[Libguestfs] Having integration tests "test the future" with QEMU

[Richard W.M. Jones]

On Wed, Mar 04, 2020 at 03:48:34PM +0100, Markus Armbruster wrote:
> I understand libguestfs comes with integration tests.  You might be
> interested in something I'm developing for QEMU 5.0 to permit "testing
> the future".  I'd like to ensure it is actually useful before I
> continue.  Let me know what you think.
> 
> = Motivation =
> 
> When layers above QEMU somehow miss feature deprecation, things continue
> to work until the deprecated interface goes away, at which point we have
> an entirely avoidable regression.
> 
> We've been trying to get better at communicating feature deprecation to
> the layers above QEMU.  An obvious first step was systematic
> documentation.
> 
> However, documentation is all too easy to miss.  Even if you don't miss
> it, you may need to continue using the deprecated feature with old
> versions of QEMU, and that logic needs to be tested.
> 
> I'm working on a way to run QEMU with the deprecated features disabled.
> This permits "testing the future".
> 
> >From my cover letter:
> 
>     This series extends QMP introspection to cover deprecation.
>     Additionally, new option -compat lets you configure what to do when
>     deprecated interfaces get used.  This is intended for testing users of
>     the management interfaces.  It is experimental.
> 
>     -compat deprecated-input=<in-policy> configures what to do when
>     deprecated input is received.  Available policies:
> 
>     * accept: Accept deprecated commands and arguments (default)
>     * reject: Reject them
>     * crash: Crash
> 
>     -compat deprecated-output=<out-policy> configures what to do when
>     deprecated output is sent.  Available output policies:
> 
>     * accept: Emit deprecated command results and events (default)
>     * hide: Suppress them
> 
>     For now, -compat covers only deprecated syntactic aspects of QMP.  We
>     may want to extend it to cover semantic aspects, CLI, and experimental
>     features.

Yes we would add a -compat option when running qemu (only when running
non-production builds of libguestfs) so that it would fail loudly when
we use a deprecated feature.  This is the best way I've heard of so
far to get advanced warning of deprecated features.

Rich.

> If you want to learn more, check out the last three commit messages in
> the series.
> 
> = Patches =
> 
>     [PATCH v2 00/30] Configurable policy for handling deprecated interfaces
>     Message-Id: <20200303163505.32041-1-armbru at redhat.com>
>     https://lists.nongnu.org/archive/html/qemu-devel/2020-03/msg00645.html
> 
> = Git =
> 
>     Tag patchew/20200303163505.32041-1-armbru at redhat.com in repository
>     https://github.com/patchew-project/qemu.git
> 
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