GNU bug report logs - #6902
23.2; indent-line-to possible devide by zero error bug

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

From: Arik Mitschang <arik.mitschang <at> gmail.com>
To: bug-gnu-emacs <at> gnu.org
Subject: 23.2; indent-line-to possible devide by zero error bug
Date: Tue, 24 Aug 2010 11:27:47 -0400

In indent.el the indent-line-to function has a comparison that devides
the current column count by tab-width, whose value is an integerp and
allowed to be zero (and in my experience is often 0). I noticed the
issue in specific with org mode which utilizes this function for a lot
of interesting tasks.

I can provide the following patch which makes a check to avoid an
arithmetic error:

=== modified file 'lisp/indent.el'
--- lisp/indent.el	2010-05-19 03:06:48 +0000
+++ lisp/indent.el	2010-08-24 15:20:39 +0000
@@ -177,7 +177,9 @@
   (back-to-indentation)
   (let ((cur-col (current-column)))
     (cond ((< cur-col column)
-	   (if (>= (- column (* (/ cur-col tab-width) tab-width)) tab-width)
+	   (if (and (> tab-width 0)
+		    (>= (- column (* (/ cur-col tab-width) tab-width)) 
+			tab-width))
 	       (delete-region (point)
 			      (progn (skip-chars-backward " ") (point))))
 	   (indent-to column))

Thanks,
~Arik

Message #8 received at 6902 <at> debbugs.gnu.org (full text, mbox):

From: Glenn Morris <rgm <at> gnu.org>
To: Arik Mitschang <arik.mitschang <at> gmail.com>
Cc: 6902 <at> debbugs.gnu.org
Subject: Re: bug#6902: 23.2; indent-line-to possible devide by zero error bug
Date: Wed, 25 Aug 2010 15:32:35 -0400

Arik Mitschang wrote:

> In indent.el the indent-line-to function has a comparison that devides
> the current column count by tab-width, whose value is an integerp and
> allowed to be zero (and in my experience is often 0).

Why do you want to set tab-width to 0?

A quick grep for tab-width shows other places that will break.

Message #11 received at 6902 <at> debbugs.gnu.org (full text, mbox):

From: Arik Mitschang <arik.mitschang <at> gmail.com>
To: Glenn Morris <rgm <at> gnu.org>
Cc: Arik Mitschang <arik.mitschang <at> gmail.com>, 6902 <at> debbugs.gnu.org
Subject: Re: bug#6902: 23.2; indent-line-to possible devide by zero error bug
Date: Wed, 25 Aug 2010 16:04:45 -0400

Glenn Morris writes:
 > Arik Mitschang wrote:
 > 
 > > In indent.el the indent-line-to function has a comparison that devides
 > > the current column count by tab-width, whose value is an integerp and
 > > allowed to be zero (and in my experience is often 0).
 > 
 > Why do you want to set tab-width to 0?

Honestly I cannot remember, but I had it set there and I assume there
was a reason.

 > 
 > A quick grep for tab-width shows other places that will break.

The fact that it's value can be zero (without even warning in the doc
string) and this value causes functionality to break doesn't seem good
to me. If nobody but me ever had it set to zero, then perhaps just
updating its doc-string to warn users away from that value would be
nice.

Thanks,
~Arik

Message #14 received at 6902 <at> debbugs.gnu.org (full text, mbox):

From: Chong Yidong <cyd <at> stupidchicken.com>
To: Arik Mitschang <arik.mitschang <at> gmail.com>
Cc: Glenn Morris <rgm <at> gnu.org>, 6902 <at> debbugs.gnu.org
Subject: Re: bug#6902: 23.2; indent-line-to possible devide by zero error bug
Date: Sun, 10 Apr 2011 16:56:52 -0400

Arik Mitschang <arik.mitschang <at> gmail.com> writes:

>  > A quick grep for tab-width shows other places that will break.
>
> The fact that it's value can be zero (without even warning in the doc
> string) and this value causes functionality to break doesn't seem good
> to me. If nobody but me ever had it set to zero, then perhaps just
> updating its doc-string to warn users away from that value would be
> nice.

I have made such an update to the doc-string.  It doesn't seem
worthwhile to try to handle zero tab-widths in all the places where it
could be problematic.  Thanks.

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