NullPointerException in generated parser

[SomMeri]

Generated parser throws NullPointerException on some syntactically incorrect inputs. Only some syntactically incorrect inputs cause an exception, others work fine. For example, attached grammar throws null pointer exception on p:not*() but not on p:not(().

The issue is reproducible on both ANTLR3.3 and ANTLR3.4.

The linked gist contains:

• Smallest ANTLR grammer I was able to create that has the issue.
• Unit test with two failing and two passing methods.
• Exception thrown by ANTLR v3.4.
• Exception thrown by ANTLR v3.3.

Gist: https://gist.github.com/2952908

[sharwell]

Here's the relevant block of generated code. In the failure cases (when the NPE is thrown), stream_c.hasNext()==true but stream_b.hasNext()==false.

while ( stream_c.hasNext() || stream_b.hasNext() ) {
    Object root_2 = (Object)adaptor.nil();
    root_2 = (Object)adaptor.becomeRoot(stream_b.nextNode(), root_2);
    adaptor.addChild(root_2, stream_c.nextTree());
    adaptor.addChild(root_1, root_2);
}

I'm actually not sure what the best way to handle this is. We would probably need to do one of the following:

1. Manufacture an error node to use in place of stream_b.nextNode() in the call to becomeRoot.
2. Change the condition to use && instead of || (this would result in the entire subtree not appearing in the AST).
3. Place the call to becomeRoot inside an if (stream_b.hasNext()). This would result in the tree returned by stream_c.nextTree() being added to the parent.