[PATCH 1/9] qapi: New special feature flag "unstable"

[Daniel P. Berrangé]

On Tue, Oct 26, 2021 at 10:22:15AM +0100, Dr. David Alan Gilbert wrote:
> * Kevin Wolf (kwolf at redhat.com) wrote:
> > Am 25.10.2021 um 07:25 hat Markus Armbruster geschrieben:
> > > By convention, names starting with "x-" are experimental.  The parts
> > > of external interfaces so named may be withdrawn or changed
> > > incompatibly in future releases.
> > > 
> > > Drawback: promoting something from experimental to stable involves a
> > > name change.  Client code needs to be updated.
> > > 
> > > Moreover, the convention is not universally observed:
> > > 
> > > * QOM type "input-barrier" has properties "x-origin", "y-origin".
> > >   Looks accidental, but it's ABI since 4.2.
> > > 
> > > * QOM types "memory-backend-file", "memory-backend-memfd",
> > >   "memory-backend-ram", and "memory-backend-epc" have a property
> > >   "x-use-canonical-path-for-ramblock-id" that is documented to be
> > >   stable despite its name.
> > > 
> > > We could document these exceptions, but documentation helps only
> > > humans.  We want to recognize "unstable" in code, like "deprecated".
> > > 
> > > Replace the convention by a new special feature flag "unstable".  It
> > > will be recognized by the QAPI generator, like the existing feature
> > > flag "deprecated", and unlike regular feature flags.
> > > 
> > > This commit updates documentation and prepares tests.  The next commit
> > > updates the QAPI schema.  The remaining patches update the QAPI
> > > generator and wire up -compat policy checking.
> > > 
> > > Signed-off-by: Markus Armbruster <armbru at redhat.com>
> > 
> > Obviously, replacing the old convention gets rid of the old drawbacks,
> > but adds a new one: While using x- makes it very obvious for a human
> > user that this is an unstable feature, a feature flag in the schema will
> > almost certainly go unnoticed in manual use.
> 
> Agreed, I'd keep the x- as well.
> 
> Having said that, the x- represents a few different things (that we
> don't currently distinguish):
>   - experimental
>   - for internal use
>   - for debugging/human use

All of those usage scenarios have the same implication though:

   Command/data format is liable to change in incompatible ways,
   or be deleted, with no prior warning.

I don't think we need to distinguish the use cases - some commands
may belong to two or three of those use cases. All that matters is
that they're considered "unstable" from an API compat POV.

Regards,
Daniel
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