Re: [PATCH] md5sum, sha*sum: only escape file names containing newlines

[Eric Blake]

On 11/01/2013 11:03 AM, Pádraig Brady wrote:

>>
>> Escape the output (marking with a leading '\' and backslash-escaping
>> both '\' and '\n') only when the file name contains a newline.
>> Before, we would do that for a file name containing either newline or 
>> backslash.
>>
>> This probably deserves a NEWS entry, since it is user-visible.
> 
> I debated that as I thought it could have no impact on anything,
> but it could actually if one was comparing old and new outputs?
> 
> newsum=$(md5sum my file set | md5sum)
> [ "$newsum" = "$(cat ./oldsum)" ] || error

Not just that, but the new format is not necessarily parseable by older
md*sum.  Your patch didn't show (but probably should be enhanced) what
happens for a file named 'a\nb'; pre-patch, it gave '\sum  a\\nb',
post-patch it gives 'sum  a\nb' - but if the older utility assumes that
the missing leading \ was a mistake and unescapes the file name, it
results in looking for a file as 3 three-byte name "a<newline>b", which
is also part of the user-visible change.

Breaking output so that older versions can't parse newer output has been
one of the reasons that I have only threatened to patch \r handling,
rather than actually doing it, because it's tricky to think about
old/new interactions and what might break.  Depending on how
conservative we are trying to be, we may need to add a command line
option that will let the user forcefully revert to the older-style
output for intentional interaction with older checksum tools regardless
of filename.  For 99% of the cases, the output is identical, since files
with \n or \\ in the name are already rare.  Thinking aloud, it may be
appropriate to have such a mode option be tri-state (old, new, or warn;
with default being warn), where the warning mode gives the new output
but ALSO flags to the user that their output may not be parseable by
older summing utilities.


-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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