Re: RFC: renameat(): Add a RENAME_REMOVE flag to unlink hardlinks

[Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org>]

On Fri, Nov 21, 2014 at 5:29 PM, Pádraig Brady <P@draigbrady.com> wrote:
> On 21/11/14 22:40, Andy Lutomirski wrote:
>> On Fri, Nov 21, 2014 at 2:30 PM, Al Viro <viro-RmSDqhL/yNMiFSDQTTA3OLVCufUGDwFn@public.gmane.org> wrote:
>>> On Fri, Nov 21, 2014 at 10:20:54PM +0000, Pádraig Brady wrote:
>>>
>>>>>> That said, would you still like me to take a stab at a proposal to the
>>>>>> POSIX folks that would relax the requirements to allow
>>>>>> implementation-defined behavior when the two arguments to rename
>>>>>> describe the same file but via different directory entries?
>>>>
>>>> I guess there is no point discussing in POSIX and adding extra
>>>> implementation options if no implementations do/will act accordingly.
>>>>
>>>> Linux can decide to do that independently, if appropriate.
>>>> This is one of those borderline cases where we balance
>>>> accretion of cruft vs incompatibility.
>>>> On consideration, I'm OK with keeping the existing
>>>> rename() behavior for compat and adding the new flag.
>>>> That said I still can't think of anything depending
>>>> rename() doing nothing with hardlinked source and dest.
>>>
>>> You do realize that it opens a very nasty can of worms for filesystems that
>>> are e.g. case-insensitive to some extent?  How do you tell links from
>>> alternative equivalent spellings of the name?
>>
>> I assume that VFS can handle this correctly if it wants to.
>
> I was assuming there was a way to distinguish directory entries,
> and that's what should be checked first, which is what my
> psuedo code patch attempted to show.
>
>> OTOH, if someone does rename("foo", "Foo"), and foo, Foo, fOO, etc.
>> are all valid spellings, then presumably they don't actually expect
>> "foo" to go away.  They may, however, want the name shown in readdir
>> to change, so maybe RENAME_HARDLINK should do that, too.
>
> That's a separate case, though also useful for case retentive file systems.
> If you did want to support that, i.e. when src and dst are the same directory entry,
> though spelled differently, then you might have another flag.
> Or you could combine both functions to a RENAME_ALIAS flag?

I like that option.

>
> thanks,
> Pádraig.



-- 
Andy Lutomirski
AMA Capital Management, LLC