etreeutils: sort attrs with matching namespaces by their NS URIs

[adamdecaf]

In several of the XML Canonicalization specifications it states that attributes with matching namespaces need to be sorted by their Namespace URIs and not the prefixes.

However, the namespace URI was selected as the primary key because
this is closer to the intent of the Namespaces in XML specification,
which is to identify namespaces by URI and local name, not by a prefix
and local name.

The specifications call out that sorting by prefix is easier, but incorrect.

Further, the specifications state:

An element's attribute nodes are sorted lexicographically with
namespace URI as the primary key and local name as the secondary
key (an empty namespace URI is lexicographically least).

Sources:

• https://www.w3.org/TR/xml-c14n11/#SortByNSURI
• https://www.w3.org/TR/2007/CR-xml-c14n11-20070621/#SortByNSURI
• https://www.w3.org/TR/2001/REC-xml-c14n-20010315#SortByNSURI

[adamdecaf]

cc @russellhaering

[russellhaering]

Thanks for catching / fixing this, looks good!