Re: [xml] XPath/Document concurrency issue

[Daniel Veillard <veillard redhat com>]

On Sat, Feb 23, 2008 at 03:05:27AM +0100, Belgabor wrote:
> Hi everybody,
>
> I have a strange problem which I can only describe as a concurrency 
> issue between the (xml) document content and the results I get from 
> XPath queries. I'm working on a project where I plan to use the 
> Model/Controller/View architecture, the model being a parsed xml tree 
> and read/write access to it is mainly done via XPath queries. Therefore 
> I'm quite worried by this issue.
>
> Here's what I do:
>
> - Read xml document
> - Create an XPath context (xmlXPathNewContext)
> - Run a query (xmlXPathEvalExpression), getting a result node
> - Create a new node (xmlNewNode), set content (xmlEncodeSpecialChars, 
> xmlNodeSetContent)
> - Add new node as child to the result node (xmlAddChild)
>
> Later:
> - Create an XPath context (xmlXPathNewContext) with the same document
> - Run a query (xmlXPathEvalExpression) which checks for the content of 
> the node added above in a predicate.
> - Get no result.
>
> The curious thing is (and the reason why I bring it here rather than 
> think it's an issue of the c++ wrapper around libxml2 I wrote) that 
> after I save the document to disk and reload it, everything works as 
> expected.

  My guess is that when you modify the tree you build it wrong (I'm guessing
you're using namespaces and not properly building your tree), the output
once serialized looks fine, but the internal tree structures are wrong.
That would lead to XPath failing on the modifed tree and but succeeding
when serializing/parsing. Triple check all the pointers and namespaces
generated when using xmlAddChild.

Daniel

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