Re: More on Hard Links (was A bold idea (Re: Carrying Attributes too Far))

[David Masover <jedi@ninja.dynup.net>]

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Alexander G. M. Smith wrote:

>David Masover wrote on Fri, 05 Dec 2003 19:54:26 -0600:
>  
>
>>>Incidentally, renaming files is a superset of the delete operation.
>>>Implement that first, and you have delete for free.
>>> 
>>>
>>>      
>>>
>>If only it was that easy.  When you rename a folder, you don't usually 
>>do a recursive rename of everything inside.  When you delete a folder -- 
>>you don't, unless it's "rmdir".  You recursively delete each file 
>>inside, and it's that kind of operation that creates problems for hard 
>>links to directories.
>>    
>>
>
>Actually, renaming also is recursive.  The obvious case is when you
>rename a file over an existing file (or directory), which implies the
>whole deletion mechanism is needed.
>  
>
Most of the time, rename over an existing file needs only a single 
deletion, and renaming over a directory is (last I checked) impossible 
with standard tools without removing or renaming the directory first.  
If I do
touch a
mkdir b
mv a b
It will move 'a' to a file inside of 'b', rather than removing 'b'.  
This isn't necessarily the best way, but it's how it's done now -- to 
get the effect you're implying, I'd have to do
rm -rf b
mv a b

>Also, at least for the AGMS link mechanism (or perhaps for any system
>that allows multiple parent directories), it has to do a graph traversal
>to find out which parent links are still valid, and which need to be
>adjusted if their parent has been changed.  If you see the example at
>the bottom of http://web.ncf.ca/au829/BeOS/AGMSLinkExplanation.txt you'll
>notice that deleting something can cause a sweeping re-evaluation of who
>is the parent to what.  The same thing would happen if you rename (move)
>a parent directory to be a subdirectory of one of its children.
>
>  
>
Can we do that?  Last I checked, the file utilities (or maybe the fs 
itself) won't let you move a directory to a child of itself, or a child 
of a child of itself.  If this "graph traversal" is what I think it is, 
it belongs in fsck, not normal fs operation.  Of course, I don't have a 
better solution, aside from either stretching the limits of reference 
counts or dropping directory hardlinks altogether.
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