`UNDEF` in `VALUES` doesn't work with `SERVICE` about jena HOT 4 CLOSED

All SERVICE execution goes through an extension point ServiceExec.exec.

One collection of extensions, focued on federated query performance, are:
https://jena.apache.org/documentation/query/service_enhancer.html

You can add your own policy via ServiceExec.exec.

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See https://www.w3.org/TR/sparql11-federated-query/#variableService

Variables in the SERVICE clause are "best effort". <https://query.wikidata.org/sparql> makes sense but an unbound variable does indicate where to execute the SERVICE.

It has to be constrained somehow. What are you expecting? If you want it to ignore, then SERVICE SILENT may do what you want - it will generate a warning but otherwise skip the execution of that step.

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In my use-case, since the data in VALUES are retrieved from external process, there is no warranty that the variables would be bound eventually, i.e, having UNDEF as value. In this case yes, I expect the BGP under SERVICE to simply be ignored. That said, SERVICE SILENT is satisfactory enough, although its usage can cause important errors to get ignored, e.g. HTTP 300-500.

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Alright, many thanks for your prompt assistance ☺️

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