[ntuple] automatic schema evolution from streamer to class field

Author: jblomer

tree/ntuple/src/RFieldMeta.cxx

@@ -457,6 +457,14 @@ std::unique_ptr<ROOT::RFieldBase> ROOT::RClassField::BeforeConnectPageSource(ROO
       const ROOT::RNTupleDescriptor &desc = descriptorGuard.GetRef();
       const auto &fieldDesc = desc.GetFieldDescriptor(GetOnDiskId());
 
+      if (fieldDesc.GetStructure() == ENTupleStructure::kStreamer) {
+         // Streamer field on disk but meanwhile the type can be represented as a class field; replace this field
+         // by a streamer field to read the data from disk.
+         auto substitute = std::make_unique<RStreamerField>(GetFieldName(), GetTypeName());
+         substitute->SetOnDiskId(GetOnDiskId());
+         return substitute;
+      }
+
       for (auto linkId : fieldDesc.GetLinkIds()) {
          const auto &subFieldDesc = desc.GetFieldDescriptor(linkId);
          regularSubfields.insert(subFieldDesc.GetFieldName());

tree/ntuple/test/StreamerField.cxx

@@ -25,3 +25,14 @@ void CustomStreamerForceNative::Streamer(TBuffer &R__b)
       R__b.WriteClassBuffer(CustomStreamerForceNative::Class(), this);
    }
 }
+
+void CustomStreamerEvolution::Streamer(TBuffer &R__b)
+{
+   if (R__b.IsReading()) {
+      UInt_t R__s, R__c;
+      Version_t R__v = R__b.ReadVersion(&R__s, &R__c);
+      R__b.ReadClassBuffer(CustomStreamerEvolution::Class(), this, R__v, R__s, R__c);
+   } else {
+      R__b.WriteClassBuffer(CustomStreamerEvolution::Class(), this);
+   }
+}

tree/ntuple/test/StreamerField.hxx

@@ -54,6 +54,11 @@ struct CustomStreamerForceStreamed {
    float a;
 };
 
+struct CustomStreamerEvolution {
+   int x;
+   ClassDefNV(CustomStreamerEvolution, 1);
+};
+
 // For the time being, RNTuple ignores the unsplit comment marker and does _not_ use an RStreamerField for such members.
 class IgnoreUnsplitComment {
    std::vector<float> v; //||

tree/ntuple/test/StreamerFieldLinkDef.h

@@ -10,6 +10,7 @@
 #pragma link C++ struct CyclicMember;
 #pragma link C++ class ClassWithStreamedMember + ;
 #pragma link C++ class CustomStreamer - ;
+#pragma link C++ class CustomStreamerEvolution - ;
 #pragma link C++ options = rntupleStreamerMode(false) class CustomStreamerForceNative - ;
 #pragma link C++ options = rntupleStreamerMode(true) class CustomStreamerForceStreamed + ;
 #pragma link C++ class IgnoreUnsplitComment + ;

tree/ntuple/test/rfield_streamer.cxx

@@ -350,6 +350,36 @@ TEST(RField, StreamerSchemaEvolution)
    EXPECT_EQ(137, ptrF->fValue);
 }
 
+TEST(RField, StreamerToClassFieldSupport)
+{
+   FileRaii fileGuard("test_ntuple_rfield_streamer_to_class_field_support.root");
+
+   {
+      auto model = RNTupleModel::Create();
+      model->AddField(std::make_unique<ROOT::RStreamerField>("f", "CustomStreamerEvolution"));
+      auto writer = RNTupleWriter::Recreate(std::move(model), "ntpl", fileGuard.GetPath());
+      auto ptr = writer->GetModel().GetDefaultEntry().GetPtr<CustomStreamerEvolution>("f");
+      ptr->x = 137;
+      writer->Fill();
+   }
+
+   auto cl = TClass::GetClass("CustomStreamerEvolution");
+   ASSERT_TRUE(cl != nullptr);
+   cl->CreateAttributeMap();
+   cl->GetAttributeMap()->AddProperty("rntuple.streamerMode", "false");
+
+   const auto inMemoryField = RFieldBase::Create("f", "CustomStreamer").Unwrap();
+   const auto inMemoryFieldPtr = inMemoryField.get(); // silence clang
+   EXPECT_EQ(typeid(*inMemoryFieldPtr), typeid(ROOT::RClassField));
+
+   auto reader = RNTupleReader::Open("ntpl", fileGuard.GetPath());
+   auto v = reader->GetView<CustomStreamerEvolution>("f");
+   const auto &onDiskField = v.GetField();
+   EXPECT_EQ(typeid(onDiskField), typeid(ROOT::RStreamerField));
+
+   EXPECT_FLOAT_EQ(137, v(0).x);
+}
+
 TEST(RField, StreamerClassMismatch)
 {
    FileRaii fileGuard("test_ntuple_rfield_streamer_class_mismatch.root");