[PATCH 2/2] daemon: Introduce the possibility for users to register custom XML validator

[Daniel P. Berrangé]

On Fri, Sep 23, 2022 at 05:42:13PM +0200, Peter Krempa wrote:
> Introduce a new config option 'xml_validator' into the daemon config
> file which will allow users to make libvirt daemons use a custom XML
> validator.
> 
> The rationale is that validators such as 'jing'[1] provide drastically
> better error specification when compared to the native libxml2 validator
> we use. A drawback though is that jing is written in Java and thus
> unusable by libvirt directly and also not a popular package present in
> distros.
> 
> For power users and developers it still is worth having this feature to
> provide better errors in a native way.

I kinda wonder if those users need it integrated in libvirt
though, as opposed to just calling jing themselves when
needed. It feels like a fairly narrow set of users benefitting
here from this change, made even smaller by the fact that those
users have to remember to reconfigure libvirt to enable this.



> An example showing the quality of the errors:
> 
> XML used:
> 
>   <vcpu placement='static' current='1'>asdf8</vcpu>
> 
> native validator:
>   error: XML document failed to validate against schema: Unable to validate doc against /home/pipo/libvirt/src/conf/schemas/domain.rng
>   Extra element vcpu in interleave
>   Invalid sequence in interleave
>   Element domain failed to validate content
> 
> jing:
>   error: XML document failed to validate against schema: Unable to validate doc against /home/pipo/libvirt/src/conf/schemas/domain.rng
>   /dev/stdin:6:52: error: character content of element "vcpu" invalid; must be an integer

No doubt the errors are way better, but a power user
can just invoke this script below themselves if they
ever want to understand a XML problem better.

> 
> Example script to make this feature work with jing:
> 
>   #!/bin/bash
>   java -jar /home/pipo/git/jing-trang/build/jing.jar $1 /dev/stdin 2>&1 || exit 1
> 
> [1] https://github.com/relaxng/jing-trang

To me this is only compelling if there's an option we can reasonably
wire up out of the box.

With regards,
Daniel
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