[JENA-908] Turtle output can be empty when graph contains cyclic blank node references

• Type: Bug
• Status: Closed
• Priority: Critical
• Resolution: Fixed
• Affects Version/s: Jena 2.13.0
• Fix Version/s: Jena 2.13.1
• Component/s: RIOT
• Labels: BNodes BlankNodes Turtle
• Assignee: Andy Seaborne
• Reporter: Rob Vesse
• Created: 31/Mar/15 13:42
• Updated: 29/Jul/15 14:52
• Resolved: 01/Apr/15 10:50

Description

Discovered a bug in writing Turtle with the latest release, haven't checked whether it applies to past versions of ARQ.

Simple test case using test data from the DAWG tests:

    @Test
    public void bnode_cycles() {
        Model m = RDFDataMgr.loadModel("testing/DAWG-Final/construct/data-ident.ttl");
        Assert.assertTrue(m.size() > 0);
        
        ByteArrayOutputStream output = new ByteArrayOutputStream();
        RDFDataMgr.write(output, m, Lang.TURTLE);
        
        ByteArrayInputStream input = new ByteArrayInputStream(output.toByteArray());
        System.out.println(new String(output.toByteArray(), Charset.forName("utf-8")));
        Model m2 = ModelFactory.createDefaultModel();
        RDFDataMgr.read(m2, input, Lang.TURTLE);
        Assert.assertTrue(m2.size() > 0);
        
        Assert.assertTrue(m.isIsomorphicWith(m2));
    }

Debugging this I can see that the problem appears to lie in TurtleShell which refuses to write any triples because each subject appears as the object of some other triple. This creates a cycle resulting in no data being output.

TurtleShell needs to detect and arbitrarily break the cycles such that it can output the data successfully.

I'll commit this as a disabled test case in TestLangTurtle to make it easy to start debugging.