Use MessageSvc to handle ROOT messages

The default ROOT error handler (the equivalent of Gaudi MessageSvc) prints messages to stderr, which is problematic with Gaudi based workflows that expect all messages to go through stdout and be filtered according to MessageSvc output level.

This MR replaces the ROOT error handler with an adapter function that redirects to MessageSvc.

There are a couple of drawbacks which the changes proposed here:

• it works only if ROOT (gROOT) has been initialized before ApplicationMgr::configure, but my tests suggest this is always the case
• every message sent by ROOT makes a call to IMessageSvc::outputLevel( source ) to decide if the message has to be printed or not, while MsgStream instances cache the thresholds; we can optimize the adapter caching internally the thresholds (e.g. with a static std::unordered_map<std::string, int> thresholds) or by using a global threshold for ROOT messages.

GaudiCoreSvc/src/ApplicationMgr/ApplicationMgr.cpp

@@ -45,6 +45,8 @@ DECLARE_OBJECT_FACTORY( ApplicationMgr )
 namespace {
   /// Pointer to the IMessageSvc instance used by ROOTErrorHandlerAdapter.
   static IMessageSvc* s_messageSvcInstance{ nullptr };
+  /// Pointer to the ROOT error handler registered before we replaced it with ROOTErrorHandlerAdapter.
+  static ErrorHandlerFunc_t s_originalRootErrorHandler{ nullptr };
 
   /// @brief Adapter to forward ROOT messages to a MessageSvc.
   void ROOTErrorHandlerAdapter( int level, Bool_t abort, const char* location, const char* msg ) {
@@ -57,8 +59,14 @@ namespace {
       if ( msgLevel >= s_messageSvcInstance->outputLevel( location ) )
         s_messageSvcInstance->reportMessage( Message{ location, msgLevel, msg }, msgLevel );
     } else {
-      // If a message is sent when we do not have an IMessageSvc, let's use a basic implementation from ROOT
-      ROOT::Internal::MinimalErrorHandler( level, abort, location, msg );
+      // If a message is sent when we do not have an IMessageSvc, let's use something else
+      if ( s_originalRootErrorHandler ) {
+        // either the original handler (if any)
+        s_originalRootErrorHandler( level, abort, location, msg );
+      } else {
+        // or some minimalistic implementation from ROOT (just not to loose the message)
+        ROOT::Internal::MinimalErrorHandler( level, abort, location, msg );
+      }
     }
   }
 
@@ -165,8 +173,8 @@ StatusCode ApplicationMgr::i_startup() {
   if ( m_useMessageSvcForROOTMessages ) {
     if ( gROOT ) {
       // if ROOT is already initialized (usually it is the case) we redirect messages to MessageSvc.
-      s_messageSvcInstance = m_messageSvc.get();
-      SetErrorHandler( ROOTErrorHandlerAdapter );
+      s_messageSvcInstance       = m_messageSvc.get();
+      s_originalRootErrorHandler = SetErrorHandler( ROOTErrorHandlerAdapter );
     } else {
       log << MSG::WARNING
           << "ROOT not yet initialized, we cannot override the error handler are requested "
@@ -649,6 +657,11 @@ StatusCode ApplicationMgr::terminate() {
     opts.set( "JobOptionsSvc.AuditFinalize", "false" );
   }
 
+  // if we have overriden it, restore the original ROOT error handler
+  if ( s_originalRootErrorHandler ) {
+    SetErrorHandler( s_originalRootErrorHandler );
+    s_originalRootErrorHandler = nullptr;
+  }
   // make sure ROOTErrorHandlerAdapter (if in use) does not try to use the MessageSvc we are about to delete
   s_messageSvcInstance = nullptr;
   // finalize MessageSvc