Saxon xquery validation fails - help needed to understand behavior

Question

Whilst i am using Saxon java api to execute the following xQuery, i am unable to understand why the following execution / validation fails ? (Interestingly , when i replace and clause with or in the if statement, the query validation is successful but am unable to understand this behaviour )

In oxygen xml validator, when i open the xQuery, i get NullPointerException-null exception and the validation fails.

And in java , i get the following area

java.lang.NullPointerException
    at net.sf.saxon.expr.parser.LoopLifter.markDependencies(LoopLifter.java:168)
    at net.sf.saxon.expr.parser.LoopLifter.gatherInfo(LoopLifter.java:112)

I am looking up for some experts to help me understand why the following fails.

Following is the xQuery,

 
    declare function local:method($input as element(input)) as element(output) 
        {
                <output>
                    <itemADetails>              
                        <service>
                        {
                        for $i in 1 to count($input/*:foo)
                        return
                            for $j in 1 to count($input/*:bar)
                            return
                                if((data($input/*:foo[$i]/*:itemB[1]/*:rangeQualifier)="A") and (data($input/*:foo[$i]/*:serviceId/*:type)="B") ) then
                                    <node></node>
                                else()          
                        }
                        </service>                                          
                    </itemADetails>                                         
                </output>
        };


    declare variable $input as element(input) external;
    local:method($input)

Saxon Verion

implementation 'net.sf.saxon:Saxon-HE:10.2'
implementation 'net.sf.saxon:saxon-xqj:9.1.0.8'

Sample Snippet that i tried

 Processor saxon = new Processor(false);
         
        // compile the query
        XQueryCompiler compiler = saxon.newXQueryCompiler();
        XQueryExecutable exec;
       
        ClassLoader classLoader = MessageProcessor.class.getClassLoader();
       
            exec = compiler.compile(new File(classLoader.getResource("Xquery.xqy").getFile()));

Source src = new StreamSource(new StringReader(Files.readString( Paths.get(ClassLoader.getSystemResource(inputfile.xml").toURI()))));
        XdmNode doc = builder.build(src);
 // instantiate the query, bind the input and evaluate
        XQueryEvaluator query = exec.load();
        query.setContextItem(doc);     
 query.setExternalVariable(new QName("input"), doc.select(child("input")).asNode());
   XdmValue result = query.evaluate();
       System.out.println(result.itemAt(0).toString());

Irrespective of the java code when i open the xquery in Oxygen XML editor (that uses Saxon-PE XQuery 9.9.1.7 engine) , i get the following validation error.

It always helps if you mention the exact version and edition of the software (e.g. Saxon) you use when you hit the problem. As you say you write Java code to use Saxon a minimal repro of that code would also be helpful. — Martin Honnen
Have updated my question with further details @MartinHonnen ! — Madhan
In the Java code, do you get the error on the compiler.compile(new File(classLoader.getResource("Xquery.xqy").getFile())); call or later on the query.evaluate() call? In the latter case, can you also include a minimal XML sample? — Martin Honnen
And please clarify, the code snippet you have shown uses or, the screenshot and? Does the problem occur with both? — Martin Honnen

Michael Kay Michael Kay · Accepted Answer · 2020-09-29T14:48:58

It is indeed a Saxon optimisation bug. The problem occurs when Saxon attempts to rewrite

for $j in 1 to count($input/bar)
return if ($input/foo[$i]/data = 'A' and $input/baz[$i]/type = 'B')
       then <result/>
       else ()

as

if ($input/foo[$i]/data = 'A' and $input/baz[$i]/type = 'B')
then for $j in 1 to count($input/bar)
     return <result/>
else ()

and you can work around the problem by doing this rewrite "by hand". The purpose of the rewrite is to prevent the unnecessary repeated evaluation of the "if" condition, which is the same each time round the loop.

The reason it's dependent on the "and" condition is that Saxon considers each term of the "and" as a separate candidate for promoting outside the loop, and when it finds that all these terms can be promoted, it reconstitutes the "and" expression from its parts, and the bug occurs during this reconstitution.

Saxon xquery validation fails - help needed to understand behavior

Saxon Verion

3 Answers