Fix an issue with namespaces when an element is unmarshalled

[IevaVasiljeva]

What

Change the logic of namespace retrieval for element unmarshalling. The new logic:

• iterates over the element and its parents;
• creates a map of prefixes to namespaces, taking into account namespace overriding (ignore a parent namespace if a namespace with the same prefix has already been registered by a child).

Also correctly apply default namespaces (previously they would have been added like xmlns:=value, now they are added as xmlns=value).

Why

A valid XML lead to the following error: expected element <Assertion> in name space urn:oasis:names:tc:SAML:2.0:assertion but have no name space

XML:

<?xml version="1.0" encoding="UTF-8"?><ns3:Response xmlns:ns3="urn:oasis:names:tc:SAML:2.0:protocol" xmlns="urn:oasis:names:tc:SAML:2.0:assertion" xmlns:ns2="http://www.w3.org/2000/09/xmldsig#" xmlns:ns4="http://www.w3.org/2001/04/xmlenc#" Destination="..." ID="..." InResponseTo="..." IssueInstant="..." Version="2.0">
  <Issuer>...</Issuer>
  <ns3:Status>
    <ns3:StatusCode Value="urn:oasis:names:tc:SAML:2.0:status:Success"/>
  </ns3:Status>
  <Assertion ID="..." IssueInstant="...." Version="2.0">
    <Issuer>...</Issuer>
    <Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
      ...
    </Signature>
     .....
  </Assertion>
</ns3:Response>

[alexanderzobnin]

It seems, there's a bit simpler solution.

	for space, uri := range namespaces {
		prefix := "xmlns:" + space
		if space == "" {
			prefix = "xmlns"
		}
		doc.Root().CreateAttr(prefix, uri)
	}

I was confused by NamespaceURI() function which says that

If the element is part of the XML default namespace, NamespaceURI returns the empty string.

But it actually returns a default namespace. So we can skip the part inside for _, childEl := range el.FindElements("//*") { and just use "xmlns" key for default namespace.

Source of the NamespaceURI(). Here's a check for Space property and if it's empty that means it's a part of default namespace.

// NamespaceURI returns the XML namespace URI associated with the element. If
// the element is part of the XML default namespace, NamespaceURI returns the
// empty string.
func (e *Element) NamespaceURI() string {
	if e.Space == "" {
		return e.findDefaultNamespaceURI()
	}
	return e.findLocalNamespaceURI(e.Space)
}

[IevaVasiljeva]

So we can skip the part inside for _, childEl := range el.FindElements("//*") { and just use "xmlns" key for default namespace.

That would solve the issue with default namespace not being correctly added to attributes, but it doesn't solve the problem with the namespaces not being picked up correctly. If we keep the for _, childEl := range el.FindElements("//*") { loop, there are two problems:

1. we are reading namespaces from all elements not only from the parent elements;
2. we are not picking up namespaces from the attributes.

[alexanderzobnin]

Ok, sounds good! Could you maybe add some comments to the namespace searching section so that will be easier to navigate and read the code in the future?

[mgyongyosi]

Could you please add a new test case for testing this functionality (elements from the default namespace)?

[IevaVasiljeva]

Added a test for it 👍

[robotdan]

Thanks!!! This is great!