Re: [xml] ID already defined in 2.9.2

[Nick Wellnhofer <wellnhofer aevum de>]

On 18/12/2014 12:37, Bruce Miller wrote:
> Hi all;
>    I've seen a bug report (https://bugzilla.gnome.org/show_bug.cgi?id=737840#c4)
> regarding a new error about ID's already defined that turned
> up with 2.9.2, but we're encountering a somewhat different manifestation.
>
> In the Perl program below (using XML::LibXML), we basically want
> to replace an element that has an ID, with another element that
> (ultimately) should have the same ID.
>
> Unfortunately, when linked to libxml2 2.9.2, this results in an error:
>    element c: validity error : ID b is already defined

Yes, this is a different manifestation.

> The documentation of (XML::LibXML::Node) unbindNode makes clear
> that it removes the node from the dom tree, but it is still
> associated with the Document. It is not unreasonable to think
> that the Document should remember that the ID is "defined";
> and in fact it's not necessarily a bad thing that libxml2
> should be checking that.
>
> However, this never resulted in an error until 2.9.2.
> So, I'm not sure whether this is necessarily a bug
> report on 2.9.2, or a request for suggestions on how
> best (via XML::LibXML) to cleanly _remove_ the record
> of the id from the Document.

To work around this error, you can simply force the unbound node to be 
destroyed by removing any references to it. In your example program, add `$b = 
undef` after `$b->unbindNode()`. Alternatively, you could remove the `id` 
attribute after unbinding a node.

Of course, this only works if you really want to remove the node. It seems 
that libxml2 doesn't provide a useful mechanism to move ID attributes from one 
document to another. I think that `xmlSetTreeDoc` should be changed to account 
for added and removed IDs.

Nick