hadd writes files with unspecified compression algorithm

[eguiraud]

Describe the bug

The following sequence of commands shows how an output file f2.root produced via hadd has compression settings equal to 1, i.e. compression level is 1 but the compressoin algorithm is 0, which corresponds to RCompressionSetting::EAlgorithm::kUseGlobal. Unfortunately kUseGlobal does not correspond to a specific compression algorithm but it refers to the "compression setting for this process; may be affected by rootrc". In other words, a user that receives a file with compression algorithm 0 has no way to figure out what the compression algorithm actually is.

~ hadd f2.root f.root
hadd Target file: f2.root
hadd compression setting for all output: 1
hadd Source file 1: f.root
hadd Target path: f2.root:/
~ file f2.root
f2.root: ROOT file Version 62301 (Compression: 1)
~ root -l -b -q f2.root -e '_file0->GetCompressionSettings()'

Attaching file f2.root as _file0...
(TFile *) 0x55a618b47f40
(int) 1

Expected behavior

The last command above should print full compression settings with non-zero compression algorithm, e.g. 101.

To Reproduce

I think any input file can be used to reproduce the problem, but e.g. this snippet produces a file "f.root" that can be used for the reproducer above:

#include <TFile.h>
#include <TTree.h>
int main() {
   TFile f("f.root", "recreate");
   TTree t("t", "t");
   int x = 42;
   t.Branch("x", &x);
   t.Fill();
   t.Write();
   f.Close();
   return 0;
}

hadd f2.root f.root then produces a file f2.root with unspecified compression algorithm.

[eguiraud]

Still present in current master.