[ntuple] RNTupleReader::Show() produces empty output if user does not impose a model

[jalopezg-git]

• Checked for duplicates

Describe the bug

Calling RNTupleReader::Show() on an instance where the model is generated from ntuple metadata on storage yields an empty JSON output.
However, as can be seen below, calling LoadEntry() triggers model generation from the ntuple descriptor and the following Show() call produces the expected output. This results in surprising behaviour.

root [] auto ntuple = ROOT::Experimental::RNTupleReader::Open(“ntuple”, “/tmp/out.ntuple”);
root [] ntuple->Show(0);
{}
root [] ntuple->LoadEntry(0);
root [] ntuple->Show(0);
{
  “i”: 42,
  “f”: 12.2313
}

Expected behavior

If LoadEntry() generates the model from the ntuple descriptor, most probably Show() should also do it.

[enirolf]

Hi Javier, I opened a PR that addresses this issue: #12026!