factor out host WDT data preparation

Author: SeverinDiederichs

include/AdePT/core/AsyncAdePTTransport.hh

@@ -94,7 +94,7 @@ public:
   std::vector<std::string> const *GetGPURegionNames() { return fGPURegionNames; }
   std::vector<std::string> const *GetCPURegionNames() { return fCPURegionNames; }
   G4HepEmState *GetHepEmState() const { return fg4hepem_state.get(); }
-  void CompleteInitialization(adeptint::VolAuxData *auxData, const adeptint::WDTHostRaw &wdtRaw,
+  void CompleteInitialization(adeptint::VolAuxData *auxData, const adeptint::WDTHostPacked &wdtPacked,
                               const std::vector<float> &uniformFieldValues);
   /// Block until transport of the given event is done.
   void Flush(int threadId, int eventId, AdePTGeant4Integration &g4Integration);

include/AdePT/core/AsyncAdePTTransport.icc

@@ -202,18 +202,24 @@ inline void AsyncAdePTTransport::Initialize(G4HepEmConfig *hepEmConfig)
 }
 
 inline void AsyncAdePTTransport::CompleteInitialization(adeptint::VolAuxData *auxData,
-                                                        const adeptint::WDTHostRaw &wdtRaw,
+                                                        const adeptint::WDTHostPacked &wdtPacked,
                                                         const std::vector<float> &uniformFieldValues)
 {
+  // This is the second half of the split initialization. A non-zero volume count was already
+  // required in Initialize() before geometry upload, and it remains a hard precondition here
+  // before uploading any geometry-derived metadata to the device.
+  const auto numVolumes = vecgeom::GeoManager::Instance().GetRegisteredVolumesCount();
+  if (numVolumes == 0)
+    throw std::runtime_error("AsyncAdePTTransport::CompleteInitialization: Number of geometry volumes is zero.");
+
   // Initialize volume auxiliary data on device
   auto &volAuxArray       = adeptint::VolAuxArray::GetInstance();
-  volAuxArray.fNumVolumes = vecgeom::GeoManager::Instance().GetRegisteredVolumesCount();
+  volAuxArray.fNumVolumes = numVolumes;
   volAuxArray.fAuxData    = auxData;
   AsyncAdePT::InitVolAuxArray(volAuxArray);
 
-  // Pack host-side WDT
-  adeptint::WDTHostPacked wdtPacked = adeptint::PackWDT(wdtRaw);
-  fHasWDTRegions                    = !wdtPacked.regions.empty(); // set the flag whether WDT regions are enabled
+  // The Geant4 side has already packed the host-side WDT metadata before handing it to transport.
+  fHasWDTRegions = !wdtPacked.regions.empty(); // set the flag whether WDT regions are enabled
 
   // Upload WDT to device
   adeptint::WDTDeviceBuffers wdtDev;

src/AdePTTrackingManager.cc

@@ -76,9 +76,10 @@ void AdePTTrackingManager::InitializeSharedAdePTTransport()
   fGeant4Integration.InitVolAuxData(auxData, fAdeptTransport->GetHepEmState(), fHepEmTrackingManager.get(),
                                     fAdePTConfiguration->GetTrackInAllRegions(),
                                     fAdePTConfiguration->GetGPURegionNames(), wdtRaw);
+  adeptint::WDTHostPacked wdtPacked = adeptint::PackWDT(wdtRaw);
 
   // Finish the shared transport initialization by uploading the prepared metadata to the device.
-  fAdeptTransport->CompleteInitialization(auxData, wdtRaw, uniformFieldValues);
+  fAdeptTransport->CompleteInitialization(auxData, wdtPacked, uniformFieldValues);
 }
 
 void AdePTTrackingManager::InitializeAdePT()