Please consider the following formatting changes to #13725

Framework/Core/include/Framework/RootArrowFilesystem.h

@@ -17,6 +17,8 @@
 #include <arrow/type_fwd.h>
 #include <memory>
 
+class TFile;
+class TBranch;
 class TTree;
 class TBufferFile;
 class TDirectoryFile;
@@ -227,11 +229,38 @@ class TTreeFileFormat : public arrow::dataset::FileFormat
     const std::shared_ptr<arrow::dataset::FileFragment>& fragment) const override;
 };
 
-// An arrow outputstream which allows to write to a ttree
+// An arrow outputstream which allows to write to a TDirectoryFile.
+// This will point to the location of the file itself. You can
+// specify the location of the actual object inside it by passing the
+// associated path to the Write() API.
+class TDirectoryFileOutputStream : public arrow::io::OutputStream
+{
+ public:
+  TDirectoryFileOutputStream(TDirectoryFile*);
+
+  arrow::Status Close() override;
+
+  arrow::Result<int64_t> Tell() const override;
+
+  arrow::Status Write(const void* data, int64_t nbytes) override;
+
+  bool closed() const override;
+
+  TDirectoryFile* GetDirectory()
+  {
+    return mDirectory;
+  }
+
+ private:
+  TDirectoryFile* mDirectory;
+};
+
+// An arrow outputstream which allows to write to a TTree. Eventually
+// with a prefix for the branches.
 class TTreeOutputStream : public arrow::io::OutputStream
 {
  public:
-  TTreeOutputStream(TTree* t);
+  TTreeOutputStream(TTree*, std::string branchPrefix);
 
   arrow::Status Close() override;
 
@@ -241,13 +270,16 @@ class TTreeOutputStream : public arrow::io::OutputStream
 
   bool closed() const override;
 
+  TBranch* CreateBranch(char const* branchName, char const* sizeBranch);
+
   TTree* GetTree()
   {
     return mTree;
   }
 
  private:
   TTree* mTree;
+  std::string mBranchPrefix;
 };
 
 } // namespace o2::framework

Framework/Core/src/RootArrowFilesystem.cxx

@@ -17,7 +17,6 @@
 #include <arrow/array/builder_nested.h>
 #include <arrow/array/builder_primitive.h>
 #include <memory>
-#include <stdexcept>
 #include <TFile.h>
 #include <TLeaf.h>
 #include <TBufferFile.h>
@@ -28,8 +27,11 @@
 #include <arrow/dataset/file_base.h>
 #include <arrow/result.h>
 #include <arrow/status.h>
+#include <arrow/util/key_value_metadata.h>
 #include <fmt/format.h>
 
+#include <stdexcept>
+#include <utility>
 
 O2_DECLARE_DYNAMIC_LOG(root_arrow_fs);
 
@@ -129,8 +131,15 @@ arrow::Result<std::shared_ptr<arrow::io::OutputStream>> TFileFileSystem::OpenOut
   const std::string& path,
   const std::shared_ptr<const arrow::KeyValueMetadata>& metadata)
 {
-  auto* t = new TTree(path.c_str(), "should put a name here");
-  auto stream = std::make_shared<TTreeOutputStream>(t);
+  if (path == "/") {
+    return std::make_shared<TDirectoryFileOutputStream>(this->GetFile());
+  }
+
+  auto* dir = dynamic_cast<TDirectoryFile*>(this->GetFile()->Get(path.c_str()));
+  if (!dir) {
+    throw runtime_error_f("Unable to open directory %s in file %s", path.c_str(), GetFile()->GetName());
+  }
+  auto stream = std::make_shared<TDirectoryFileOutputStream>(dir);
   return stream;
 }
 
@@ -286,13 +295,46 @@ arrow::Result<std::shared_ptr<arrow::dataset::FileFragment>> TTreeFileFormat::Ma
 }
 
 // An arrow outputstream which allows to write to a ttree
-TTreeOutputStream::TTreeOutputStream(TTree* t)
-  : mTree(t)
+TDirectoryFileOutputStream::TDirectoryFileOutputStream(TDirectoryFile* f)
+  : mDirectory(f)
+{
+}
+
+arrow::Status TDirectoryFileOutputStream::Close()
+{
+  mDirectory->GetFile()->Close();
+  return arrow::Status::OK();
+}
+
+arrow::Result<int64_t> TDirectoryFileOutputStream::Tell() const
+{
+  return arrow::Result<int64_t>(arrow::Status::NotImplemented("Cannot move"));
+}
+
+arrow::Status TDirectoryFileOutputStream::Write(const void* data, int64_t nbytes)
+{
+  return arrow::Status::NotImplemented("Cannot write raw bytes to a TTree");
+}
+
+bool TDirectoryFileOutputStream::closed() const
+{
+  return mDirectory->GetFile()->IsOpen() == false;
+}
+
+// An arrow outputstream which allows to write to a ttree
+// @a branch prefix is to be used to identify a set of branches which all belong to
+// the same table.
+TTreeOutputStream::TTreeOutputStream(TTree* f, std::string branchPrefix)
+  : mTree(f),
+    mBranchPrefix(std::move(branchPrefix))
 {
 }
 
 arrow::Status TTreeOutputStream::Close()
 {
+  if (mTree->GetCurrentFile() == nullptr) {
+    return arrow::Status::Invalid("Cannot close a tree not attached to a file");
+  }
   mTree->GetCurrentFile()->Close();
   return arrow::Status::OK();
 }
@@ -309,9 +351,18 @@ arrow::Status TTreeOutputStream::Write(const void* data, int64_t nbytes)
 
 bool TTreeOutputStream::closed() const
 {
+  // A standalone tree is never closed.
+  if (mTree->GetCurrentFile() == nullptr) {
+    return false;
+  }
   return mTree->GetCurrentFile()->IsOpen() == false;
 }
 
+TBranch* TTreeOutputStream::CreateBranch(char const* branchName, char const* sizeBranch)
+{
+  return mTree->Branch((mBranchPrefix + "/" + branchName).c_str(), (char*)nullptr, (mBranchPrefix + sizeBranch).c_str());
+}
+
 char const* rootSuffixFromArrow(arrow::Type::type id)
 {
   switch (id) {
@@ -411,8 +462,24 @@ class TTreeFileWriter : public arrow::dataset::FileWriter
     : FileWriter(schema, options, destination, destination_locator)
   {
     // Batches have the same number of entries for each column.
+    auto directoryStream = std::dynamic_pointer_cast<TDirectoryFileOutputStream>(destination_);
     auto treeStream = std::dynamic_pointer_cast<TTreeOutputStream>(destination_);
-    TTree* tree = treeStream->GetTree();
+
+    if (directoryStream.get()) {
+      TDirectoryFile* dir = directoryStream->GetDirectory();
+      dir->cd();
+      auto* tree = new TTree(destination_locator_.path.c_str(), "");
+      treeStream = std::make_shared<TTreeOutputStream>(tree, "");
+    } else if (treeStream.get()) {
+      // We already have a tree stream, let's derive a new one
+      // with the destination_locator_.path as prefix for the branches
+      // This way we can multiplex multiple tables in the same tree.
+      auto tree = treeStream->GetTree();
+      treeStream = std::make_shared<TTreeOutputStream>(tree, destination_locator_.path);
+    } else {
+      // I could simply set a prefix here to merge to an already existing tree.
+      throw std::runtime_error("Unsupported backend.");
+    }
 
     for (auto i = 0u; i < schema->fields().size(); ++i) {
       auto& field = schema->field(i);
@@ -427,23 +494,23 @@ class TTreeFileWriter : public arrow::dataset::FileWriter
           valueTypes.push_back(field->type()->field(0)->type());
           sizesBranches.push_back(nullptr);
           std::string leafList = fmt::format("{}[{}]{}", field->name(), listSizes.back(), rootSuffixFromArrow(valueTypes.back()->id()));
-          branches.push_back(tree->Branch(field->name().c_str(), (char*)nullptr, leafList.c_str()));
+          branches.push_back(treeStream->CreateBranch(field->name().c_str(), leafList.c_str()));
         } break;
         case arrow::Type::LIST: {
           valueTypes.push_back(field->type()->field(0)->type());
           listSizes.back() = 0; // VLA, we need to calculate it on the fly;
           std::string leafList = fmt::format("{}[{}_size]{}", field->name(), field->name(), rootSuffixFromArrow(valueTypes.back()->id()));
           std::string sizeLeafList = field->name() + "_size/I";
-          sizesBranches.push_back(tree->Branch((field->name() + "_size").c_str(), (char*)nullptr, sizeLeafList.c_str()));
-          branches.push_back(tree->Branch(field->name().c_str(), (char*)nullptr, leafList.c_str()));
+          sizesBranches.push_back(treeStream->CreateBranch((field->name() + "_size").c_str(), sizeLeafList.c_str()));
+          branches.push_back(treeStream->CreateBranch(field->name().c_str(), leafList.c_str()));
           // Notice that this could be replaced by a better guess of the
           // average size of the list elements, but this is not trivial.
         } break;
         default: {
           valueTypes.push_back(field->type());
           std::string leafList = field->name() + rootSuffixFromArrow(valueTypes.back()->id());
           sizesBranches.push_back(nullptr);
-          branches.push_back(tree->Branch(field->name().c_str(), (char*)nullptr, leafList.c_str()));
+          branches.push_back(treeStream->CreateBranch(field->name().c_str(), leafList.c_str()));
         } break;
       }
     }
@@ -463,11 +530,18 @@ class TTreeFileWriter : public arrow::dataset::FileWriter
     }
 
     // Batches have the same number of entries for each column.
+    auto directoryStream = std::dynamic_pointer_cast<TDirectoryFileOutputStream>(destination_);
+    TTree* tree = nullptr;
+    if (directoryStream.get()) {
+      TDirectoryFile* dir = directoryStream->GetDirectory();
+      tree = (TTree*)dir->Get(destination_locator_.path.c_str());
+    }
     auto treeStream = std::dynamic_pointer_cast<TTreeOutputStream>(destination_);
-    TTree* tree = treeStream->GetTree();
 
-    // Caches for the vectors of bools.
-    std::vector<std::shared_ptr<arrow::UInt8Array>> caches;
+    if (!tree) {
+      // I could simply set a prefix here to merge to an already existing tree.
+      throw std::runtime_error("Unsupported backend.");
+    }
 
     for (auto i = 0u; i < batch->columns().size(); ++i) {
       auto column = batch->column(i);
@@ -484,24 +558,12 @@ class TTreeFileWriter : public arrow::dataset::FileWriter
           auto list = std::static_pointer_cast<arrow::ListArray>(column);
           valueArrays.back() = list;
         } break;
-        default:
-          valueArrays.back() = column;
-      }
-    }
-
-    int64_t pos = 0;
-    while (pos < batch->num_rows()) {
-      for (size_t bi = 0; bi < branches.size(); ++bi) {
-        auto* branch = branches[bi];
-        auto* sizeBranch = sizesBranches[bi];
-        auto array = batch->column(bi);
-        auto& field = batch->schema()->field(bi);
-        auto& listSize = listSizes[bi];
-        auto valueType = valueTypes[bi];
-        auto valueArray = valueArrays[bi];
+        case arrow::Type::BOOL: {
+          // In case of arrays of booleans, we need to go back to their
+          // char based representation for ROOT to save them.
+          valueArrays.back() = std::make_shared<arrow::UInt8Array>();
+          auto boolArray = std::static_pointer_cast<arrow::BooleanArray>(column);
 
-        if (field->type()->id() == arrow::Type::BOOL) {
-          auto boolArray = std::static_pointer_cast<arrow::BooleanArray>(array);
           int64_t length = boolArray->length();
           arrow::UInt8Builder builder;
           auto ok = builder.Reserve(length);
@@ -516,11 +578,24 @@ class TTreeFileWriter : public arrow::dataset::FileWriter
               auto ok = builder.AppendNull();
             }
           }
-
-          ok = builder.Finish(&caches[bi]);
-          branch->SetAddress((void*)(caches[bi]->values()->data()));
-          continue;
+          ok = builder.Finish(&valueArrays.back());
         }
+        default:
+          valueArrays.back() = column;
+      }
+    }
+
+    int64_t pos = 0;
+    while (pos < batch->num_rows()) {
+      for (size_t bi = 0; bi < branches.size(); ++bi) {
+        auto* branch = branches[bi];
+        auto* sizeBranch = sizesBranches[bi];
+        auto array = batch->column(bi);
+        auto& field = batch->schema()->field(bi);
+        auto& listSize = listSizes[bi];
+        auto valueType = valueTypes[bi];
+        auto valueArray = valueArrays[bi];
+
         switch (field->type()->id()) {
           case arrow::Type::LIST: {
             auto list = std::static_pointer_cast<arrow::ListArray>(array);
@@ -769,8 +844,12 @@ arrow::Result<std::shared_ptr<arrow::io::OutputStream>> TTreeFileSystem::OpenOut
   const std::string& path,
   const std::shared_ptr<const arrow::KeyValueMetadata>& metadata)
 {
-  auto stream = std::make_shared<TTreeOutputStream>(GetTree({path, shared_from_this()}));
-  return stream;
+  arrow::dataset::FileSource source{path, shared_from_this()};
+  auto prefix = metadata->Get("branch_prefix");
+  if (prefix.ok()) {
+    return std::make_shared<TTreeOutputStream>(GetTree(source), *prefix);
+  }
+  return std::make_shared<TTreeOutputStream>(GetTree(source), "");
 }
 
 TBufferFileFS::TBufferFileFS(TBufferFile* f)