Re: XML::LibXML XPath fails to find xmlns:xml="http://www.w3.org/XML/1998/namespace"

Petr Pajas wrote:
> Hi Emmanuel, 
>
> you are right, but is it worth fixing? 
Probably not. My initial aim was to understand why there was a 
difference between the results returned by Perl and xmllint (C). Once I 
found the reason I figured that I might as well share it.
> The namespace:: axis is problematic and 
> not fully implemented even in libxml2 itself. The problem is that namespace 
> nodes are in libxml2 represented by a very simple structure (compared to 
> other nodes); unlike for instance ordinary attribute nodes, they do not know 
> their owner element (e.g. namespace::*/parent::* won't work), sibling 
> attributes, etc. 
I've noticed that by looking at the internals of XML::LibXML and libxml2.
> Also their life cycle in libxml2 is different and quite 
> complicated (a NS node can be attached to no element, yet still there may be 
> elements refering to it as their namespace). For this reason, XML::LibXML's 
> memory management is quite elementary for NS nodes compared to other nodes. 
> We currently create a copy of xmlNs struct every time we expose a namespace 
> to Perl. It is simple but safe.
>
> What libxml2 does when it needs to instantiate a NS node for the 'xml' prefix 
> is that it creates the xmlNs struct by hand (avoiding the xmlNewNs 
> constructor):
>
>         xmlNsPtr ns;
>         ns = (xmlNsPtr) xmlMalloc(sizeof(xmlNs));
>         if (ns == NULL) {
>             xmlTreeErrMemory(
>                 "allocating the XML namespace");
>             return (NULL);
>         }
>         memset(ns, 0, sizeof(xmlNs));
>         ns->type = XML_LOCAL_NAMESPACE;
>         ns->href = xmlStrdup(XML_XML_NAMESPACE);
>         ns->prefix = xmlStrdup((const xmlChar *)"xml");
>
>   
I was wondering how libxml2 was dealing with this because its XML parser 
is adding the XML namespaces to the DOM tree without problem.
> I could of course do the same, but I don't feel very comfortable adding such 
> an unencapsulated code to XML::LibXML, but I'm willing to if you insist.
>   
Based on the explanations that I have received I don't think that this 
is truly a bug. Probably no one is going to be missing the xmlns:xml 
namespace in their results.

Cheers,
Emmanuel

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