VoxelSource may cause potential double free in MT

[tontyoutoure]

At GateVoxelSource.cpp:45

  ll.fSPS->SetPosGenerator(fVoxelPositionGenerator);

which effectively does:

void GateSingleParticleSource::SetPosGenerator(GateSPSPosDistribution *pg) {
  fPositionGenerator = pg;
  ...
}

and in GateSingleParticleSource's destructor there is

delete fPositionGenerator;

So if the GateSingleParticleSource's destructor is called in worker thread (luckily or not, it isn't right now), a double free will occur. Solution is of course putting fVoxelPositionGenerator into a G4Cache. Of course there is also a bit of memory leak here, but I think that's not a very big deal.

[tontyoutoure]

I got it. The fix is not that simple. Currently the initialization of GateSPSVoxelsPosDistribution is done at python side at initialization time. Therefore it only affacts the master thread. I tried to create a G4Cache and copy the master thread's GateSPSVoxelsPosDistribution to each worker thread, turns out that the GateSPSVoxelsPosDistribution cannot be copy assgined, due to:

Object of type 'GateSPSVoxelsPosDistribution' cannot be assigned because its copy assignment operator is implicitly deletedclang(ovl_deleted_special_oper)
GateSPSVoxelsPosDistribution.h(17, 38): Copy assignment operator of 'GateSPSVoxelsPosDistribution' is implicitly deleted because base class 'GateSPSPosDistribution' has a deleted copy assignment operator
GateSPSPosDistribution.h(14, 32): Copy assignment operator of 'GateSPSPosDistribution' is implicitly deleted because base class 'G4SPSPosDistribution' has a deleted copy assignment operator
G4SPSPosDistribution.hh(219, 13): Copy assignment operator of 'G4SPSPosDistribution' is implicitly deleted because field 'a_mutex' has a deleted copy assignment operator
std_mutex.h(108, 12): 'operator=' has been explicitly marked deleted here

There's no way of fixing it modify change the implementation of GateSPSVoxelsPosDistribution. It might need to provide interface of transfering internal states to another object.