detect if the session is up and opened, lock PlacementRef else

src/proc/mobject/placement-ref.hpp

@@ -204,7 +204,8 @@ namespace mobject {
       bool
       checkValidity ()  const
         {
-          return session::SessionServiceFetch::isRegisteredID (this->id_);
+          return session::SessionServiceFetch::isAccessible() // session interface opened?
+              && session::SessionServiceFetch::isRegisteredID (this->id_);
         }
       
       static void

src/proc/mobject/session.hpp

@@ -127,6 +127,9 @@ namespace mobject {
     class SessManager : private boost::noncopyable
       {
       public:
+        /** diagnostics: session interface opened? */
+        virtual bool isUp ()  =0;
+        
         /** clear current session contents 
          *  without resetting overall session config.
          *  Afterwards, the session will contain only one 

src/proc/mobject/session/scope-path.cpp

@@ -119,7 +119,7 @@ namespace session {
   
   
   /** constant \em invalid path token. Created by locating an invalid scope */
-  const ScopePath ScopePath::INVALID = ScopePath(Scope());
+  const ScopePath ScopePath::INVALID = ScopePath(Scope());                 ///////////////////////TODO can this initialisation be deferred? it causes creation of an default session very early
   
   
   /** a \em valid path consists of more than just the root element.

src/proc/mobject/session/sess-manager-impl.cpp

@@ -91,6 +91,13 @@ namespace session {
   { }
   
   
+  
+  bool
+  SessManagerImpl::isUp ()
+  {
+    return bool(pImpl_);
+  }
+  
   /** @note no transactional behaviour.
    *        may succeed partial.
    */

src/proc/mobject/session/sess-manager-impl.hpp

@@ -51,6 +51,8 @@ namespace session {
       virtual void load ()  ;
       virtual void save ()  ;
       
+      virtual bool isUp ()  ;
+      
       
     public:
       /* ==== proc layer internal API ==== */

src/proc/mobject/session/session-service-fetch.hpp

@@ -63,6 +63,8 @@ namespace session {
     public:
       static PlacementMO& resolveID (PlacementMO::ID const&) ;
       static bool    isRegisteredID (PlacementMO::ID const&) ;
+      
+      static bool isAccessible() ;
     };
   
   

src/proc/mobject/session/session-services.cpp

@@ -33,6 +33,15 @@
 namespace mobject {
 namespace session {
   
+  /** is the element-fetch service usable?
+   *  Effectively this means: is the session up?
+   */
+  bool
+  SessionServiceFetch::isAccessible ()
+  {
+    return Session::current.isUp();
+  }
+  
   /** verify the given placement-ID (hash) is valid,
    *  by checking if it refers to a Placement instance
    *  currently registered with the PlacementIndex of the