[ntuple] automatic schema evolution of fixed-size array to std::vector

tree/ntuple/inc/ROOT/RField/RFieldSequenceContainer.hxx

@@ -200,6 +200,7 @@ public:
 /// The generic field for a (nested) `std::vector<Type>` except for `std::vector<bool>`
 /// The field can be constructed as untyped collection through CreateUntyped().
 class RVectorField : public RFieldBase {
+   friend class RArrayAsVectorField; // to get access to the RVectorDeleter
    friend std::unique_ptr<RFieldBase> Internal::CreateEmulatedVectorField(std::string_view fieldName,
                                                                           std::unique_ptr<RFieldBase> itemField,
                                                                           std::string_view emulatedFromType);
@@ -222,6 +223,10 @@ class RVectorField : public RFieldBase {
    ROOT::Internal::RColumnIndex fNWritten;
    std::unique_ptr<RDeleter> fItemDeleter;
 
+   // Ensures that the std::vector pointed to by vec has at least nItems valid elements.
+   static void ResizeVector(void *vec, std::size_t nItems, std::size_t itemSize, const RFieldBase &itemField,
+                            RDeleter *itemDeleter);
+
 protected:
    /// Creates a possibly-untyped VectorField.
    /// If `emulatedFromType` is not nullopt, the field is untyped. If the string is empty, it is a "regular"
@@ -241,6 +246,7 @@ protected:
    std::size_t AppendImpl(const void *from) final;
    void ReadGlobalImpl(ROOT::NTupleSize_t globalIndex, void *to) final;
 
+   std::unique_ptr<RFieldBase> BeforeConnectPageSource(ROOT::Internal::RPageSource &pageSource) final;
    void ReconcileOnDiskField(const RNTupleDescriptor &desc) final;
 
    void CommitClusterImpl() final { fNWritten = 0; }
@@ -275,6 +281,9 @@ template <>
 class RField<std::vector<bool>> final : public RFieldBase {
 private:
    ROOT::Internal::RColumnIndex fNWritten{0};
+   /// If schema-evolved from an std::array, fOnDiskNRepetition is > 0 and there will be no
+   /// principal column.
+   std::size_t fOnDiskNRepetitions = 0;
 
 protected:
    std::unique_ptr<RFieldBase> CloneImpl(std::string_view newName) const final
@@ -291,6 +300,7 @@ protected:
 
    std::size_t AppendImpl(const void *from) final;
    void ReadGlobalImpl(ROOT::NTupleSize_t globalIndex, void *to) final;
+   void ReadInClusterImpl(ROOT::RNTupleLocalIndex localIndex, void *to) final;
 
    void ReconcileOnDiskField(const RNTupleDescriptor &desc) final;
 
@@ -363,6 +373,50 @@ public:
    void AcceptVisitor(ROOT::Detail::RFieldVisitor &visitor) const final;
 };
 
+/**
+\class ROOT::RArrayAsVectorField
+\brief A field for fixed-size arrays that are represented as std::vector in memory.
+\ingroup NTuple
+This class is used only for reading. In particular, it helps for schema evolution of fixed-size arrays into vectors.
+*/
+class RArrayAsVectorField final : public RFieldBase {
+private:
+   std::unique_ptr<RDeleter> fItemDeleter; /// Sub field deleter or nullptr for simple fields
+   std::size_t fItemSize;                  /// The size of a child field's item
+   std::size_t fArrayLength;               /// The length of the arrays in this field
+
+protected:
+   std::unique_ptr<RFieldBase> CloneImpl(std::string_view newName) const final;
+
+   void GenerateColumns() final;
+   using RFieldBase::GenerateColumns;
+
+   void ConstructValue(void *where) const final { new (where) std::vector<char>(); }
+   /// Returns an RVectorField::RVectorDeleter
+   std::unique_ptr<RDeleter> GetDeleter() const final;
+
+   void ReadGlobalImpl(ROOT::NTupleSize_t globalIndex, void *to) final;
+   void ReadInClusterImpl(RNTupleLocalIndex localIndex, void *to) final;
+
+   void ReconcileOnDiskField(const RNTupleDescriptor &desc) final;
+
+public:
+   /// The `itemField` argument represents the inner item of the on-disk array,
+   /// i.e. for an `std::array<float>` it is the `float`
+   RArrayAsVectorField(std::string_view fieldName, std::unique_ptr<RFieldBase> itemField, std::size_t arrayLength);
+   RArrayAsVectorField(const RArrayAsVectorField &other) = delete;
+   RArrayAsVectorField &operator=(const RArrayAsVectorField &other) = delete;
+   RArrayAsVectorField(RArrayAsVectorField &&other) = default;
+   RArrayAsVectorField &operator=(RArrayAsVectorField &&other) = default;
+   ~RArrayAsVectorField() final = default;
+
+   size_t GetValueSize() const final { return sizeof(std::vector<char>); }
+   size_t GetAlignment() const final { return std::alignment_of<std::vector<char>>(); }
+
+   std::vector<RFieldBase::RValue> SplitValue(const RFieldBase::RValue &value) const final;
+   void AcceptVisitor(ROOT::Detail::RFieldVisitor &visitor) const final;
+};
+
 } // namespace ROOT
 
 #endif

tree/ntuple/inc/ROOT/RFieldVisitor.hxx

@@ -61,6 +61,7 @@ public:
    virtual void VisitFieldZero(const ROOT::RFieldZero &field) { VisitField(field); }
    virtual void VisitArrayField(const ROOT::RArrayField &field) { VisitField(field); }
    virtual void VisitArrayAsRVecField(const ROOT::RArrayAsRVecField &field) { VisitField(field); }
+   virtual void VisitArrayAsVectorField(const ROOT::RArrayAsVectorField &field) { VisitField(field); }
    virtual void VisitAtomicField(const ROOT::RAtomicField &field) { VisitField(field); }
    virtual void VisitBitsetField(const ROOT::RBitsetField &field) { VisitField(field); }
    virtual void VisitBoolField(const ROOT::RField<bool> &field) { VisitField(field); }
@@ -235,6 +236,7 @@ public:
    void VisitCardinalityField(const ROOT::RCardinalityField &field) final;
    void VisitArrayField(const ROOT::RArrayField &field) final;
    void VisitArrayAsRVecField(const ROOT::RArrayAsRVecField &field) final;
+   void VisitArrayAsVectorField(const ROOT::RArrayAsVectorField &field) final;
    void VisitClassField(const ROOT::RClassField &field) final;
    void VisitTObjectField(const ROOT::RField<TObject> &field) final;
    void VisitStreamerField(const ROOT::RStreamerField &field) final;