Mike Beckerle created DAFFODIL-2324:
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Summary: Unparser choice branch selection not conformant to DFDL
standard
Key: DAFFODIL-2324
URL: https://issues.apache.org/jira/browse/DAFFODIL-2324
Project: Daffodil
Issue Type: Bug
Components: Back End
Affects Versions: 2.5.0
Reporter: Mike Beckerle
Fix For: 3.0.0
Erratum 5.60 of the DFDL spec clarifies how choice branch selection works.
Our current schemas that use presence bits flags use a structure like this:
{code:java}
<choice>
<sequence>
<sequence dfdl:hiddenGroupRef="PI_true"/>
<sequence>
<element name="foo" minOccurs="0" ...../>
<element name="bar" minOccurs="0" ..../>
</sequence>
<sequence dfdl:hiddenGroupRef="PI_false"/>
</choice>{code}
We depend on the fact that by default this chooses the PI_false hidden group
when the incoming infoset event is not for a foo nor bar element.
The DFDL spec says (with the Erratum/Clarification) that if we want PI_false to
be chosen, then we must flip the order of these two choice branches.
Daffodil is currently preferring the "more empty" of the two branches, and so
seleting PI_false, rather than just taking the first possibly empty branch.
Once we fix this behavior to conform to the spec., we will have to update DFDL
schemas that use this behavior, and actually flip these branches.
Based on that, we may want to put a compatibility flag into daffodil. Or
perhaps a feature to generate a warning whenever there is more than one empty
choice branch that *could* be selected, if the current functionality selects a
default branch that is not the first possibly empty branch thena warning would
be issued indicating that the branches should be reordered.
The only schema I know of that is sensitive to this bug is mil-std-6017/VMF
(and variants thereof).
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