A similar process is used in the GeometryNavigator without an apparent problem.

I note that the initialization is a little different in that case. Specifically

G4Navigator* _navigator = new G4Navigator();
_navigator->SetWorldVolume(this->_global_volume);

And there is a method that uses this precise method GeometryNavigator::_setPoint(G4ThreeVector pos) which Jan requires (G4Navigator::LocateGlobalPointAndSetup()).

If I use this method in MAUSGeant4Manager immediately after the definition of the geometry exactly as Jan does, there is no problem. If I use a point inside one of the quadrupole volumes I get the logical volume back --- no errors. If I pick a position outside of the geometry then I get a null volume; I query the output volume name and it returns nothing. Could it be possible that Jan is asking for a point outside of the defined geometry and getting a NULL pointer as a result? In that case it should be noted that the geometry is only well defined for z > 0 and no volume will register for abs(x) > 2000 mm and abs(y) > 2000 mm.

To summarize, my guess is that Jan is trying to access a portion of the geometry where no G4LogicalVolume. The segfault can probably be avoided rather simply by checking that the output pointer does exist.

Happily, I have now been able to reproduce Jan's error using the merge branch targeting a test point that sits in the world volume of the geometry, but not in any sub volume. I was able to circumvent the seg fault by requiring that the G4LogicalVolume is not NULL and the simulation proceeded as normal. It seems that the world volume in the GDML parser returns a NULL pointer to a logical volume --- I don't know what wise person thought that this was a good idea, but it is annoying. As it stands it is just something that the programmer needs to be aware of and take steps to mitigate.

Somehow I can't seem to be able to reproduce the error now. Not even if I force the position vector to be outside of any subvolumes... I'll keep my eyes open in case it pops up again. I'm not sure how checking the volume to not be null would help though because the segfault occured while running LocateGlobalPointAndSetup(), not later, and I'm not actually doing anything with the returned volume.

I did some digging. It looks like the issue arises due to some problem in Geant4 handling of trackers:

1. G4ParameterisedNavigation::LevelLocate(G4NavigationHistory& is a navigator class for parameterised volumes as in the tracker
2. I removed trackers by hand from the GDML files and everything worked okay

Obviously G4 itself can get through trackers okay, so maybe some problem with the way the geometry is being dealt with? Will keep digging...

Looking at the code and stack trace with G4 in debug; for some reason G4LogicalVolume::GetSmartVoxelHeader() is returning NULL for the mother of the parameterised volumes at line 588 of G4ParameterisedNavigation.cc, i.e. motherVoxelHeader = motherLogical->GetVoxelHeader();. This is something to do with the meshing of the parameterised volume mother, so not sure why that would not work out.

A bit more digging... the geometry close procedure does not really finalise the geometry. There is some more set up that needs to be done. Either we can call G4RunManager::ReOptimize(G4VLogicalVolume*) to finalise a specific logical volume object or G4RunManager::BeamOn() to finish closing the entire geometry, as per patch:

=== modified file 'src/common_cpp/Simulation/DetectorConstruction.cc'
--- src/common_cpp/Simulation/DetectorConstruction.cc    2015-11-16 10:57:06 +0000
+++ src/common_cpp/Simulation/DetectorConstruction.cc    2016-03-22 13:23:06 +0000
@@ -289,9 +289,9 @@
   // close the geometry
   G4GeometryManager::GetInstance()->CloseGeometry();
   G4RunManager::GetRunManager()->GeometryHasBeenModified();
-  G4RunManager::GetRunManager()->Initialize();  // makes a G4 segv
-  // std::cout << "Dumping new geometry" << std::endl;  // need verbose level 0
-  // G4RunManager::GetRunManager()->DumpRegion();
+  G4RunManager::GetRunManager()->Initialize();
+  // Ack! G4 does some setup at RunBeam time
+  G4RunManager::GetRunManager()->BeamOn(0);
 }

 void DetectorConstruction::AddDaughter

I think this is okay, but will continue digging just in case there is something else.

Interestingly, the patch described in comment 7 does not work. The following patch does. Not sure why. I have not checked with valgrind:

=== modified file 'src/common_cpp/Simulation/GeometryNavigator.cc'
--- src/common_cpp/Simulation/GeometryNavigator.cc    2015-06-19 16:33:31 +0000
+++ src/common_cpp/Simulation/GeometryNavigator.cc    2016-03-22 14:23:24 +0000
@@ -20,6 +20,7 @@

 #include <string>

+#include "Geant4/G4RunManager.hh" 
 #include "CLHEP/Units/SystemOfUnits.h" 
 #include "src/common_cpp/Utils/Exception.hh" 

@@ -63,6 +64,8 @@
     }
     _navigator = new G4Navigator();
     _navigator->SetWorldVolume(this->_global_volume);
+  // Ack! G4 does some setup at BeamOn time
+  G4RunManager::GetRunManager()->BeamOn(0);
     this->_setPoint(G4ThreeVector(0.0, 0.0, 0.0));
   }