[TREEPROCMT] Add gtest with many input files

tree/treeplayer/test/CMakeLists.txt

@@ -2,3 +2,4 @@ ROOT_ADD_UNITTEST_DIR(TreePlayer)
 add_custom_command(TARGET treetreeplayertestUnit POST_BUILD
                    COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_CURRENT_SOURCE_DIR}/data.h data.h)
 
+ROOT_ADD_GTEST(treeprocessormt_manyfiles treeprocmt/treeprocessormt_manyfiles.cxx LIBRARIES TreePlayer)

tree/treeplayer/test/treeprocmt/treeprocessormt_manyfiles.cxx

+#include <atomic>
+#include <chrono>
+#include <random>
+#include <string>
+#include <thread>
+
+#include <TFile.h>
+#include <TTree.h>
+#include <TSystem.h>
+#include <TTreeReader.h>
+#include <TTreeReaderValue.h>
+#include <ROOT/TTreeProcessorMT.hxx>
+
+#include "gtest/gtest.h"
+
+void WriteFiles(const std::string &treename, const std::vector<std::string> &filenames)
+{
+   int v = 0;
+   for (const auto &f : filenames) {
+      TFile file(f.c_str(), "recreate");
+      TTree t(treename.c_str(), treename.c_str());
+      t.Branch("v", &v);
+      for (auto i = 0; i < 10; ++i) {
+         ++v;
+         t.Fill();
+      }
+      t.Write();
+   }
+}
+
+void DeleteFiles(const std::vector<std::string> &filenames)
+{
+   for (const auto &f : filenames)
+      gSystem->Unlink(f.c_str());
+}
+
+TEST(TreeProcessorMT, ManyFiles)
+{
+   const auto nFiles = 100u;
+   const std::string treename = "t";
+   std::vector<std::string> filenames;
+   for (auto i = 0u; i < nFiles; ++i)
+      filenames.emplace_back("treeprocmt_" + std::to_string(i) + ".root");
+
+   WriteFiles(treename, filenames);
+
+   std::atomic_int sum(0);
+   std::atomic_int count(0);
+   auto sumValues = [&sum, &count](TTreeReader &r) {
+      TTreeReaderValue<int> v(r, "v");
+      std::random_device seed;
+      std::default_random_engine eng(seed());
+      std::uniform_int_distribution<> rand(1, 100);
+      while (r.Next()) {
+         std::this_thread::sleep_for(std::chrono::nanoseconds(rand(eng)));
+         sum += *v;
+         ++count;
+      }
+   };
+
+   // TTreeProcMT requires a vector<string_view>
+   std::vector<std::string_view> fnames;
+   for (const auto &f : filenames)
+      fnames.emplace_back(f);
+
+   ROOT::TTreeProcessorMT proc(fnames, treename);
+   proc.Process(sumValues);
+   
+   EXPECT_EQ(count.load(), int(nFiles*10)); // 10 entries per file
+   EXPECT_EQ(sum.load(), 500500); // sum 1..nFiles*10
+
+   DeleteFiles(filenames);
+}