Segfault when I fail to declare variable in SatisfactionLink inside LambdaLink

[luketpeterson]

The following Scheme code crashes the process running OpenCog (guile in my case)

.. code-block:: scheme

(use-modules (opencog) (opencog exec))

(EvaluationLink
    (PredicateNode "_obj")
    (ListLink
        (ConceptNode "make")
        (ConceptNode "pottery")))

(DefineLink
    (DefinedPredicateNode "can_be_made?")
    (LambdaLink
        (Variable "test_object")
        (SatisfactionLink
            (AndLink
                (EvaluationLink
                    (PredicateNode "_obj")
                    (ListLink
                        (ConceptNode "make")
                        (Variable "predicate_object")
                    ) )        
                (EqualLink
                    (Variable "test_object")
                    (Variable "predicate_object")
                ) ) ) ) )

(cog-evaluate!
    (Evaluation
        (DefinedPredicate "can_be_made?")
        (ConceptNode "pottery")
    ) )

I can work around the issue by declaring (Variable "predicate_object") in the SatisfactionLink. Regardless of whether the undeclared variable is valid, I would expect an error and not a crash.

I don't know how I access the relevant OpenCog / Atomspace version. I am running the OpenCog Docker images, which are based on Ubuntu 18.04.4 LTS.

[linas]

I tried the above, and I don't get a crash, I get a different error (which should not occur)

ice-9/boot-9.scm:1669:16: In procedure raise-exception:
Throw to key `C++-EXCEPTION' with args `("cog-evaluate!" "Transient space memleak! (/home/linas/src/novamente/src/atomspace-git/opencog/atomspace/Transient.cc:89)\nFunction args:\n((EvaluationLink\n  (DefinedPredicateNode \"can_be_made?\")\n  (ConceptNode \"pottery\"))\n)")'.

[linas]

Digging just a bit deeper, it appears that there's some inf-loop in there, which, for me, is terminated after exceeding the max count ...

[linas]

Oh. Hah. The inf loop is due to the variables being untyped. So, somewhere during matching, it decides that it should explore the DefinedPredicateNode as a possible grounding. So it expands the definition. Then somehow its decides its runnable, so it tries to run it, which just causes it to loop.

At the moment, its not obvious to me why, exactly, it tried to loop. However, this is a generic problem of untyped variables: during pattern matching, the worst-case assumption is to try to match everything, which is both a performance hit, and often leads to obscure bugs like this.

The following works:

(DefineLink
    (DefinedPredicateNode "can_be_made?")
    (LambdaLink
        (Variable "test_object")   ;; Note, I left this untyped. That's fine. Its the return value.
        (SatisfactionLink
            ; Force the search to look only for ConceptNodes.
            (TypedVariable (Variable "predicate_object") (Type 'ConceptNode))
            (AndLink
                (EvaluationLink
                    (PredicateNode "_obj")
                    (ListLink
                        (ConceptNode "make")
                        (Variable "predicate_object")
                    ) )
                (EqualLink
                    (Variable "test_object")
                    (Variable "predicate_object")
                ) ) ) ) )

[linas]

Oh, ahh Hmm I have to eat some of my words. The following also works:

(DefineLink
    (DefinedPredicateNode "can_be_made?")
    (LambdaLink
        (Variable "test_object") 
        (SatisfactionLink
            (Variable "predicate_object") 
            (AndLink
                (EvaluationLink
                    (PredicateNode "_obj")
                    (ListLink
                        (ConceptNode "make")
                        (Variable "predicate_object")
                    ) )
                (EqualLink
                    (Variable "test_object")
                    (Variable "predicate_object")
                ) ) ) ) )

You are right -- the definition for the (Variable "predicate_object") should not have been needed, and the SatisfactionLink should have worked exactly the same way with and without this definition. So that's weird.