ::asio::io_context::strand > strand )
            :   base_type{ std::move(dispatcher) }
            ,   m_strand{ strand }
            {}

        ::asio::io_context::strand &
        strand() noexcept { return m_strand.get(); }

    private :
        //! Strand to be used with this event_queue.
        outliving_reference_t< ::asio::io_context::strand > m_strand;
    };

//
// binder_with_own_strand_t
//
/*!
 * \brief An implementation of binder that uses an own strand object.
 *
 * This own strand object will be a part of the binder object.
 *
 * \since
 * v.1.3.0
 */
class binder_with_own_strand_t final
    : public binder_template_t< binder_with_own_strand_t >
    {
        using base_type = binder_template_t< binder_with_own_strand_t >;

    public :
        binder_with_own_strand_t(
            actual_dispatcher_shptr_t dispatcher )
            :   base_type{ std::move(dispatcher) }
            ,   m_strand{ m_dispatcher->io_context() }
            {}

        ::asio::io_context::strand &
        strand() noexcept { return m_strand; }

    private :
        //! Strand to be used with this event_queue.
        ::asio::io_context::strand m_strand;
    };

//
// basic_dispatcher_skeleton_t
//
/*!
 * \brief Basic stuff for all implementations of dispatcher.
 *
 * Derived classes should implement the following virtual methods:
 * - data_source();
 * - launch_work_threads();
 * - wait_work_threads().
 *
 * \since
 * v.1.0.2
 */
class basic_dispatcher_skeleton_t : public actual_dispatcher_iface_t
    {
    protected :
        class disp_data_source_t;
        friend class disp_data_source_t;

    public:
        basic_dispatcher_skeleton_t(
            ::so_5::environment_t & env,
            disp_params_t params )
            :   m_thread_count( params.thread_count() )
            ,   m_io_context( params.io_context() )
            ,   m_thread_factory(
                    ::so_5::disp::reuse::actual_work_thread_factory_to_use(
                            params,
                            env )
                )
            {
            }

        [[nodiscard]]
        disp_binder_shptr_t
        binder_with_external_strand(
            ::asio::io_context::strand & strand ) override
            {
                return { std::make_shared< binder_with_external_strand_t >(
                        shared_from_this(),
                        outliving_mutable(strand) )
                };
            }

        [[nodiscard]]
        disp_binder_shptr_t
        binder_with_own_strand() override
            {
                return { std::make_shared< binder_with_own_strand_t >(
                        shared_from_this() )
                    };
            }

        ::asio::io_context &
        io_context() const noexcept override { return *m_io_context; }

        void
        agent_bound() noexcept override
            {
                ++m_agents_bound;
            }

        void
        agent_unbound() noexcept override
            {
                --m_agents_bound;
            }

        demands_counter_t &
        demands_counter() noexcept override
            {
                return m_demands_counter;
            }

    protected :
        void
        start(
            environment_t & env,
            std::string_view data_sources_name_base )
            {
                data_source().set_data_sources_name_base( data_sources_name_base );
                data_source().start( outliving_mutable(env.stats_repository()) );

                ::so_5::details::do_with_rollback_on_exception(
                        [&] { launch_work_threads(env); },
                        [this] { data_source().stop(); } );
            }

        void
        shutdown()
            {
                ::so_5::details::invoke_noexcept_code( [this] {
                        // Stopping Asio IO service.
                        m_io_context->stop();
                    } );
            }

        void
        wait()
            {
                ::so_5::details::invoke_noexcept_code( [this] {
                        // Waiting for complete stop of all work threads.
                        wait_work_threads();
                        // Stopping data source.
                        data_source().stop();
                    } );
            }

    protected :
        /*!
         * \brief Get the count of work threads to be created.
         */
        [[nodiscard]]
        std::size_t
        thread_count() const noexcept { return m_thread_count; }

        /*!
         * \brief Get access to actual data source instance for that
         * dispatcher.
         */
        [[nodiscard]]
        virtual disp_data_source_t &
        data_source() noexcept = 0;

        /*!
         * \brief Get access to thread factory to be used for that dispatcher.
         *
         * \since v.1.5.0
         */
        [[nodiscard]]
        ::so_5::disp::abstract_work_thread_factory_shptr_t
        thread_factory() const noexcept { return m_thread_factory; }

#if defined(__clang__)
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wnon-virtual-dtor"
#endif

        /*!
         * \brief Data source for run-time monitoring of whole dispatcher.
         *
         * \since
         * v.1.0.2
         */
        class disp_data_source_t
            :   public ::so_5::stats::source_t
            {
                //! Dispatcher to work with.
                basic_dispatcher_skeleton_t & m_dispatcher;

                //! Basic prefix for data sources.
                ::so_5::stats::prefix_t m_base_prefix;

                //! Data source repository.
                /*!
                 * Will receive actual value in start() method.
                 *
                 * \since
                 * v.1.3.0
                 */
                ::so_5::stats::repository_t * m_stats_repo{ nullptr };

            protected :
                //! Access to data source prefix for derived classes.
                const ::so_5::stats::prefix_t &
                base_prefix() const noexcept { return m_base_prefix; }

            public :
                disp_data_source_t( basic_dispatcher_skeleton_t & disp )
                    :   m_dispatcher( disp )
                    {}

                virtual void
                distribute( const mbox_t & mbox ) override
                    {
                        const auto agents_count = m_dispatcher.m_agents_bound.load(
                                std::memory_order_acquire );

                        const auto demands_count = m_dispatcher.m_demands_counter.load(
                                std::memory_order_acquire );

                        send< ::so_5::stats::messages::quantity< std::size_t > >(
                                mbox,
                                m_base_prefix,
                                ::so_5::stats::suffixes::agent_count(),
                                agents_count );

                        // Note: because there is no way to detect on which thread a
                        // demand will be handled, the total number of waiting
                        // demands is destributed for the whole dispatcher.
                        send< ::so_5::stats::messages::quantity< std::size_t > >(
                                mbox,
                                m_base_prefix,
                                ::so_5::stats::suffixes::work_thread_queue_size(),
                                demands_count );
                    }

                void
                set_data_sources_name_base(
                    std::string_view name_base )
                    {
                        using namespace ::so_5::disp::reuse;

                        m_base_prefix = make_disp_prefix(
                                "ext-asio-tp",
                                name_base,
                                &m_dispatcher );
                    }

                void
                start( ::so_5::stats::repository_t & repo )
                    {
                        repo.add( *this );
                        m_stats_repo = &repo;
                    }

                void
                stop() noexcept
                    {
                        m_stats_repo->remove( *this );
                    }
            };

#if defined(__clang__)
#pragma clang diagnostic pop
#endif

    private:
        //! Count of work threads.
        const std::size_t m_thread_count;

        //! IO Service to work with.
        const std::shared_ptr< ::asio::io_context > m_io_context;

        /*!
         * \brief Thread factory to be used with this dispatcher.
         *
         * \since v.1.5.0
         */
        const ::so_5::disp::abstract_work_thread_factory_shptr_t m_thread_factory;

        //! Count of agents bound to that dispatcher.
        std::atomic< std::size_t > m_agents_bound{ 0u };

        //! Count of waiting demands.
        demands_counter_t m_demands_counter;

        //! Start all working threads.
        virtual void
        launch_work_threads(
            //! SObjectizer Environment for which threads will be created.
            environment_t & env ) = 0;

        //! Wait for finish of all threads.
        /*!
         * It is a blocking call. The current thread will be stopped until
         * all work thread will finish their work.
         */
        virtual void
        wait_work_threads() noexcept = 0;
    };

//
// dispatcher_skeleton_without_thread_activity_tracking_t
//
/*!
 * \brief Extension of basic dispatcher skeleton for the case when
 * work thread activity is not collected.
 *
 * This class contains disp_data_source_t instance and implements
 * virtual method data_source() for accessing this instance.
 *
 * It also provides static method run_work_thread() which must be called
 * at the beginnig of work thread.
 *
 * \since
 * v.1.0.2
 */
class dispatcher_skeleton_without_thread_activity_tracking_t
    :   public basic_dispatcher_skeleton_t
    {
    public :
        dispatcher_skeleton_without_thread_activity_tracking_t(
            environment_t & env,
            disp_params_t params )
            :   basic_dispatcher_skeleton_t( env, std::move(params) )
            {}

    protected :
        virtual disp_data_source_t &
        data_source() noexcept { return m_data_source; }

        //! Implementation of main function for a work thread.
        static void
        run_work_thread(
            environment_t & env,
            ::asio::io_context & io_svc,
            dispatcher_skeleton_without_thread_activity_tracking_t & /*self*/,
            std::size_t /*index*/ )
            {
                work_thread_t::run< work_thread_without_activity_tracking_t >(
                        env, io_svc );
            }

    private :
        //! Actual data source instance.
        disp_data_source_t m_data_source{ *this };
    };

//
// dispatcher_skeleton_with_thread_activity_tracking_t
//
/*!
 * \brief Extension of basic dispatcher skeleton for the case when
 * work thread activity must be collected.
 *
 * This class defines its own actual_disp_data_source_t type and
 * contains an instance of that type. There is also implementation
 * of data_source() virtual method for accessing this instance.
 *
 * It provides static method run_work_thread() which must be called
 * at the beginnig of work thread.
 *
 * \since
 * v.1.0.2
 */
class dispatcher_skeleton_with_thread_activity_tracking_t
    :   public basic_dispatcher_skeleton_t
    {
#if defined(__clang__)
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wnon-virtual-dtor"
#endif
        /*!
         * \brief Actual data source type for dispatcher with
         * work thread activity tracking.
         *
         * \since
         * v.1.0.2
         */
        class actual_disp_data_source_t
            :   public disp_data_source_t
            {
            private :
                //! Collectors for run-time stats for every thread.
                std::vector<
                            std::unique_ptr< work_thread_activity_collector_t > >
                    m_collectors;

            public :
                actual_disp_data_source_t(
                    basic_dispatcher_skeleton_t & disp,
                    std::size_t thread_count )
                    :   disp_data_source_t( disp )
                    ,   m_collectors( thread_count )
                    {
                        for( auto & c : m_collectors )
                            c = std::make_unique< work_thread_activity_collector_t >();
                    }

                virtual void
                distribute( const mbox_t & mbox ) override
                    {
                        disp_data_source_t::distribute( mbox );

                        for( std::size_t i = 0; i != m_collectors.size(); ++i )
                            distribute_stats_for_work_thread_at( mbox, i );
                    }

                /*!
                 * \note \a index is not checked for validity!
                 */
                work_thread_activity_collector_t &
                collector_at( std::size_t index ) noexcept
                    {
                        return *(m_collectors[index]);
                    }

            private :
                void
                distribute_stats_for_work_thread_at(
                    const mbox_t & mbox,
                    std::size_t index )
                    {
                        std::ostringstream ss;
                        ss << base_prefix().c_str() << "/wt-" << index;

                        const ::so_5::stats::prefix_t prefix{ ss.str() };
                        auto & collector = collector_at( index );

                        so_5::send< ::so_5::stats::messages::work_thread_activity >(
                                mbox,
                                prefix,
                                ::so_5::stats::suffixes::work_thread_activity(),
                                collector.thread_id(),
                                collector.take_activity_stats() );
                    }
            };

#if defined(__clang__)
#pragma clang diagnostic pop
#endif

    public :
        dispatcher_skeleton_with_thread_activity_tracking_t(
            environment_t & env,
            disp_params_t params )
            :   basic_dispatcher_skeleton_t( env, params )
            ,   m_actual_data_source( *this, params.thread_count() )
            {}

    protected :
        virtual disp_data_source_t &
        data_source() noexcept override { return m_actual_data_source; }

        //! Implementation of main function for a work thread.
        static void
        run_work_thread(
            //! SObjectizer Environment for which the work thread is created.
            environment_t & env,
            //! Asio IoService to be used.
            ::asio::io_context & io_svc,
            //! Dispatcher who owns this thread.
            dispatcher_skeleton_with_thread_activity_tracking_t & self,
            //! Ordinal number of this thread.
            std::size_t index )
            {
                work_thread_t::run< work_thread_with_activity_tracking_t >(
                        env,
                        io_svc,
                        outliving_mutable(
                                self.m_actual_data_source.collector_at(index) ) );
            }

    private :
        //! Data source instance.
        actual_disp_data_source_t m_actual_data_source;
    };

//
// dispatcher_template_t
//
/*!
 * \brief Template-based implementation of dispatcher.
 *
 * Implements virual methods launch_work_threads() and wait_work_threads()
 * from basic_dispatcher_skeleton_t.
 *
 * \tparam Traits Traits-type to be used.
 * \tparam Basic_Skeleton A specific skeleton to be used as base type.
 * It expected to be dispatcher_skeleton_with_thread_activity_tracking_t or
 * dispatcher_skeleton_without_thread_activity_tracking_t.
 *
 * \since
 * v.1.0.2
 */
template<
    typename Traits,
    typename Basic_Skeleton >
class dispatcher_template_t final : public Basic_Skeleton
    {
    public:
        dispatcher_template_t(
            //! SObjectizer Environment to work in.
            outliving_reference_t< environment_t > env,
            //! Value for creating names of data sources for
            //! run-time monitoring.
            std::string_view data_sources_name_base,
            //! Parameters for the dispatcher.
            disp_params_t params )
            :   Basic_Skeleton{ env.get(), std::move(params) }
            {
                this->start( env.get(), data_sources_name_base );
            }

        ~dispatcher_template_t() noexcept override
            {
                this->shutdown();
                this->wait();
            }

    private:
        //! An alias for thread_holder.
        using thread_holder_t = ::so_5::disp::work_thread_holder_t;

        //! Working threads.
        std::vector< thread_holder_t > m_threads;

        virtual void
        launch_work_threads(
            environment_t & env ) override
            {
                using namespace std;

                m_threads.resize( this->thread_count() );

                ::so_5::details::do_with_rollback_on_exception( [&] {
                        for( std::size_t i = 0u; i != this->thread_count(); ++i )
                            {
                                m_threads[ i ] = this->make_work_thread( env, i );
                            }
                    },
                    [&] {
                        ::so_5::details::invoke_noexcept_code( [&] {
                            this->io_context().stop();

                            // Shutdown all started threads.
                            for( auto & t : m_threads )
                                if( t )
                                    {
                                        t.unchecked_get().join();
                                        t = thread_holder_t{};
                                    }
                                else
                                    // No more started threads.
                                    break;
                        } );
                    } );
            }

        virtual void
        wait_work_threads() noexcept override
            {
                for( auto & t : m_threads )
                    {
                        t.unchecked_get().join();
                        t = thread_holder_t{};
                    }
            }

        thread_holder_t
        make_work_thread(
            environment_t & env,
            std::size_t index )
            {
                Basic_Skeleton * self = this;
                thread_holder_t work_thread{
                        this->thread_factory()->acquire( env ),
                        this->thread_factory()
                    };
                work_thread.unchecked_get().start(
                    [&env, io_svc = &this->io_context(), self, index]()
                    {
                        Basic_Skeleton::run_work_thread( env, *io_svc, *self, index );
                    } );

                return work_thread;
            }
    };

//
// dispatcher_handle_maker_t
//
class dispatcher_handle_maker_t
    {
    public :
        [[nodiscard]]
        static dispatcher_handle_t
        make( actual_dispatcher_shptr_t disp ) noexcept
            {
                return { std::move( disp ) };
            }
    };

} /* namespace impl */

//
// default_thread_pool_size
//
/*!
 * \brief A helper function for detecting default thread count for
 * thread pool.
 *
 * \since
 * v.1.0.2
 */
inline std::size_t
default_thread_pool_size()
    {
        auto c = std::thread::hardware_concurrency();
        if( !c )
            c = 2;

        return c;
    }

//
// default_traits_t
//
/*!
 * \brief Default traits of %asio_thread_pool dispatcher.
 *
 * \note
 * This type is empty in v.1.5.0.
 * It is left empty intentionally to have a possibility to extend it later, in
 * some future version.
 *
 * \since
 * v.1.0.2
 */
struct default_traits_t
    {
    };

//
// make_dispatcher
//
/*!
 * \brief A function for creation an instance of %asio_thread_pool dispatcher.
 *
 * Usage examples:
 * \code
 * // Dispatcher which uses own Asio IoContext and default traits.
 * namespace asio_tp = so_5::extra::disp::asio_thread_pool;
 * asio_tp::disp_params_t params;
 * params.use_own_io_context(); // Asio IoContext object will be created here.
 *      // This object will be accessible later via
 *      // dispatcher_handle_t::io_context() method.
 * auto disp = asio_tp::make_dispatcher(
 *  env,
 *  "my_asio_tp",
 *  std::move(disp_params) );
 *
 *
 * // Dispatcher which uses external Asio IoContext and default traits.
 * asio::io_context & io_svc = ...;
 * namespace asio_tp = so_5::extra::disp::asio_thread_pool;
 * asio_tp::disp_params_t params;
 * params.use_external_io_context( io_svc );
 * auto disp = asio_tp::make_dispatcher(
 *  env,
 *  "my_asio_tp",
 *  std::move(disp_params) );
 * \endcode
 *
 * \par Requirements for traits type
 * The Traits-type is empty in v.1.5.0. There was a possibitily to specify
 * a custom thread type in previous versions of so_5_extra, but since v.1.5.0
 * custom threads are supported via standard SObjectizer's mechanism based
 * on `abstract_work_thread_t`/`abstract_work_thread_factory_t` interfaces.
 * But the Traits-type might be extended by some content in future versions.
 *
 * \tparam Traits Type with traits for a dispatcher. For the requirements
 * for \a Traits type see the section "Requirements for traits type" above.
 *
 * \since
 * v.1.0.2
 */
template< typename Traits = default_traits_t >
[[nodiscard]]
inline dispatcher_handle_t
make_dispatcher(
    //! SObjectizer Environment to work in.
    environment_t & env,
    //! Value for creating names of data sources for
    //! run-time monitoring.
    const std::string_view data_sources_name_base,
    //! Parameters for the dispatcher.
    disp_params_t disp_params )
    {
        const auto io_svc_ptr = disp_params.io_context();
        if( !io_svc_ptr )
            SO_5_THROW_EXCEPTION(
                    errors::rc_io_context_is_not_set,
                    "io_context is not set in disp_params" );

        if( !disp_params.thread_count() )
            disp_params.thread_count( default_thread_pool_size() );

        using so_5::stats::activity_tracking_stuff::create_appropriate_disp;
        auto disp = create_appropriate_disp<
                // Type of result pointer.
                impl::actual_dispatcher_iface_t,
                // Actual type of dispatcher without thread activity tracking.
                impl::dispatcher_template_t<
                        Traits,
                        impl::dispatcher_skeleton_without_thread_activity_tracking_t >,
                // Actual type of dispatcher with thread activity tracking.
                impl::dispatcher_template_t<
                        Traits,
                        impl::dispatcher_skeleton_with_thread_activity_tracking_t > >(
            outliving_mutable(env),
            data_sources_name_base,
            std::move(disp_params) );

        return impl::dispatcher_handle_maker_t::make( std::move(disp) );
    }

} /* namespace asio_thread_pool */

} /* namespace disp */

} /* namespace extra */

} /* namespace so_5 */