Seed Geant4 through a function installed by the Geant4 configuration

This replaces the just-in-time compiled Geant4 seeding with a function that the Geant4 configuration installs.

- The JIT code needed CLHEP/Random/Random.h on ROOT_INCLUDE_PATH, which the alienv module environment does not provide.
- g4Config.C links Geant4 and installs the same seeding recipe through VMCSeederService::setGeant4Seeder.

https://its.cern.ch/jira/browse/O2-7305

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: sawenzel

Detectors/Base/include/DetectorsBase/VMCSeederService.h

@@ -42,11 +42,19 @@ class VMCSeederService
 
   typedef std::function<void(TRandom*)> SeederFcn;
 
+  /// installs the function that seeds Geant4 from a generator; called by the Geant4 configuration, which links Geant4
+  static void setGeant4Seeder(SeederFcn fcn) { geant4Seeder() = std::move(fcn); }
+
  private:
   VMCSeederService();
+  static SeederFcn& geant4Seeder()
+  {
+    static SeederFcn fcn;
+    return fcn;
+  }
   void initSeederFunction(TVirtualMC const*);
 
-  SeederFcn mSeederFcn;                                  // the just-in-time compiled function talking to the VMC engines
+  SeederFcn mSeederFcn;                                  // the function talking to the VMC engine
   mutable std::atomic<unsigned long long> mSeedCount{0}; // number of setSeed() calls
 };
 

Detectors/Base/src/VMCSeederService.cxx

@@ -12,9 +12,7 @@
 #include "DetectorsBase/VMCSeederService.h"
 #include "DetectorsBase/SimRandom.h"
 #include "TVirtualMC.h"
-#include <fairlogger/Logger.h>                    // for FairLogger
-#include <CommonUtils/ConfigurationMacroHelper.h> // for ROOT JIT helpers
-#include <TInterpreter.h>
+#include <fairlogger/Logger.h> // for FairLogger
 #include <TRandom.h>
 
 using namespace o2::base;
@@ -25,13 +23,11 @@ void VMCSeederService::initSeederFunction(TVirtualMC const* vmc)
     // Geant3 doesn't need anything special in our context
     mSeederFcn = [](TRandom*) {};
   } else if (strcmp(vmc->GetName(), "TGeant4") == 0) {
-    // Seed the Geant4 engine of the calling thread from the given generator, with the same recipe as
-    // TG4RunManager::SetRandomSeed (which always reads gRandom). JIT-compiled to avoid linking against Geant4.
-    if (!gInterpreter->Declare("#include <CLHEP/Random/Random.h>")) {
-      LOG(fatal) << "CLHEP/Random/Random.h not found on ROOT_INCLUDE_PATH; cannot seed Geant4";
+    // installed by the Geant4 configuration, so that this library does not link against Geant4
+    mSeederFcn = geant4Seeder();
+    if (!mSeederFcn) {
+      LOG(fatal) << "No Geant4 seeding function installed (VMCSeederService::setGeant4Seeder)";
     }
-    std::string G4func("std::function<void(TRandom*)> G4func() { gSystem->Load(\"libgeant4vmc\"); return [](TRandom* r){ long seeds[10]; seeds[0] = r->GetSeed(); seeds[1] = r->GetSeed(); seeds[2] = 0; CLHEP::HepRandom::setTheSeeds(seeds); };}");
-    mSeederFcn = o2::conf::JITAndEvalFunction<SeederFcn>(G4func, "G4func()", "std::function<void(TRandom*)>", "VMCSEEDERFUNC123");
   } else {
     LOG(warn) << "Unknown VMC engine or unimplemented VMC seeding function";
     mSeederFcn = [](TRandom*) {};

Detectors/gconfig/g4Config.C

@@ -68,6 +68,9 @@ R__LOAD_LIBRARY(libgeant4vmc)
 #include <unistd.h>
 #include "SimSetup/G4RunConfiguration.h"
 #include "SimSetup/VecGeomNavigation.h"
+#include "DetectorsBase/VMCSeederService.h"
+#include "CLHEP/Random/Random.h"
+#include "TRandom.h"
 #endif
 #include "commonConfig.C"
 
@@ -153,6 +156,15 @@ void Config()
   TGeant4* geant4 = new TGeant4("TGeant4", "The Geant4 Monte Carlo", runConfiguration);
   std::cout << "Geant4 has been created." << std::endl;
 
+  // seeds the Geant4 engine of the calling thread from a ROOT generator, as TG4RunManager::SetRandomSeed does from gRandom
+  o2::base::VMCSeederService::setGeant4Seeder([](TRandom* r) {
+    long seeds[10];
+    seeds[0] = r->GetSeed();
+    seeds[1] = r->GetSeed();
+    seeds[2] = 0;
+    CLHEP::HepRandom::setTheSeeds(seeds);
+  });
+
   if (g4Params.navmode == o2::conf::EG4Nav::kVecGeom) {
     o2::simsetup::installVecGeomNavigator();
   }