Remove empty directories from TFileMerger's output

core/base/inc/TDirectory.h

@@ -180,6 +180,7 @@ can be replaced with the simpler and exception safe:
    static  void        AddDirectory(Bool_t add=kTRUE);
    static  Bool_t      AddDirectoryStatus();
    virtual void        Append(TObject *obj, Bool_t replace = kFALSE);
+   /// Append object to this directory. \see Append(TObject*, Bool_t)
    virtual void        Add(TObject *obj, Bool_t replace = kFALSE) { Append(obj,replace); }
    virtual Int_t       AppendKey(TKey *) {return 0;}
            void        Browse(TBrowser *b) override;

io/io/inc/TFileMerger.h

@@ -109,6 +109,7 @@ class TFileMerger : public TObject {
    void        SetMergeOptions(const TString &options) { fMergeOptions = options; }
    void        SetMergeOptions(const std::string_view &options) { fMergeOptions = options; }
    void        SetIOFeatures(ROOT::TIOFeatures &features) { fIOFeatures = &features; }
+   /// Add object names for PartialMerge().
    void        AddObjectNames(const char *name) {fObjectNames += name; fObjectNames += " ";}
    const char *GetObjectNames() const {return fObjectNames.Data();}
    void        ClearObjectNames() {fObjectNames.Clear();}

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;
@@ -936,16 +943,17 @@ Bool_t TFileMerger::MergeRecursive(TDirectory *target, TList *sourcelist, Int_t
 /// the file "FileMerger.root" in the working directory. Returns true
 /// on success, false in case of error.
 /// The type is defined by the bit values in EPartialMergeType:
-///   kRegular        : normal merge, overwritting the output file
-///   kIncremental    : merge the input file with the content of the output file (if already exising) (default)
-///   kResetable      : merge only the objects with a MergeAfterReset member function.
-///   kNonResetable   : merge only the objects without a MergeAfterReset member function.
-///   kDelayWrite     : delay the TFile write (to reduce the number of write when reusing the file)
-///   kAll            : merge all type of objects (default)
-///   kAllIncremental : merge incrementally all type of objects.
-///   kOnlyListed     : merge only the objects specified in fObjectNames list
-///   kSkipListed     : skip objects specified in fObjectNames list
-///   kKeepCompression: keep compression level unchanged for each input
+///
+///     kRegular        : normal merge, overwriting the output file
+///     kIncremental    : merge the input file with the content of the output file (if already exising) (default)
+///     kResetable      : merge only the objects with a MergeAfterReset member function.
+///     kNonResetable   : merge only the objects without a MergeAfterReset member function.
+///     kDelayWrite     : delay the TFile write (to reduce the number of write when reusing the file)
+///     kAll            : merge all type of objects (default)
+///     kAllIncremental : merge incrementally all type of objects.
+///     kOnlyListed     : merge only the objects specified in fObjectNames list
+///     kSkipListed     : skip objects specified in fObjectNames list
+///     kKeepCompression: keep compression level unchanged for each input
 ///
 /// If the type is not set to kIncremental, the output file is deleted at the end of this operation.
 

io/io/test/TFileMergerTests.cxx

@@ -221,3 +221,61 @@ TEST(TFileMerger, ChangeFile)
    gSystem->Unlink("file6640mergerinput_2.root");
    gSystem->Unlink("file6640mergeroutput.root");
 }
+
+TEST(TFileMerger, SelectiveMergeWithDirectories)
+{
+   constexpr auto input1 = "selectiveMerge_input_1.root";
+   constexpr auto input2 = "selectiveMerge_input_2.root";
+   constexpr auto output = "selectiveMerge_output.root";
+   for (auto const &filename : {input1, input2}) {
+      TH1F histo("histo", "Histo", 2, 0, 1);
+      TFile infile(filename, "recreate");
+      auto dir = infile.mkdir("A");
+      dir->WriteObject(&histo, "Histo_A1");
+      if (filename == input1)
+         dir->WriteObject(&histo, "Histo_A2");
+      if (filename == input2)
+         dir->WriteObject(&histo, "Histo_A3");
+
+      dir = infile.mkdir("B");
+      dir->WriteObject(&histo, "Histo_B");
+
+      dir = infile.mkdir("C")->mkdir("D");
+      dir->WriteObject(&histo, "Histo_D");
+      dir = infile.mkdir("E")->mkdir("F");
+      dir->WriteObject(&histo, "Histo_F1");
+      if (filename == input1)
+         dir->WriteObject(&histo, "Histo_F2");
+      if (filename == input2)
+         dir->WriteObject(&histo, "Histo_F3");
+   }
+
+   {
+      TFileMerger fileMerger(false);
+      fileMerger.AddFile(input1);
+      fileMerger.AddFile(input2);
+      fileMerger.AddObjectNames("A");
+      fileMerger.AddObjectNames("Histo_F1 Histo_F2 Histo_F3");
+      fileMerger.OutputFile(output);
+      fileMerger.PartialMerge(TFileMerger::kOnlyListed | TFileMerger::kAll | TFileMerger::kRegular);
+   }
+
+   TFile outfile(output);
+   auto dir = outfile.Get<TDirectory>("A");
+   ASSERT_NE(dir, nullptr);
+   for (auto name : {"Histo_A1", "Histo_A2", "Histo_A3"})
+      EXPECT_NE(dir->Get<TH1F>(name), nullptr) << name;
+
+   EXPECT_EQ(outfile.Get("B"), nullptr);
+   EXPECT_EQ(outfile.Get("C"), nullptr);
+
+   dir = outfile.Get<TDirectory>("E");
+   ASSERT_NE(dir, nullptr);
+   dir = dir->Get<TDirectory>("F");
+   ASSERT_NE(dir, nullptr);
+   for (auto name : {"Histo_F1", "Histo_F2", "Histo_F3"})
+      EXPECT_NE(dir->Get<TH1F>(name), nullptr) << name;
+
+   for (auto name : {input1, input2, output})
+      gSystem->Unlink(name);
+}