proposal: encoding/xml: access to ns short codes

[dhubler]

Proposal Details

I would like access to the namespace short codes to long form mappings used during an unmarshaling. This is useful for processing custom element and attribute values that reference the short codes like xpaths in IETF RFC6241 NETCONF

Example:

<x xmlns:z="zee">
  <y xpath="z:banana/foo/z:apple"></y>
</x>

Here I need to parse xpath and lookup that z's full namespace is zee

Fix: to encoding/xml/xml.go

// LookupNs will return the full namespace string give a short code. This will
// only be useful from implementations of Unmarshaler while the namespaces
// are still in scope
func (d *Decoder) LookupNs(shortcode string) string {
	return d.ns[shortcode]
}

Example:

func TestXpath(t *testing.T) {
	x := struct {
		Y *Y `xml:"y"`
	}{}
	orig := `<x xmlns:z="zee">
	  <y xpath="z:banana/foo/z:apple"></y>
	</x>`
	d := xml.NewDecoder(strings.NewReader(orig))
	if err := d.Decode(&x); err != nil {
		t.Fail()
	}
	if x.Y.XPathLookup != "z=zee" {
		t.Error(x.Y.XPathLookup)
	}
}

type Y struct {
	XPath       string
	XPathLookup string
}

func (y *Y) UnmarshalXML(d *xml.Decoder, start xml.StartElement) error {
	copy := struct {
		XPath string
	}{}
	if err := d.DecodeElement(&copy, &start); err != nil {
		return err
	}
	y.XPath = copy.XPath
	y.XPathLookup = fmt.Sprintf("z=%s", d.LookupNs("z"))
	return nil
}