lumiera_/src/lib/depend2.hpp

/*
  DEPEND.hpp  -  access point to singletons and dependencies

  Copyright (C)         Lumiera.org
    2013,               Hermann Vosseler <Ichthyostega@web.de>
    2018,               Hermann Vosseler <Ichthyostega@web.de>

  This program is free software; you can redistribute it and/or
  modify it under the terms of the GNU General Public License as
  published by the Free Software Foundation; either version 2 of
  the License, or (at your option) any later version.

  This program is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.
 
  You should have received a copy of the GNU General Public License
  along with this program; if not, write to the Free Software
  Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.

*/


/** @file depend.hpp
 ** Singleton services and Dependency Injection.
 ** The <b>Singleton Pattern</b> provides a single access point to a class or
 ** service and exploits this ubiquitous access point to limit the number of objects
 ** of this type to a single shared instance. Within Lumiera, we mostly employ a
 ** factory template for this purpose; the intention is to use on-demand initialisation
 ** and a standardised lifecycle. In the default configuration, this `Depend<TY>` factory
 ** maintains a singleton instance of type `TY`. The possibility to install other factory
 ** functions allows for subclass creation and various other kinds of service management.
 ** 
 ** 
 ** # Why Singletons? Inversion-of-Control and Dependency Injection
 ** 
 ** Singletons are frequently over-used, and often they serve as disguised
 ** global variables to support a procedural programming style. As a remedy, typically
 ** the use of a »Dependency Injection Container« is promoted. And -- again typically --
 ** these DI containers tend to evolve into heavyweight universal tools and substitute
 ** the original problem by metadata hell.
 ** 
 ** Thus, for Lumiera, the choice to use Singletons was deliberate: we understand the
 ** Inversion-of-Control principle, yet we want to stay just below the level of building
 ** a central application manager core. At the usage site, we access a factory for some
 ** service *by name*, where the »name« is actually the type name of an interface or
 ** facade. Singleton is used as an _implementation_ of this factory, when the service
 ** is self-contained and can be brought up lazily.
 ** 
 ** ## Conventions, Lifecycle and Unit Testing
 ** 
 ** Usually we place an instance of the singleton factory (or some other kind of factory)
 ** as a static variable within the interface class describing the service or facade.
 ** As a rule, everything accessible as Singleton is sufficiently self-contained to come
 ** up any time -- even prior to `main()`. But at shutdown, any deregistration must be done
 ** explicitly using a lifecycle hook. In Lumiera, destructors aren't allowed to do
 ** _any significant work_ beyond releasing references, and we acknowledge that
 ** singletons can be released in _arbitrary order_.
 ** 
 ** Lifecycle and management of dependencies is beyond the scope of this access mechanism
 ** exposed here. However, the actual product to be created or exposed lazily can be
 ** configured behind the scenes, as long as this configuration is performed _prior_
 ** to the first access. This configuration is achieved with the help of the "sibling"
 ** template lib::DependInject, which is declared friend within `Depend<T>` for type `T`
 ** - a service with distinct lifecycle can be exposed through the `Depend<T>` front-end
 ** - it is possible to create a mock instance, which temporarily shadows what
 **   Depend<T> delivers on access.
 ** 
 ** ## Implementation and performance
 ** 
 ** Due to this option for flexible configuration, the implementation can not be built
 ** as Meyer's Singleton. Rather, Double Checked Locking of a Mutex is combined with an
 ** std::atomic to work around the known (rather theoretical) problems of this pattern.
 ** Microbenchmarks indicate that this implementation technique ranges close to the
 ** speed of a direct access to an already existing object; in the fully optimised
 ** variant it was found to be roughly at ≈ 1ns and thus about 3 to 4 times slower
 ** than the comparable unprotected direct access without lazy initialisation.
 ** This is orders of magnitude better than any flavour of conventional locking.
 ** 
 ** @see depend-inject.hpp
 ** @see lib::DependInject
 ** @see Singleton_test
 ** @see DependencyConfiguration_test
 */


#ifndef WIP_LIB_DEPEND_H
#define WIP_LIB_DEPEND_H


#include "lib/error.hpp"
#include "lib/nocopy.hpp"
#include "lib/nobug-init.hpp"
#include "lib/sync-classlock.hpp"
#include "lib/meta/util.hpp"

#include <type_traits>
#include <functional>
#include <atomic>
#include <memory>


namespace lib {
  namespace error = lumiera::error;
  
  
  
  
  
  
  /**
   * @internal access point to reconfigure dependency injection on a per type base
   * @see depend-inject.hpp
   */
  template<class SRV>
  class DependInject;
  
  
  /**
   * Access point to singletons and other kinds of dependencies designated *by type*.
   * Actually this is a Factory object, which is typically placed into a static field
   * of the Singleton (target) class or some otherwise suitable interface.
   * @tparam SRV the class of the Service or Singleton instance
   * @note uses static fields internally, so all factory configuration is shared per type
   * @remarks
   *  - threadsafe lazy instantiation implemented by Double Checked Locking with std::atomic.
   *  - by default, without any explicit configuration, this template creates a singleton.
   *  - a per-type factory function can be configured with the help of lib::DependInject<SRV>
   *  - singletons will be destroyed when the embedded static InstanceHolder is destroyed.
   */
  template<class SRV>
  class Depend
    {
      using Factory = std::function<SRV*()>;
      using Lock = ClassLock<SRV, NonrecursiveLock_NoWait>;
      
      static std::atomic<SRV*> instance;
      static Factory            factory;
      
      friend class DependInject<SRV>;
      
      
    public:
      /** Interface to be used by clients for retrieving the service instance.
       *  Manages the instance creation, lifecycle and access in multithreaded context.
       *  @return instance of class `SRV`. When used in default configuration,
       *          this service instance is a singleton
       */
      SRV&
      operator() ()
        {
          SRV* object = instance.load (std::memory_order_acquire);
          if (!object)
            {
              Lock guard;
              
              object = instance.load (std::memory_order_relaxed);
              if (!object)
                {
                  if (!factory)
                    {
                      object = buildInstance<SRV>();
                      deleter = []{
                                    destroy (instance);
                                    instance = nullptr;
                                  };
                    }
                  else
                    object = factory();
                  factory = disabledFactory;
                }
              instance.store (object, std::memory_order_release);
            }
          ENSURE (object);
          return *object;
        }
      
      
    private:
      /** @internal preconfigured factory to block any (further) on-demand instance creation */
      static SRV*
      disabledFactory()
        {
          throw error::Fatal("Service not available at this point of the Application Lifecycle"
                            ,error::LUMIERA_ERROR_LIFECYCLE);
        }
      
      template<class ABS>
      static     meta::enable_if<std::is_abstract<ABS>,
      ABS*                      >
      buildInstance()
        {
          throw error::Fatal("Attempt to create a singleton instance of an abstract class. "
                             "Application architecture or lifecycle is seriously broken.");
        }
      template<class TAR>
      static     meta::disable_if<std::is_abstract<TAR>,
      TAR*                      >
      buildInstance()
        {
          return new TAR;
        }
      
      static void
      destroy (SRV* obj)
        {
          if (obj)
            delete obj;
        }
      
      class Deleter
        {
          std::function<void(void)> cleanUp_;
          
        public:
         ~Deleter()
            {
              if (cleanUp_)
                cleanUp_();
            }
          template<typename DEL>
          void operator= (DEL&& fun)
            {
              cleanUp_ = std::forward<DEL> (fun);
            }
        };
      
      static Deleter deleter;
    };
  
  
  
  
  /* === allocate Storage for static per type instance management === */
  template<class SRV>
  std::atomic<SRV*> Depend<SRV>::instance;
  
  template<class SRV>
  typename Depend<SRV>::Factory Depend<SRV>::factory;
  
  template<class SRV>
  typename Depend<SRV>::Deleter Depend<SRV>::deleter;
  
  
  
} // namespace lib
#endif