Made file access in SimHitPrinter thread safe

Since multiple instances of SimHitPrinter all share the same output
file via a static and those instances can be running on different
threads the file has to be protected by a mutex. In addition,
the creation of the file itself is done in a thread safe way.

SimG4CMS/Muon/interface/SimHitPrinter.h

@@ -18,6 +18,7 @@
 #include "DataFormats/GeometryVector/interface/LocalPoint.h"
 
 #include<fstream>
+#include <atomic>
 
 class SimHitPrinter {
 public:
@@ -34,7 +35,7 @@ class SimHitPrinter {
   void printLocal(LocalPoint,LocalPoint) const;
   void printGlobal(GlobalPoint) const;
 private:
-  static std::ofstream * theFile;
+  static std::atomic<std::ofstream*> theFile;
 };
 
 #endif

SimG4CMS/Muon/src/SimHitPrinter.cc

@@ -2,13 +2,25 @@
 
 #include<iomanip>
 #include<iostream>
+#include <memory>
+#include <mutex>
 
-std::ofstream * SimHitPrinter::theFile(0);
+std::atomic<std::ofstream *> SimHitPrinter::theFile(nullptr);
+
+namespace {
+  std::mutex fileMutex;
+}
 
 SimHitPrinter::SimHitPrinter(std::string filename){
   if (theFile) return;
   const char* theName = filename.c_str();
-  theFile = new std::ofstream(theName, std::ios::out);
+  auto f = std::make_unique<std::ofstream>(theName, std::ios::out);
+
+  std::ofstream* previous = nullptr;
+  if(theFile.compare_exchange_strong(previous,f.get()) ) {
+    //this thread was the first one to try to set the value
+    f.release();
+  }
 }
 
 SimHitPrinter::~SimHitPrinter(){
@@ -21,6 +33,8 @@ void SimHitPrinter::startNewSimHit(std::string s){
   std::cout.setf(std::ios::scientific,std::ios::floatfield);
   std::cout.precision(6);
   std::cout << "SimHit in "<<s<<std::endl;
+
+  std::lock_guard<std::mutex> guard{fileMutex};
   (*theFile).width(10);
   (*theFile).setf(std::ios::right,std::ios::adjustfield);
   (*theFile).setf(std::ios::scientific|std::ios::uppercase|std::ios::showpos,std::ios::floatfield);
@@ -29,11 +43,13 @@ void SimHitPrinter::startNewSimHit(std::string s){
 }
 
 void SimHitPrinter::startNewEvent(int num){
+  std::lock_guard<std::mutex> guard{fileMutex};
   (*theFile) << "Event "<<num<<std::endl;
 }
 
 void SimHitPrinter::printId(int id) const{
   std::cout << " Id: "<<id<<std::endl;
+  std::lock_guard<std::mutex> guard{fileMutex};
   (*theFile) << " id ";
   (*theFile).width(10);
   (*theFile).setf(std::ios::right,std::ios::adjustfield);
@@ -42,6 +58,7 @@ void SimHitPrinter::printId(int id) const{
 
 void SimHitPrinter::printTrack(int id) const{
   std::cout << " Track: "<<id<<std::endl;
+  std::lock_guard<std::mutex> guard{fileMutex};
   (*theFile) << " trk ";
   (*theFile).width(10);
   (*theFile).setf(std::ios::right,std::ios::adjustfield);
@@ -50,34 +67,38 @@ void SimHitPrinter::printTrack(int id) const{
 
 void SimHitPrinter::printPabs(float pabs) const{
   std::cout << " Pabs: "<<pabs<<std::endl;
+  std::lock_guard<std::mutex> guard{fileMutex};
   (*theFile) << " p "<<pabs;
 }
 
 void SimHitPrinter::printEloss(float eloss) const{
   std::cout << " Eloss: "<<eloss<<std::endl;
+  std::lock_guard<std::mutex> guard{fileMutex};
   (*theFile) << " e "<<eloss;
 }
 
 void SimHitPrinter::printLocal(LocalPoint localen,LocalPoint localex) const{
+  std::cout << " Local(en/ex): "<<localen.x()<<" "<< localen.y()<<" "
+       <<localen.z()<<" / "<<localex.x()<<" "<< localex.y()<<" "
+       <<localex.z()<<std::endl;
+  std::lock_guard<std::mutex> guard{fileMutex};
   (*theFile).width(10);
   (*theFile).setf(std::ios::right,std::ios::adjustfield);
   (*theFile).setf(std::ios::floatfield);
   (*theFile).precision(6);
-  std::cout << " Local(en/ex): "<<localen.x()<<" "<< localen.y()<<" "
-       <<localen.z()<<" / "<<localex.x()<<" "<< localex.y()<<" "
-       <<localex.z()<<std::endl;
   (*theFile) << " en/ex "<<localen.x()<<" "<< localen.y()<<" "
        <<localen.z()<<" / "<<localex.x()<<" "<< localex.y()<<" "
        <<localex.z();
 }
 
 void SimHitPrinter::printGlobal(GlobalPoint global) const {
+  std::cout << " Global(en): "<<global.x()<<" "<< global.y()<<" "
+	     <<global.z()<<std::endl;
+  std::lock_guard<std::mutex> guard{fileMutex};
   (*theFile).width(10);
   (*theFile).setf(std::ios::right,std::ios::adjustfield);
   (*theFile).setf(std::ios::floatfield);
   (*theFile).precision(6);
-  std::cout << " Global(en): "<<global.x()<<" "<< global.y()<<" "
-	     <<global.z()<<std::endl;
   (*theFile) << " gl "<<global.x()<<" "<< global.y()<<" "
 	     <<global.z()<<std::endl;
 }