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Re: PUT and Lock

From: Greg Stein <gstein@lyra.org>
Date: Fri, 8 Dec 2000 18:55:05 -0800
To: w3c-dist-auth@w3.org
Message-ID: <20001208185504.U30644@lyra.org>
On Fri, Dec 08, 2000 at 03:09:03PM -0000, Hall, Shaun wrote:
> Greg Stein wrote:
>...
> > A precondition with a no-tag-list 
> > refers to all
> > URIs involved in the operation. In this case: the Request-URI 
> > and the parent
> > collection. The no-tag-list is asserting a lock token on 
> > *both* resources,
> > the but the Request-URI does not have any locks associated 
> > with it (it is a
> > null resource).
> 
> Err where does it state that a no-tag-list refers to all URIs involved in
> the operation?
> 
> Section 7.6 "...the If header for all locked resources that a method may
> interact with or the method MUST fail."
> 
> Well, the null-resource isn't locked, so this statement doesn't apply.

You took that out of context. The statement indicates that you must supply
locktokens "in the If header ...". The statement does not indicate what the
If: header *applies* to.

> Section 9.4.1 No-tag-list states "If a method, due to the presence of a
> Depth or Destination header is applied to multiple resources, then the
> no-tag-list MUST be applied to each resource ...".
> 
> We don't need Depth or Destination headers with a PUT. So how do you
> conclude that all URIs are involved?

I consider the spec to be deficient. You didn't quite quote it properly, and
the commas and complete sentence are important:

  "If a method, due to the presence of a Depth or Destination header, is
   applied to multiple resources then the no-tag-list MUST be applied to
   each resource the method is applied to."

I read "..., due to the presence of a Depth or Destination header, ..." as
an appositive. It is suggesting a way that multiple resources are involved,
but I do not view it as the canonical/complete list. For example, a MOVE
applies to a whole tree of resources, even when a Depth: header is not
present.

Second, the last phrase "... each resource the method is applied to." While
the "null resource" is not truly a resource, so it makes the above phrase a
bit confusing to tell whether it applies or not (e.g. it isn't a valid
resource, so it doesn't apply; or a null resource is a *type* of resource,
so it does apply), I consider that the PUT method *does* apply to the null
resource in question.

> I interpreted (IMHO) the no-tag-list to be applied to just the parent in
> this scenario.

To use your argument ... where does it say it must be applied to the parent?
hehe :-)

If you submit a request with an If: header, then I've got to believe that it
applies to the resource (or lack thereof) identified by the Request-URI.

[ the parent thing does come up in S7.5, actually; it just isn't stated or
  clarified in the If: header description ]

> Perhaps 9.4.1. should state "...MUST be applied to each non-null resource"?

Or the spec should state "null resources have no associated locks." I tend
to believe this statement (it *is* "null" after all!), and this is why a
no-tag-list fails against a null resource in mod_dav.

I think it would be a mistake to change the phrasing to refer only to
non-null resources. Consider the case where I issue a PUT to an existing
resource, and I use the If: header to assert an etag on that resource, using
the no-tag-list format. But! Somebody has gone and deleted the resource. I
want my PUT to fail because the If: gets applied, and the etag assertion
fails. (under the premise that a null resource has no locks and no etags)

We could restate the above problem using a shared lock on the resource (not
the parent), and I assert a shared lock token via a no-tag-list. I want the
PUT to fail because the asserted lock token no longer exists.

[ note that it would fail for two reasons: the no-tag-list would apply to
  the parent on a PUT to a null resource and the parent doesn't have that
  locktoken, and it would fail on the null resource because it, too, does
  not have the lock token ]

> The IF header is meant to have similar functionality as the If-Match header,
> which RFC2616 states "is to allow effecient updates" and "prevent
> inadvertent modification of the wrong version of a resource". Well since the
> PUT is being attempted on a null-resource, it can't be the wrong version and
> the user obviously wants to udpate the null-resource, therefore "effecient
> update" should be interpreted as "I cannot be effecient because it ain't
> there, so go ahead".

The last paragrph of RFC2616, S14.25 implies that the If-Match header
applies to the resource identified by the Request-URI. I would expect the
If: header to also apply to the resource identified by the Request-URI.

> As you've guessed, I interpreted it to be Kevin's Choice #1 (201).

Yup :-) ... and all in-between-the-lines reading of the exact choice of
words in RFC 2518 is beside the point, in my mind. I simply feel that the
intuitive operation of a no-tag-list in the If: header should apply to the
(possibly null) resource identified by the Request-URI.

IMO, there should be a clarification somewhere that a null resource has no
etags and no locks. That has a particular meaning/influence/purpose when
using If: headers and operations that affect more than the resource
identified by the Request-URI.

> PS Greg, do you return 412 for other operations that can also be performed
> in this PUT-like scenario on a null-resource i.e. MKCOL and LOCK (LNR) ?

For a null resource, the If: header must match the resource *and* its
parent. For a locknull resource, the If: header must match the resource
only. Operations on the locknull resource don't check the parent because
that was checked when the locknull was created (the creation modified the
parent, but the later operation has no further effect on the parent).

My suggestion for a PUT/MKCOL/LOCK against a null resource with a lock
parent is to use a tagged-list specifying the parent and its locktoken.

Cheers,
-g

-- 
Greg Stein, http://www.lyra.org/
Received on Friday, 8 December 2000 21:52:24 GMT

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