[df] Restore all attributes of std::cout after printing progress

tree/dataframe/src/RDFHelpers.cxx

@@ -199,16 +199,16 @@ std::pair<std::size_t, std::chrono::seconds> ProgressHelper::RecordEvtCountAndTi
 namespace {
 
 struct RestoreStreamState {
-   RestoreStreamState(std::ostream &stream) : fStream(stream), fFlags(stream.flags()), fFillChar(stream.fill()) {}
+   RestoreStreamState(std::ostream &stream) : fStream(stream) {
+      fPreservedState.copyfmt(stream);
+   }
    ~RestoreStreamState()
    {
-      fStream.flags(fFlags);
-      fStream.fill(fFillChar);
+      fStream.copyfmt(fPreservedState);
    }
 
    std::ostream &fStream;
-   std::ios_base::fmtflags fFlags;
-   std::ostream::char_type fFillChar;
+   std::ios fPreservedState{nullptr};
 };
 
 /// Format std::chrono::seconds as `1:30m`.

tree/dataframe/test/dataframe_helpers.cxx

@@ -9,6 +9,7 @@
 
 #include <algorithm>
 #include <deque>
+#include <iomanip>
 #include <vector>
 #include <string>
 
@@ -812,6 +813,48 @@ TEST(RDFHelpers, Cleanup_After_Exception)
       << "The Finalize method should have changed the value of testVal during the post-exception cleanup." << std::endl;
 }
 
+TEST(RDFHelpers, ProgressBarRestorePrecision)
+{
+   // Regression test for https://github.com/root-project/root/issues/21165
+   std::vector<double> values{13922869, 13599, 277, 40003186, 68.5966, 999.008, 40107.1};
+   // To ensure that all values above are printed with full precision
+   std::cout << std::setprecision(8);
+
+   struct StreamRAII{
+      std::streambuf* fOldStreamBuf;
+      std::ostringstream fStrBuf;
+      StreamRAII() : fOldStreamBuf(std::cout.rdbuf()) {
+         std::cout.rdbuf(fStrBuf.rdbuf());
+      }
+      ~StreamRAII() {
+         std::cout.rdbuf(fOldStreamBuf);
+      }
+   };
+
+   {
+      StreamRAII raii;
+      for (auto v : values)
+         std::cout << v << ", ";
+      EXPECT_EQ(raii.fStrBuf.str(), "13922869, 13599, 277, 40003186, 68.5966, 999.008, 40107.1, ");
+   }
+
+   {
+      // Just to run with the progress bar activated
+      StreamRAII raii;
+      ROOT::RDF::RNode d = ROOT::RDataFrame(1);
+      ROOT::RDF::Experimental::AddProgressBar(d);
+      d.Count().GetValue();
+   }
+
+   {
+      // Values after running with the progress bar should be printed the same way as before
+      StreamRAII raii;
+      for (auto v : values)
+         std::cout << v << ", ";
+      EXPECT_EQ(raii.fStrBuf.str(), "13922869, 13599, 277, 40003186, 68.5966, 999.008, 40107.1, ");
+   }
+}
+
 // The code below is a unit test for a function called `ProgressHelper_Existence_MT` in the `RDFHelpers` class.
 
 #ifdef R__USE_IMT