[DF] Add test for Define+nested parallelism

tree/dataframe/test/CMakeLists.txt

@@ -18,6 +18,10 @@ ROOT_ADD_GTEST(dataframe_leaves dataframe_leaves.cxx LIBRARIES ROOTDataFrame)
 ROOT_ADD_GTEST(dataframe_vecops dataframe_vecops.cxx LIBRARIES ROOTDataFrame)
 ROOT_ADD_GTEST(dataframe_resptr dataframe_resptr.cxx LIBRARIES ROOTDataFrame)
 
+if (imt)
+   ROOT_ADD_GTEST(dataframe_concurrency dataframe_concurrency.cxx LIBRARIES ROOTDataFrame)
+endif()
+
 ROOT_ADD_GTEST(datasource_more datasource_more.cxx LIBRARIES ROOTDataFrame)
 #ROOT_ADD_GTEST(datasource_root datasource_root.cxx LIBRARIES ROOTDataFrame)
 ROOT_ADD_GTEST(datasource_trivial datasource_trivial.cxx LIBRARIES ROOTDataFrame)

tree/dataframe/test/dataframe_concurrency.cxx

+#include <ROOT/RDataFrame.hxx>
+#include <ROOT/TThreadExecutor.hxx>
+#include <atomic>
+#include <chrono>
+#include <thread>
+
+#include "gtest/gtest.h"
+
+#ifdef R__USE_IMT
+TEST(RDFConcurrency, NestedParallelismBetweenDefineCalls)
+{
+   ROOT::EnableImplicitMT();
+
+   // this Define will return unique values from 0 to nEntries - 1 (over all threads)
+   const auto nEntries = 100000u;
+   std::atomic_int i(0);
+   auto df = ROOT::RDataFrame(nEntries).Define("i", [&] { return i++; });
+
+   // this lambda will be used to introduce nested parallelism via a dummy Filter
+   auto manysleeps = [&] {
+      ROOT::TThreadExecutor().Foreach(
+         [] { std::this_thread::sleep_for(std::chrono::milliseconds(std::rand() / RAND_MAX * 200)); }, 8);
+      return true;
+   };
+
+   // first Take triggers the computation of i, then the Filter executes, then Take accesses the cached value of i
+   // hopefully it won't have changed in the meanwhile!
+   auto res1 = df.Take<int>("i");
+   auto res2 = df.Filter(manysleeps).Take<int>("i");
+
+   // check both Takes return vectors with all numbers from 0 to nEntries - 1
+   auto checkAllNumbersAreThere = [](std::vector<int> &vec) {
+      std::sort(vec.begin(), vec.end());
+      const int s = vec.size();
+      for (auto i = 0; i < s; ++i)
+         EXPECT_EQ(vec[i], i);
+   };
+
+   checkAllNumbersAreThere(*res1);
+   checkAllNumbersAreThere(*res2);
+
+   ROOT::DisableImplicitMT();
+}
+#endif