[io] Bypass TTree::ChangeFile when using TFileMerger

Fixes root-project#6640

Author: ferdymercury

io/io/inc/TFile.h

@@ -273,7 +273,11 @@ class TFile : public TDirectoryFile {
       kWriteError    = BIT(14),
       kBinaryFile    = BIT(15),
       kRedirected    = BIT(16),
-      kReproducible  = BIT(17)
+      kReproducible  = BIT(17),
+      // Set this bit to block an unwanted automatic call to TTree::ChangeFile
+      // e.g. during TFileMerger operations (bypassing any set MaxTreeSize)
+      // This is a transient bit and has no relevance for I/O.
+      kCannotChange  = BIT(18)
    };
    enum ERelativeTo { kBeg = 0, kCur = 1, kEnd = 2 };
    enum { kStartBigFile  = 2000000000 };

io/io/src/TFileMerger.cxx

@@ -978,6 +978,7 @@ Bool_t TFileMerger::PartialMerge(Int_t in_type)
    }
 
    fOutputFile->SetBit(kMustCleanup);
+   fOutputFile->SetBit(TFile::kCannotChange);
 
    TDirectory::TContext ctxt;
 
@@ -1039,6 +1040,7 @@ Bool_t TFileMerger::PartialMerge(Int_t in_type)
       Clear();
    } else {
       fOutputFile->ResetBit(kMustCleanup);
+      fOutputFile->ResetBit(TFile::kCannotChange);
       SafeDelete(fOutputFile);
    }
    return result;

io/io/test/TFileMergerTests.cxx

@@ -160,3 +160,65 @@ TEST(TFileMerger, MergeBranches)
    EXPECT_EQ(a0tree.FindBranch("a")->GetEntries(), 2);
    file3->Write();
 }
+
+
+#include <memory>
+#include <TFile.h>
+#include <TFileMerger.h>
+#include <TTree.h>
+
+// https://github.com/root-project/root/issues/6640
+TEST(TFileMerger, ChangeFile)
+{
+   {
+      TFile f{"file6640mergerinput.root", "RECREATE"};
+   
+      TTree t{"T", "SetMaxTreeSize(1000)", 99, &f};
+      int x;
+      auto nentries = 20000;
+   
+      t.Branch("x", &x, "x/I");
+   
+      // Call function to forcedly trigger TTree::ChangeFile.
+      // This will produce in total 3 files:
+      // * file6640mergerinput.root
+      // * file6640mergerinput_1.root
+      // * file6640mergerinput_2.root
+      TTree::SetMaxTreeSize(1000);
+   
+      for (auto i = 0; i < nentries; i++)
+      {
+        x = i;
+        t.Fill();
+      }
+   
+      // Write last file to disk
+      auto cf = t.GetCurrentFile();
+      cf->Write();
+      cf->Close();
+   }
+   {
+      TFileMerger filemerger{false, false};
+      filemerger.OutputFile(std::unique_ptr<TFile>{TFile::Open("file6640mergeroutput.root", "RECREATE")});
+
+      TFile fin{"file6640mergerinput.root", "READ"};
+      TFile fin1{"file6640mergerinput_1.root", "READ"};
+      TFile fin2{"file6640mergerinput_2.root", "READ"};
+
+      filemerger.AddAdoptFile(&fin);
+      filemerger.AddAdoptFile(&fin1);
+      filemerger.AddAdoptFile(&fin2);
+
+      // Before the fix, TTree::ChangeFile was called during Merge
+      // in the end deleting the TFileMerger's output file and leading to a crash.
+      filemerger.Merge();
+   }
+   {
+      TFile fout{"file6640mergeroutput.root", "READ"};
+      EXPECT_EQ(fout.Get<TTree>("T")->GetEntries(), 20000);
+   }
+   gSystem->Unlink("file6640mergerinput.root");
+   gSystem->Unlink("file6640mergerinput_1.root");
+   gSystem->Unlink("file6640mergerinput_2.root");
+   gSystem->Unlink("file6640mergeroutput.root");
+}

tree/tree/src/TTree.cxx

@@ -2742,6 +2742,10 @@ bool TTree::EnableCache()
 
 TFile* TTree::ChangeFile(TFile* file)
 {
+   // Changing file clashes with the design of TMemFile and derivates, see #6523.
+   // as well as with TFileMerger operations, see #6640
+   if ((dynamic_cast<TMemFile *>(file)) || file->TestBit(TFile::kCannotChange))
+      return file;
    file->cd();
    Write();
    Reset();
@@ -2787,7 +2791,7 @@ TFile* TTree::ChangeFile(TFile* file)
          break;
       }
       ++nus;
-      Warning("ChangeFile", "file %s already exist, trying with %d underscores", fname, nus+1);
+      Warning("ChangeFile", "file %s already exists, trying with %d underscores", fname, nus+1);
    }
    Int_t compress = file->GetCompressionSettings();
    TFile* newfile = TFile::Open(fname, "recreate", "chain files", compress);
@@ -4779,9 +4783,7 @@ Int_t TTree::Fill()
    if (fDirectory)
       if (TFile *file = fDirectory->GetFile())
          if (static_cast<TDirectory *>(file) == fDirectory && (file->GetEND() > fgMaxTreeSize))
-            // Changing file clashes with the design of TMemFile and derivates, see #6523.
-            if (!(dynamic_cast<TMemFile *>(file)))
-               ChangeFile(file);
+            ChangeFile(file);
 
    return nerror == 0 ? nbytes : -1;
 }