Honour O2SIM_RESTRICT_EVENTPART and skip the master hit buffers in MT

This aligns the multithreaded worker with the forked one in two details.

- O2SIM_RESTRICT_EVENTPART now selects the sub-event to transport, as in the forked workers.
- The master no longer creates hit buffers in multithreaded mode, since only the thread clones fill them.
- A named flag replaces the missing data channel as the sign of a thread's first sub-event.

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

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

Author: sawenzel

Steer/include/Steer/O2MCApplication.h

@@ -123,6 +123,7 @@ class O2MCApplication : public O2MCApplicationBase
   void fetchWorkerChunk();
   bool mIsMTWorker = false;                   //! this is the application of a multithreaded Geant4 worker
   bool mIsEndOfWork = false;                  //! the current event is the empty one ending the worker's loop
+  bool mWorkerInitialized = false;            //! this worker's detectors and data channel are set up
   o2::data::SubEventInfo mWorkerSubEventInfo; //! sub-event being transported by this worker
   std::unique_ptr<TRandom> mWorkerRandom;     //! this worker's transport random generator
 

Steer/src/O2MCApplication.cxx

@@ -9,6 +9,7 @@
 // granted to it by virtue of its status as an Intergovernmental Organization
 // or submit itself to any jurisdiction.
 
+#include <cstdio>
 #include <cstdlib>
 
 #include <Steer/O2MCApplication.h>
@@ -1847,6 +1848,9 @@ void O2MCApplication::initLate()
     }
   }
   o2::utils::ShmManager::Instance().occupySegment(fRun && fRun->IsMT());
+  if (fRun && fRun->IsMT()) {
+    return; // the hit buffers belong to the detector clones of the worker threads
+  }
   for (auto det : listActiveDetectors) {
     if (dynamic_cast<o2::base::Detector*>(det)) {
       ((o2::base::Detector*)det)->initializeLate();
@@ -1999,13 +2003,14 @@ void O2MCApplication::BeginEvent()
 void O2MCApplication::fetchWorkerChunk()
 {
   auto& hooks = o2::base::MTWorkerHooks::instance();
-  if (!mSimDataChannel) {
+  if (!mWorkerInitialized) { // the first sub-event of this thread
     mSimDataChannel = hooks.dataChannel();
     for (auto det : listActiveDetectors) {
       if (auto o2det = dynamic_cast<o2::base::Detector*>(det)) {
         o2det->initializeLate();
       }
     }
+    mWorkerInitialized = true;
   }
   auto chunk = hooks.fetchChunk();
   mIsEndOfWork = !chunk;
@@ -2018,7 +2023,15 @@ void O2MCApplication::fetchWorkerChunk()
     return;
   }
   mWorkerSubEventInfo = chunk->mSubEventInfo;
-  setPrimaries(chunk->mParticles);
+  // as in a worker process, O2SIM_RESTRICT_EVENTPART="event:part" transports only that sub-event
+  int focusEvent = -1, focusPart = -1;
+  auto selection = getenv("O2SIM_RESTRICT_EVENTPART");
+  if (selection && (std::sscanf(selection, "%d:%d", &focusEvent, &focusPart) != 2 || focusEvent != mWorkerSubEventInfo.eventID || focusPart != mWorkerSubEventInfo.part)) {
+    setPrimaries(std::vector<TParticle>{});
+    LOG(info) << "This chunk will be skipped";
+  } else {
+    setPrimaries(chunk->mParticles);
+  }
   setSubEventInfo(&mWorkerSubEventInfo);
   LOG(info) << "Processing " << mPrimaries.size() << " primary particles for event " << mWorkerSubEventInfo.eventID << "/"
             << mWorkerSubEventInfo.maxEvents << " part " << mWorkerSubEventInfo.part << "/" << mWorkerSubEventInfo.nparts;