[ntuple] Fix attribute writing with RMiniFile

[silverweed]

WriteTFileNTupleKey was making 2 mistakes:

• incorrectly writing the TFile's epilogue even for Hidden clones
  (which led to files containing 1 extra StreamerInfo/FreeList/KeysList
  per attribute set);
• writing the wrong anchor location for attribute RNTuples.

This was not caught by tests because we never actually added tests for
attribute writing through RMiniFile.

[github-actions]

Test Results

    23 files      23 suites   3d 15h 26m 13s ⏱️
 3 864 tests  3 864 ✅ 0 💤 0 ❌
78 706 runs  78 706 ✅ 0 💤 0 ❌

Results for commit 36e6a23.

♻️ This comment has been updated with latest results.

[jblomer]

Thanks! Perhaps check or add a comment to prevent the use of the minifile with only hidden ntuples (I think we need a main one to produce a correct file).

[silverweed]

Perhaps check or add a comment to prevent the use of the minifile with only hidden ntuples (I think we need a main one to produce a correct file).

This should be already mandated by the current public API (you cannot Recreate a RMiniFileWriter which is hidden through public methods: only Append to one or Clone one); though it doesn't hurt to point that out (where is it best to do it? In the private ctor perhaps?)

[jblomer]

Perhaps check or add a comment to prevent the use of the minifile with only hidden ntuples (I think we need a main one to produce a correct file).

This should be already mandated by the current public API (you cannot Recreate a RMiniFileWriter which is hidden through public methods: only Append to one or Clone one); though it doesn't hurt to point that out (where is it best to do it? In the private ctor perhaps?)

Private constructor sounds good.