rootcling --genreflex ignores <field... transient="true"/> in selection XML

[greenc-FNAL]

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Description

After following advice to transition from genreflex to rootcling --genreflex, we discovered that IO dictionaries generated for classes with transient (non-persistent) data members actually included stream information for those data members, in contrast both with the behavior of genreflex and our desire.

Reproducer

1. Download, unpack and cd into the top level directory of rootcling-genreflex-transient-bug.tgz
2. ./makeit good
3. ./makeit

The output of the final ROOT -b command should be the same down to the checksum reported if the problem were not present.

ROOT version

ROOT 6.28/04@30598a8430

Installation method

UPS

Operating system

SL7

Additional context

Compiled with GCC 13.1.0/C++20.

May be related to the (insufficiently detailed) #13361.

[greenc-FNAL]

Bug is reproduced on 4f0450b.

[pcanal]

The problem is solved by passing the option --reflex to rootcling.

[pcanal]

@vgvassilev It is a bit surprising that rootlcing --reflex ... selection.xml works but rootcling ... selection.xml partially works but silently fails. The current code will parse and use the selection.xml but a few internal part (of the selection.xml handling) are disabled without the --reflex. See rootcling_impl.cxx:406 for example.

[vgvassilev]

@vgvassilev It is a bit surprising that rootlcing --reflex ... selection.xml works but rootcling ... selection.xml partially works but silently fails. The current code will parse and use the selection.xml but a few internal part (of the selection.xml handling) are disabled without the --reflex. See rootcling_impl.cxx:406 for example.

Ah, yes, that makes sense. We have seen that on several occasions actually and that’s why we added this flag. Do you want to turn that logic on when we see selection.xml file? Is there a use case where we would want rootcling only behavior and still pass a selection.xml file?

[pcanal]

Is there a use case where we would want rootcling only behavior and still pass a selection.xml file?

So far I don't see it. Worst the behavior as-is is very surprising because part of the selection.xml is used (the part selection classes) and the other is ignored. (I suppose that if this behavior is useful it should be opt-in (eg --ignore-xml-attibutres-but-keep-the-selection rather than the default).

[greenc-FNAL]

I can confirm that replacing --genreflex with --reflex resolves my immediate issue. I would be in favor of removing the option in favor of activating the same behavior upon the presence of selection XML, however.