From 841fc89fc53d18df2ec0ee3cd77e4fe7e658179d Mon Sep 17 00:00:00 2001
From: Vincenzo Eduardo Padulano <vincenzo.eduardo.padulano@cern.ch>
Date: Thu, 23 Oct 2025 12:37:51 +0200
Subject: [PATCH] [df] Fix logic to check when implicit MT is enabled

RTTreeDS needs to act differently w.r.t. TTreeReader creation whether implicit
MT is enabled or not. Fix the logic to use the more appropriate
ROOT::IsImplicitMTEnabled rather than just checking for the number of active
slots.
---
 tree/dataframe/src/RTTreeDS.cxx         | 17 +++++-----
 tree/dataframe/test/datasource_tree.cxx | 44 +++++++++++++++++++++++++
 2 files changed, 52 insertions(+), 9 deletions(-)

diff --git a/tree/dataframe/src/RTTreeDS.cxx b/tree/dataframe/src/RTTreeDS.cxx
index cf2a4c5c4c8ab..6d5836b594286 100644
--- a/tree/dataframe/src/RTTreeDS.cxx
+++ b/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.");
    }
 }
 
diff --git a/tree/dataframe/test/datasource_tree.cxx b/tree/dataframe/test/datasource_tree.cxx
index 6acabbcf60935..3c8235d315f5e 100644
--- a/tree/dataframe/test/datasource_tree.cxx
+++ b/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