CommonMessaging: fix unitialized retrieval of MsgSvc

if the MsgSvc is retrived by a client via the msgSvc() interface before the MsgStream is created, it will return a null ptr. This reverts the change that created the MsgSvc when needed.

GaudiKernel/GaudiKernel/CommonMessaging.h

@@ -68,11 +68,16 @@ public:
   virtual ~CommonMessagingBase() = default;
   /// cold functionality
   virtual void create_msgStream() const = 0;
-
+  virtual void create_msgSvc() const    = 0;
+  
   /** The standard message service.
    *  Returns a pointer to the standard message service.
    */
-  inline const SmartIF<IMessageSvc>& msgSvc() const { return m_msgsvc; }
+  inline const SmartIF<IMessageSvc>& msgSvc() const
+  {
+    if ( UNLIKELY( !m_msgsvc ) ) create_msgSvc();
+    return m_msgsvc;
+  }
 
   /// Return an uninitialized MsgStream.
   inline MsgStream& msgStream() const
@@ -145,7 +150,7 @@ private:
   MSG::Level m_level = MSG::NIL;
 
   /// Pointer to the message service;
-  SmartIF<IMessageSvc> m_msgsvc;
+  mutable SmartIF<IMessageSvc> m_msgsvc;
 };
 
 template <typename BASE>
@@ -159,6 +164,13 @@ public:
 
 private:
   // out-of-line 'cold' functions -- put here so as to not blow up the inline 'hot' functions
+
+  void create_msgSvc() const override final
+  {
+    // Get default implementation of the message service.
+    m_msgsvc = this->serviceLocator();
+  }
+  
   void create_msgStream() const override final { m_msgStream.reset( new MsgStream( msgSvc(), this->name() ) ); }
 
 protected:
@@ -167,8 +179,7 @@ protected:
   {
     if ( !m_commonMessagingReady ) {
       if ( !m_msgsvc ) {
-        // Get default implementation of the message service.
-        m_msgsvc = this->serviceLocator();
+        create_msgSvc();
       }
       create_msgStream();
       m_level = MSG::Level( m_msgStream.get() ? m_msgStream->level() : MSG::NIL );