[df] Fix warning about untriggered lazy RNTuple snapshot

Use the same conditions as TTree, looking at fOutputFile instead of
the data source.

Author: hahnjo

tree/dataframe/src/RDFSnapshotHelpers.cxx

@@ -829,7 +829,7 @@ ROOT::Internal::RDF::UntypedSnapshotRNTupleHelper &ROOT::Internal::RDF::UntypedS
 
 ROOT::Internal::RDF::UntypedSnapshotRNTupleHelper::~UntypedSnapshotRNTupleHelper()
 {
-   if (!fNTupleName.empty() && !fOutputLoopManager->GetDataSource() && fOptions.fLazy)
+   if (!fNTupleName.empty() /* not moved from */ && !fOutputFile /* did not run */ && fOptions.fLazy)
       Warning("Snapshot", "A lazy Snapshot action was booked but never triggered.");
 }
 

tree/dataframe/test/dataframe_snapshot_ntuple.cxx

@@ -72,6 +72,18 @@ TEST(RDFSnapshotRNTuple, FromScratch)
    }
 }
 
+TEST(RDFSnapshotRNTuple, LazyTriggered)
+{
+   FileRAII fileGuard{"RDFSnapshotRNTuple_lazy.root"};
+   auto d = ROOT::RDataFrame(1);
+   ROOT::RDF::RSnapshotOptions opts;
+   opts.fOutputFormat = ROOT::RDF::ESnapshotOutputFormat::kRNTuple;
+   opts.fLazy = true;
+   auto r = d.Snapshot("t", fileGuard.GetPath(), {"rdfentry_"}, opts);
+   *r;
+   r = {};
+}
+
 void BookLazySnapshot(std::string_view filename)
 {
    auto d = ROOT::RDataFrame(1);