protect AsgMessagingForward in AthenaMT

In MT the MsgStream object is thread-local.  So instead of retrieving
the MsgStream& object once, this will retrieve it on every use.

Control/AthToolSupport/AsgMessaging/AsgMessaging/AsgMessagingForward.h

@@ -12,6 +12,8 @@
 #include "AsgMessaging/MsgStream.h"
 #include "AsgMessaging/MsgStreamMacros.h"
 
+#include <functional>
+
 namespace asg
 {
   /// \brief base class to forward messages to another class
@@ -65,15 +67,21 @@ namespace asg
     //
 
     /// \brief the message stream we use
+    ///
+    /// This used to be a simple pointer to the `MsgStream` itself,
+    /// but in AthenaMT the actual object used is local to the thread.
+    /// So instead of pointing to it directly we are now using a
+    /// function to look it up, which will get the thread-local
+    /// object.
   private:
-    MsgStream *m_msg {nullptr};
+    std::function<MsgStream& ()> m_msg;
   };
 
 
 
   template<typename T>  AsgMessagingForward ::
   AsgMessagingForward( T *owner )
-    : m_msg (&owner->msg())
+    : m_msg ([owner] () -> MsgStream& {return owner->msg();})
   {}
 }
 

Control/AthToolSupport/AsgMessaging/Root/AsgMessagingForward.cxx

+
 /*
   Copyright (C) 2002-2019 CERN for the benefit of the ATLAS collaboration
 */
@@ -9,9 +10,10 @@ namespace asg
 {
   bool AsgMessagingForward::msgLvl( const MSG::Level lvl ) const
   {
-    if (m_msg->level() <= lvl)
+    MsgStream& msg = m_msg();
+    if (msg.level() <= lvl)
     {
-      (*m_msg) << lvl;
+      msg << lvl;
       return true;
     } else
     {
@@ -21,12 +23,13 @@ namespace asg
 
   MsgStream& AsgMessagingForward::msg() const
   {
-    return *m_msg;
+    return m_msg();
   }
 
   MsgStream& AsgMessagingForward::msg( const MSG::Level lvl ) const
   {
-    (*m_msg) << lvl;
-    return *m_msg;
+    MsgStream& msg = m_msg ();
+    msg << lvl;
+    return msg;
   }
 }