[io] add test bed for streaming large objects

io/io/src/TBufferFile.cxx

@@ -323,8 +323,13 @@ void TBufferFile::SetByteCount(ULong64_t cntpos, Bool_t packInVersion)
         && (fBufCur >= fBuffer)
         && static_cast<ULong64_t>(fBufCur - fBuffer) <= std::numeric_limits<UInt_t>::max()
         && "Byte count position is after the end of the buffer");
-   const UInt_t cnt = UInt_t(fBufCur - fBuffer) - UInt_t(cntpos) - sizeof(UInt_t);
+
    char  *buf = (char *)(fBuffer + cntpos);
+   if ((fBufCur - fBuffer - cntpos - sizeof(UInt_t)) >= kMaxMapCount) {
+      tobuf(buf, 0 | kByteCountMask);
+      return;
+   }
+   const UInt_t cnt = UInt_t(fBufCur - fBuffer) - UInt_t(cntpos) - sizeof(UInt_t);
 
    // if true, pack byte count in two consecutive shorts, so it can
    // be read by ReadVersion()

tree/ntuple/test/CMakeLists.txt

@@ -105,6 +105,11 @@ ROOT_GENERATE_DICTIONARY(StreamerFieldDict ${CMAKE_CURRENT_SOURCE_DIR}/StreamerF
                          MODULE rfield_streamer LINKDEF StreamerFieldLinkDef.h OPTIONS -inlineInputHeader
                          DEPENDENCIES RIO)
 
+ROOT_ADD_GTEST(rfield_streamer_beyond rfield_streamer_beyond.cxx StreamerBeyond.cxx LIBRARIES ROOTNTuple)
+ROOT_GENERATE_DICTIONARY(StreamerBeyondDict ${CMAKE_CURRENT_SOURCE_DIR}/StreamerBeyond.hxx
+                         MODULE rfield_streamer_beyond LINKDEF StreamerBeyondLinkDef.h OPTIONS -inlineInputHeader
+                         DEPENDENCIES RIO)
+
 if(MSVC)
   set(command ${CMAKE_COMMAND} -E env "ROOTIGNOREPREFIX=1" $<TARGET_FILE:genreflex>)
 else()

tree/ntuple/test/StreamerBeyond.cxx

@@ -0,0 +1 @@
+#include "StreamerBeyond.hxx"

tree/ntuple/test/StreamerBeyond.hxx

@@ -0,0 +1,14 @@
+#ifndef ROOT_RNTuple_Test_StreamerBeyond
+#define ROOT_RNTuple_Test_StreamerBeyond
+
+#include <Rtypes.h>
+
+#include <cstdint>
+#include <vector>
+
+struct StreamerBeyond {
+   std::vector<std::int64_t> fOne;
+   std::vector<std::int64_t> fTwo;
+};
+
+#endif

tree/ntuple/test/StreamerBeyondLinkDef.h

@@ -0,0 +1,5 @@
+#ifdef __CLING__
+
+#pragma link C++ options=rntupleStreamerMode(true) struct StreamerBeyond+;
+
+#endif

tree/ntuple/test/rfield_streamer_beyond.cxx

@@ -0,0 +1,70 @@
+#include <ROOT/RFieldBase.hxx>
+#include <ROOT/RNTupleModel.hxx>
+#include <ROOT/RNTupleReader.hxx>
+#include <ROOT/RNTupleWriter.hxx>
+
+#include <cstdio>
+#include <string>
+#include <utility>
+
+#include "StreamerBeyond.hxx"
+#include "gtest/gtest.h"
+
+namespace {
+
+class FileRaii {
+private:
+   std::string fPath;
+   bool fPreserveFile = false;
+
+public:
+   explicit FileRaii(const std::string &path) : fPath(path) {}
+   FileRaii(FileRaii &&) = default;
+   FileRaii(const FileRaii &) = delete;
+   FileRaii &operator=(FileRaii &&) = default;
+   FileRaii &operator=(const FileRaii &) = delete;
+   ~FileRaii()
+   {
+      if (!fPreserveFile)
+         std::remove(fPath.c_str());
+   }
+   std::string GetPath() const { return fPath; }
+
+   // Useful if you want to keep a test file after the test has finished running
+   // for debugging purposes. Should only be used locally and never pushed.
+   void PreserveFile() { fPreserveFile = true; }
+};
+
+} // anonymous namespace
+
+TEST(RField, StreamerBeyond)
+{
+   FileRaii fileGuard("test_ntuple_rfield_streamer_beyond.root");
+
+   {
+      auto model = ROOT::RNTupleModel::Create();
+      auto f = ROOT::RFieldBase::Create("f", "StreamerBeyond").Unwrap();
+      EXPECT_TRUE(dynamic_cast<ROOT::RStreamerField *>(f.get()));
+      model->AddField(std::move(f));
+      auto writer = ROOT::RNTupleWriter::Recreate(std::move(model), "ntpl", fileGuard.GetPath());
+
+      auto ptr = writer->GetModel().GetDefaultEntry().GetPtr<StreamerBeyond>("f");
+      ptr->fOne = std::vector<std::int64_t>(100000000, -1);
+      ptr->fTwo = std::vector<std::int64_t>(100000000, -2);
+
+      writer->Fill();
+   }
+
+   auto reader = ROOT::RNTupleReader::Open("ntpl", fileGuard.GetPath());
+   ASSERT_EQ(1u, reader->GetNEntries());
+   StreamerBeyond sb;
+   auto view = reader->GetView("f", &sb, "StreamerBeyond");
+
+   view(0);
+
+   auto ptr = view.GetValue().GetPtr<StreamerBeyond>();
+   EXPECT_EQ(100000000u, ptr->fOne.size());
+   EXPECT_EQ(-1, ptr->fOne.at(1000));
+   EXPECT_EQ(100000000u, ptr->fTwo.size());
+   EXPECT_EQ(-2, ptr->fTwo.at(2000));
+}