GNU bug report logs - #31335
unexpected cp -f behavior

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Message #22 received at 31335 <at> debbugs.gnu.org (full text, mbox):

From: Pádraig Brady <P <at> draigBrady.com>
To: Paul Eggert <eggert <at> cs.ucla.edu>, 31335 <at> debbugs.gnu.org, erjoalgo <at> gmail.com
Subject: Re: bug#31335: Fwd: bug#31335: unexpected cp -f behavior
Date: Wed, 16 May 2018 00:03:00 -0700

[Message part 1 (text/plain, inline)]

On 15/05/18 17:11, Paul Eggert wrote:
> On 05/15/2018 10:05 AM, Pádraig Brady wrote:
>>> It's also what I would expect, if I use -f, I expect cp will do everything
>>> it can to force the operation and succeed if all possible.
>> Maybe, though that's worth further consideration.
> 
> POSIX is arguably ambiguous about what 'cp -f S D' should do if D is a 
> symlink to itself. POSIX says that if D "exists", then cp must try to 
> unlink and then re-create D; and that if D does not "exist", cp must 
> fail. So, if one considers a self-symlink as "existing", then GNU cp 
> doesn't conform to POSIX; if one considers such a symlink as not 
> "existing", then GNU cp conforms. Unfortunately, as far as I can tell 
> POSIX never exactly defines what "exists" means in this context.

Well `test -e self-symlink` => does not exist

> That being said, POSIX uses nearly the same wording for 'ln -f' that it 
> does for 'cp -f', which implies that cp should be consistent with ln, 
> and GNU ln (like most ln implementations) treat self-symlinks as 
> "existing" in this case. Also, other implementations of cp seem act like 
> ln does here, so they interpret the ambiguity in POSIX the opposite way 
> that GNU cp does. Furthermore, I think that users by and large expect 
> the non-GNU interpretation where 'cp -f' is like 'ln -f'. For all these 
> reasons, I'm inclined to think that GNU cp should fall into line here.

Agreed. I can't think of a case where replacing a self symlink
is not what you'd want to happen.

Proposed patch is attached.

cheers,
Pádraig

[cp-f-eloop.patch (text/x-patch, attachment)]

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