GNU bug report logs - #63196
sxml or guile bug?

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Christopher Lam <christopher.lck <at> gmail.com>
To: guile-user <guile-user <at> gnu.org>, bug-guile <at> gnu.org
Subject: sxml or guile bug?
Date: Mon, 1 May 2023 10:32:26 +0800

[Message part 1 (text/plain, inline)]

Hi, we've identified an unusual behaviour in writing a test in gnucash. Can
anyone reproduce and confirmit?

I paste a simple .scm file, and its input test-file.html. Now, in
sxml-bug.scm, modify the <tr> index - from -1 to -2 i.e. access the
last/penultimate row etc.

Odd negative indices work, even negative indices lead to error. Odd (haha)
bug. See report for (tr -2) for example, pasted below.

==== sxml-bug.scm =====
(use-modules (sxml simple))
(use-modules (sxml xpath))
(use-modules (ice-9 textual-ports))
(define html (call-with-input-file "/tmp/test-file.html" get-string-all))
(define sx (xml->sxml html))

;; note this path: try different indices for the tr tag:
;; odd numbers -1 -3 -5 are ok
;; even numbers -2 -4 -6 will fail
(define path '(// (table 1) // (tr -1)))

(format #t "html=[~s]\n" html)
(format #t "sxpath is [~s]\n" path)
(format #t "result is [~s]\n" ((sxpath path) sx))
====

And the backtrace with -2

=====
Backtrace:
In ice-9/boot-9.scm:
  1752:10 18 (with-exception-handler _ _ #:unwind? _ # _)
In unknown file:
          17 (apply-smob/0 #<thunk 7f7fd530d2e0>)
In ice-9/boot-9.scm:
    724:2 16 (call-with-prompt _ _ #<procedure default-prompt-handle…>)
In ice-9/eval.scm:
    619:8 15 (_ #(#(#<directory (guile-user) 7f7fd5312c80>)))
In ice-9/boot-9.scm:
   2836:4 14 (save-module-excursion _)
  4388:12 13 (_)
In /home/user/sources/sxml-bug.scm:
    20:30 12 (_)
In sxml/xpath.scm:
   254:18 11 (map-union #<procedure 7f7fd18e8280 at sxml/xpath.scm:…> …)
   254:18 10 (map-union #<procedure 7f7fd18e8280 at sxml/xpath.scm:…> …)
   254:18  9 (map-union #<procedure 7f7fd18e8280 at sxml/xpath.scm:…> …)
   254:18  8 (map-union #<procedure 7f7fd18e8280 at sxml/xpath.scm:…> …)
   254:18  7 (map-union #<procedure 7f7fd18e8280 at sxml/xpath.scm:…> …)
   252:22  6 (map-union #<procedure 7f7fd18e8280 at sxml/xpath.scm:…> …)
   252:22  5 (map-union #<procedure 7f7fd18f2ea0 at sxml/xpath.scm:…> …)
   254:18  4 (map-union #<procedure 7f7fd1901420 at sxml/xpath.scm:…> …)
   254:18  3 (map-union #<procedure 7f7fd1901420 at sxml/xpath.scm:…> …)
   252:22  2 (map-union #<procedure 7f7fd1901420 at sxml/xpath.scm:…> …)
   484:19  1 (loop _ ((tr -6)))
   181:24  0 (_ _)

sxml/xpath.scm:181:24: yikes!
=====

[Message part 2 (text/html, inline)]

[test-file.html (text/html, attachment)]

Message #8 received at 63196 <at> debbugs.gnu.org (full text, mbox):

From: Christopher Lam <christopher.lck <at> gmail.com>
To: 63196 <at> debbugs.gnu.org
Subject: Further on this error message
Date: Mon, 1 May 2023 10:44:18 +0800

[Message part 1 (text/plain, inline)]

The source seems to be sxpath.scm -- see "yikes" error which triggers when
n is -2 -4 -6 etc. I don't know how to build guile from sources and cannot
debug further.


(define (node-pos n)
  (lambda (nodeset)
    (cond
     ((not (nodeset? nodeset)) '())
     ((null? nodeset) nodeset)
     ((eqv? n 1) (list (car nodeset)))
     ((negative? n) ((node-pos (+ n 1 (length nodeset))) nodeset))
     (else
      (or (positive? n) (error "yikes!"))
      ((node-pos (1- n)) (cdr nodeset))))))

[Message part 2 (text/html, inline)]

Message #11 received at 63196 <at> debbugs.gnu.org (full text, mbox):

From: <tomas <at> tuxteam.de>
To: Christopher Lam <christopher.lck <at> gmail.com>
Cc: 63196 <at> debbugs.gnu.org
Subject: Re: bug#63196: Further on this error message
Date: Sat, 6 May 2023 14:38:06 +0200

[Message part 1 (text/plain, inline)]

On Mon, May 01, 2023 at 10:44:18AM +0800, Christopher Lam wrote:
> The source seems to be sxpath.scm -- see "yikes" error which triggers when
> n is -2 -4 -6 etc. I don't know how to build guile from sources and cannot
> debug further.
> 
> 
> (define (node-pos n)
>   (lambda (nodeset)
>     (cond
>      ((not (nodeset? nodeset)) '())
>      ((null? nodeset) nodeset)
>      ((eqv? n 1) (list (car nodeset)))
>      ((negative? n) ((node-pos (+ n 1 (length nodeset))) nodeset))
>      (else
>       (or (positive? n) (error "yikes!"))
>       ((node-pos (1- n)) (cdr nodeset))))))

Hm. It seems that calling `node-pos' with n == 0 runs straight into the
"yikes" case (unless `nodeset' is #f or null, that is).

And we would get this when n ==-2 and (length nodeset) equals 1 on the
next recursive call. Or when n == -3 and (length nodeset) == 2, and so
on -- i.e. when n == -1 - (length nodeset).

We reach such a point again when n == 2*(-1 - (length nodeset)): it runs
into the case (negative? n), retries with n' = (+ n 1 (length nodeset))
which is still negative, next try is 0 => yikes.

So in your case I guess your (length nodeset) is 1, because you have
a cycle length of two :-)

Reading between the lines in the code above for the positive case, I
gather that the intention is to return an empty nodeset (i.e. '())
when n runs off the nodeset. So I'd extend the ((negative? n) ...)
case like so:

  ((negative? n)
    (let ((nn (+ 1 (length nodeset))))
      (if (positive? nn) (node-pos nn) '())))

Comments?

Cheers
-- 
t

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