[libvirt PATCH 16/17] meson: drop check for runtime binary dependencies

[Daniel P. Berrangé]

On Mon, Apr 19, 2021 at 06:22:18PM +0200, Pavel Hrdina wrote:
> On Mon, Apr 19, 2021 at 05:05:56PM +0100, Daniel P. Berrangé wrote:
> > On Mon, Apr 19, 2021 at 06:01:42PM +0200, Andrea Bolognani wrote:
> > > On Mon, 2021-04-19 at 13:35 +0100, Daniel P. Berrangé wrote:
> > > > On Fri, Apr 16, 2021 at 09:12:45PM +0200, Pavel Hrdina wrote:
> > > > > -  if not get_option('storage_iscsi').disabled() and iscsiadm_prog.found()
> > > > > +  if not get_option('storage_iscsi').disabled()
> > > > >      use_storage = true
> > > > >      conf.set('WITH_STORAGE_ISCSI', 1)
> > > > 
> > > > So this now enables iSCSI even on platforms which don't support iscsiadm
> > > > tools.
> > > > 
> > > > > +    use_storage = true
> > > > > +    conf.set('WITH_STORAGE_LVM', 1)
> > > > 
> > > > And enables LVM on all platforms, even though this is Linux only..
> > > > 
> > > > Overall this patch  makes it so that our meson rules don't
> > > > "do the right thing" as a default behaviour, and just enable
> > > > everything whether you have it installed or not.
> > > 
> > > Can't we address that by implementing explicit OS checks?
> > 
> > For some of the that's easy, if they're obviously Linux or FreeBSD
> > specific. Others though the platform supportability is not so
> > clear, at least not unless someone wants to check the tools and
> > what platforms they genuinely claim to support.
> > 
> > > Right now we're using "the iscsiadm command happens to be installed
> > > on the build host" as a proxy for "the iscsiadm command will be
> > > available on the deployment host", which is not a bad approximation
> > > but is not quite right either.
> > 
> > Both approaches are approximations - checking the binary may miss out
> > platforms where the feature is possible but not installed by the user.
> > Checking the OS platform may miss out platforms where libvirt devs
> > didn't realize there was support for the 3rd party tool.  The former
> > was more the favoured approach as it removes libvirt devs from being
> > the problem, and makes the users' be the problem :-)
> 
> In addition it allows devs to compile that part of code without
> installing the binaries or in some cases without using hacks like this:
> 
>     ln -s /required/bin /usr/bin/true
> 
> So I would prefer checking binaries instead of OS checks.

If we only check when the meson feature config is "auto", then we dont
need the link hacks - just set  "-Dfeature=on" when running meson.

Once we only use the checks for the "auto" case, then it is less
critical if our check is wrong, as it can be overridden. Given
this, I think we could have a mixture of binary or OS checks
depending on what is best suited.

eg LVM could easily be a linux OS check, whle sheepdog still best
as a binary check.

Regards,
Daniel
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