GNU bug report logs - #13438
24.2.92; Infinite loop in python imenu support

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From: Fabián Ezequiel Gallina <fabian <at> anue.biz>
To: Glenn Morris <rgm <at> gnu.org>
Cc: 13438 <at> debbugs.gnu.org, Stefan Monnier <monnier <at> iro.umontreal.ca>
Subject: bug#13438: /srv/bzr/emacs/emacs-24 r111196: * imenu.el (imenu-default-create-index-function): Remove useless
Date: Fri, 25 Jan 2013 10:55:57 -0300

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On 01/25/2013 05:34 AM, Glenn Morris wrote:
>>> Why is this issue (apparently) only seen in Python mode
> Answering my own question, it happens in any mode that sets
> imenu-prev-index-position-function and
> imenu-extract-index-name-function, but there are very few of those.
>
> Eg prolog mode. With buffer contents:
>
> ------
> fac(0,1).
> fac(N,F) :- N > 0, M is N - 1,
>         fac(M,Fm), F is N * Fm.
> ------
>
> and point at point-min, switching to prolog-mode causes the same issue.
>
> How about taking the more cautious approach with:
>
> (when (and (= pos (point))
>             (not (bobp)))
>     (error "Infinite loop... ))
>
> (IIUC, this was actually the suggestion in
> http://lists.gnu.org/archive/html/emacs-devel/2013-01/msg00510.html ?)
I ran into the same thing, and checking bobp is not enough, try adding a 
newline at the beginning of file to the recipe and call imenu with point 
at beginning of defun and you'll see the infinite error check will trigger.

I proposed the following patch that works:

=== modified file 'lisp/imenu.el'
--- lisp/imenu.el       2013-01-01 09:11:05 +0000
+++ lisp/imenu.el       2013-01-22 18:24:45 +0000
@@ -683,7 +683,14 @@
           (goto-char (point-max))
           ;; Search for the function
           (while (funcall imenu-prev-index-position-function)
-             (when (= pos (point))
+             (when (and (= pos (point))
+                        (= pos
+                           (save-excursion
+                             ;; The infinite loop is true if there's
+                             ;; another index position but point keeps
+                             ;; itself in the same place. bug#13438
+                             (if (funcall imenu-prev-index-position-
function)
+                                 (point) 0))))

But finally after some mails with Stefan we decided the best thing will 
be to remove this check altogether.

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