Suppress empty directories from TFileMerger's partial merge.

Partial merging creates a full directory hierarchy in an output file,
even if the user didn't ask for the directories to be merged.

Fix #19330.

Author: hageboeck

io/io/src/TFileMerger.cxx

@@ -591,8 +591,15 @@ Bool_t TFileMerger::MergeOne(TDirectory *target, TList *sourcelist, Int_t type,
       // GetPath(), so we can still figure out where we are in the recursion
 
       // If this folder is a onlyListed object, merge everything inside.
-      if (onlyListed) type &= ~kOnlyListed;
-      status = MergeRecursive(newdir, sourcelist, type);
+      const auto mergeType = onlyListed ? type &= ~kOnlyListed : type;
+      status = MergeRecursive(newdir, sourcelist, mergeType);
+
+      if ((type & kOnlyListed) && !(type & kIncremental) && !onlyListed && newdir->GetNkeys() == 0) {
+         // None of the children were merged, and the directory is not listed
+         delete newdir;
+         newdir = nullptr;
+         target->rmdir(obj->GetName());
+      }
       // Delete newdir directory after having written it (merged)
       if (!(type&kIncremental)) delete newdir;
       if (onlyListed) type |= kOnlyListed;

io/io/test/TFileMergerTests.cxx

@@ -221,3 +221,35 @@ TEST(TFileMerger, ChangeFile)
    gSystem->Unlink("file6640mergerinput_2.root");
    gSystem->Unlink("file6640mergeroutput.root");
 }
+
+TEST(TFileMerger, SelectiveMergeWithDirectories)
+{
+   constexpr auto input = "selectiveMerge_input.root";
+   constexpr auto output = "selectiveMerge_output.root";
+   {
+      TH1F histo("histo", "Histo", 2, 0, 1);
+      TFile infile(input, "recreate");
+      auto dir = infile.mkdir("A");
+      dir->WriteObject(&histo, "Histo_A");
+      dir = infile.mkdir("B");
+      dir->WriteObject(&histo, "Histo_B");
+   }
+
+   {
+      TFileMerger fileMerger(false);
+      fileMerger.AddFile(input);
+      fileMerger.AddObjectNames("A");
+      fileMerger.OutputFile(output);
+      fileMerger.PartialMerge(TFileMerger::kOnlyListed | TFileMerger::kAll | TFileMerger::kRegular);
+   }
+
+   TFile outfile(output);
+   auto dir_A = dynamic_cast<TDirectory *>(outfile.Get("A"));
+   ASSERT_NE(dir_A, nullptr);
+   EXPECT_NE(dir_A->Get("Histo_A"), nullptr);
+
+   EXPECT_EQ(outfile.Get("B"), nullptr);
+
+   std::remove(input);
+   std::remove(output);
+}