[6.36] Fix logic to check when implicit MT is enabled

tree/dataframe/src/RTTreeDS.cxx

@@ -399,15 +399,14 @@ void ROOT::Internal::RDF::RTTreeDS::Finalize()
 
 void ROOT::Internal::RDF::RTTreeDS::Initialize()
 {
-   if (fNSlots == 1) {
-      assert(!fTreeReader);
-      fTreeReader = std::make_unique<TTreeReader>(fTree.get(), fTree->GetEntryList(), /*warnAboutLongerFriends*/ true);
-      if (fGlobalEntryRange.has_value() && fGlobalEntryRange->first <= std::numeric_limits<Long64_t>::max() &&
-          fGlobalEntryRange->second <= std::numeric_limits<Long64_t>::max() && fTreeReader &&
-          fTreeReader->SetEntriesRange(fGlobalEntryRange->first, fGlobalEntryRange->second) !=
-             TTreeReader::kEntryValid) {
-         throw std::logic_error("Something went wrong in initializing the TTreeReader.");
-      }
+   if (ROOT::IsImplicitMTEnabled())
+      return;
+   assert(!fTreeReader);
+   fTreeReader = std::make_unique<TTreeReader>(fTree.get(), fTree->GetEntryList(), /*warnAboutLongerFriends*/ true);
+   if (fGlobalEntryRange.has_value() && fGlobalEntryRange->first <= std::numeric_limits<Long64_t>::max() &&
+       fGlobalEntryRange->second <= std::numeric_limits<Long64_t>::max() && fTreeReader &&
+       fTreeReader->SetEntriesRange(fGlobalEntryRange->first, fGlobalEntryRange->second) != TTreeReader::kEntryValid) {
+      throw std::logic_error("Something went wrong in initializing the TTreeReader.");
    }
 }
 

tree/dataframe/test/datasource_tree.cxx

@@ -56,3 +56,47 @@ TEST(RTTreeDS, BranchWithNestedSameName)
 
    expect_vec_eq(branchNames, expectedBranchNames);
 }
+
+#ifdef R__USE_IMT
+struct Dataset20164RAIII {
+   const char *fTreeName{"tree_20164"};
+   const char *fFileName{"tree_20164.root"};
+
+   Dataset20164RAIII()
+   {
+      std::unique_ptr<TFile> ofile{TFile::Open(fFileName, "recreate")};
+
+      std::unique_ptr<TTree> myTree = std::make_unique<TTree>(fTreeName, fTreeName);
+
+      int v1{42};
+
+      myTree->Branch("v1", &v1);
+      myTree->Fill();
+      ofile->Write();
+
+      // It's important to use 1 as the logic in RTTreeDS was faulty only for the case fNSlots==1
+      ROOT::EnableImplicitMT(1);
+   }
+
+   ~Dataset20164RAIII()
+   {
+      ROOT::DisableImplicitMT();
+      std::remove(fFileName);
+   }
+};
+
+TEST(RTTreeDS, Regression20164)
+{
+   // Test that TTree thread tasks coherently use the TTreeReader created for the specific task and do not access
+   // a TTreeReader from the RTTreeDS.
+   Dataset20164RAIII dataset{};
+
+   auto chain = std::make_unique<TChain>(dataset.fTreeName);
+   chain->Add(dataset.fFileName);
+
+   auto val = ROOT::RDataFrame(*chain).Sum<int>("v1").GetValue();
+
+   ASSERT_EQ(val, 42);
+}
+
+#endif