From ed4373d97a84efdebda63c43553b142a144918c2 Mon Sep 17 00:00:00 2001
From: Marino Missiroli <m.missiroli@cern.ch>
Date: Sun, 30 Apr 2023 10:58:40 +0200
Subject: [PATCH] guard HLTRecHitInAllL1RegionsProducer<T> against empty
 collection of L1T candidates

---
 .../EgammaHLTProducers/plugins/BuildFile.xml   |  2 ++
 .../plugins/HLTRecHitInAllL1RegionsProducer.cc | 18 +++++++++++++++++-
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/RecoEgamma/EgammaHLTProducers/plugins/BuildFile.xml b/RecoEgamma/EgammaHLTProducers/plugins/BuildFile.xml
index 316e3225f8d14..82da66d4300bb 100644
--- a/RecoEgamma/EgammaHLTProducers/plugins/BuildFile.xml
+++ b/RecoEgamma/EgammaHLTProducers/plugins/BuildFile.xml
@@ -1,5 +1,7 @@
 <use name="FWCore/Framework"/>
+<use name="FWCore/MessageLogger"/>
 <use name="FWCore/ParameterSet"/>
+<use name="FWCore/Utilities"/>
 <use name="DataFormats/EgammaCandidates"/>
 <use name="Geometry/CaloGeometry"/>
 <use name="RecoEcal/EgammaCoreTools"/>
diff --git a/RecoEgamma/EgammaHLTProducers/plugins/HLTRecHitInAllL1RegionsProducer.cc b/RecoEgamma/EgammaHLTProducers/plugins/HLTRecHitInAllL1RegionsProducer.cc
index fd82e24904568..b24ac1caa0aca 100644
--- a/RecoEgamma/EgammaHLTProducers/plugins/HLTRecHitInAllL1RegionsProducer.cc
+++ b/RecoEgamma/EgammaHLTProducers/plugins/HLTRecHitInAllL1RegionsProducer.cc
@@ -7,8 +7,9 @@
 #include "FWCore/Framework/interface/MakerMacros.h"
 #include "FWCore/Framework/interface/ConsumesCollector.h"
 #include "FWCore/Framework/interface/ESHandle.h"
-
+#include "FWCore/MessageLogger/interface/MessageLogger.h"
 #include "FWCore/ParameterSet/interface/ParameterSet.h"
+#include "FWCore/Utilities/interface/TypeDemangler.h"
 
 // Reco candidates (might not need)
 #include "DataFormats/RecoCandidate/interface/RecoCandidate.h"
@@ -80,6 +81,10 @@ class L1RegionData : public L1RegionDataBase {
                         const edm::EventSetup&,
                         std::vector<RectangularEtaPhiRegion>&) const override;
   template <typename T2>
+  bool isEmpty(const T2& coll) const {
+    return coll.empty();
+  }
+  template <typename T2>
   static typename T2::const_iterator beginIt(const T2& coll) {
     return coll.begin();
   }
@@ -88,6 +93,10 @@ class L1RegionData : public L1RegionDataBase {
     return coll.end();
   }
   template <typename T2>
+  bool isEmpty(const BXVector<T2>& coll) const {
+    return (coll.size() == 0 or coll.isEmpty(0));
+  }
+  template <typename T2>
   static typename BXVector<T2>::const_iterator beginIt(const BXVector<T2>& coll) {
     return coll.begin(0);
   }
@@ -286,6 +295,13 @@ void L1RegionData<L1CollType>::getEtaPhiRegions(const edm::Event& event,
   edm::Handle<L1CollType> l1Cands;
   event.getByToken(token_, l1Cands);
 
+  if (isEmpty(*l1Cands)) {
+    LogDebug("HLTRecHitInAllL1RegionsProducerL1RegionData")
+        << "The input collection of L1T candidates is empty (L1CollType = \""
+        << edm::typeDemangle(typeid(L1CollType).name()) << "\"). No regions selected.";
+    return;
+  }
+
   for (auto l1CandIt = beginIt(*l1Cands); l1CandIt != endIt(*l1Cands); ++l1CandIt) {
     if (l1CandIt->et() >= minEt_ && l1CandIt->et() < maxEt_) {
       double etaLow = l1CandIt->eta() - regionEtaMargin_;