[libvirt] Limiting old version back compat for language bindings ?

[Andrea Bolognani]

On Mon, 2019-10-07 at 13:35 +0100, Daniel P. Berrangé wrote:
> On Mon, Oct 07, 2019 at 01:58:10PM +0200, Andrea Bolognani wrote:
> > Can't we follow the same policy as the main library? That would make
> > it more straightforward to reason about. Also note that our CI only
> > runs jobs on the platforms targeted by the main library, which means
> > RHEL 6 and Ubuntu 14.04 are out already...
> 
> I don't think this is the same kind of situation at play, because of how
> it interacts with application developers expressing their dependancies
> for the language bindings.  If an app expresses a dep on the oldest
> version of libvirt-python they support they can't use APIs newer than
> that.  If an app expresses a dep on the newest version of libvirt-python
> they can use, then they can conditionally use the new APIs while still
> being compatible with the older ihnstalls.  This works regardless of
> our support policy wrt the main libvirt EOL.
> 
> If we put the same EOL policy on the language bindings, we're either
> forcing the application onto the same support policy as libvirt, or
> making their build / deployment process more complicated, neither of
> which I think are reasonable.
> 
> With main libvirt library our EOL policy is a great benefit so us as
> it dramatically lowers our maint burden. This makes it worth the cost
> for people who might wish to deploy libvirt on older systems. 
> 
> The language bindings do not have a high maint cost from supporting
> old versions, so it doesn't justify creating pain for application
> developers by dropping support so aggressively as for main libvirt.
> 
> A time based scheme for dropping old versions in language bindings
> is very easy to describe to people & apply ourselves, more so than
> our main policy which needs us to research versions across distros
> every time we change something.

It seems to me that people who want to run the latest version of
whatever application will also use a non-obsolete operating system,
and conversely people stuck with an old OS will rather also stick to
the vendor-provided (and -supported) versions of the various
components rather than installing newer ones from source.

It's basically the same argument we used to justify libvirt dropping
support for old operating systems and old QEMU versions, and I think
it still applies when you take it one layer up the stack.

If anything, the higher you go up the stack and the more developers
are okay with having tighter coupling between their application and
libvirt/QEMU versions, so I'd say it actually applies even more to
them.

-- 
Andrea Bolognani / Red Hat / Virtualization