[ntuple] fix handling of I/O rules with empty source

roottest/root/ntuple/evolution/CMakeLists.txt

@@ -0,0 +1,38 @@
+ROOTTEST_GENERATE_DICTIONARY(
+  ntplevolv_v2_dict
+  ${CMAKE_CURRENT_SOURCE_DIR}/NtplEvolv_v2.hxx
+  LINKDEF ${CMAKE_CURRENT_SOURCE_DIR}/NtplEvolv_v2_LinkDef.h
+  NO_ROOTMAP NO_CXXMODULE
+  FIXTURES_SETUP generated_ntplevolv_v2_dictionary
+)
+
+ROOTTEST_GENERATE_EXECUTABLE(
+  write_ntplevolv
+  write_ntplevolv.cxx ntplevolv_v2_dict.cxx
+  LIBRARIES Core RIO ROOTNTuple
+  FIXTURES_REQUIRED generated_ntplevolv_v2_dictionary
+  FIXTURES_SETUP write_ntplevolv_excutable)
+
+ROOTTEST_ADD_TEST(write_ntplevolv
+                  EXEC ./write_ntplevolv
+                  FIXTURES_REQUIRED write_ntplevolv_excutable
+                  FIXTURES_SETUP write_ntplevolv_done)
+
+ROOTTEST_GENERATE_DICTIONARY(
+  ntplevolv_v3_dict
+  ${CMAKE_CURRENT_SOURCE_DIR}/NtplEvolv_v3.hxx
+  LINKDEF ${CMAKE_CURRENT_SOURCE_DIR}/NtplEvolv_v3_LinkDef.h
+  NO_ROOTMAP NO_CXXMODULE
+  FIXTURES_SETUP generated_ntplevolv_v3_dictionary
+)
+
+ROOTTEST_GENERATE_EXECUTABLE(
+  read_ntplevolv
+  read_ntplevolv.cxx ntplevolv_v3_dict.cxx
+  LIBRARIES Core RIO ROOTNTuple
+  FIXTURES_REQUIRED generated_ntplevolv_v3_dictionary
+  FIXTURES_SETUP read_ntplevolv_excutable)
+
+ROOTTEST_ADD_TEST(read_ntplevolv
+                  EXEC ./read_ntplevolv
+                  FIXTURES_REQUIRED read_ntplevolv_excutable write_ntplevolv_done)

roottest/root/ntuple/evolution/NtplEvolv_v2.hxx

@@ -0,0 +1,12 @@
+#ifndef NTPL_EVOLV_V2_H
+#define NTPL_EVOLV_V2_H
+
+#include <RtypesCore.h>
+
+struct NtplEvolv {
+   int fA = 0;
+
+   ClassDefNV(NtplEvolv, 2)
+};
+
+#endif // NTPL_EVOLV_V2_H

roottest/root/ntuple/evolution/NtplEvolv_v2_LinkDef.h

@@ -0,0 +1,5 @@
+#ifdef __ROOTCLING__
+
+#pragma link C++ class NtplEvolv+;
+
+#endif

roottest/root/ntuple/evolution/NtplEvolv_v3.hxx

@@ -0,0 +1,13 @@
+#ifndef NTPL_EVOLV_V3_H
+#define NTPL_EVOLV_V3_H
+
+#include <RtypesCore.h>
+
+struct NtplEvolv {
+   int fA = 0;
+   int fB = 0;
+
+   ClassDefNV(NtplEvolv, 3)
+};
+
+#endif // NTPL_EVOLV_V3_H

roottest/root/ntuple/evolution/NtplEvolv_v3_LinkDef.h

@@ -0,0 +1,7 @@
+#ifdef __ROOTCLING__
+
+#pragma link C++ class NtplEvolv+;
+
+#pragma read sourceClass="NtplEvolv" version="[1-]" source="" targetClass="NtplEvolv" target="fA" code = "{ fA = 13; }"
+
+#endif

roottest/root/ntuple/evolution/read_ntplevolv.cxx

@@ -0,0 +1,17 @@
+#include <ROOT/RNTupleReader.hxx>
+
+#include "NtplEvolv_v3.hxx"
+
+#include <iostream>
+
+int main()
+{
+   auto reader = ROOT::RNTupleReader::Open("ntpl", "root_test_ntpl_evolution.root");
+
+   reader->LoadEntry(0);
+
+   auto a = reader->GetModel().GetDefaultEntry().GetPtr<NtplEvolv>("event")->fA;
+   std::cout << "Result of event.fA: " << a << std::endl;
+
+   return a != 13;
+}

roottest/root/ntuple/evolution/write_ntplevolv.cxx

@@ -0,0 +1,18 @@
+#include <ROOT/RNTupleModel.hxx>
+#include <ROOT/RNTupleWriter.hxx>
+
+#include "NtplEvolv_v2.hxx"
+
+#include <memory>
+
+int main()
+{
+   auto model = ROOT::RNTupleModel::Create();
+   auto event = model->MakeField<NtplEvolv>("event");
+   auto writer = ROOT::RNTupleWriter::Recreate(std::move(model), "ntpl", "root_test_ntpl_evolution.root");
+
+   event->fA = 1;
+   writer->Fill();
+
+   return 0;
+}

tree/ntuple/src/RFieldMeta.cxx

@@ -541,22 +541,28 @@ std::unique_ptr<ROOT::RFieldBase> ROOT::RClassField::BeforeConnectPageSource(ROO
          }
       }
 
-      if (!rules.empty()) {
+      const bool hasSources = std::any_of(rules.begin(), rules.end(), [](const auto &r) {
+         return r->GetSource() && (r->GetSource()->GetEntries() > 0);
+      });
+
+      // A staging class (conversion streamer info) only exists if there is at least one rule that has an
+      // on disk source member defined.
+      if (hasSources) {
          SetStagingClass(fieldDesc.GetTypeName(), fieldDesc.GetTypeVersion());
          PrepareStagingArea(rules, desc, fieldDesc);
          for (auto &[_, si] : fStagingItems) {
             Internal::CallConnectPageSourceOnField(*si.fField, pageSource);
             si.fField = std::move(static_cast<RFieldZero *>(si.fField.get())->ReleaseSubfields()[0]);
          }
+      }
 
-         // Remove target member of read rules from the list of regular members of the underlying on-disk field
-         for (const auto rule : rules) {
-            if (!rule->GetTarget())
-               continue;
+      // Remove target member of read rules from the list of regular members of the underlying on-disk field
+      for (const auto rule : rules) {
+         if (!rule->GetTarget())
+            continue;
 
-            for (const auto target : ROOT::Detail::TRangeStaticCast<const TObjString>(*rule->GetTarget())) {
-               regularSubfields.erase(std::string(target->GetString()));
-            }
+         for (const auto target : ROOT::Detail::TRangeStaticCast<const TObjString>(*rule->GetTarget())) {
+            regularSubfields.erase(std::string(target->GetString()));
          }
       }
    }

tree/ntuple/test/CustomStructLinkDef.h

@@ -142,6 +142,9 @@
 #pragma link C++ options = version(3) class NewName < int> + ;
 #pragma link C++ options = version(3) class NewName < NewName < int>> + ;
 #pragma read sourceClass = "OldName<OldName<int>>" targetClass = "NewName<OldName<int>>" version = "[3]"
+#pragma link C++ options = version(4) class OldName<float>+;
+#pragma link C++ options = version(5) class NewName<float>+;
+#pragma read sourceClass = "OldName<float>" targetClass = "NewName<float>"
 
 #pragma link C++ struct SourceStruct + ;
 #pragma link C++ struct StructWithSourceStruct + ;

tree/ntuple/test/rfield_class.cxx

@@ -260,12 +260,14 @@ TEST(RNTuple, TClassReadRules)
       auto ptrOldCoord = model->MakeField<OldCoordinates>("oldCoord");
       auto ptrLowPrecisionFloat = model->MakeField<LowPrecisionFloatWithIORules>("lowPrecisionFloat");
       auto ptrOldName = model->MakeField<OldName<OldName<int>>>("rename");
+      auto ptrOldNameVersionChange = model->MakeField<OldName<float>>("renameVersionChange");
       auto ptrWithSource = model->MakeField<StructWithSourceStruct>("withSource");
       ptrCoord->fX = ptrOldCoord->fOldX = 1.0;
       ptrCoord->fY = ptrOldCoord->fOldY = 1.0;
       ptrLowPrecisionFloat->fFoo = 1.0;
       ptrLowPrecisionFloat->fLast8BitsZero = last8BitsZero;
       ptrOldName->fValue.fValue = 42;
+      ptrOldNameVersionChange->fValue = 1.0;
       // The following two members are transient and should not be stored.
       ptrWithSource->fSource.fTransient = 1;
       ptrWithSource->fTransient = 2;
@@ -322,6 +324,10 @@ TEST(RNTuple, TClassReadRules)
 
    auto viewRename = reader->GetView<NewName<OldName<int>>>("rename");
    EXPECT_EQ(42, viewRename(0).fValue.fValue);
+
+   EXPECT_NE(ROOT::RField<OldName<float>>("").GetTypeVersion(), ROOT::RField<NewName<float>>("").GetTypeVersion());
+   auto viewRenameVersionChange = reader->GetView<NewName<float>>("renameVersionChange");
+   EXPECT_FLOAT_EQ(1.0, viewRenameVersionChange(0).fValue);
 }
 
 // Adjusted from