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Re: [libvirt] [PATCH 0/8] Add XML validation to the APIs

On Wed, Nov 19, 2014 at 09:45:39AM +0000, Daniel P. Berrange wrote:
> On Wed, Nov 19, 2014 at 08:23:22AM +0100, Martin Kletzander wrote:
> > On Tue, Nov 18, 2014 at 05:59:47PM +0000, Daniel P. Berrange wrote:
> > >This proof of concept patch extends the virDomainDefineXML
> > >and virDomainCreateXML APIs so that they can validate
> > >the user supplied XML document against the RNG schemas.
> > >
> > >The virsh command will enable validation by default, it
> > >must be turned off with --skip-validation if desired.
> > >
> > >This series is not complete
> > >
> > >- The network, interface, storage pool, etc APIs are
> > >  not wired up to support validation.
> > >- Only the QEMU virt driver is wired up to validate
> > >- The virsh edit command is not wired up to validate
> > >
> > >It is enough to demonstrate it working with 'virsh define'
> > >and the QEMU driver though.
> > >
> > >The biggest problem I see is the really awful error
> > >messages we get back from libxml2 when validation
> > >fails :-( They are essentially useless :-(

> > >
> > 
> > This is one of the things why I'm not convinced this work is worth
> > it.  It may be nice if we tell the user their XML is invalid instead
> > of silently losing information.  But error message similar to "invalid
> > element in interleave" doesn't help much when you are adding 100-line
> > XML.  There are some better validators, but requiring those would be
> > too cumbersome.
> At least when using 'virsh edit' you would know what element you
> just changed / added. So if you got get a generic 'validation failed'
> error you have a pretty good idea of where in teh document you made
> the mistake. So I think it'd be useful in that scenario. The error
> reporting is more of a problem for the apps where they're passing in
> a big XML document to define the guest and basically anything could
> be wrong.

So, it seems not all of the error messages are so awful. It does a bad
job of reporting unknown elements, but if you have an unknown attribute
it does better:

  "Invalid attribute foo for element name"

If you give an invalid value for an attribute which is an enum it
is semi-usefull

  "Element domain failed to validate attributes"

So this does seem somewhat more useful to have in libvirt

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