bug#23404: Infinite recursion in GOOPS in Guile 2.2

[Christopher Allan Webber <cwebber@dustycloud.org>]

Heya all,

So I've been building something with GOOPS, and I decided to try it out
with Guile 2.2.  Unfortunately, I hit a pretty nasty bug.  You can try
it yourself... the code is pretty short:

  (use-modules (oop goops)
               (srfi srfi-9))
  (define-class <some-class> ())
  (define-method (time-to-break (some-class <some-class>) not-a-class)
    (display "We're never going home are we?\n"))

  ;; And now, call it
  (time-to-break (make <some-class>) 1)

This will loop forever and *never* complete.  Why?

Turns out this is why, from goops.scm:

  (define (single-arity-cache-dispatch cache nargs cache-miss)
    (match cache
      (() cache-miss)
      (((typev . cmethod) . cache)
       (cond
        ((eqv? nargs (vector-length typev))
         (let ((cache-miss (single-arity-cache-dispatch cache nargs cache-miss)))

I'm not sure what was supposed to happen (presumably the cache was
supposed to be narrowed down somehow), but what happens here is that
single-arity-cache-dispatch keeps calling single-arity-cache-dispatch
forever.

scheme@(guile-user) [1]> ,bt
In oop/goops.scm:
   1437:41939 (cache-miss #<<hive> e8a1e0> #<<message> id: "5c787e65bbeb34b27ffd16eead286fc6:1" to: ("8394701db8…>)
   1486:41938 (memoize-effective-method! _ _ _)
  1466:131937 (recompute-generic-function-dispatch-procedure! #<<generic> hive-process-message (1)>)
  1452:161936 (compute-generic-function-dispatch-procedure _)
  1378:251935 (single-arity-cache-dispatch _ 2 _)
  1378:251934 (single-arity-cache-dispatch _ 2 _)
  1378:251933 (single-arity-cache-dispatch _ 2 _)
  1378:251932 (single-arity-cache-dispatch _ 2 _)
  1378:251931 (single-arity-cache-dispatch _ 2 _)
  1378:251930 (single-arity-cache-dispatch _ 2 _)
  1378:251929 (single-arity-cache-dispatch _ 2 _)
  1378:251928 (single-arity-cache-dispatch _ 2 _)
  1378:251927 (single-arity-cache-dispatch _ 2 _)
  1378:251926 (single-arity-cache-dispatch _ 2 _)
  1378:251925 (single-arity-cache-dispatch _ 2 _)
  1378:251924 (single-arity-cache-dispatch _ 2 _)
  ... etc ...

(g?)Oops!

 - Chris