[JENA-471] Regression in Algebra.toQuadForm() for nested sub-query with GRAPH clause

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: Jena 2.10.1
• Fix Version/s: Jena 2.11.0
• Component/s: ARQ
• Assignee: Andy Seaborne
• Reporter: Rob Vesse
• Created: 14/Jun/13 22:06
• Updated: 17/Jun/13 18:54
• Resolved: 15/Jun/13 10:24

Description

Our internal testing has flagged up a regression in behaviour between 2.10.0 and 2.10.1 with regards to Algebra.toQuadForm() when applied to sub-queries nested within a GRAPH clause

The query in question is as follows:

SELECT ?x 
WHERE 
  { GRAPH ?g 
      { { SELECT ?x 
          WHERE 
            { ?x ?p ?g } 
        } 
      } 
  } 

With 2.10.0 this produced the following algebra:

(project (?x)
  (project (?x)
    (quadpattern (quad ?g ?x ?p ?g))))

With 2.10.1 this now produces an exception:

com.hp.hpl.jena.sparql.ARQInternalErrorException
	at com.hp.hpl.jena.sparql.algebra.OpVars$OpVarsPattern.visit(OpVars.java:197)
	at com.hp.hpl.jena.sparql.algebra.op.OpProject.visit(OpProject.java:48)
	at com.hp.hpl.jena.sparql.algebra.OpWalker$WalkerVisitor.visit1(OpWalker.java:86)
	at com.hp.hpl.jena.sparql.algebra.OpVisitorByType.visitModifer(OpVisitorByType.java:42)
	at com.hp.hpl.jena.sparql.algebra.OpVisitorByType.visit(OpVisitorByType.java:154)
	at com.hp.hpl.jena.sparql.algebra.op.OpProject.visit(OpProject.java:48)
	at com.hp.hpl.jena.sparql.algebra.OpWalker.walk(OpWalker.java:43)
	at com.hp.hpl.jena.sparql.algebra.OpWalker.walk(OpWalker.java:33)
	at com.hp.hpl.jena.sparql.algebra.OpVars.mentionedVars(OpVars.java:70)
	at com.hp.hpl.jena.sparql.algebra.OpVars.mentionedVars(OpVars.java:62)
	at com.hp.hpl.jena.sparql.algebra.AlgebraQuad$Pusher.visit(AlgebraQuad.java:93)
	at com.hp.hpl.jena.sparql.algebra.op.OpGraph.visit(OpGraph.java:46)
	at com.hp.hpl.jena.sparql.algebra.OpWalker$WalkerVisitor.before(OpWalker.java:64)
	at com.hp.hpl.jena.sparql.algebra.OpWalker$WalkerVisitor.visit1(OpWalker.java:84)
	at com.hp.hpl.jena.sparql.algebra.OpVisitorByType.visit(OpVisitorByType.java:110)
	at com.hp.hpl.jena.sparql.algebra.op.OpGraph.visit(OpGraph.java:46)
	at com.hp.hpl.jena.sparql.algebra.OpWalker$WalkerVisitor.visit1(OpWalker.java:85)
	at com.hp.hpl.jena.sparql.algebra.OpVisitorByType.visitModifer(OpVisitorByType.java:42)
	at com.hp.hpl.jena.sparql.algebra.OpVisitorByType.visit(OpVisitorByType.java:154)
	at com.hp.hpl.jena.sparql.algebra.op.OpProject.visit(OpProject.java:48)
	at com.hp.hpl.jena.sparql.algebra.OpWalker.walk(OpWalker.java:43)
	at com.hp.hpl.jena.sparql.algebra.OpWalker.walk(OpWalker.java:38)
	at com.hp.hpl.jena.sparql.algebra.Transformer.applyTransformation(Transformer.java:156)
	at com.hp.hpl.jena.sparql.algebra.Transformer.transformation(Transformer.java:149)
	at com.hp.hpl.jena.sparql.algebra.Transformer.transformation(Transformer.java:138)
	at com.hp.hpl.jena.sparql.algebra.Transformer.transformation(Transformer.java:132)
	at com.hp.hpl.jena.sparql.algebra.Transformer.transform(Transformer.java:57)
	at com.hp.hpl.jena.sparql.algebra.Transformer.transformSkipService(Transformer.java:87)
	at com.hp.hpl.jena.sparql.algebra.AlgebraQuad.quadize(AlgebraQuad.java:56)
	at com.hp.hpl.jena.sparql.algebra.Algebra.toQuadForm(Algebra.java:89)

I assume this regression happens because of the change to AlgebraQuad to use OpVars.mentionedVars() in the 2.10.1 dev cycle

While the 2.10.0 algebra may have actually been incorrect it didn't throw a stack trace and I assume it is possible to make a valid quad transform on this algebra.