o2-sim: Provide standalone MC header information

Extracting (without breaking backward compatibility) the MC header
information into a separate file.

This file will be small and can quickly copied/provided to unblock
start of signal transport in cases of embedding.
The motivation is background event caching on the GRID. The headers
file can be copied quickly and the rest of kinematics + hits may
follow asyncronously  since they are only needed at much later
stages (digitization).

Author: sawenzel

DataFormats/Detectors/Common/include/DetectorsCommonDataFormats/NameConf.h

@@ -49,7 +49,7 @@ class NameConf
     return o2::utils::concat_string(prefix, "_", DIGITS_STRING, d.getName(), ".root");
   }
 
-  // Filename to store kinematics + TrackRefs
+  // Filename to store general run parameters (GRP)
   static std::string getGRPFileName(const std::string_view prefix = STANDARDSIMPREFIX)
   {
     return o2::utils::concat_string(prefix, "_", GRP_STRING, ".root");
@@ -61,6 +61,12 @@ class NameConf
     return o2::utils::concat_string(prefix, "_", KINE_STRING, ".root");
   }
 
+  // Filename to store kinematics + TrackRefs
+  static std::string getMCHeadersFileName(const std::string_view prefix = STANDARDSIMPREFIX)
+  {
+    return o2::utils::concat_string(prefix, "_", MCHEADER_STRING, ".root");
+  }
+
   // Filename to store final MC configuration file
   static std::string getMCConfigFileName(const std::string_view prefix = STANDARDSIMPREFIX)
   {
@@ -104,6 +110,7 @@ class NameConf
   static constexpr std::string_view DIGITS_STRING = "Digits"; // hardcoded
   static constexpr std::string_view GRP_STRING = "grp";       // hardcoded
   static constexpr std::string_view KINE_STRING = "Kine";     // hardcoded
+  static constexpr std::string_view MCHEADER_STRING = "MCHeader"; // hardcoded
   static constexpr std::string_view GEOM_FILE_STRING = "geometry";
   static constexpr std::string_view CUT_FILE_STRING = "proc-cut";
   static constexpr std::string_view CONFIG_STRING = "configuration";

DataFormats/simulation/include/SimulationDataFormat/MCEventHeader.h

@@ -85,6 +85,9 @@ class MCEventHeader : public FairMCEventHeader
 
   MCEventStats& getMCEventStats() { return mEventStats; }
 
+  /// create a standalone ROOT file/tree with only the MCHeader branch
+  static void extractFileFromKinematics(std::string_view kinefilename, std::string_view targetfilename);
+
  protected:
   std::string mEmbeddingFileName;
   Int_t mEmbeddingEventIndex = 0;

DataFormats/simulation/src/MCEventHeader.cxx

@@ -12,6 +12,8 @@
 
 #include "SimulationDataFormat/MCEventHeader.h"
 #include "FairRootManager.h"
+#include <TFile.h>
+#include <TTree.h>
 
 namespace o2
 {
@@ -32,6 +34,32 @@ void MCEventHeader::Reset()
   mEmbeddingEventIndex = 0;
 }
 
+void MCEventHeader::extractFileFromKinematics(std::string_view kinefilename, std::string_view targetfilename)
+{
+  auto oldfile = TFile::Open(kinefilename.data());
+  auto kinetree = (TTree*)oldfile->Get("o2sim");
+  // deactivate all branches
+  kinetree->SetBranchStatus("*", 0);
+  // activate the header branch
+  kinetree->SetBranchStatus("MCEventHeader*", 1);
+  // create a new file + a clone of old tree header. Do not copy events
+  auto newfile = TFile::Open(targetfilename.data(), "RECREATE");
+  auto newtree = kinetree->CloneTree(0);
+  // here we copy the branches
+  newtree->CopyEntries(kinetree, kinetree->GetEntries());
+  newtree->SetEntries(kinetree->GetEntries());
+  // flush to disk
+  newtree->Write();
+  newfile->Close();
+  delete newfile;
+
+  // clean
+  if (oldfile) {
+    oldfile->Close();
+    delete oldfile;
+  }
+}
+
 /*****************************************************************/
 /*****************************************************************/
 

macro/migrateSimFiles.C

@@ -75,6 +75,11 @@ void migrateSimFiles(const char* filebase = "o2sim")
   auto kinematicsfile = o2::base::NameConf::getMCKinematicsFileName(filebase);
   copyBranch(originalfilename.c_str(), kinematicsfile.c_str(), o2::detectors::SimTraits::KINEMATICSBRANCHES);
 
+  // split off additional MCHeaders file
+  std::vector<std::string> headerbranches = {"MCEventHeader"};
+  auto headersfile = o2::base::NameConf::getMCHeadersFileName(filebase);
+  copyBranch(originalfilename.c_str(), headersfile.c_str(), headerbranches);
+
   // loop over all possible detectors
   for (auto detid = o2::detectors::DetID::First; detid <= o2::detectors::DetID::Last; ++detid) {
     if (!grp->isDetReadOut(detid)) {

run/o2sim_parallel.cxx

@@ -29,6 +29,7 @@
 #include "Framework/FreePortFinder.h"
 #include <sys/types.h>
 #include "DetectorsCommonDataFormats/NameConf.h"
+#include "SimulationDataFormat/MCEventHeader.h"
 
 #include "rapidjson/document.h"
 #include "rapidjson/writer.h"
@@ -458,6 +459,18 @@ int main(int argc, char* argv[])
   // (mainly useful for continuous integration / automated testing suite)
   auto returncode = checkresult();
   if (returncode == 0) {
+    // Extract a single file for MCEventHeaders
+    // This file will be small and can quickly unblock start of signal transport (in embedding).
+    // This is useful when we cache background events on the GRID. The headers file can be copied quickly
+    // and the rest of kinematics + Hits may follow asyncronously since they are only needed at much
+    // later stages (digitization).
+
+    auto& conf = o2::conf::SimConfig::Instance();
+    // easy check: see if we have number of entries in output tree == number of events asked
+    std::string kinefilename = o2::base::NameConf::getMCKinematicsFileName(conf.getOutPrefix().c_str());
+    std::string headerfilename = o2::base::NameConf::getMCHeadersFileName(conf.getOutPrefix().c_str());
+    o2::dataformats::MCEventHeader::extractFileFromKinematics(kinefilename, headerfilename);
+
     LOG(INFO) << "SIMULATION RETURNED SUCCESFULLY";
   }