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Re: [Xen-devel] [xen-unstable bisection] complete build-i386-libvirt

On Mon, 2014-06-30 at 15:13 +0100, Ian Jackson wrote:
> Ian Campbell writes ("Re: [Xen-devel] [xen-unstable bisection] complete 
> build-i386-libvirt"):
> > libvirt doesn't use the LIBXL_API_VERSION mechanism but instead uses the
> > LIBXL_HAVE stuff to retain compatibility.
> This means that either:
>  (a) Anyone who makes a backwards-incompatible[1] change to the libxl
>      API must ensure that the corresponding libvirt change is in the
>      libvirt tree and through its osstest push gate before the libxl
>      change is committed;
> or
>  (b) We need to stop gating xen.git pushes on libvirt.
> [1] Of course if you use LIBXL_API_VERSION there is no such thing as a
> backwards-incompatible change.  I mean an change that breaks
> non-VERSION using callers.
> Neither (a) nor (b) is particularly palatable.
> Perhaps we can avoid this by having the libvirt builds #define
> LIBXL_API_VERSION but only some of the time.  As a configure option
> perhaps.  (The configure option would specify not the version to use,
> but rather just that VERSION ought to be defined rather than not.)

We could probably arrange for this ourselves in the harness, using
whatever CFLAGS-appending scheme the libvirt build system supports.

However we currently insist that LIBXL_API_VERSION is one of a known
bunch of values if it is set, so defining it without a specific value
won't currently work.

What is the intended semantics of setting it to nothing? Latest or
oldest interface? I'm not sure how either would help -- it seems to me
we'd need a Do What I Need option ;-) I think specifically it would need
to be the latest version of Xen that libvirt currently knows how to deal
with, which seems tricky to arrange in general.

Another wrinkle is that like SONAMES we only currently bump
LIBXL_API_VERSION once per release cycle (if at all).

> We could supply that option in the osstest push gates for things that
> aren't libvirt.
> Ian.

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