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RE: Notes on xml-exc-c14n rev 1.21

From: John Boyer <JBoyer@PureEdge.com>
Date: Thu, 10 Jan 2002 10:22:46 -0800
Message-ID: <7874BFCCD289A645B5CE3935769F0B520C34FE@tigger.PureEdge.com>
To: <reagle@w3.org>, "Thomas Maslen" <tmaslen@wedgetail.com>, <w3c-ietf-xmldsig@w3.org>


Hi all, 

Please see comments marked <jb> below.

-----Original Message-----
From: Joseph Reagle [mailto:reagle@w3.org]
Sent: Wednesday, January 09, 2002 3:33 PM
To: Thomas Maslen; w3c-ietf-xmldsig@w3.org; John Boyer
Subject: Re: Notes on xml-exc-c14n rev 1.21


OPEN

On Wednesday 09 January 2002 04:28, Thomas Maslen wrote:
>     (e)	A paragraph in section 1.1 says "The namespace axis of
an element
> 	contains nodes for all namespace declarations [...]".  If this
is
> 	meant to be consistent with XPath semantics, it should mention
the
> 	absence of a node for xmlns="".

John, can you confirm and provide text?

<jb>
Confirmed.  Proposed new text:

The namespace axis of an element contains nodes for all non-default
namespace declarations made within the element as well as non-default
namespace declarations inherited from ancestors of the element. The
namespace axis also contains a node representing the default namespace
if it is not the empty string, whether the default namespace was
declared within the element or by an ancestor of the element.  Any
subset of the nodes in a namespace axis can be included in a document
subset.
</jb>


>     (2)	Where is the exc-c14n behaviour of the default namespace
> specified?
>
> 	The default namespace ("xmlns") has various funny properties
that
> 	have to be dealt with in definitions, particularly
>
> 	      -	since the default namespace doesn't have a namespace
prefix,
> 		phrases like "For namespace prefixes ..." don't apply to
it,
>
> 	      -	since XPath very thoughtfully indicates xmlns="" by the
> 		absence of a namespace node, phrases like "each
namespace
> 		node" don't do the job either.
>
> 	The Canonical XML recommendation jumped through the appropriate
> 	hoops to correctly define the behaviour of the default namespace
> 	(despite XPath), but I don't think that the exc-c14n draft does.
>
> 	Section 1.1 of exc-c14n is fine:  the definition of "visibly
> 	utilizes" does have a sentence that accounts for the default
> 	namespace [well, assuming that it is *not* using XPath
semantics,
> 	i.e. the incredible disappearing xmlns="" node].

Right, that's this bit, "which occurs if E has no namespace prefix".

<jb>Actually, it is accounted for by the whole sentence that ends with
the dependent clause given by Joseph above. </jb>

> 	Section 3 contains two definitions of exc-c14n, and I don't
think
> 	that either of them really addresses the default namespace:
>
>
> 	      -	the first definition is "Canonical XML, with these three
> 		exceptions".
>
> 		The wording in the exceptions (2 and 3) talks about
> 		"namespace prefixes", so it doesn't include the default
> 		namespace -- so presumably the default namespace just
> 		inherits the Canonical XML behaviour, i.e. it uses
> 		inclusive c14n?
>
> 		Is that the intent?  (I would have guessed that the
> 		default namespace was meant to be handled exclusively).
>
>
> 	      -	the second definition is the pseudocode algorithm.
>
> 		Step 3 of the pseudocode talks about "namespace nodes"
> 		in the XPath sense, so implicitly [accidentally?  Or
> 		deliberately?] it applies to xmlns="mumble" and will
> 		treat it exclusively -- c.f. the first definition,
> 		above -- but it does not handle xmlns="" at all.
>
>
> 	I think that there are two options for the spec that would give
> 	consistent results:
>
> 	    (I)	state that the default namespace is always treated
> 		inclusively, i.e. effectively the InclusiveNamespaces
> 		PrefixList invisibly contains the default namespace
> 		(which, of course, doesn't have a prefix)
>
> 	   (II)	modify Section 3 (I haven't figured out how) so that
both
> 		xmlns="mumble" and xmlns="" are canonicalized
exclusively,
> 		i.e. they only show up when they are visibly utilized
>
> 	Of these, I definitely prefer (II), because I think it produces
> 	the less surprising behaviour.
>
> 	[Or is there something I haven't realized about exc-c14n that
> 	makes this all a silly question, e.g. element names are always
> 	prefixed?]

No, I think you identified a blind spot. I've modified 3.1.3 to read:

  For every /+namespace node with a+/ prefix that does not appear in
 the InclusiveNamespacePrefix List, a namespace declaration is output
 at every output element where that prefix is visibly utilized and that
 prefix and its value is not already in an output ancestor.

However, I wonder what if their is a semantic difference between a
missing 
InclusiveNamespace PrefixList and one equal to "" ?

<jb>
Yes, there should be, though it is no longer specified.  In the 5 July
2001 draft, I had the following text for #3 (Pertinent meant that it
would be in the output):

The Exclusive XML Canonicalization method receives an additional string
parameter UnsuppressedNamespacePrefixList containing a comma separated
list of namespace prefixes that are not to be suppressed (e.g.
"ns1,ns2,ns3" with no intervening whitespace). Any namespace node that
declares a namespace prefix in this list is automatically pertinent. If
there is an empty entry in the UnsuppressedNamespacePrefixList (e.g.
",ns1" or "ns1," or "ns1,,ns2"), then default namespace nodes are
automatically pertinent. 
</jb>

<jb>
Some of the parts of Section 3 don't sound quite right:

1) The third exception says "For every namespace node with a prefix that
does not appear in the InclusiveNamespacePrefix List, a namespace
declaration is output at every output element where that prefix is
visibly utilized and that prefix and its value is not already in an
output ancestor."  Shouldn't it be the case that the *nearest* output
ancestor has to have the same value?

2) In implementation note 3.2, shouldn't it say that when the recursion
returns, then the namepsace nodes should be popped from the stack?
Otherwise, doesn't an element get interference from the namespaces at
play by the last processed descendant of the element's preceding
sibling?

3) In the Note after the implementation method, it says that "many XPath
implementations do not distinguish namespace nodes from attribute
nodes."  This is actually an incorrect implementation of XPath.  The
next sentence should start with something other than "This is
necessary..." such as "It is necessary to distinguish namespace nodes
from attribute nodes because ... [same ending]".

Cheers,
John Boyer
PureEdge Solutions Inc.
</jb>
Received on Thursday, 10 January 2002 13:23:31 GMT

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