[ntuple] Add GetNEntries() to RPageSink

This is needed by the RNTupleMerger to properly know the initial
number of entries in the destination sink in case of incremental
merging. Since the Descriptor's NEntries is not updated until the
first cluster group is committed - and since we don't commit a cluster
group in InitFromDescriptor() - it cannot be used for that purpose.

Author: silverweed

tree/ntuple/v7/inc/ROOT/RPageNullSink.hxx

@@ -48,6 +48,8 @@ public:
       return descriptor;
    }
 
+   NTupleSize_t GetNEntries() const final { return 0; }
+
    void ConnectFields(const std::vector<RFieldBase *> &fields, NTupleSize_t firstEntry)
    {
       auto connectField = [&](RFieldBase &f) { CallConnectPageSinkOnField(f, *this, firstEntry); };

tree/ntuple/v7/inc/ROOT/RPageSinkBuf.hxx

@@ -133,6 +133,8 @@ public:
 
    const RNTupleDescriptor &GetDescriptor() const final;
 
+   NTupleSize_t GetNEntries() const final { return fInnerSink->GetNEntries(); }
+
    void InitImpl(RNTupleModel &model) final;
    void UpdateSchema(const RNTupleModelChangeset &changeset, NTupleSize_t firstEntry) final;
    void UpdateExtraTypeInfo(const RExtraTypeInfoDescriptor &extraTypeInfo) final;

tree/ntuple/v7/inc/ROOT/RPageStorage.hxx

@@ -317,6 +317,8 @@ public:
    /// Return the RNTupleDescriptor being constructed.
    virtual const RNTupleDescriptor &GetDescriptor() const = 0;
 
+   virtual NTupleSize_t GetNEntries() const = 0;
+
    /// Physically creates the storage container to hold the ntuple (e.g., a keys a TFile or an S3 bucket)
    /// Init() associates column handles to the columns referenced by the model
    void Init(RNTupleModel &model)
@@ -519,6 +521,8 @@ public:
 
    const RNTupleDescriptor &GetDescriptor() const final { return fDescriptorBuilder.GetDescriptor(); }
 
+   NTupleSize_t GetNEntries() const final { return fPrevClusterNEntries; }
+
    /// Updates the descriptor and calls InitImpl() that handles the backend-specific details (file, DAOS, etc.)
    void InitImpl(RNTupleModel &model) final;
    void UpdateSchema(const RNTupleModelChangeset &changeset, NTupleSize_t firstEntry) final;

tree/ntuple/v7/src/RNTupleParallelWriter.cxx

@@ -74,6 +74,8 @@ class RPageSynchronizingSink : public RPageSink {
 
    const RNTupleDescriptor &GetDescriptor() const final { return fInnerSink->GetDescriptor(); }
 
+   NTupleSize_t GetNEntries() const final { return fInnerSink->GetNEntries(); }
+
    ColumnHandle_t AddColumn(DescriptorId_t, RColumn &) final { return {}; }
    void InitImpl(RNTupleModel &) final {}
    void UpdateSchema(const RNTupleModelChangeset &, NTupleSize_t) final

tree/ntuple/v7/test/ntuple_endian.cxx

@@ -49,6 +49,8 @@ class RPageSinkMock : public RPageSink {
       return descriptor;
    }
 
+   NTupleSize_t GetNEntries() const final { return 0; }
+
    void InitImpl(RNTupleModel &) final {}
    void UpdateSchema(const ROOT::Experimental::Internal::RNTupleModelChangeset &, NTupleSize_t) final {}
    void UpdateExtraTypeInfo(const ROOT::Experimental::RExtraTypeInfoDescriptor &) final {}

tree/ntuple/v7/test/ntuple_storage.cxx

@@ -41,6 +41,8 @@ class RPageSinkMock : public RPageSink {
       return descriptor;
    }
 
+   NTupleSize_t GetNEntries() const final { return 0; }
+
    void InitImpl(RNTupleModel &) final {}
    void UpdateSchema(const ROOT::Experimental::Internal::RNTupleModelChangeset &, NTupleSize_t) final {}
    void UpdateExtraTypeInfo(const ROOT::Experimental::RExtraTypeInfoDescriptor &) final {}