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Date: Wed, 30 Nov 2011 17:12:41 -0600 (CST)
From: Tim McDaniel <tmcd AT panix DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: ash is wrong about [ -w Temp ], so rebaseall fails
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On Wed, 30 Nov 2011, Eric Blake wrote:
> On 11/30/2011 03:17 PM, Tim McDaniel wrote:
>> $ /bin/ash -c ' [ -w /Users/tmcdaniel/AppData/Local/Temp ] && echo yes
>> || echo no'
>> no
>>
>> So bash and ash disagree on whether this Temp directory is writable.
>
> Known limitation in dash - it is going off of just st_mode bits instead
> of using faccessat() and honoring ACLs.

I think that's the case.

> I've been meaning to do a new build of dash (aka ash), and to force
> the use of faccessat as part of that build; I just haven't had the
> time to get to it yet.

In the meantime, though, rebaseall as distributed did not work for me.

There's a general principle that, if you want to find out whether you
can do an operation, you should not check permissions bits, but
instead just try to do what you want to do and check for errors -- for
this very reason, that your test may not be accurate, and also because
if it's so critical an operation, you should be checking for errors
anyway when you do it.

So I suggest that

# Validate temp directory
if [ ! -d "$TmpDir" ]
then
     echo "$ProgramName: '$TmpDir' is not a directory"
     exit 2
fi
if [ ! -w "$TmpDir" ]
then
     echo "$ProgramName: '$TmpDir' is not writable"
     exit 2
fi

be removed, and that the check instead be done just before writing it
for real:

# Create rebase list
# This creates an empty file if you have permission to do so,
# and outputs an error message if not.
if ! > "$TmpFile"; then
     echo "$ProgramName: cannot write to temporary file in '$TmpDir'" 1>&2
     exit 2
fi
case $Platform in
...

I've tested this proposal briefly and it appears to work, though I
can't really try it for real, because I'd have to close this window.

Even better might be to check every command that tries to > or >> on
TmpFile.

-- 
Tim McDaniel

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