Keep only known PDG codes as Stack prototypes

A code missing from TDatabasePDG may still be added at run time, e.g. an ion added by geant4_vmc, so it is no longer cached as unknown.

https://its.cern.ch/jira/browse/O2-7305

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: sawenzel

Detectors/Base/src/Stack.cxx

@@ -27,7 +27,6 @@
 #include "CommonUtils/ConfigurationMacroHelper.h"
 
 #include "TLorentzVector.h" // for TLorentzVector
-#include "TDatabasePDG.h"   // for TDatabasePDG
 #include "TParticle.h"      // for TParticle
 #include "TRefArray.h"      // for TRefArray
 #include "TVirtualMC.h"     // for VMC
@@ -37,7 +36,6 @@
 #include <cassert>
 #include <cstddef> // for NULL
 #include <cmath>
-#include <optional>
 #include <unordered_map>
 #include "DetectorsBase/SimRandom.h"
 
@@ -48,24 +46,20 @@ using namespace o2::data;
 
 namespace
 {
-// a TParticle whose PDG record is looked up once per thread and PDG code, since every TDatabasePDG lookup takes ROOT's global lock
+// a TParticle copied from a per-thread prototype per PDG code, avoiding the ROOT-locked PDG lookup
 TParticle makeParticle(int pdg, int status, int mother1, int mother2, int daughter1, int daughter2,
                        double px, double py, double pz, double e, double vx, double vy, double vz, double t)
 {
-  // an empty optional marks a PDG code that is not in TDatabasePDG
-  thread_local std::unordered_map<int, std::optional<TParticle>> prototypes;
+  thread_local std::unordered_map<int, TParticle> prototypes;
   auto it = prototypes.find(pdg);
   if (it == prototypes.end()) {
-    std::optional<TParticle> proto;
-    if (TDatabasePDG::Instance()->GetParticle(pdg)) {
-      proto.emplace(pdg, 0, -1, -1, -1, -1, 0., 0., 0., 0., 0., 0., 0., 0.);
+    TParticle p(pdg, status, mother1, mother2, daughter1, daughter2, px, py, pz, e, vx, vy, vz, t);
+    if (p.GetPDG(1)) { // unknown codes may still be added to TDatabasePDG later
+      prototypes.emplace(pdg, p);
     }
-    it = prototypes.emplace(pdg, std::move(proto)).first;
+    return p;
   }
-  if (!it->second) { // the mass then comes from the 4-momentum
-    return TParticle(pdg, status, mother1, mother2, daughter1, daughter2, px, py, pz, e, vx, vy, vz, t);
-  }
-  TParticle p(*it->second); // the copy keeps the PDG record and the mass
+  TParticle p(it->second); // the copy keeps the PDG record and the mass
   p.SetStatusCode(status);
   p.SetFirstMother(mother1);
   p.SetLastMother(mother2);