Made statics in NuclearInteractionFTFSimulator thread safe

The static analyzer identified a thread-safety issue with the static
class members of NuclearInteractionFTFSimulator. Using an std::call_once
makes sure the array is safely initialized.

FastSimulation/MaterialEffects/src/NuclearInteractionFTFSimulator.cc

@@ -1,3 +1,6 @@
+// system headers
+#include <mutex>
+
 // Framework Headers
 #include "FWCore/MessageLogger/interface/MessageLogger.h"
 
@@ -61,8 +64,9 @@
 #include "G4PhysicsLogVector.hh"
 #include "G4SystemOfUnits.hh"
 
-const G4ParticleDefinition* NuclearInteractionFTFSimulator::theG4Hadron[] = {0};
-int NuclearInteractionFTFSimulator::theId[] = {0};
+static std::once_flag initializeOnce;
+[[cms::thread_guard("initializeOnce")]] const G4ParticleDefinition* NuclearInteractionFTFSimulator::theG4Hadron[] = {0};
+[[cms::thread_guard("initializeOnce")]] int NuclearInteractionFTFSimulator::theId[] = {0};
 
 const double fact = 1.0/CLHEP::GeV;
 
@@ -133,7 +137,7 @@ NuclearInteractionFTFSimulator::NuclearInteractionFTFSimulator(
   theBertiniCascade = new G4CascadeInterface();
 
   // Geant4 particles and cross sections
-  if(!theG4Hadron[0]) {
+  std::call_once(initializeOnce, [this] () {
     theG4Hadron[0] = G4Proton::Proton();
     theG4Hadron[1] = G4Neutron::Neutron();
     theG4Hadron[2] = G4PionPlus::PionPlus();
@@ -177,7 +181,7 @@ NuclearInteractionFTFSimulator::NuclearInteractionFTFSimulator(
     for(int i=0; i<numHadrons; ++i) {
       theId[i] = theG4Hadron[i]->GetPDGEncoding();
     }
-  }
+  }); 
 
   // local objects
   vect = new G4PhysicsLogVector(npoints-1,100*MeV,TeV);