[ntuple] Add new merger test

This test checks two things:

1. that the fix applied by 14ade04 works
   properly (this is already checked by MergeDeferredAdvanced but that
   might not be the case anymore if we update the merger to change the
   column encodings depending on the output compression - in which case
   that test would not be adding a late extended column anymore);
2. that the merger properly handles fields with the same cardinality
   but different number of column representations

Author: silverweed

tree/ntuple/test/ntuple_merger.cxx

@@ -2729,6 +2729,93 @@ TEST(RNTupleMerger, MergeDeferredAdvanced)
    }
 }
 
+TEST(RNTupleMerger, MergeDeferredAdvanced2)
+{
+   // Like MergeDeferredAdvanced, but make sure the fields have different column types so that the merging produces
+   // a late-extended column.
+   FileRaii fileGuard1("test_ntuple_merge_deferred_adv2_in_1.root");
+   FileRaii fileGuard2("test_ntuple_merge_deferred_adv2_in_2.root");
+
+   // First RNTuple with late model extended field "flt" (column is not deferred because it's still entry 0)
+   {
+      auto model1 = RNTupleModel::Create();
+      auto wopts = RNTupleWriteOptions();
+      wopts.SetCompression(0);
+      auto tfile = TFile::Open(fileGuard1.GetPath().c_str(), "RECREATE");
+      auto writer1 = RNTupleWriter::Append(std::move(model1), "ntuple", *tfile, wopts);
+      auto updater = writer1->CreateModelUpdater();
+      auto field = std::make_unique<RField<float>>("flt");
+      field->SetHalfPrecision();
+      updater->BeginUpdate();
+      updater->AddField(std::move(field));
+      updater->CommitUpdate();
+      auto pFlt1 = writer1->GetModel().GetDefaultEntry().GetPtr<float>("flt");
+      for (int i = 0; i < 10; ++i) {
+         *pFlt1 = i;
+         writer1->Fill();
+      }
+   }
+
+   // Second RNTuple with late model extended field "flt"
+   {
+      auto model2 = RNTupleModel::Create();
+      // Add a non-late model extended field so we can write some entries before we extend the model and obtain
+      // actual deferred columns in the extension header.
+      auto pInt = model2->MakeField<int>("int");
+      auto wopts = RNTupleWriteOptions();
+      wopts.SetCompression(0);
+      auto writer2 = RNTupleWriter::Recreate(std::move(model2), "ntuple", fileGuard2.GetPath(), wopts);
+      for (int i = 0; i < 5; ++i) {
+         *pInt = 10 + i;
+         writer2->Fill();
+      }
+      auto updater = writer2->CreateModelUpdater();
+      auto field = std::make_unique<RField<float>>("flt");
+      field->SetColumnRepresentatives({{ROOT::ENTupleColumnType::kReal32}});
+      updater->BeginUpdate();
+      updater->AddField(std::move(field));
+      updater->CommitUpdate();
+      auto pFlt2 = writer2->GetModel().GetDefaultEntry().GetPtr<float>("flt");
+      for (int i = 5; i < 10; ++i) {
+         *pInt = 10 + i;
+         *pFlt2 = 10 + i;
+         writer2->Fill();
+      }
+   }
+
+   // Now merge them
+   std::vector<std::unique_ptr<RPageSource>> sources;
+   sources.push_back(RPageSource::Create("ntuple", fileGuard1.GetPath(), RNTupleReadOptions()));
+   sources.push_back(RPageSource::Create("ntuple", fileGuard2.GetPath(), RNTupleReadOptions()));
+   std::vector<RPageSource *> sourcePtrs;
+   for (const auto &s : sources) {
+      sourcePtrs.push_back(s.get());
+   }
+
+   FileRaii fileGuardOut("test_ntuple_merge_deferred_adv2_out.root");
+   auto wopts = RNTupleWriteOptions();
+   wopts.SetCompression(0);
+   auto destination = std::make_unique<RPageSinkFile>("ntuple", fileGuardOut.GetPath(), wopts);
+   RNTupleMerger merger{std::move(destination)};
+   auto opts = RNTupleMergeOptions();
+   opts.fMergingMode = ENTupleMergingMode::kUnion;
+   auto res = merger.Merge(sourcePtrs, opts);
+   ASSERT_TRUE(bool(res));
+
+   auto reader = RNTupleReader::Open("ntuple", fileGuardOut.GetPath());
+   EXPECT_EQ(reader->GetNEntries(), 20);
+
+   auto pInt = reader->GetModel().GetDefaultEntry().GetPtr<int>("int");
+   auto pFlt = reader->GetModel().GetDefaultEntry().GetPtr<float>("flt");
+   for (auto i : reader->GetEntryRange()) {
+      reader->LoadEntry(i);
+      float expectedFlt = (i >= 10 && i < 15) ? 0 : i;
+      EXPECT_FLOAT_EQ(*pFlt, expectedFlt);
+      int expectedInt = (i >= 10 && i < 20) * i;
+      EXPECT_EQ(*pInt, expectedInt);
+   }
+}
+
 TEST(RNTupleMerger, MergeIncrementalLMExt)
 {
    // Create the input files: