add forgotten rcu.h header

git-svn-id: svn://localhost/ardour2/trunk@708 d708f5d6-7413-0410-9779-e7cbd77b26cf

libs/pbd/pbd/rcu.h

@@ -0,0 +1,120 @@
+#ifndef __pbd_rcu_h__
+#define __pbd_rcu_h__
+
+#include "boost/shared_ptr.hpp"
+#include "glibmm/thread.h"
+ 
+#include <list> 
+ 
+template<class T>
+class RCUManager
+{
+public:
+ 
+	RCUManager (T* new_rcu_value)
+		: m_rcu_value(new_rcu_value)
+	{
+ 
+	}
+ 
+	virtual ~RCUManager() { }
+ 
+	boost::shared_ptr<T> reader () const { return m_rcu_value; }
+ 
+	// should be private
+	virtual boost::shared_ptr<T> write_copy () = 0;
+ 
+	// should be private
+	virtual void update (boost::shared_ptr<T> new_value) = 0;
+ 
+protected:
+ 
+	boost::shared_ptr<T> m_rcu_value;
+ 
+ 
+};
+ 
+ 
+template<class T>
+class SerializedRCUManager : public RCUManager<T>
+{
+public:
+ 
+	SerializedRCUManager(T* new_rcu_value)
+		: RCUManager<T>(new_rcu_value)
+	{
+ 
+	}
+ 
+	virtual boost::shared_ptr<T> write_copy ()
+	{
+		m_lock.lock();
+ 
+		// I hope this is doing what I think it is doing :)
+		boost::shared_ptr<T> new_copy(new T(*RCUManager<T>::m_rcu_value));
+ 
+		// XXX todo remove old copies with only 1 reference from the list.
+ 
+		return new_copy;
+	}
+ 
+	virtual void update (boost::shared_ptr<T> new_value)
+	{
+		// So a current reader doesn't hold the only reference to
+		// the existing value when we assign it a new value which 
+		// should ensure that deletion of old values doesn't
+		// occur in a reader thread.
+		boost::shared_ptr<T> old_copy = RCUManager<T>::m_rcu_value;
+ 
+		// we hold the lock at this point effectively blocking
+		// other writers.
+		RCUManager<T>::m_rcu_value = new_value;
+ 
+ 
+		// XXX add the old value to the list of old copies.
+ 
+		m_lock.unlock();
+	}
+ 
+private:
+	Glib::Mutex			m_lock;
+ 
+	std::list<boost::shared_ptr<T> > m_old_values;
+};
+ 
+template<class T>
+class RCUWriter
+{
+public:
+ 
+	RCUWriter(RCUManager<T>& manager)
+		: m_manager(manager)
+	{
+		m_copy = m_manager.write_copy();	
+	}
+ 
+	~RCUWriter()
+	{
+		// we can check here that the refcount of m_copy is 1
+ 
+		if(m_copy.use_count() == 1) {
+			m_manager.update(m_copy);
+		} else {
+ 
+			// critical error.
+		}
+ 
+	}
+ 
+	// or operator boost::shared_ptr<T> ();
+	boost::shared_ptr<T> get_copy() { return m_copy; }
+ 
+private:
+ 
+	RCUManager<T>& m_manager;
+ 
+	// preferably this holds a pointer to T
+	boost::shared_ptr<T> m_copy;
+};
+
+#endif /* __pbd_rcu_h__ */