Modification of BkgCat behaviour to not throw exception

Addresses https://its.cern.ch/jira/browse/LHCBPS-1844, see there for description and discussion

This is the first change to the Cat's logic for a while, and I'm fairly nervous because the test coverage here is not that broad (essentially we are chasing corner cases) and I haven't looked at the code in ages. @pkoppenb could you have a look and see if the output makes sense to you using a couple of other analysis use cases? I.e. that in situations where there was no problem and this MR should therefore not change anything, indeed nothing changes? If you want to put some options and input files on eos I will happily make some tests from them for the future.

cc @erodrigu @jonrob

Phys/DaVinciMCTools/src/BackgroundCategory.cpp

 /*****************************************************************************\
-* (c) Copyright 2000-2018 CERN for the benefit of the LHCb Collaboration      *
+* (c) Copyright 2000-2019 CERN for the benefit of the LHCb Collaboration      *
 *                                                                             *
 * This software is distributed under the terms of the GNU General Public      *
 * Licence version 3 (GPL Version 3), copied verbatim in the file "COPYING".   *
@@ -73,6 +73,11 @@ BackgroundCategory::category( const LHCb::Particle* reconstructed_mother,
 //all the categories mean, please visit IBackgroundCategory.h
 {
 
+  //Set the global variable used to jump out with status Undefined to false
+  //this is used because the places where we may need to signal Undefined are
+  //in recursive code which is difficult to exit elegantly without a global flag
+  m_undefined = false;
+ 
   //Check if the tool was actually given a particle to categorise
   if ( !reconstructed_mother )
   {
@@ -163,6 +168,8 @@ BackgroundCategory::category( const LHCb::Particle* reconstructed_mother,
     // "physics" backgrounds (meaning we have reconstructed some actual decay
     // that occured, even if not the one we were after).
     if (isTheDecayFullyReconstructed(mc_particles_linked_to_decay) ) {
+      if (m_undefined)
+        return {Undefined, LogicError};
       if (msgLevel(MSG::VERBOSE)) verbose() << "Checked if fully reconstructed" << endmsg;
       if (areAllFinalStateParticlesCorrectlyIdentified(particles_in_decay,
                                                        mc_particles_linked_to_decay) ) {
@@ -170,10 +177,14 @@ BackgroundCategory::category( const LHCb::Particle* reconstructed_mother,
         if (isTheMotherCorrectlyIdentified(reconstructed_mother) ) {
           if (msgLevel(MSG::VERBOSE)) verbose() << "Checked if correct mother" << endmsg;
           if (!wereAnyResonancesMissed(reconstructed_mother) ) {
+            if (m_undefined) 
+              return {Undefined, LogicError};
             if (msgLevel(MSG::VERBOSE)) verbose() << "This is signal" << endmsg;
             //This is a signal decay
             return {Signal, Good};
           } else {
+            if (m_undefined)
+              return {Undefined, LogicError};
             if (msgLevel(MSG::VERBOSE)) verbose() << "This is quasi-signal" << endmsg;
             //This is a quasi-signal decay
             return {QuasiSignal, Good};
@@ -191,6 +202,8 @@ BackgroundCategory::category( const LHCb::Particle* reconstructed_mother,
     }
     else
     {
+      if (m_undefined)
+        return {Undefined, LogicError};
       if (msgLevel(MSG::VERBOSE)) verbose() << "Is it an inclusive decay?" << endmsg;
       if ( m_inclusiveDecay )
       {
@@ -381,8 +394,13 @@ int BackgroundCategory::topologycheck(const LHCb::MCParticle* topmother)
   //Check for an MCMother with a null endvertex -- really shouldn't happen
   //here though, but the Cat is often betrayed by other pieces of code...
   const SmartRefVector<LHCb::MCVertex>& motherEndVertices = topmother->endVertices();
-  if ( motherEndVertices.empty() )
-    Exception("The Cat found a common MC mother but it has no daughters, please report this as a bug.");
+  if ( motherEndVertices.empty() ) {
+    Warning("The Cat found a common MC mother but it has no daughters!"\
+            "You may be trying to process an input file which does not"\
+            "contain the necessary information, returning Undefined.");
+    m_undefined = true;
+    return -1;
+  }
   //Assuming all went well...
   //We need to find the first end vertex which is actually a decay vertex
   SmartRefVector<LHCb::MCVertex>::const_iterator iV;
@@ -522,8 +540,13 @@ BackgroundCategory::create_finalstatedaughterarray_for_mcmother(const LHCb::MCPa
   //Check for an MCMother with a null endvertex -- really shouldn't happen
   //here though, but the Cat is often betrayed by other pieces of code...
   const SmartRefVector<LHCb::MCVertex>& motherEndVertices = topmother->endVertices();
-  if (motherEndVertices.empty() )
-    Exception("The Cat found a common MC mother but it has no daughters, please report this as a bug.");
+  if (motherEndVertices.empty() ) {
+    Warning("The Cat found a common MC mother but it has no daughters!"\
+            "You may be trying to process an input file which does not"\
+            "contain the necessary information, returning Undefined.");
+    m_undefined = true;
+    return finalstateproducts;
+  }
   //Assuming all went well...
   //We need to find the first end vertex which is actually a decay vertex
   SmartRefVector<LHCb::MCVertex>::const_iterator iV;
@@ -947,7 +970,11 @@ bool BackgroundCategory::isTheDecayFullyReconstructed(const MCParticleVector & m
   int neutrinosFound = 0;
   MCParticleVector finalstateproducts = create_finalstatedaughterarray_for_mcmother(m_commonMother);
   MCParticleVector::const_iterator iPP = finalstateproducts.begin();
-  if (finalstateproducts.empty() ) Exception("Condition B : No final states, please report the bug.");
+  if (finalstateproducts.empty() ) {
+    Warning("Common MC mother has empty daughters array, returning Undefined").ignore();
+    m_undefined = true;
+    return false;
+  }
   MCParticleVector::const_iterator iP = mc_particles_linked_to_decay.begin();
 
   //Because of the special case of the merged pi0, the only non stable particle made without any
@@ -973,6 +1000,11 @@ bool BackgroundCategory::isTheDecayFullyReconstructed(const MCParticleVector & m
       //this is only relevant for checking condition B, which is why
       //we use a temp array to do it
       merged_pi0_daughters = create_finalstatedaughterarray_for_mcmother(*iP);
+      if (merged_pi0_daughters.empty() ) {
+        Warning("Merged Pi0 has empty daughters array, returning Undefined").ignore();
+        m_undefined = true;
+        return false;
+      }
       mc_particles_linked_to_decay_without_pi0.insert(
                                                       mc_particles_linked_to_decay_without_pi0.end(),
                                                       merged_pi0_daughters.begin(),