[ntuple] Add some Internal helpers needed for Attributes

[silverweed]

• const version of GetProjectedFieldsOfModel()
• RPageSink::GetUnderlyingDirectory()

These functionalities are needed in the RNTuple Attribute prototype (they are both Internal so there is no long-term implication of adding them).

Checklist:

• tested changes locally
• updated the docs (if necessary)

[hahnjo]

This breaks the abstraction of RPageSink quite heavily, since not all sinks are a file and even of those not all have a TDirectory. I'm not sure this makes sense to ask a TDirectory from all page sinks...

[hahnjo]

I guess the wanted functionality is to open a new page sink that ends up in the same storage? Maybe we can have an interface virtual std::unique_ptr<RPageSink> CreateAtSameLocation(std::string_view ntupleName) that throws if not implemented by a sink, or if the sink is unable to provide that functionality (simple RMiniFile, for the moment)?

[silverweed]

True, but for the prototype Attributes only work with TFile-based RNTuples and breaking the abstraction makes it much easier to deal with...at this point maybe the prototype should not be merged in master yet, until there is a more finalized design?
Either that or we merge it and later change it (making sure everything is in Internal/Experimental so we can do that easily)

[hahnjo]

For the prototype itself, I think we cannot merge until we are sure the on-disk format is right because it requires bumping (at least) the minor version. In the past, we did merge interface extensions to make work-in-progress prototypes easier (my work on distributed parallel writing, the RNTupleProcessor while it was / is still evolving), but I think we should still strive to get the interfaces right (my 2 cents at least).

[silverweed]

Good point on the binary format...we definitely cannot merge that until we're sure.

[github-actions]

Test Results

    18 files      18 suites   3d 17h 10m 29s ⏱️
 2 856 tests  2 856 ✅ 0 💤 0 ❌
49 966 runs  49 966 ✅ 0 💤 0 ❌

Results for commit 34a5563.