o2-sim: Improvements for media initialization order and strong track-seeding for Geant4

Detectors/Base/include/DetectorsBase/VMCSeederService.h

@@ -35,13 +35,17 @@ class VMCSeederService
 
   void setSeed() const; // will propagate seed to the VMC engines
 
+  /// how often a seed was propagated; lets callers detect a silent no-op
+  unsigned long long getSeedCount() const { return mSeedCount; }
+
   typedef std::function<void()> SeederFcn;
 
  private:
   VMCSeederService();
   void initSeederFunction(TVirtualMC const*);
 
-  SeederFcn mSeederFcn; // the just-in-time compiled function talking to the VMC engines
+  SeederFcn mSeederFcn;                     // the just-in-time compiled function talking to the VMC engines
+  mutable unsigned long long mSeedCount{0}; // number of setSeed() calls
 };
 
 } // namespace base

Detectors/Base/src/VMCSeederService.cxx

@@ -50,4 +50,5 @@ void VMCSeederService::setSeed() const
   // This is ok since in any case gRandom->SetSeed(seed); gRandom->GetSeed() != seed;
   gRandom->Rndm();
   mSeederFcn();
+  ++mSeedCount;
 }

Steer/include/Steer/O2MCApplicationBase.h

@@ -68,6 +68,11 @@ class O2MCApplicationBase : public FairMCApplication
                                                   // keeping track of volumeIds and volume names
 
   double mLongestTrackTime = 0;
+  bool mTrackSeedWarned{false}; // whether we already complained that seeding never fired
+
+  /// whether this engine needs per-track seeding in PreTrack (Geant3 seeds at
+  /// stack-pop time instead, see O2MCApplicationBase::seedsInPreTrack)
+  bool seedsInPreTrack() const;
   /// some common parts of finishEvent
   void finishEventCommon();
   TrackRefFcn mTrackRefFcn; // a function hook that gets (optionally) called during Stepping

Steer/src/O2MCApplication.cxx

@@ -35,6 +35,9 @@
 #include <filesystem>
 #include <CommonUtils/FileSystemUtils.h>
 #include "SimConfig/GlobalProcessCutSimParam.h"
+#include <FairRunSim.h>
+#include <FairField.h> // full type: FairField derives from TVirtualMagField
+#include <TVirtualMC.h>
 #include "DetectorsBase/GeometryManagerParam.h"
 #include <TGeoParallelWorld.h>
 #include <TGeoVolume.h>
@@ -43,6 +46,10 @@
 #include <DetectorsBase/O2Tessellated.h>
 #include <unordered_set>
 #include "SimConfig/G4Params.h"
+#include "DetectorsBase/VMCSeederService.h" // per-track seeding of the engine
+#include <TLorentzVector.h>
+#include <TRandom.h>
+#include <cstring>
 
 namespace o2
 {
@@ -116,14 +123,98 @@ void O2MCApplicationBase::Stepping()
   FairMCApplication::Stepping();
 }
 
+namespace
+{
+// Hash of a track's initial state (vertex, global time, momentum, PDG). Used as
+// the random seed for that track, so that a track's random stream depends only
+// on the track itself and not on how many randoms earlier tracks happened to
+// consume.
+//
+// The values are read from the transport engine, not from
+// o2::data::Stack::GetCurrentTrack(): under Geant4 the stack's "current track"
+// is only meaningful for primaries -- Stack::SetCurrentTrack() falls back to
+// mCurrentParticle0 (the last particle *pushed*) for anything beyond the
+// primary array, so every secondary would hash the wrong particle. Both engines
+// have the track's initial state loaded by the time PreTrack is called (Geant4
+// sets the step to kVertex first; Geant3 calls GLTRAC before GUTRAK).
+ULong_t hashCurrentTrack(TVirtualMC* vmc)
+{
+  auto asLong = [](double x) {
+    ULong_t l;
+    std::memcpy(&l, &x, sizeof(l));
+    return l;
+  };
+
+  TLorentzVector pos, mom;
+  vmc->TrackPosition(pos);
+  vmc->TrackMomentum(mom);
+
+  ULong_t hash = asLong(pos.X());
+  hash ^= asLong(pos.Y());
+  hash ^= asLong(pos.Z());
+  hash ^= asLong(pos.T());
+  hash ^= asLong(mom.Px());
+  hash ^= asLong(mom.Py());
+  hash ^= asLong(mom.Pz());
+  hash += (ULong_t)vmc->TrackPid();
+  return hash;
+}
+} // namespace
+
+bool O2MCApplicationBase::seedsInPreTrack() const
+{
+  // Geant3 seeds at stack-pop time, in o2::data::Stack::PopNextTrack(). That is
+  // strictly earlier than its PreTrack hook (gutrak) and measurably stronger:
+  // with the TOF module removed from an otherwise identical setup, pop-time
+  // seeding keeps all 603 ITS hits bit-identical, PreTrack seeding only 68 %.
+  // Do not seed Geant3 here as well -- it is already covered, and reseeding a
+  // second time mid-track would undo the first.
+  static const bool inPreTrack = [this]() {
+    const char* name = (fMC != nullptr) ? fMC->GetName() : "";
+    return strncmp(name, "TGeant3", 7) != 0;
+  }();
+  return inPreTrack;
+}
+
 void O2MCApplicationBase::PreTrack()
 {
-  // dispatch first to function in FairRoot
+  if (mCutParams.trackSeed && seedsInPreTrack()) {
+    // Per-track seeding for engines that do not go through
+    // o2::data::Stack::PopNextTrack(). Geant4 is one: it takes primaries via
+    // PopPrimaryForTracking and keeps secondaries internally, so the stack hook
+    // never fires and this is the only per-track hook available. It is called
+    // for primaries and secondaries alike
+    // (TG4TrackingAction::PreUserTrackingAction), and only on a track's first
+    // step, so a suspended track is not reseeded mid-flight.
+    auto hash = hashCurrentTrack(fMC);
+    // TRandom::SetSeed(0) means "seed from the clock" -- never let that happen.
+    gRandom->SetSeed(hash == 0 ? 1 : hash);
+    o2::base::VMCSeederService::instance().setSeed();
+  }
+
+  // dispatch now to function in FairRoot
   FairMCApplication::PreTrack();
 }
 
 void O2MCApplicationBase::ConstructGeometry()
 {
+  // The transport engine constructs the geometry from inside its own
+  // constructor, long before FairMCApplication::InitMC() attaches the magnetic
+  // field to it. The media built below read the field through
+  // Detector::initFieldTrackingParams(), so without this they all silently fall
+  // back to hardcoded defaults. The run has known the field since
+  // build_geometry.C, which runs before Init() -- hand it over now.
+  if (auto* vmc = TVirtualMC::GetMC(); vmc != nullptr && vmc->GetMagField() == nullptr) {
+    auto* run = FairRunSim::Instance();
+    if (run != nullptr && run->GetField() != nullptr) {
+      vmc->SetMagField(run->GetField());
+      LOG(info) << "Magnetic field attached to the engine before media creation";
+    } else {
+      LOG(warn) << "No magnetic field available at geometry construction; media "
+                   "will be initialised with default tracking parameters";
+    }
+  }
+
   // fill the mapping
   mModIdToName.clear();
   o2::detectors::DetID::mask_t dmask{};
@@ -289,6 +380,17 @@ void O2MCApplicationBase::finishEventCommon()
   header->setDetId2HitBitLUT(o2::base::Detector::getDetId2HitBitIndex());
 
   static_cast<o2::data::Stack*>(GetStack())->updateEventStats();
+
+  // Per-track seeding used to be wired to a stack callback that one of the two
+  // engines never invoked, and it failed silently. Never again: if it was asked
+  // for and nothing was seeded, say so.
+  if (mCutParams.trackSeed && o2::base::VMCSeederService::instance().getSeedCount() == 0 &&
+      !mTrackSeedWarned) {
+    mTrackSeedWarned = true;
+    LOG(warn) << "Per-track seeding (SimCutParams.trackSeed) was requested but not a single track "
+                 "was seeded -- neither the stack nor the PreTrack hook fired for this engine. "
+                 "Seeding is NOT active.";
+  }
 }
 
 void O2MCApplicationBase::FinishEvent()