[bugfix] fn:transform: Conversion and treeIndex problems

[nverwer]

Description

The fn:transform function sometimes produced NullPointerExceptions. This happened for documents containing nodes of type org.exist.dom.memtree.ElementImpl and other subtypes of org.exist.dom.memtree.NodeImpl.

The function org.exist.xquery.functions.fn.transform.Convert.ToSaxon.ofNode(Node) uses org.exist.xquery.functions.fn.transform.TreeUtils.treeIndex(Node) to get a 'path' (as a list of indexes) of the Node in its containing document. This 'path' is used to find the node in a newly built document that is suitable for use by Saxon.
Using the 'path' that the treeIndex function produces assumes that it starts at the document node.

However, some nodes do not have a containing document. Specifically, in org.exist.dom.memtree.NodeImpl.getParentNode():

if (parent.getNodeType() == DOCUMENT_NODE && !((DocumentImpl) parent).isExplicitlyCreated()) {
            /*
                All nodes in the MemTree will return an Owner document due to how the MemTree is implemented,
                however the explicitlyCreated flag tells us whether there "really" was a Document Node or not.
                See https://github.com/eXist-db/exist/issues/1463
             */
            return null;

In this case, the 'path' returned by treeIndex does not contain the index (0) for the root node, and org.exist.xquery.functions.fn.transform.TreeUtils.xdmNodeAtIndex(XdmNode, List<Integer>) will return null when a node is expected.

The proposed fix tests for this case. Going up the ancestor chain, when a node that is not a document node, and that does not have a parent is found, the index 0 for this root element is inserted in the 'path'.

Testing

The NullPointerException happened in a rather complicated XQuery with documents composed from several sources.
I have not yet been able to make a simple test case, but I will keep trying.

[dizzzz]

@wolfgangmm please can you check?

[nverwer]

It appears there is another (different) problem with the way fn:transform handles its input, see #5682.

[nverwer]

I have added two more commits.

• Show the location when an error occurs during an XSL transformation (because my client who uses fn:transform wanted this).
• Fix an error when transforming a document containing 'NodeProxy` nodes into an XdmValue for Saxon.

The second commit fixes another bug in the handling of NodeProxy instances.

[nverwer]

There was another problem with the the 'path' returned by treeIndex.
The indexes are determined by counting previous-siblings.
It turns out that a org.exist.dom.persistent.StoredNode returns attributes of an element as previous siblings of the element's children.
When later the tree is traversed using the indexes, attributes are not in the children list.

I think that what StoredNode does is wrong, but decided to not try to solve it there (yet).
Instead, I made a work-around in TreeUtils.

[line-o]

Thanks @nverwer again for your contribution. We will pull it in once we have at least one positive and one negative test for it.
Any pointers you could give how to test this scenario would be much appreciated.

[line-o]

Or maybe it is easier to test in XQuery?