LArSoft

It appears that the changes in v09_00_00 involving larsim/LegacyLArG4/CustomPhysicsLArSoft.h conflict with the way GEANT4 finds its physics processes.
The net result is that if you pick a GEANT4 configuration and ask for the "Optical" process (as opposed to "FastOptical") you get rich

Setting up Configurable Physics List.
Custom physics list enabled, contents:
  Em
  SynchrotronAndGN
  Ion
  Hadron
  Decay
  HadronElastic
  Stopping
  Optical

ERROR: physics not available: Optical

-- Available physics:
  ChargeExchange
  Decay
  Em
  FastOptical
  Hadron
  HadronElastic
  HadronElasticHP
  HadronElasticPHP
  HadronHP
  Ion
  LowEnergyEm
  NeutronHP
  NeutronTrackingCut
  Stopping
  SynchrotronAndGN
--

ERROR: Throwing exception!

You can reproduce it by running for example standard_g4_icarus.fcl from icaruscode v09_00_01 with the override line: services.LArG4Parameters.EnabledPhysics: [ "Em", "SynchrotronAndGN", "Ion", "Hadron", "Decay", "HadronElastic", "Stopping", "Optical" ].
A suitable input file is attached to this ticket.

Needless to say, the bugs in LightSource before v09_00_01 and the ones in LArG4 _after v09_00_00 leave us with no option for some essential light propagation speed simulation, hence the raised priority.