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Date: Wed, 13 Oct 2010 23:42:48 +0200
From: Jilles Tjoelker <jilles@stack.nl>
To: obrien@freebsd.org, freebsd-current@freebsd.org
Message-ID: <20101013214247.GA36354@stack.nl>
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Subject: Re: [PATCH] fix shell bug in ${var%pattern} expansion
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On Mon, Oct 11, 2010 at 07:19:14PM -0700, David O'Brien wrote:
> At $WORK we hit a bug where ${var%/*} was not producing the correct
> expansion.  There are two patches to fix this.  I prefer the first
> as I feel it is cleaner from an API perspective.  I've also added
> a regression testcase that shows the problem.

> Thoughts?

Thank you, I think this is the mysterious bug I encountered a while ago
while making changes to arithmetic expansion, heavily using the stack
string. It only failed when /etc/rc.d/nfsserver was called from /etc/rc
on boot.

Your patch looks good (the first one) and works for me. The second patch
indeed seems a hack rather than fixing the problem properly.

Style bug:
> +growstrstackblock(int n) {
The opening brace should be on its own line.

Your test is too fragile: it often fails to detect the bug. Calling like
  sh -c '. expansion/trim4.0'
gives the correct output even with a buggy sh. I propose something like
this, or perhaps with an additional string comparison:

# $FreeBSD$

v1=/homes/SOME_USER
v2=
v3=C123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789

while [ ${#v2} -lt 2000 ]; do
	v4="${v2} ${v1%/*} $v3"
	if [ ${#v4} -ne $((${#v2} + ${#v3} + 8)) ]; then
		echo bad: ${#v4} -ne $((${#v2} + ${#v3} + 8))
	fi
	v2=x$v2
	v3=y$v3
done

-- 
Jilles Tjoelker