Re: [xml] libxml2 xmlNode management

[Alex Bligh <alex alex org uk>]

Daniel,

> > So, questions:
> >
> > 1. Can an xmlNode that has children in document tree be "pruned" from
> >   one document and added to another together with its children? Is this
> >   a simple matter of doing an xmlAddChild? (ie are operations like
> >   xmlAddChild safe with xmlNodes that already form part of the same
> >   or a different document).
>
>   no in general it's more complex in the presence of namespace, entities
> and if using document dictionaries.

Thanks. On this occasion, I can guarantee there are no namespaces,
there is no DTD (and hence no entities) and no dictionary. The
tree consists solely of node elements and attributes.

I am doing an xmlUnlinkNode() from the current document before
doing (e.g.) an AddChild(). Is this safe in this case?

> > 2. xmlNodes can be created without adding them to an xmlDocument.
> >   Can such an xmlNode have children? If so how do I prune a subtree
> >   of xmlNodes off an existing tree without adding them to a new
> >   one (i.e. how do I reverse xmlAddChild)? I think this is
> >   xmlUnlinkNode though the documentation refers to this as
> >   unlinking it from its "current context". Does that mean it is
> >   unlinked from just its parent? Or its children too?
>
> just from the parent and siblings, it will still reference the document,
> namespaces, entities and possibly use the document dictionary

Thanks.

I resorted to reading the code at one point. It does at least appear
that you are meant to be able to link in nodes to a different document.

> > 3. If I want to prune a node and its children and delete them
> >   how can I do that without risking a memory leak? Does xmlUnlinkNode
> >   followed by xmlFreeNode (which is described as recursive) do that. If
> >   the node is unlinked, how can it have children?
> >
> > 4. Is it safe to xmlFreeNode a node which is part of a tree?
> >
> > I get the feeling some calls (e.g. xmlAddChild) will deal intelligently
> > with the node's existing parents or children, but some (perhaps e.g.
> > xmlFreeNode) will not. Is there a list somewhere, or perhaps better
> > an idiots guide to how these tree manipulation operations work?
>
>   Unfortunately the tree calls are not fool-proof, you really need
> to understand the document structure to do copy paste of subtrees.
> The problem is that it's easy to made on simple document, but suddenly
> if the document start using namespace or entities the application may
> fail brutally.

Fortunately I can guarantee the document is simple. I am now following
using the following heuristics:
* Commands to add stuff from a tree should be done with nodes without
 parents
* Commands to remove stuff from a tree should be done with nodes with
 parents
* UnlinkNode removes things from the tree
* AddChild / ReplaceNode adds things to the tree
* Provided the document is simple (see above), it's safe to move nodes
 between documents.
* Nothing will automatically xmlFreeNode for you. So if you set the
 root element (for example) to something else, or xmlReplaceNode
 a node, the old root / replaced node is unlinked, but not freed.

Does that sound fair?

--
Alex Bligh