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+/*
+ *  Copyright (C) 2005-2018 Team Kodi
+ *  This file is part of Kodi - https://kodi.tv
+ *
+ *  SPDX-License-Identifier: GPL-2.0-or-later
+ *  See LICENSES/README.md for more information.
+ */
+
+#pragma once
+
+#include <memory>
+
+/**
+ * This file contains the pattern for moving "globals" from the BSS Segment to the heap.
+ * A note on usage of this pattern for globals replacement:
+ *
+ * This pattern uses a singleton pattern and some compiler/C preprocessor sugar to allow
+ * "global" variables to be lazy instantiated and initialized and moved from the BSS segment
+ * to the heap (that is, they are instantiated on the heap when they are first used rather
+ * than relying on the startup code to initialize the BSS segment). This eliminates the
+ * problem associated with global variable dependencies across compilation units.
+ *
+ * Reference counting from the BSS segment is used to destruct these globals at the time the
+ * last compilation unit that knows about it is finalized by the post-main shutdown. The book
+ * keeping is done by smuggling a smart pointer into every file that references a particular
+ * "global class" through the use of a 'static' declaration of an instance of that smart
+ * pointer in the header file of the global class (did you ever think you'd see a file scope
+ * 'static' variable in a header file - on purpose?)
+ *
+ * There are two different ways to use this pattern when replacing global variables.
+ * The selection of which one to use depends on whether or not there is a possibility
+ * that the code in the .cpp file for the global can be executed from a static method
+ * somewhere. This may take some explanation.
+ *
+ * The (at least) two ways to do this:
+ *
+ * 1) You can use the reference object std::shared_ptr to access the global variable.
+ *
+ * This would be the preferred means since it is (very slightly) more efficient than
+ * the alternative. To use this pattern you replace standard static references to
+ * the global with access through the reference. If you use the C preprocessor to
+ * do this for you can put the following code in the header file where the global's
+ * class is declared:
+ *
+ * static std::shared_ptr<GlobalVariableClass> g_globalVariableRef(xbmcutil::GlobalsSingleton<GlobalVariableClass>::getInstance());
+ * #define g_globalVariable (*(g_globalVariableRef.get()))
+ *
+ * Note what this does. In every file that includes this header there will be a *static*
+ * instance of the std::shared_ptr<GlobalVariableClass> smart pointer. This effectively
+ * reference counts the singleton from every compilation unit (ie, object code file that
+ * results from a compilation of a .c/.cpp file) that references this global directly.
+ *
+ * There is a problem with this, however. Keep in mind that the instance of the smart pointer
+ * (being in the BSS segment of the compilation unit) is ITSELF an object that depends on
+ * the BSS segment initialization in order to be initialized with an instance from the
+ * singleton. That means, depending on the code structure, it is possible to get into a
+ * circumstance where the above #define could be exercised PRIOR TO the setting of the
+ * value of the smart pointer.
+ *
+ * Some reflection on this should lead you to the conclusion that the only way for this to
+ * happen is if access to this global can take place through a static/global method, directly
+ * or indirectly (ie, the static/global method can call another method that uses the
+ * reference), where that static is called from initialization code exercised prior to
+ * the start of 'main.'
+ *
+ * Because of the "indirectly" in the above statement, this situation can be difficult to
+ * determine beforehand.
+ *
+ * 2) Alternatively, when you KNOW that the global variable can suffer from the above described
+ * problem, you can restrict all access to the variable to the singleton by changing
+ * the #define to:
+ *
+ * #define g_globalVariable (*(xbmcutil::Singleton<GlobalVariableClass>::getInstance()))
+ *
+ * A few things to note about this. First, this separates the reference counting aspect
+ * from the access aspect of this solution. The smart pointers are no longer used for
+ * access, only for reference counting. Secondly, all access is through the singleton directly
+ * so there is no reliance on the state of the BSS segment for the code to operate
+ * correctly.
+ *
+ * This solution is required for g_Windowing because it's accessed (both directly and
+ * indirectly) from the static methods of CLog which are called repeatedly from
+ * code exercised during the initialization of the BSS segment.
+ */
+
+namespace xbmcutil
+{
+  /**
+   * This class is an implementation detail of the macros defined below and
+   *  is NOT meant to be used as a general purpose utility. IOW, DO NOT USE THIS
+   *  CLASS to support a general singleton design pattern, it's specialized
+   *  for solving the initialization/finalization order/dependency problem
+   *  with global variables and should only be used via the macros below.
+   *
+   * Currently THIS IS NOT THREAD SAFE! Why not just add a lock you ask?
+   *  Because this singleton is used to initialize global variables and
+   *  there is an issue with having the lock used prior to its
+   *  initialization. No matter what, if this class is used as a replacement
+   *  for global variables there's going to be a race condition if it's used
+   *  anywhere else. So currently this is the only prescribed use.
+   *
+   * Therefore this hack depends on the fact that compilation unit global/static
+   *  initialization is done in a single thread.
+   */
+  template <class T> class GlobalsSingleton
+  {
+    /**
+     * This thing just deletes the shared_ptr when the 'instance'
+     * goes out of scope (when the bss segment of the compilation unit
+     * that 'instance' is sitting in is deinitialized). See the comment
+     * on 'instance' for more information.
+     */
+    template <class K> class Deleter
+    {
+    public:
+      K* guarded;
+      ~Deleter() { if (guarded) delete guarded; }
+    };
+
+    /**
+     * Is it possible that getInstance can be called prior to the shared_ptr 'instance'
+     *  being initialized as a global? If so, then the shared_ptr constructor would
+     *  effectively 'reset' the shared pointer after it had been set by the prior
+     *  getInstance call, and a second instance would be created. We really don't
+     *  want this to happen so 'instance' is a pointer to a smart pointer so that
+     *  we can deterministically handle its construction. It is guarded by the
+     *  Deleter class above so that when the bss segment that this static is
+     *  sitting in is deinitialized, the shared_ptr pointer will be cleaned up.
+     */
+    static Deleter<std::shared_ptr<T> > instance;
+
+    /**
+     * See 'getQuick' below.
+     */
+    static T* quick;
+  public:
+
+    /**
+     * Retrieve an instance of the singleton using a shared pointer for
+     *  reference counting.
+     */
+    inline static std::shared_ptr<T> getInstance()
+    {
+      if (!instance.guarded)
+      {
+        if (!quick)
+          quick = new T;
+        instance.guarded = new std::shared_ptr<T>(quick);
+      }
+      return *(instance.guarded);
+    }
+
+    /**
+     * This is for quick access when using form (2) of the pattern. Before 'mdd' points
+     * it out, this might be a case of 'solving problems we don't have' but this access
+     * is used frequently within the event loop so any help here should benefit the
+     * overall performance and there is nothing complicated or tricky here and not
+     * a lot of code to maintain.
+     */
+    inline static T* getQuick()
+    {
+      if (!quick)
+        quick = new T;
+
+      return quick;
+    }
+
+  };
+
+  template <class T> typename GlobalsSingleton<T>::template Deleter<std::shared_ptr<T> > GlobalsSingleton<T>::instance;
+  template <class T> T* GlobalsSingleton<T>::quick;
+
+  /**
+   * This is another bit of hackery that will act as a flag for
+   *  whether or not a global/static has been initialized yet. An instance
+   *  should be placed in the cpp file after the static/global it's meant to
+   *  monitor.
+   */
+  class InitFlag {  public:  explicit InitFlag(bool& flag) { flag = true; }  };
+}
+
+/**
+ * For pattern (2) above, you can use the following macro. This pattern is safe to
+ * use in all cases but may be very slightly less efficient.
+ *
+ * Also, you must also use a #define to replace the actual global variable since
+ * there's no way to use a macro to add a #define. An example would be:
+ *
+ * XBMC_GLOBAL_REF(CWinSystemWin32DX, g_Windowing);
+ * #define g_Windowing XBMC_GLOBAL_USE(CWinSystemWin32DX)
+ *
+ */
+#define XBMC_GLOBAL_REF(classname,g_variable) \
+  static std::shared_ptr<classname> g_variable##Ref(xbmcutil::GlobalsSingleton<classname>::getInstance())
+
+/**
+ * This declares the actual use of the variable. It needs to be used in another #define
+ * of the form:
+ *
+ * #define g_variable XBMC_GLOBAL_USE(classname)
+ */
+#define XBMC_GLOBAL_USE(classname) (*(xbmcutil::GlobalsSingleton<classname>::getQuick()))