Saxon

Some further analysis showed that this might occur while compiling the code in Q{de/bottlecaps/railroad/xq/ast-to-svg.xq}render-vertical-pass2.

I was able to build a minimal repro based on that. It does not make any sense, but it preserves the NPE. It can be run from command line.

declare variable $center-choice := false();

declare function local:render-vertical-pass2($max-width, $todo)
{
	if ($todo) then
		let $extension-length := $max-width
		let $ext1 := if ($center-choice) then $extension-length else 42
		let $ext2 := $extension-length - $ext1
		return <line x2="{$ext1}"/>[$ext2 != 0]
	else
	    ()
};

()

I guess that the problem is related to the condition on $center-choice, which is declared as false() statically.

Thanks. I've reproduced this - it fails in HE only (or in EE with no license file).

The NPE arises when the downwards (depth-first) search of the expression tree looking for candidates for loop-lifting finds an expression whose parent pointer in the tree is wrong - specifically, is an expression that hasn't been encountered during the depth-first search. The expression in question is a variable reference to $max-width, and this appears to arise because the redundant variable $extension-length has been eliminated.

The compiled expression is

if ($todo) then
   (let $ext1 := if (false()) 
                      then ($max-width) 
                      else if (true()) then (42) 
   return (let $ext2 := (convertUntyped(atomizeSingleton($max-width))) - (convertUntyped(atomizeSingleton($ext1))) 
               return ((<line {FixedAttribute(string-join(convert(data($ext1)), " "))}/>)[$ext2 != 0])))

and the "bad" variable reference is the second one, inside the atomizeSingleton() call, which started life as a reference to $extension-length.

Both the variable references for $max-width are represented by the same LocalVariableReference object, which means that one of them has the wrong parent expression. This arises from a failure to copy the LocalVariableReference.

The offending code seems to be in FLWORExpression.optimize() line 520, where ExpressionTool.replaceVariableReferences() is called with mustCopy=false in the case where oneRef=true.

I'm now getting a failure "A license is needed to use custom serialization {http://saxon.sf.net/}indent-spaces". This is because I'm running Saxon-EE without a license, rather than running HE. EnterpriseConfiguration.makeXQueryExpression() should create an instance of XQueryExpression rather than XQueryExpressionEE under these circumstances. But the actual failure is that the Query command line, when preparing the "explain" output destination (at line 365) is testing whether edition="HE" rather than testing whether it is licensed.

Thanks for the fix - this works fine on Saxon-HE 10.2.