Re: [libvirt PATCH] libxl: Fix build with recent Xen that introduces new disk backend type

[Daniel P. Berrangé <berrange@xxxxxxxxxx>]

On Mon, Aug 01, 2022 at 09:51:11AM +0100, Julien Grall wrote:
> Hi Michal,
> 
> On 01/08/2022 09:23, Michal Prívozník wrote:
> > On 7/29/22 17:50, Oleksandr Tyshchenko wrote:
> > > From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> > > 
> > > Xen toolstack has gained basic Virtio support recently which becides
> > > adding various virtio related stuff introduces new disk backend type
> > > LIBXL_DISK_BACKEND_STANDALONE [1].
> > > 
> > > Unfortunately, this caused a regression in libvirt build with Xen support
> > > enabled, reported by the osstest today [2]:
> > > 
> > > CC       libxl/libvirt_driver_libxl_impl_la-xen_xl.lo
> > > ../../src/libxl/xen_xl.c: In function 'xenParseXLDisk':
> > > ../../src/libxl/xen_xl.c:779:17: error: enumeration value 
> > > 'LIBXL_DISK_BACKEND_STANDALONE'
> > >     not handled in switch [-Werror=switch-enum]
> > >                   switch (libxldisk->backend) {
> > >                   ^~~~~~
> > > cc1: all warnings being treated as errors
> > > 
> > > The interesting fact is that switch already has a default branch (which 
> > > ought
> > > to cover such new addition), but the error is triggered as -Wswitch-enum
> > > gives a warning about an omitted enumeration code even if there is a 
> > > default
> > > label.
> > 
> > This is expected and in fact working correctly. We want compiler to warn
> > us about enum members that are not handled in a switch() statement.
> 
> For us this is treated as an error. Is it intended?

Yes & no, but mostly yes.

You can choose to build with -Werror or not. If building from .git
then it defaults to enabled, but can be disabled if desired.

Generally we want to see errors triggered from new enums arriving,
as it can be a sign that libvirt code needs a semantic change in
order to continue operating correctly.  It isn't always correct
to assume that the 'default' case gives the correct behaviour.

With regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|