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RE: Status code for creating lock-null resource

From: Lisa Dusseault <lisa@xythos.com>
Date: Thu, 28 Jun 2001 09:36:44 -0700
To: "Alan Kent" <ajk@mds.rmit.edu.au>, <w3c-dist-auth@w3.org>
Alan wrote:
> What about the case
> where an application wants to create a temporary file with a new unique
> file name. It does a 'LOCK' on a name. If it fails, it trinother
> unique name. It keeps trying until it succeeds.
> Without the current LOCK semantics, you would have to do a PUT to
> create an empty file and then LOCK it. Does PUT have an OVERWRITE Y/N
> flag? (I dont have the spec handy). If it always overwrites, then
> there is the potential for a race condition.

In a sense, PUT does have overwrite semantics.  You can use the
"If-None-Match: *" header to make sure that there are no entity tags at the
URL destination.  We might want to mention it so (a) servers know they must
support it and (b) clients know they can use it, and what for.  RFC2616 does
state it quite clearly though (section 14.26): "It is also used to prevent a
method (e.g. PUT) from inadvertently modifying an existing resource when the
client believes that the resource does not exist."

Let's assume the "If-None-Match: *" feature can reliably be used, though I'd
want to check that before setting anything in stone.  Can we just remove the
lock-null feature?  Some useful interactions would still be lost:
 - A client can be the creator of a file which they subsequently lose
control of.  This is because they would have to create the file with PUT
then set permissions, leaving a window where the file is unlocked and some
other user can set ACLs before the first user has a chance to prevent.
 - A client can PUT a file and then find themselves unable to LOCK it if
somebody else has locked it.  They can't create the body, change properties,
or delete it.  They may not be able to unlock it.  What kind of problems
might this present to a user interface client?
 - Similar scenarios with MKCOL.

So let's imagine we could transform lock-null resources into locked empty
resources -- the only requirement on the server would now be that the server
would allow the creation of an empty resource through use of the LOCK
request.  With a locked empty resource, it's a regular resource, it appears
in listings, and it doesn't magically go away if no PUT request is received
before UNLOCK.  Would this help us out?

A locked empty resource as I described would solve the PUT-control problem,
but not the MKCOL problem, unless the server were required to be able to
support MKCOL on a locked empty resource.  So if I have write and
permissions access to a repository where other users have write and
permissions access, I could create a directory called "lisa" - and there's a
risk that somebody else can change permissions before I have a chance to.
The malicious user could lock me out of my own directory.  On the other
hand, presumably the malicious user could create a directory called "lisa"
in the first place and not have to worry about waiting for me to create it.

Some servers will solve this problem by assigning special permissions to the
creator of a resource, however I don't think we can rely on this being
always the case. There are certainly use cases for other permissions models.
We would want to think about whether it's valid for a server to grant
permissions permission on newly created resources to principals that did not
either create the new resource, or own the parent resource.

Received on Thursday, 28 June 2001 12:38:55 UTC

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