[ntuple] allow for returning field substitute in BeforeConnectPageSource()

Author: jblomer

tree/ntuple/inc/ROOT/RField.hxx

@@ -207,7 +207,7 @@ protected:
    void ReadGlobalImpl(ROOT::NTupleSize_t globalIndex, void *to) final;
    void ReadInClusterImpl(RNTupleLocalIndex localIndex, void *to) final;
 
-   void BeforeConnectPageSource(ROOT::Internal::RPageSource &pageSource) final;
+   std::unique_ptr<RFieldBase> BeforeConnectPageSource(ROOT::Internal::RPageSource &pageSource) final;
    void ReconcileOnDiskField(const RNTupleDescriptor &desc) final;
 
 public:
@@ -262,7 +262,7 @@ protected:
    // Returns the list of seen streamer infos
    ROOT::RExtraTypeInfoDescriptor GetExtraTypeInfo() const final;
 
-   void BeforeConnectPageSource(ROOT::Internal::RPageSource &source) final;
+   std::unique_ptr<RFieldBase> BeforeConnectPageSource(ROOT::Internal::RPageSource &source) final;
    void ReconcileOnDiskField(const RNTupleDescriptor &desc) final;
 
 public:

tree/ntuple/inc/ROOT/RFieldBase.hxx

@@ -514,7 +514,11 @@ protected:
    /// Called by ConnectPageSource() before connecting; derived classes may override this as appropriate, e.g.
    /// for the application of I/O rules. In the process, the field at hand or its subfields may be marked as
    /// "artifical", i.e. introduced by schema evolution and not backed by on-disk information.
-   virtual void BeforeConnectPageSource(ROOT::Internal::RPageSource & /* source */) {}
+   /// May return a field substitute that fits the on-disk schema as a replacement for the field at hand
+   virtual std::unique_ptr<RFieldBase> BeforeConnectPageSource(ROOT::Internal::RPageSource & /* source */)
+   {
+      return nullptr;
+   }
 
    /// For non-artificial fields, check compatibility of the in-memory field and the on-disk field. In the process,
    /// the field at hand may change its on-disk ID or perform other tasks related to automatic schema evolution.

tree/ntuple/src/RFieldMeta.cxx

@@ -440,7 +440,7 @@ void ROOT::RClassField::AddReadCallbacksFromIORule(const TSchemaRule *rule)
    });
 }
 
-void ROOT::RClassField::BeforeConnectPageSource(ROOT::Internal::RPageSource &pageSource)
+std::unique_ptr<ROOT::RFieldBase> ROOT::RClassField::BeforeConnectPageSource(ROOT::Internal::RPageSource &pageSource)
 {
    std::vector<const TSchemaRule *> rules;
    // On-disk members that are not targeted by an I/O rule; all other sub fields of the in-memory class
@@ -507,6 +507,8 @@ void ROOT::RClassField::BeforeConnectPageSource(ROOT::Internal::RPageSource &pag
          CallSetArtificialOn(*field);
       }
    }
+
+   return nullptr;
 }
 
 void ROOT::RClassField::ReconcileOnDiskField(const RNTupleDescriptor &desc)
@@ -979,9 +981,10 @@ void ROOT::RStreamerField::GenerateColumns(const ROOT::RNTupleDescriptor &desc)
    GenerateColumnsImpl<ROOT::Internal::RColumnIndex, std::byte>(desc);
 }
 
-void ROOT::RStreamerField::BeforeConnectPageSource(ROOT::Internal::RPageSource &source)
+std::unique_ptr<ROOT::RFieldBase> ROOT::RStreamerField::BeforeConnectPageSource(ROOT::Internal::RPageSource &source)
 {
    source.RegisterStreamerInfos();
+   return nullptr;
 }
 
 void ROOT::RStreamerField::ReconcileOnDiskField(const RNTupleDescriptor &desc)