@doriantaylor I'm not opposed to this in principle. Would you be willing to draft a pull request and seeing what (if any) tests are failing? And maybe seeing what's involved to get them to pass?

Can you say more about your use case? I've never really run into a situation where I didn't know the object was a Document as opposed to a non-document Node. If you could share some real-world code it would help me understand your motivation.

While libxml2 also sets a document's parent to the document itself, I consider that a design mistake. Conceptually, the chain of ancestors should be a linked list and setting doc.parent = doc introduces a cycle that can lead to infinite loops if you're not careful. I'd suggest to make doc.parent return nil, so something like the following works:

while node
    ...
    node = node.parent
end

You're right; my original use case was a function that took an arbitrary node as an argument but didn't have to care about what kind of node it was, and had logic to orient itself, but I had written it with libxml2 in my head.

I went and double-checked the DOM spec (well, MDN because WHATWG is a dog's breakfast) the following turns out to be the case for parentNode or ownerDocument. For parity with DOM, both those properties should be present on a document node, but return nil. (The current behaviour, ostensibly, is doc.parent raises a NoMethodError despite being in the documentation, and doc.document returns self.)

@doriantaylor Again, I'm open to changing this behavior if we agree (following the advice of @nwellnhof who's the libxml2 maintainer above) that the desired behavior is for Document#parent to return nil. Would you be interested in drafting a pull request to do this?

I've created #3481 for this change.

@doriantaylor I also just want to respond to the fact that you (twice!) mentioned that Document#parent is documented ... it's not. You are referencing docs generated by yard, and not rdoc, and yard doesn't seem to recognize when methods have been undefed. I can't control what's on the internet!

Official docs, generated by rdoc, do not currently include #parent: https://nokogiri.org/rdoc/Nokogiri/XML/Document.html