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Make calls to TH2F::Projection thread-safe #21995

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Jan 30, 2018
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Make calls to TH2F::Projection thread-safe #21995

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13 changes: 10 additions & 3 deletions RecoVertex/BeamSpotProducer/src/PVFitter.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -36,9 +36,11 @@ ________________________________________________________________**/
#include "Minuit2/MnMigrad.h"
#include "Minuit2/MnPrint.h"
#include "TF1.h"
#include "TDirectory.h"
#include "TMinuitMinimizer.h"

#include <iostream>
#include <sstream>
using namespace std ;

PVFitter::PVFitter(const edm::ParameterSet& iConfig,
Expand Down Expand Up @@ -318,9 +320,14 @@ bool PVFitter::runFitter() {

if ( pvStore_.size() <= minNrVertices_ ) return false;

TH1F *h1PVx = (TH1F*) hPVx->ProjectionX("h1PVx", 0, -1, "e");
TH1F *h1PVy = (TH1F*) hPVy->ProjectionX("h1PVy", 0, -1, "e");
TH1F *h1PVz = (TH1F*) hPVx->ProjectionY("h1PVz", 0, -1, "e");
//need to create a unique histogram name to avoid ROOT trying to re-use one
// also tell ROOT to hide its global state
TDirectory::TContext guard{nullptr};
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@Dr15Jones : mainly for my own education, could you please explain what this line is meant to? Thank you!
Also the class documentation in root doesn't look very explanatory to me:
https://root.cern.ch/doc/master/classTDirectory_1_1TContext.html

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This temporarily sets the global gDirectory pointer to nullptr which avoids accidental sharing across threads. When TDirectory::TContext is deleted, it rests gDirectory back to what it was before the constructor was called.

std::ostringstream str;
str <<this;
std::unique_ptr<TH1D> h1PVx{ hPVx->ProjectionX( (std::string("h1PVx")+str.str()).c_str(), 0, -1, "e") };
std::unique_ptr<TH1D> h1PVy{ hPVy->ProjectionX( (std::string("h1PVy")+str.str()).c_str(), 0, -1, "e") };
std::unique_ptr<TH1D> h1PVz{ hPVx->ProjectionY( (std::string("h1PVz")+str.str()).c_str(), 0, -1, "e") };

//Use our own copy for thread safety
// also do not add to global list of functions
Expand Down