From 56a5ac94b19cdb236a7d75d337a8566e4eb58329 Mon Sep 17 00:00:00 2001
From: Christopher Jones <chrisdjones15@gmail.com>
Date: Tue, 18 Aug 2020 16:11:20 -0500
Subject: [PATCH] Fix undefined behavior in EgammaTowerIsolation

Avoid doing iterator arithmetic on an empty container.
---
 .../interface/EgammaTowerIsolation.h                  | 11 ++++++-----
 .../EgammaIsolationAlgos/src/EgammaTowerIsolation.cc  | 11 ++++++-----
 .../EgammaIsolationAlgos/test/EgammaTowerIso_t.cpp    |  3 ++-
 3 files changed, 14 insertions(+), 11 deletions(-)

diff --git a/RecoEgamma/EgammaIsolationAlgos/interface/EgammaTowerIsolation.h b/RecoEgamma/EgammaIsolationAlgos/interface/EgammaTowerIsolation.h
index b47b9763edad6..078efb9f6bbe1 100644
--- a/RecoEgamma/EgammaIsolationAlgos/interface/EgammaTowerIsolation.h
+++ b/RecoEgamma/EgammaIsolationAlgos/interface/EgammaTowerIsolation.h
@@ -53,13 +53,13 @@ class EgammaTowerIsolationNew {
 
   ~EgammaTowerIsolationNew() { delete[] mem; }
 
-  void compute(
-      bool et, Sum& sum, reco::Candidate const& cand, CaloTowerDetId const* first, CaloTowerDetId const* last) const {
+  template <typename I>
+  void compute(bool et, Sum& sum, reco::Candidate const& cand, I first, I last) const {
     reco::SuperCluster const& sc = *cand.get<reco::SuperClusterRef>().get();
     return compute(et, sum, sc, first, last);
   }
-  void compute(
-      bool et, Sum& sum, reco::SuperCluster const& sc, CaloTowerDetId const* first, CaloTowerDetId const* last) const;
+  template <typename I>
+  void compute(bool et, Sum& sum, reco::SuperCluster const& sc, I first, I last) const;
 
   void setRadius(float const extRadius[NC], float const intRadius[NC]) {
     for (std::size_t i = 0; i != NCuts; ++i) {
@@ -153,8 +153,9 @@ inline EgammaTowerIsolationNew<NC>::EgammaTowerIsolationNew(float extRadius[NC],
 }
 
 template <unsigned int NC>
+template <typename I>
 inline void EgammaTowerIsolationNew<NC>::compute(
-    bool et, Sum& sum, reco::SuperCluster const& sc, CaloTowerDetId const* first, CaloTowerDetId const* last) const {
+    bool et, Sum& sum, reco::SuperCluster const& sc, I first, I last) const {
   if (nt == 0)
     return;
 
diff --git a/RecoEgamma/EgammaIsolationAlgos/src/EgammaTowerIsolation.cc b/RecoEgamma/EgammaIsolationAlgos/src/EgammaTowerIsolation.cc
index 2fb5dfb2b89f6..f7e9c99b11ab2 100644
--- a/RecoEgamma/EgammaIsolationAlgos/src/EgammaTowerIsolation.cc
+++ b/RecoEgamma/EgammaIsolationAlgos/src/EgammaTowerIsolation.cc
@@ -58,11 +58,12 @@ double EgammaTowerIsolation::getSum(bool et,
   tls.newAlgo->setRadius(&extRadius, &intRadius);
 
   EgammaTowerIsolationNew<1>::Sum sum;
-  tls.newAlgo->compute(et,
-                       sum,
-                       sc,
-                       (detIdToExclude == nullptr) ? nullptr : &((*detIdToExclude).front()),
-                       (detIdToExclude == nullptr) ? nullptr : (&(*detIdToExclude).back()) + 1);
+  if (detIdToExclude == nullptr) {
+    tls.newAlgo->compute(
+        et, sum, sc, static_cast<CaloTowerDetId const*>(nullptr), static_cast<CaloTowerDetId const*>(nullptr));
+  } else {
+    tls.newAlgo->compute(et, sum, sc, detIdToExclude->cbegin(), detIdToExclude->cend());
+  }
 
   switch (depth_) {
     case AllDepths:
diff --git a/RecoEgamma/EgammaIsolationAlgos/test/EgammaTowerIso_t.cpp b/RecoEgamma/EgammaIsolationAlgos/test/EgammaTowerIso_t.cpp
index 7f79769579244..8693427f284f6 100644
--- a/RecoEgamma/EgammaIsolationAlgos/test/EgammaTowerIso_t.cpp
+++ b/RecoEgamma/EgammaIsolationAlgos/test/EgammaTowerIso_t.cpp
@@ -9,7 +9,8 @@ int main() {
 
   CaloTower cand;
   EgammaTowerIsolationNew<2>::Sum sum;
-  iso.compute(true, sum, cand, nullptr, nullptr);
+  iso.compute(
+      true, sum, cand, static_cast<CaloTowerDetId const*>(nullptr), static_cast<CaloTowerDetId const*>(nullptr));
 
   return 0;
 }