Workaround feature in HepMC3::GenVertex::position()

In HepMC3, if a GenVertex is positioned at the origin, then it is treated as not having its position set. In such a case, the method will loop over the production vertices of particles incoming to the current vertex and return their positions if possible.

In the case of events generated at the origin, then many of the GenVertex objects end up being flagged as not having their positions set.

Once the position of a GenVertex is reset by the vertex positioning code, calling position() on any "downstream" GenVertex objects which were positioned at the origin, then returns the "upstream" GenVertex (already-shifted) position, the shift is then applied to this causing the positions of these "downstream" GenVertex objects to effectively be shifted twice, and so on.

The workaround is to avoid calling GenVertex::position() in the case that GenVertex::has_set_position() returns false and directly use HepMC::FourVector::ZERO_VECTOR() as the GenVertex position.

Tagging @averbyts, @tlari, @mbandier

Simulation/ISF/ISF_Core/ISF_Services/src/TruthSvc.cxx

@@ -267,7 +267,11 @@ void ISF::TruthSvc::recordIncidentToMCTruth( ISF::ITruthIncident& ti, bool passW
       // 2) A new GenVertex for the intermediate interaction should be
       // added.
 #ifdef HEPMC3
-      auto newVtx = HepMC::newGenVertexPtr( vtx->position(), vtx->status());
+      // NB Doing this check to explicitly avoid the fallback mechanism in
+      // HepMC3::GenVertex::position() to return the position of
+      // another GenVertex in the event if the position isn't set (or is set to zero)!
+      const HepMC::FourVector &posVec = (vtx->has_set_position()) ? vtx->position() : HepMC::FourVector::ZERO_VECTOR();
+      auto newVtx = HepMC::newGenVertexPtr( posVec, vtx->status());
       HepMC::GenEvent *mcEvent = parentBeforeIncident->parent_event();
       auto tmpVtx = newVtx;
       mcEvent->add_vertex( newVtx);
@@ -448,7 +452,14 @@ HepMC::GenVertexPtr  ISF::TruthSvc::createGenVertexFromTruthIncident( ISF::ITrut
       this->deleteChildVertex(oldVertex);
     }
     else {
+#ifdef HEPMC3
+      // NB Doing this check to explicitly avoid the fallback mechanism in
+      // HepMC3::GenVertex::position() to return the position of
+      // another GenVertex in the event if the position isn't set (or is set to zero)!
+      const HepMC::FourVector &old_pos = (oldVertex->has_set_position()) ? oldVertex->position() : HepMC::FourVector::ZERO_VECTOR();
+#else
       const auto& old_pos=oldVertex->position();
+#endif
       const auto& new_pos=ti.position();
       double diffr=std::sqrt(std::pow(new_pos.x()-old_pos.x(),2)+std::pow(new_pos.y()-old_pos.y(),2)+std::pow(new_pos.z()-old_pos.z(),2));
       //AV The comparison below is not portable.