[ntuple] Make RNTupleWriter::Append(RFile) a free function

Just for the time being, to avoid exposing experimental API as a public
method of the RNTupleWriter, use a free friend function in the Experimental
namespace. When we settle on the RFile/RNTuple writing semantics we will
remove this function and replace it with a factory method.

Author: silverweed

io/io/test/rfile.cxx

@@ -713,7 +713,7 @@ TEST(RFile, RNTuple)
    const auto kNEntries = 100;
    for (int i = 0; i < kNDatasets; ++i) {
       const auto ntuplePath = std::string("data_") + (i + 1);
-      const auto writer = ROOT::RNTupleWriter::Append(model->Clone(), ntuplePath, *file);
+      const auto writer = ROOT::Experimental::RNTupleWriter_Append(model->Clone(), ntuplePath, *file);
       for (int j = 0; j < kNEntries; ++j) {
          *pX = j;
          pY->clear();

tree/ntuple/inc/ROOT/RNTupleWriter.hxx

@@ -39,7 +39,19 @@ class RNTupleWriteOptions;
 
 namespace Experimental {
 class RFile;
-}
+
+/// Creates an RNTupleWriter that writes into the given `file`, appending to it. The RNTuple is written under the
+/// path `ntuplePath`.
+/// `ntuplePath` may have the form `"path/to/ntuple"`, in which case the ntuple's name will be `"ntuple"` and it will
+/// be stored under the given `ntuplePath` in the RFile.
+/// Throws an exception if the model is null.
+/// NOTE: this is a temporary, experimental API that will be replaced by an overload of RNTupleWriter::Append in the
+/// future.
+std::unique_ptr<RNTupleWriter>
+RNTupleWriter_Append(std::unique_ptr<ROOT::RNTupleModel> model, std::string_view ntuplePath,
+                     ROOT::Experimental::RFile &file,
+                     const ROOT::RNTupleWriteOptions &options = ROOT::RNTupleWriteOptions());
+} // namespace Experimental
 
 namespace Internal {
 // Non-public factory method for an RNTuple writer that uses an already constructed page sink
@@ -106,6 +118,9 @@ class RNTupleWriter {
    friend ROOT::RNTupleModel::RUpdater;
    friend std::unique_ptr<RNTupleWriter>
       Internal::CreateRNTupleWriter(std::unique_ptr<ROOT::RNTupleModel>, std::unique_ptr<Internal::RPageSink>);
+   friend std::unique_ptr<RNTupleWriter>
+   Experimental::RNTupleWriter_Append(std::unique_ptr<ROOT::RNTupleModel> model, std::string_view ntuplePath,
+                                      ROOT::Experimental::RFile &file, const ROOT::RNTupleWriteOptions &options);
 
 private:
    RNTupleFillContext fFillContext;
@@ -156,12 +171,6 @@ public:
    static std::unique_ptr<RNTupleWriter> Append(std::unique_ptr<ROOT::RNTupleModel> model, std::string_view ntupleName,
                                                 TDirectory &fileOrDirectory,
                                                 const ROOT::RNTupleWriteOptions &options = ROOT::RNTupleWriteOptions());
-   /// Throws an exception if the model is null.
-   /// `ntuplePath` may have the form `"path/to/ntuple"`, in which case the ntuple's name will be `"ntuple"` and it will
-   /// be stored under the given `ntuplePath` in the RFile.
-   static std::unique_ptr<RNTupleWriter> Append(std::unique_ptr<ROOT::RNTupleModel> model, std::string_view ntuplePath,
-                                                ROOT::Experimental::RFile &file,
-                                                const ROOT::RNTupleWriteOptions &options = ROOT::RNTupleWriteOptions());
 
    RNTupleWriter(const RNTupleWriter &) = delete;
    RNTupleWriter &operator=(const RNTupleWriter &) = delete;

tree/ntuple/src/RNTupleWriter.cxx

@@ -114,15 +114,6 @@ ROOT::RNTupleWriter::Append(std::unique_ptr<ROOT::RNTupleModel> model, std::stri
    return Create(std::move(model), std::move(sink), options);
 }
 
-std::unique_ptr<ROOT::RNTupleWriter>
-ROOT::RNTupleWriter::Append(std::unique_ptr<ROOT::RNTupleModel> model, std::string_view ntupleName,
-                            ROOT::Experimental::RFile &file, const ROOT::RNTupleWriteOptions &options)
-{
-   auto [ntupleDir, ntupleBasename] = ROOT::Experimental::Detail::DecomposePath(ntupleName);
-   auto sink = std::make_unique<Internal::RPageSinkFile>(ntupleBasename, file, ntupleDir, options);
-   return Create(std::move(model), std::move(sink), options);
-}
-
 void ROOT::RNTupleWriter::CommitClusterGroup()
 {
    if (GetNEntries() == fLastCommittedClusterGroup)
@@ -155,3 +146,12 @@ ROOT::Internal::CreateRNTupleWriter(std::unique_ptr<ROOT::RNTupleModel> model,
 {
    return std::unique_ptr<ROOT::RNTupleWriter>(new ROOT::RNTupleWriter(std::move(model), std::move(sink)));
 }
+
+std::unique_ptr<ROOT::RNTupleWriter>
+ROOT::Experimental::RNTupleWriter_Append(std::unique_ptr<ROOT::RNTupleModel> model, std::string_view ntupleName,
+                                         ROOT::Experimental::RFile &file, const ROOT::RNTupleWriteOptions &options)
+{
+   auto [ntupleDir, ntupleBasename] = ROOT::Experimental::Detail::DecomposePath(ntupleName);
+   auto sink = std::make_unique<ROOT::Internal::RPageSinkFile>(ntupleBasename, file, ntupleDir, options);
+   return ROOT::RNTupleWriter::Create(std::move(model), std::move(sink), options);
+}