shutdowner.hpp

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/*!
 * \file
 * \brief Implementation of shutdowner-related stuff.
 */

#pragma once

#include <so_5_extra/error_ranges.hpp>

#include <so_5/rt/impl/h/msg_tracing_helpers.hpp>
#include <so_5/rt/impl/h/message_limit_internals.hpp>
#include <so_5/rt/impl/h/agent_ptr_compare.hpp>

#include <so_5/details/h/sync_helpers.hpp>

#include <so_5/rt/h/mbox.hpp>
#include <so_5/rt/h/send_functions.hpp>

#include <so_5/h/outliving.hpp>

namespace so_5 {

namespace extra {

namespace shutdowner {

namespace errors {

/*!
 * \brief An attempt to use illegal message type.
 *
 * For example: shutdowner mbox allows subscription only to
 * shutdown_initiated message.
 */
const int rc_illegal_msg_type =
        so_5::extra::errors::shutdowner_errors;

/*!
 * \brief Subscription to shutdowner mbox when shutdown in progess
 * is prohibited.
 */
const int rc_subscription_disabled_during_shutdown =
        so_5::extra::errors::shutdowner_errors + 1;

} /* namespace errors */

/*!
 * \brief A message to be used to inform about start of shutdown operation.
 *
 * \note
 * This is a message, not a signal. This message is empty now but it
 * can be extended in future versions of so_5_extra.
 */
struct shutdown_initiated_t : public so_5::message_t {};

namespace details {

/*!
 * \brief Implementation of stop_guard for shutdowner's purposes.
 *
 * This implementation sends shutdown_initiated_t message to the specified
 * mbox.
 */
class shutdowner_guard_t : public so_5::stop_guard_t
    {
        //! Mbox to which shutdown_initiated must be sent.
        const so_5::mbox_t m_notify_mbox;

    public :
        //! Initializing constructor.
        shutdowner_guard_t( so_5::mbox_t notify_mbox )
            :   m_notify_mbox( std::move(notify_mbox) )
            {}

        virtual void
        stop() noexcept override
            {
                so_5::send< shutdown_initiated_t >( m_notify_mbox );
            }
    };

/*!
 * \brief A signal which is used to limit time of shutdown operation.
 */
struct shutdown_time_elapsed_t final : public so_5::message_t {};

/*!
 * \brief Special mbox to receive and handle a signal about time limit.
 *
 * This mbox implements just one method: do_deliver_message(). A std::abort()
 * is called in this method if shutdown operation is not finished yet.
 */
class time_elapsed_mbox_t
    : public abstract_message_box_t
    {
    public :
        //! Initializing constructor.
        time_elapsed_mbox_t(
            //! SObjectizer environment to work in.
            //! It is necessary to access error_logging before calling std::abort().
            outliving_reference_t< environment_t > env,
            //! Unique ID of this mbox.
            mbox_id_t id )
            :   m_env( env )
            ,   m_id( id )
            {}

        virtual mbox_id_t
        id() const override
            {
                return m_id;
            }

        virtual void
        subscribe_event_handler(
            const std::type_index & /*type_index*/,
            const message_limit::control_block_t * /*limit*/,
            agent_t * /*subscriber*/ ) override
            {
                SO_5_THROW_EXCEPTION( rc_not_implemented,
                        "subscribe_event_handler is not implemented for "
                        "time_elapsed_mbox" );
            }

        virtual void
        unsubscribe_event_handlers(
            const std::type_index & /*type_index*/,
            agent_t * /*subscriber*/ ) override
            {
                SO_5_THROW_EXCEPTION( rc_not_implemented,
                        "subscribe_event_handler is not implemented for "
                        "time_elapsed_mbox" );
            }

        virtual std::string
        query_name() const override
            {
                std::ostringstream ss;
                ss << "<mbox:type=MPSC:shutdowner_time_elapsed:id=" << m_id << ">";
                return ss.str();
            }

        virtual mbox_type_t
        type() const override
            {
                return mbox_type_t::multi_producer_single_consumer;
            }

        virtual void
        do_deliver_message(
            const std::type_index & /*msg_type*/,
            const message_ref_t & /*message*/,
            unsigned int /*overlimit_reaction_deep*/ ) const override
            {
                m_env.get().error_logger().log(
                        __FILE__, __LINE__,
                        "Time of shutdown operation is elapsed. "
                        "Application will be terminated." );
                std::abort();
            }

        virtual void
        do_deliver_service_request(
            const std::type_index & /*msg_type*/,
            const message_ref_t & /*message*/,
            unsigned int /*overlimit_reaction_deep*/ ) const override
            {
                SO_5_THROW_EXCEPTION( rc_not_implemented,
                        "do_deliver_service_request is not implemented for "
                        "time_elapsed_mbox" );
            }

        virtual void
        set_delivery_filter(
            const std::type_index & /*msg_type*/,
            const delivery_filter_t & /*filter*/,
            agent_t & /*subscriber*/ ) override
            {
                SO_5_THROW_EXCEPTION( rc_not_implemented,
                        "set_delivery_filter is not implemented for "
                        "time_elapsed_mbox" );
            }

        virtual void
        drop_delivery_filter(
            const std::type_index & /*msg_type*/,
            agent_t & /*subscriber*/ ) noexcept override
            {
                /* Nothing to do. */
            }

    private :
        //! SOEnv to work in.
        outliving_reference_t< environment_t > m_env;
        //! Unique ID of that mbox.
        const mbox_id_t m_id;
    };

//
// subscriber_info_t
//
/*!
 * \brief Description of one subscriber.
 */
struct subscriber_info_t
    {
        //! Actual subscriber.
        /*!
         * Can't be nullptr.
         */
        agent_t * m_subscriber;
        //! Message limit for that subscriber.
        /*!
         * Can be nullptr if message limit is not used.
         */
        const message_limit::control_block_t * m_limits;

        //! Initializing constructor.
        subscriber_info_t(
            agent_t * subscriber,
            const message_limit::control_block_t * limits )
            :   m_subscriber(subscriber)
            ,   m_limits(limits)
            {}

        //! Comparison operator.
        /*!
         * Compares only pointers to m_subscriber with respects
         * to agent's priority.
         */
        bool
        operator<( const subscriber_info_t & o ) const
            {
                return ::so_5::impl::special_agent_ptr_compare(
                        *m_subscriber, *(o.m_subscriber) );
            }
    };

//
// subscriber_container_t
//
/*!
 * \brief Type of subscriber's container.
 */
using subscriber_container_t = std::vector< subscriber_info_t >;

namespace status {

//! Avaliable statuses of shutdown operation.
enum class value_t
    {
        //! Shutdown is not started yet.
        not_started,
        //! Shutdown is started but there are some pending subscribers.
        started,
        //! All subscribers are unsubscribed.
        //! Shutdown can and should be completed.
        must_be_completed
    };

const value_t not_started = value_t::not_started;
const value_t started = value_t::started;
const value_t must_be_completed = value_t::must_be_completed;

//! Which action must be performed after updating status of shutdown operation.
enum class deferred_action_t
    {
        //! No action is required.
        do_nothing,
        //! Shutdown action must be completed.
        complete_shutdown
    };

//! Special object which holds status.
/*!
 * Updates for the status are enabled only to instances of updater_t class.
 */
class holder_t
    {
        friend class updater_t;

        value_t m_status = value_t::not_started;

    public :
        value_t
        current() const { return m_status; }
    };

//! Special object to change the status and detect deferred action to be performed.
class updater_t
    {
        outliving_reference_t< holder_t > m_status;
        deferred_action_t m_action = deferred_action_t::do_nothing;

    public :
        updater_t(
            outliving_reference_t< holder_t > status )
            :   m_status( status )
            {}

        value_t
        current() const { return m_status.get().current(); }

        void
        update( value_t new_status )
            {
                m_status.get().m_status = new_status;
                if( value_t::must_be_completed == new_status )
                    m_action = deferred_action_t::complete_shutdown;
                else
                    m_action = deferred_action_t::do_nothing;
            }

        deferred_action_t
        action() const { return m_action; }
    };

} /* namespace status */

//
// notify_mbox_data_t
//
/*!
 * \brief An internal data of notify_mbox.
 *
 * It is collected in separate struct to provide ability to mark
 * object of that struct as mutable. It is necessary because some
 * of abstract_message_box_t's methods are declared as const by
 * historical reasons.
 */
struct notify_mbox_data_t
{
    //! Status of the shutdown operation.
    status::holder_t m_status;

    //! List of actual subscribers.
    subscriber_container_t m_subscribers;

    //! Mbox to be used for delayed shutdown_time_elapsed message.
    mbox_t m_time_elapsed_mbox;
    //! A time for shutdown operation.
    const std::chrono::steady_clock::duration m_max_shutdown_time;

    //! Timer ID for shutdown_time_elapsed message.
    /*!
     * Will be used for canceling of delayed message when shutdown is
     * completed.
     *
     * Will receive actual value only when shutdown operation started.
     */
    timer_id_t m_shutdown_timer;

    //! Initializing constructor.
    notify_mbox_data_t(
        mbox_t time_elapsed_mbox,
        std::chrono::steady_clock::duration max_shutdown_time )
        :   m_time_elapsed_mbox( std::move(time_elapsed_mbox) )
        ,   m_max_shutdown_time( max_shutdown_time )
        {}
};

//
// notify_mbox_t
//
/*!
 * \brief A special mbox which must be used for notification about
 * shutdown operation.
 *
 * \tparam Lock_Type type of lock to be used for thread safety
 * \tparam Tracing_Base type that implements message tracing stuff
 * It is expected to be so_5::impl::msg_tracing_helpers::tracing_enabled_base
 * or so_5::impl::msg_tracing_helpers::tracing_disabled_base
 */
template< typename Lock_Type, typename Tracing_Base >
class notify_mbox_t
    : public abstract_message_box_t
    , private ::so_5::details::lock_holder_detector<Lock_Type>::type
    , private Tracing_Base
    {
    public :
        template< typename... Tracing_Base_Args >
        notify_mbox_t(
            //! SObjectizer Environment to work in.
            outliving_reference_t< environment_t > env,
            //! A mbox for delayed message shutdown_time_elapsed_t.
            mbox_t time_elapsed_mbox,
            //! Time for the whole shutdown operation.
            std::chrono::steady_clock::duration max_shutdown_time,
            //! Unique ID of that mbox.
            mbox_id_t id,
            //! Params for tracing stuff.
            Tracing_Base_Args && ...tracing_args )
            :   Tracing_Base( std::forward<Tracing_Base_Args>(tracing_args)... )
            ,   m_env( env )
            ,   m_id( id )
            ,   m_data( std::move(time_elapsed_mbox), max_shutdown_time )
            {
                // Now we can create and install shutdowner_guard to
                // prevent SObjectizer from shutdown.
                auto guard = std::make_shared< shutdowner_guard_t >( mbox_t(this) );
                m_env.get().setup_stop_guard( guard );
                m_shutdowner_guard = std::move(guard);
            }

        virtual mbox_id_t
        id() const override
            {
                return m_id;
            }

        virtual void
        subscribe_event_handler(
            const std::type_index & type_index,
            const message_limit::control_block_t * limit,
            agent_t * subscriber ) override
            {
                ensure_valid_message_type( type_index );
                this->lock_and_perform( [&] {
                    do_event_subscription( limit, subscriber );
                } );
            }

        virtual void
        unsubscribe_event_handlers(
            const std::type_index & type_index,
            agent_t * subscriber ) override
            {
                ensure_valid_message_type( type_index );
                const auto action = this->lock_and_perform( [&] {
                    return do_event_unsubscripton( subscriber );
                } );

                if( status::deferred_action_t::complete_shutdown == action )
                    complete_shutdown();
            }

        virtual std::string
        query_name() const override
            {
                std::ostringstream ss;
                ss << "<mbox:type=MPMC:shutdowner:id=" << m_id << ">";
                return ss.str();
            }

        virtual mbox_type_t
        type() const override
            {
                return mbox_type_t::multi_producer_multi_consumer;
            }

        virtual void
        do_deliver_message(
            const std::type_index & msg_type,
            const message_ref_t & message,
            unsigned int /*overlimit_reaction_deep*/ ) const override
            {
                ensure_valid_message_type( msg_type );
                const auto action = do_initiate_shutdown( msg_type, message );
                if( status::deferred_action_t::complete_shutdown == action )
                    complete_shutdown();
            }

        virtual void
        do_deliver_service_request(
            const std::type_index & msg_type,
            const message_ref_t & /*message*/,
            unsigned int /*overlimit_reaction_deep*/ ) const override
            {
                ensure_valid_message_type( msg_type );
                SO_5_THROW_EXCEPTION( rc_not_implemented,
                        "do_deliver_service_request is not implemented for "
                        "shutdowner mbox" );
            }

        virtual void
        set_delivery_filter(
            const std::type_index & msg_type,
            const delivery_filter_t & /*filter*/,
            agent_t & /*subscriber*/ ) override
            {
                ensure_valid_message_type( msg_type );
                SO_5_THROW_EXCEPTION( rc_not_implemented,
                        "unable to set delivery filter to shutdowner mbox" );
            }

        virtual void
        drop_delivery_filter(
            const std::type_index & /*msg_type*/,
            agent_t & /*subscriber*/ ) noexcept override
            {
                /* Nothing to do. */
            }

    private :
        //! SObjectizer Environment to work in.
        outliving_reference_t< environment_t > m_env;

        //! Stop_guard which prevents SObjectizer from shutdown.
        stop_guard_shptr_t m_shutdowner_guard;

        //! Unique ID of that mbox.
        const mbox_id_t m_id;

        //! Actual mbox data.
        /*!
         * \attention
         * It is marked as `mutable` because some methods of
         * abstract_message_box_t are const by historical reasons.
         * But there is a necessity to change mbox's data inside such methods.
         */
        mutable notify_mbox_data_t m_data;

        //! Check for valid type of message to be handled.
        /*!
         * \note
         * Only shutdown_initiated_t message can be handled by that mbox type.
         */
        static void
        ensure_valid_message_type( const std::type_index & type_index )
            {
                if( type_index != typeid(shutdown_initiated_t) )
                    SO_5_THROW_EXCEPTION(
                            ::so_5::extra::shutdowner::errors::rc_illegal_msg_type,
                            "only shutdown_initiated_t message type is allowed to "
                            "be used with shutdowner mbox" );
            }

        //! Main subscription actions.
        /*!
         * May throw if shutdown is in progress or already completed.
         */
        void
        do_event_subscription(
            const message_limit::control_block_t * limit,
            agent_t * subscriber )
            {
                if( status::not_started != m_data.m_status.current() )
                    SO_5_THROW_EXCEPTION(
                            ::so_5::extra::shutdowner::errors::rc_subscription_disabled_during_shutdown,
                            "a creation of new subscription is disabled during "
                            "shutdown procedure" );

                subscriber_info_t info{ subscriber, limit };
                auto it = std::lower_bound(
                        std::begin(m_data.m_subscribers),
                        std::end(m_data.m_subscribers),
                        info );
                m_data.m_subscribers.insert( it, info );
            }

        //! Main unsubscription actions.
        /*!
         * Returns the action to be performed (shutdown completion may be
         * necessary).
         */
        status::deferred_action_t
        do_event_unsubscripton( agent_t * subscriber )
            {
                status::updater_t status_updater(
                        outliving_mutable( m_data.m_status ) );

                subscriber_info_t info{ subscriber, nullptr };
                auto it = std::lower_bound(
                        std::begin(m_data.m_subscribers),
                        std::end(m_data.m_subscribers),
                        info );
                if( it != std::end(m_data.m_subscribers) &&
                        it->m_subscriber == subscriber )
                    {
                        m_data.m_subscribers.erase( it );

                        if( status::started == status_updater.current() )
                            {
                                if( m_data.m_subscribers.empty() )
                                    {
                                        status_updater.update(
                                                status::must_be_completed );
                                    }
                            }
                    }

                return status_updater.action();
            }

        //! Do all necessary actions for completion of shutdown.
        /*!
         * \note
         * This method is marked as const because is may be called from
         * const virtual methods.
         */
        void
        complete_shutdown() const
            {
                m_data.m_shutdown_timer.release();

                m_env.get().remove_stop_guard( m_shutdowner_guard );
            }

        //! Do all necessary actions for start of shutdown operation.
        /*!
         * Returns the action to be performed (shutdown completion may be
         * necessary).
         *
         * \note
         * This method is marked as const because is may be called from
         * const virtual methods.
         */
        status::deferred_action_t
        do_initiate_shutdown(
            const std::type_index & msg_type,
            const message_ref_t & message ) const
            {
                status::updater_t updater(
                        outliving_mutable( m_data.m_status ) );

                if( status::not_started == updater.current() )
                    {
                        updater.update( status::started );
                        if( !m_data.m_subscribers.empty() )
                            {
                                send_shutdown_initated_to_all( msg_type, message );
                                start_shutdown_clock();
                            }
                        else
                            {
                                updater.update( status::must_be_completed );
                            }
                    }

                return updater.action();
            }

        //! Send shutdown_initiated message to all actual subscribers.
        /*!
         * \note
         * This method is marked as const because is may be called from
         * const virtual methods.
         */
        void
        send_shutdown_initated_to_all(
            const std::type_index & msg_type,
            const message_ref_t & message ) const
            {
                constexpr unsigned int overlimit_reaction_deep = 0u;

                typename Tracing_Base::deliver_op_tracer tracer{
                        *this, // as Tracing_Base
                        *this, // as abstract_message_box_t
                        "deliver_message",
                        msg_type, message, overlimit_reaction_deep };

                for( const auto & subscriber : m_data.m_subscribers )
                    {
                        using namespace so_5::message_limit::impl;

                        try_to_deliver_to_agent(
                                m_id,
                                invocation_type_t::event,
                                *(subscriber.m_subscriber),
                                subscriber.m_limits,
                                msg_type,
                                message,
                                overlimit_reaction_deep,
                                tracer.overlimit_tracer(),
                                [&] {
                                    tracer.push_to_queue( subscriber.m_subscriber );

                                    agent_t::call_push_event(
                                            *(subscriber.m_subscriber),
                                            subscriber.m_limits,
                                            m_id,
                                            msg_type,
                                            message );
                                } );
                    }
            }

        //! Initiate delayed shutdown_time_elapsed message.
        /*!
         * \note
         * This method is marked as const because is may be called from
         * const virtual methods.
         */
        void
        start_shutdown_clock() const
            {
                m_data.m_shutdown_timer = so_5::send_periodic< shutdown_time_elapsed_t >(
                        m_env,
                        m_data.m_time_elapsed_mbox,
                        m_data.m_max_shutdown_time,
                        std::chrono::milliseconds::zero() );
            }
    };

} /* namespace details */

//
// layer_t
//
/*!
 * \brief An interface of shutdowner layer.
 *
 * This is a public interface of actual layer. An user should use only
 * that interface when it want to work with shutdown layer.
 *
 * For example, to subscribe to shutdown_initiated_t message it is neccessary
 * to receive a pointer/reference to layer_t and call notify_mbox() method:
 * \code
 * // To make a subscription to shutdown notification.
 * class my_agent final : public so_5::agent_t {
 * public :
 *  my_agent(context_t ctx) : so_5::agent_t(std::move(ctx)) {
 *      // Long way:
 *      so_5::extra::shutdowner::layer_t * s =
 *          so_environment().query_layer<so_5::extra::shutdowner::layer_t>();
 *      so_subscribe(s->notify_mbox()).event(&my_agent::on_shutdown);
 *
 *      // Shortest way:
 *      so_subscribe(so_5::extra::shutdowner::layer(so_environment()).notify_mbox())
 *              .event(&my_agent::on_shutdown);
 *      ...
 *  }
 *  ...
 * private :
 *  void on_shutdown(mhood_t<so_5::extra::shutdowner::shutdown_initiated_t>)
 *  {...}
 * };
 * \endcode
 *
 * To initiate shutdown it is necessary to call so_5::environment_t::stop()
 * method:
 * \code
 * so_environment().stop();
 * \endcode
 */
class layer_t : public ::so_5::layer_t
    {
    public :
        //! Get a mbox which can be used for subscription to
        //! shutdown_initiated message.
        virtual mbox_t
        notify_mbox() const = 0;
    };

namespace details {

//
// layer_template_t
//
/*!
 * \brief A template-based implementation of shutdowner layer.
 *
 * \note
 * It creates all shutdowner-related stuff in start() method.
 */
template< typename Lock_Type >
class layer_template_t : public ::so_5::extra::shutdowner::layer_t
    {
    public :
        //! Initializing constructor.
        layer_template_t(
            //! Maximum time for the shutdown operation.
            std::chrono::steady_clock::duration shutdown_time )
            :   m_shutdown_time( shutdown_time )
            {}

        virtual mbox_t
        notify_mbox() const override
            {
                return m_notify_mbox;
            }

        virtual void
        start() override
            {
                auto time_elapsed_mbox = so_environment().make_custom_mbox(
                        [&]( const mbox_creation_data_t & data ) {
                            return do_make_time_elapsed_mbox( data );
                        } );

                m_notify_mbox = so_environment().make_custom_mbox(
                        [&]( const mbox_creation_data_t & data ) {
                            return do_make_notification_mbox(
                                    data,
                                    std::move( time_elapsed_mbox ) );
                        } );
            }

    private :
        //! Maximum time for the shutdown operation.
        const std::chrono::steady_clock::duration m_shutdown_time;

        //! Notification mbox.
        /*!
         * Will be created in start() method. Until then it is a null pointer.
         */
        mbox_t m_notify_mbox;

        //! Create mbox for delayed shutdown_time_elapsed message.
        mbox_t
        do_make_time_elapsed_mbox(
            //! Parameters for the new mbox.
            const mbox_creation_data_t & data )
            {
                return mbox_t( new details::time_elapsed_mbox_t(
                            outliving_mutable( so_environment() ),
                            data.m_id ) );
            }

        //! Create notification mbox.
        /*!
         * A new mbox will be created with respect to message tracing stuff.
         */
        mbox_t
        do_make_notification_mbox(
            //! Parameters for the new mbox.
            const mbox_creation_data_t & data,
            //! A mbox to be used for delayed shutdown_time_elapsed message.
            mbox_t time_elapsed_mbox )
            {
                mbox_t result;

                if( data.m_tracer.get().is_msg_tracing_enabled() )
                    {
                        using T = details::notify_mbox_t<
                                Lock_Type,
                                ::so_5::impl::msg_tracing_helpers::tracing_enabled_base >;
                        result = mbox_t( new T(
                                    outliving_mutable( so_environment() ),
                                    std::move( time_elapsed_mbox ),
                                    m_shutdown_time,
                                    data.m_id,
                                    data.m_tracer.get() ) );
                    }
                else
                    {
                        using T = details::notify_mbox_t<
                                Lock_Type,
                                ::so_5::impl::msg_tracing_helpers::tracing_disabled_base >;
                        result = mbox_t( new T(
                                    outliving_mutable( so_environment() ),
                                    std::move( time_elapsed_mbox ),
                                    m_shutdown_time,
                                    data.m_id ) );
                    }

                return result;
            }
    };

} /* namespace details */

//
// make_layer
//
/*!
 * \brief Main function to create an instance of shutdowner layer.
 *
 * Usage example:
 * \code
 * // Creation of layer with default mutex type.
 * so_5::launch([](so_5::environment_t & env) {...},
 *  [](so_5::environment_params_t & params) {
 *      params.add_layer( so_5::extra::shutdowner::make_layer<>(30s) );
 *  });
 *
 * // Creation of layer with user-provided spinlock type.
 * so_5::launch([](so_5::environment_t & env) {...},
 *  [](so_5::environment_params_t & params) {
 *      params.add_layer( so_5::extra::shutdowner::make_layer<my_spinlock_t>(30s));
 *  });
 *
 * // Creation of layer with null_mutex.
 * // Note: null_mutex must be used only for non thread-safe environments.
 * so_5::launch([](so_5::environment_t & env) {...},
 *  [](so_5::environment_params_t & params) {
 *      // Use single-threaded and not thread-safe environment.
 *      params.infrastructure_factory(
 *          so_5::env_infrastructures::simple_not_mtsafe::factory());
 *      // Shutdowner layer with null_mutex can be used in that environment.
 *      params.add_layer( so_5::extra::shutdowner::make_layer<so_5::null_mutex_t>(30s));
 *  });
 * \endcode
 *
 * \tparam Lock_Type type of lock to be used for thread safety.
 */
template< typename Lock_Type = std::mutex >
std::unique_ptr< ::so_5::extra::shutdowner::layer_t >
make_layer(
    //! A maximum time for timeout operation.
    std::chrono::steady_clock::duration shutdown_max_time )
    {
        std::unique_ptr< ::so_5::extra::shutdowner::layer_t > result(
                new details::layer_template_t< Lock_Type >(
                        shutdown_max_time ) );

        return result;
    }

//
// layer
//
/*!
 * \brief A helper function to receive a reference to shutdowner layer.
 *
 * Usage example:
 * \code
 * // To make a subscription to shutdown notification.
 * class my_agent final : public so_5::agent_t {
 * public :
 *  my_agent(context_t ctx) : so_5::agent_t(std::move(ctx)) {
 *      auto & s = so_5::extra::shutdowner::layer( so_environment() );
 *      so_subscribe( s.notify_mbox() ).event( &my_agent::on_shutdown );
 *      ...
 *  }
 *  ...
 * private :
 *  void on_shutdown(mhood_t<so_5::extra::shutdowner::shutdown_initiated_t>)
 *  {...}
 * };
 *
 * // To initiate shutdown.
 * so_5::extra::shutdowner::layer(env).initiate_shutdown();
 * \endcode
 *
 * \note May throw an exception if shutdowner layer is not defined.
 */
inline ::so_5::extra::shutdowner::layer_t &
layer( ::so_5::environment_t & env )
    {
        return *(env.query_layer< ::so_5::extra::shutdowner::layer_t >());
    }

} /* namespace shutdowner */

} /* namespace extra */

} /* namespace so_5 */